Introduction

The major problems in the ongoing decade are the conservation of energy and search for alternative energy sources. In the coming decades, it is anticipated that conventional energy sources will be depleted because of their ongoing use. To overcome these difficulties, some researchers have focused on substituting renewable energy sources with conventional ones. By contrast, other researchers have focused on using creative approaches to improve the energy performance of devices. Research on the development of energy-efficient devices has resulted in numerous subfields. Among the numerous methods, miniaturization of conventional devices is one. Several industries rely on heat exchangers and use conventional heat exchangers with a high thermal resistance and low efficiency. Therefore, the development of heat exchangers is required to lower thermal resistance, boost convective heat transfer, and enhance efficiency. A heat exchanger is designed to transfer heat from one fluid to another without losing heat to the surrounding environment. Two significant phenomena occur in a heat exchanger: fluid movement in passages and the transfer of energy between the channel walls and fluids. Hence, these devices can be made more efficient by enhancing the performances of these two phenomena. Because the heat transfer rate is dependent on the ratio of the surface area to the volume, smaller channel dimensions result in a higher heat transfer coefficient. Conventional heat exchangers employ tubes with a diameter of ≥ 6 mm; however, microchannels with a size of ≤ 1 mm are the next phase in the development of heat sink and heat exchangers. Owing to their high rate of heat transfer potential and lightweight, as well as their ability to save energy, space, and materials compared with conventional heat exchangers, this field of study is the subject of significant attention and analysis.

Microchannels are defined as narrow flow tubes of size 1 mm or less, that allow for heat transfer surface densities of 10,000 m2 m−3 or higher [1], in contrast to conventional channels with a surface density of 700 m2 m−3. Micro and minichannels differ from normal channels in terms of the channel hydraulic diameter (Dh). The classification methods proposed by Mehendafe et al. [2] and Kandlikar et al. [3], the latter of which is becoming increasingly popular, are typically adopted. Table 1 lists the terminology used by these authors.

Table 1 Channel taxonomy based on hydraulic diameter
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The amount of heat that can flow through the microchannels depends on the surface area which can facilitate heat transfer, which is determined by the hydraulic diameter (Dh) of the channel. By contrast, the flow rate is determined by the cross-sectional area of the channel, which is proportional to D 2h . Therefore, as Dh decreases, the surface-area-to-volume ratio increases, indicating that the surface area of the channel relative to its volume increases. The flow within the microchannels is typically laminar, and the local heat transfer coefficient varies inversely with Dh. Consequently, a decrease in Dh was necessary to enhance the heat transfer coefficient. Tuckerman and Pease [4] suggested an initial study of the MCHS approximately 40 years ago. They assumed that reducing the Dh of the channel would increase heat transfer. This invention has strengthened the electronics industry, which has dealt with difficulties such as high heat dissipation in compact areas. Kandlikar et al. [5] studied the thermohydraulic performance of minichannels and microchannels in their literature. In the initial stages of microchannel development, there were significantly fewer publications. However, microchannels have slowly become the most crucial field of research, as shown in Fig. 1 (references are taken as per Scopus).

Fig. 1
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Publications on microchannel heat exchanger as per Scopus

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Microchannel heat sink (MCHS) and its applications

A heat sink is a device that uses extended surfaces to collect heat from a source and effectively disperses it into the environment effectively as shown in Fig. 2. The capacity of traditional channel heat sinks to dissipate heat is limited even with the use of forced convective cooling. However, as technology has progressed, concerns regarding high heat dissipation in small spaces have increased. Micro- and minichannels have provided appropriate solutions to meet these objectives. Heat sinks are classified as passive, active, liquid-cooled, or phase-change heat sinks depending on the cooling technology used for a specific application as shown in Table 2. The MCHS is particularly effective for removing high heat fluxes and maintaining electronic component temperatures within acceptable limits. Unlike regular channels, microchannels can dissipate power densities up to 1000 W cm−2, whereas conventional channels can only dissipate heat fluxes up to 20 W cm−2. Xiang et al. [6] conducted a series of comparative experiments to assess the heat transfer capabilities of a microchannel heat sink in comparison with a conventional metal solid heat sink. The results indicate that the microchannel heat sink outperforms the conventional metal solid heat sink in terms of heat dissipation performance. This makes them an excellent choice for applications involving high power LEDs. To cool the electronic devices, Xiong et al. [7] proposed the use of microchannels as liquid-cooled thermospreaders linked to gas-cooled heat sinks. In addition to integrated circuits, these channels have been used in many other applications. Micropumps, microturbines, micromotors, microvalves, and microreactors are devices that are based on this technology [8]. Additionally, microscale development has spawned a new field of microfluidics.

Fig. 2
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MCHS design & boundary parameters. (a) Front view (b) detailed view

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Table 2 Classification of heat sinks
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Micro-/minichannel heat exchangers and applications

With respect to heat exchangers, Little et al. [9] were the first to realize the possibilities of micro- and minichannels. The authors suggested that these narrow channels could be used in small-scale Joule–Thomson refrigeration systems. Swift et al. [10] were the first to submit a patent application for the manufacturing process of microchannel heat exchangers (MCHE) with cross-flow. These heat exchangers can be used as energy-efficient alternatives to conventional HVAC (heating, ventilation, and air conditioning) systems. The additional benefit of MCHE in these systems is that they consume less refrigerant and have a greater overall system efficiency. However, owing to their high manufacturing costs and lack of “precise performance prediction tools,” their commercial use is limited. MCHE, on the other hand, has found widespread use in refrigeration [11] and air-conditioning systems[12]. window air conditioning[13], split air conditioners[14], chillers, air-cooled ammonia condensers[15], vapor compression refrigeration systems [16], and heat pumps[17] are just a few examples. A schematic of the various MCHE types is shown in Fig. 3, along with information about their uses, types, construction materials, and fabrication methods.

Fig. 3
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represent the classification of MCHE

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Classification of microchannel design

Numerous attempts have been made to reduce the thermal resistance of MCHS and enhance their heat transfer ability. Steinke and Kandlikar [5] examined the use of passive and active strategies to improve the thermal properties of mini- and microchannels. Active methods for improving the heat transfer rely on additional vibrational, magnetic, and electric flux forces. In contrast to active techniques, which often depend on external factors, passive techniques enhance heat transfer by means such as flow disruption, changing the geometry, and altering the working fluid of the heat sink. In addition, they performed a comprehensive analysis and assessed the feasibility of using these methodologies in novel MCHS applications. This review discusses the essential heat transfer enhancement methods used in microchannel design, as shown in Fig. 4. Microchannel designs can be categorized based on several factors, such as the types of working fluids utilized in microchannels, materials utilized to construct microchannels, techniques employed in microchannels to disrupt flow to improve heat transfer, and microchannel geometries.

Fig. 4
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Heat transfer enhancement methods in microchannel design

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Working fluid

Microchannels in the twentieth century used various working fluids. The most frequently used fluid is water, followed by air and various gases. Other fluids include liquid and gaseous nitrogen, common refrigerants, alcohol, and lubricants. Water has been commonly employed as the primary working fluid in earlier research; however, its limited thermal conductivity in comparison with metals restricts its applicability in microchannels. Nanofluids have received considerable attention over the last two decades because of their exceptional thermophysical properties. Owing to their exceptional thermal properties, nanofluids can be used in various applications such as solar energy devices, aerospace, the automobile industry, electronic devices, medical applications, and manufacturing sector as shown in Fig. 5.

Fig. 5
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Various applications of nanofluids

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Nanofluids

A nanofluid is a mixture of a base fluid and suspended solid particles on a nanometer scale. Suspending tiny solid particles in energy-transfer fluids can significantly improve their thermal conductivity and provide a cost-effective novel technique for improving their thermal characteristics. The concept of “nanofluid” was coined by Choi [18] to describe a mixture of base fluids such as water, glycerine, ethylene glycol, and oil with nanoparticles. This combination resulted in a significant enhancement of the thermal characteristics of the fluid. Standard fluids such as water and ethylene glycol have low thermal conductivities, but adding solid particles to the fluid can enhance the thermal conductivity because solid materials typically have a higher thermal conductivity than fluids. It is possible to employ metallic or nonmetallic solid particles. However, large micro- and macro-sized particles may block flow channels and have poor stability, making their use unjustifiable. Table 3 lists the metallic, metallic oxide, and carbonaceous materials and their corresponding thermal conductivities. Compared to commonly used heat transfer fluids such as water, ethylene glycol, and various oils, these materials exhibit significantly higher thermal conductivities.

Table 3 Thermal conductivity of the various base fluid and base materials of nanofluid
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Researchers have conducted several studies on the use of nanofluid in MCHS, necessitating a review of prior and current research to identify and carry out future research. The most important criterion before using a nanomaterial as a nanofluid in a microchannel is the material selection. The possible factors for selecting a nanomaterial for use in the creation of nanofluids for heat transfer in microchannels are the thermal characteristics, chemical stability, safety, compatibility with the base fluid, affordability, and accessibility [19, 20]. Figure 6 presents an overview of the aspects to be considered when selecting a nanomaterial for nanofluid creation.

Fig. 6
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Classification of nanofluids based on selection criteria

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Advances in nanofluid preparation

  • Synthesis Techniques Different techniques have been developed to create stable nanofluids with controlled particle dispersion. These include chemical processes, physical processes (such as milling or laser ablation), and more contemporary approaches, such as electrochemical synthesis and green synthesis.

  • Characterization Techniques Techniques like transmission electron microscopy (TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD) were used to obtain a clear picture of the size, shape, distribution, and physical characteristics of the nanoparticles. These approaches aid in understanding the behavior and stability of nanofluids.

  • Surface Modification Nanoparticles can be functionalized via surface modification methods, improving their dispersion stability and compatibility with the base fluid. Surfactant coating, polymer grafting, or chemical processes can be used to modify the surfaces.

  • Stability Enhancement Researchers are working to find ways to make nanofluid more stable over time. The most common methods are the use of stabilizing agents, magnetic fields, and external fields such as ultrasound and electric fields to prevent nanoparticle agglomeration and sedimentation.

Effect of nanomaterial on the stability of nanofluids

Nanofluid are suspensions that remain in a stable equilibrium state when subjected to different forces, such as van der Waals, electrostatic, and gravitational forces. The ability to maintain this state is known as the stability of the nanofluid, and this ability of nanofluids is a crucial characteristic to consider when employing in many applications, including MCHS and MCHE. The stability of nanofluids can be influenced by several factors, including the choice of nanomaterial, base fluid properties, nanomaterial concentration, surface chemistry, and temperature [21, 22]. The stability of a nanofluid can be evaluated by various methods, such as:

  • Zeta potential analysis The zeta potential analysis evaluates the stability of nanofluids through the observation of the electrophoretic behavior of the fluid. Higher absolute zeta potential values (either positive or negative) indicate greater repulsion between the particles, leading to enhanced stability.

  • Electron microscopy methods The particle size distribution of the nanofluid was measured using transmission electron microscopy (TEM) or scanning electron microscopy (SEM). SEM & TEM are widely used by researchers to investigate particle shape, size, and aggregation, as shown in Fig. 7.

  • Sedimentation photograph capturing method In this method, the volume of agglomerated nanoparticles in a nanofluid is observed under an external force. This was done by placing a sample of the prepared nanofluid in a transparent glass vial; and the formation of sediments was observed by capturing photographs of the vial at equal intervals of time using a camera. The captured images were then compared to analyze the stability of the nanofluid. Thus, the characterized nanofluid is considered stable when the particle size and dispersity remain constant over time.

  • Centrifugation method In this method, nanofluid sedimentation was performed using a dispersion analyzer centrifuge.

Fig. 7
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SEM and TEM image Ref. [23]

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Achieving long-term stability in nanofluids is crucial for their practical application, as it ensures consistent performance and prevents issues such as clogging in microchannels or sedimentation during storage. Researchers continue to explore methods for enhancing and maintaining the stability of nanofluids to maximize their efficacy in heat transfer and other relevant applications. Various methods can be used to improve the stability are:

  • Surfactant stabilization Surfactants are chemical compounds that can be added to nanofluids to provide electrostatic or steric stabilization. They form a protective layer around the nanoparticles, preventing agglomeration. Surfactants can be anionic, cationic, or nonionic, and their selection depends on the nature of the nanoparticles and base fluid.

  • Surface modification Nanoparticle surfaces can be modified to enhance their stability. Surface functionalization involves attaching molecules or polymers to the nanoparticle surface, which can provide repulsive forces between the particles, reducing agglomeration. Functionalization can be achieved through various methods, such as silane coupling agents, polymer coating, or ligand exchange.

  • pH adjustment Changing the pH of the nanofluid can affect its stability by altering the surface charge of the nanoparticles. The choice of pH adjustment depends on the nanoparticle material and surface chemistry. By adjusting the pH, repulsive forces can be enhanced, hindering agglomeration.

  • Ultrasonication Ultrasonication involves subjecting a nanofluid to high-frequency sound waves, typically in the range of 20–100 kHz. The acoustic cavitation generated during ultrasonication helps disperse and deagglomerate nanoparticles. This process breaks down the large agglomerates into smaller particles, enhancing their stability.

  • Mechanical stirring Mechanical stirring or mixing with the nanofluid can help distribute nanoparticles evenly throughout the base fluid. Agitation disrupts agglomerates and promotes the uniform dispersion of nanoparticles. The intensity and duration of stirring should be optimized to prevent excessive energy input, which can lead to excessive heating.

Characterization of nanofluids

The process of determining and understanding the properties, characteristics, and behavior is referred to as characterization. The characterization of nanofluids involves measuring various physical and chemical properties such as thermal conductivity, viscosity, density, surface tension, composition, surface morphology, and stability. It is crucial to understand these properties as they affect the thermo-fluidic behavior of nanofluids in microchannels. Some common methods used for nanofluid characterization are as follows:

  1. 1.

    Scanning electron microscopy (SEM)—is used to study the distribution of materials on a surface.

  2. 2.

    SEM & EDS—is used to measure the elemental composition in a sample.

  3. 3.

    Transmission electron microscopy (TEM)—which is used to visualize the smallest structures in matter, produces high-resolution images.

  4. 4.

    FTIR analysis—was used to identify unknown compounds, determine the purity of the sample, and monitor the chemical reactions in real-time.

  5. 5.

    Transient hot wire—determines the thermal conductivity

  6. 6.

    Laser flash method—determines the thermal conductivity

  7. 7.

    Rheometer—was used to study fluid properties such as dynamic viscosity, flow behavior (Newtonian/non-Newtonian).

  8. 8.

    DSC—To measure the specific heat capacity of the nanofluid as a function of temperature.

  9. 9.

    Oscillation method—is used to measure the density of nanofluid.

  10. 10.

    Pendant drop method—is used to study the surface tension of nanofluids, which is a property of the interface between the fluid and surrounding air or another medium.

Therefore, characterization of nanofluids is essential for understanding their behavior. Table 4 presents various characterizations of the nanofluids and instruments used for their measurements.

Table 4 Characterizations techniques of nanofluids
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Base working fluid used in microchannel

Water, oils, glycols, and glycol–water combinations are commonly used as base fluids to create nanofluids. Figure 8 shows that among the different base fluids, water-based nanofluids have received the most attention from researchers, likely because of their lower cost and higher thermal conductivity. Owing to their potential as high heat transfer fluids in microchannels, glycol-based nanofluids have also attracted attention.

Fig. 8
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Base fluid use in microchannels for nanofluids formation

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Nanomaterial used for nanofluid formation

The nanoparticles commonly used for nanofluid creation are metals, metal oxides, carbonaceous materials, and hybrid nanoparticles. Figure 9 shows the distribution of publications on nanofluids employing various nanomaterials in microchannels. Although metal oxides exhibit lower thermal conductivities than metals, they appear to be the most suitable option for creating nanofluids. This is because metal oxides are chemically stable and are resistant to oxidation. Moreover, certain metal oxides have a lower density than their corresponding metals, which could lead to fewer particle-settling problems in nanofluid formulations. Owing to its high thermal conductivity and low density, alumina is widely used as a metal oxide for nanofluid formation. Based on the nanomaterial used for nanofluid creation in the present study. The authors reviewed all the journals on nanofluids and categorized them into four main types.

Fig. 9
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Nanofluids used in various microchannel geometries

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(a) Nanofluids used in various microchannel geometries suspended with metallic nanoparticles.

Metallic nanofluids (e.g., Ag, Cu, Fe, Au, etc.) generate higher effective thermal conductivity values. Therefore, various researchers have focused on using metallic nanomaterial for nanofluid creation, as presented in Table 5. Mghari et al. [24] conducted a numerical analysis of the heat transfer in a single horizontal smooth square tube. According to the findings of the research, either elevating the mass flux from 80 to 110 kg m−2-s or increasing the concentration of copper nanoparticles by 5% has the potential to increase the heat transfer coefficient by 20%.

Table 5 Effect of metallic nanomaterial in MCHS and MCHE on various geometry
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Abbassi and Aghanajafi [25] examined the incorporation of Cu nanoparticles into MCHS. The results of their research showed that employing a nanofluid significantly improved the heat transfer in the MCHS, and this advantage became even greater as the Re and particle concentration increased. Additionally, they demonstrated that if the flow regime transitions into the turbulent domain, the enhancement in heat transfer could be greatly amplified.

Diao et al. [26] analyzed the performance of a microchannel surface in a vapor chamber using Cu-R141b as the working fluid. The authors recommended that with a volume concentration of 0.001–0.01 percent, R141b-based Cu nanofluids may enhance the thermal performance of microchannel surfaces, especially at lower operating pressures.

Boudouh et al. [27] analyzed the convective boiling heat transfer in a vertical rectangular channel using copper–water nanofluids and deionized water. They found that in comparison with the temperature and pressure of the base fluid, the Cu–water nanofluid exhibited a lower surface temperature and local heat flux at the same flow rate.

Using porous media and the least square method, Hatami and Ganji [28] investigated the thermal performance of a MCHS employing a nanofluid composed of copper and water. They observed that raising the volume fraction of nanoparticles encouraged Brownian motion and facilitated greater heat transfer.

Simsek et al. [29] employed suspensions of silver nanowires to enhance thermal performance in a MCHS for the first time. According to the outcomes, the utilization of silver nanowire suspensions in MCHS could lead to a rise in heat transfer coefficient of up to 56%, while causing only a minor increment in pumping power.

Tehrani et al. [30] conducted a numerical analysis on flow and heat transfer of an Ag–water nanofluid under varying heat flux, while considering different volume concentrations and Hartmann numbers. They suggested that because the Re of nanofluids inside microchannels is often low, “utilizing a higher volume fraction to enhance the Nusselt number (Nu) would be counterproductive in micro and nanoflows.” Shahsavar et al. [31] conducted a numerical investigation to enhance the performance of a liquid-cooled heat sink. In this study, a water/silver nanofluid was used as the coolant. The researchers focused on two operational variables: nanoparticle concentration and Reynolds number. They also examined the impact of a structural parameter, specifically rifling the inlet tube of the coolant. The results revealed that the overall hydrothermal performance of the heat sink with a rifled inlet was 1.073–1.541 times higher compared to the heat sink with a plain inlet.

Table 5 indicates that copper and silver are the most commonly used nanomaterials for preparing working fluids in research, likely due to their high thermal conductivity compared to other metallic materials. Based on the research on metallic nanofluids, it can be concluded that increasing particle concentration results in nearly a 50% improvement in heat transfer in the microchannel in most cases. However, this may also cause an increase in pressure drop.

(b) Nanofluids used in various microchannel geometries suspended with metallic oxides nanoparticles

Despite their poorer thermal conductivities than those of metals, metal oxides have been frequently employed in the formulation of nanofluids. Metal oxides (e.g., Al2O3, CuO, TiO2, SiO2, etc.) have several benefits over metal nanoparticles, including oxidation resistance and chemical stability. Furthermore, some metal oxides have lower densities than metals, resulting in less sedimentation when they are used in nanofluid formulations. This concept is a popular research topic because it is a viable solution for satisfying the temperature management requirements of the MCHS and MCHE. Tables 6–11 represent the literature on metallic oxide nanoparticles for the formation of nanofluids used in various microchannel geometries.

Table 6 Numerical, experimental, and analytical analysis of the effect of Al2O3 and water base nanofluid in a rectangular cross section microchannel
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Table 7 Numerical and experimental analysis on the effect of Al2O3 and water base nanofluid in a circular microchannel
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Table 8 Numerical, experimental, and analytical analysis on the effect of Al2O3 and water base nanofluid in a parallel plate MCHS & MCHE
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Table 9 Numerical and experimental investigation of the effect of CuO nanofluid in a rectangular microchannel
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Table 10 Effect of TiO-based nanofluid in a rectangular cross section microchannel
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Table 11 Investigation on the effect of various nanofluids on the thermal performance of various MCHS and MCHE geometry
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The study conducted by Bhattacharya et al. [38] on the heat transfer properties of an Al2O3–H2O nanofluid flowing through a rectangular MCHS showed that an increase in nanoparticle concentration resulted in a more pronounced improvement in MCHS performance with the use of the nanofluid.

In a study conducted by Ali et al. [39], the thermal performance of a circular MCHS was evaluated in the range of 100 ≤ Re ≤ 350, using swirl flow and nanofluid. The coolant was a water–Al2O3 nanofluid with nanoparticle volume fractions ranging from 0 to 3%. The results indicated that the MCHS with swirl flow and maximum nanoparticle volume fraction had the lowest thermal resistance and contact temperature.

Meanwhile, Heidarshenas et al. [40] utilized an ionic liquid–Al2O3 nanofluid to enhance heat transfer in a cylindrical MCHS. Their findings showed that the use of ionic liquid–Al2O3 nanofluid increased the Nusselt number by up to 40%.

The thermal performance of a trapezoidal microchannel was studied by Li and Kleinstreuer [41] using pure water and CuO–water with volume fractions of 1% and 4%, respectively. They found that with a modest increase in pumping power, the nanofluids enhanced the thermal performance of the microchannel.

Yang et al. [42] investigated a trapezoidal MCHS utilizing a CuO–water nanofluid as the cooling fluid. The results indicated that the two-phase model was more accurate than the single-phase model.

Duangthongsuk and Wongwises [43] conducted an experimental investigation of the heat transfer and pressure drop in a zigzag MCHS. The study found that there was a 2–6% improvement in heat transfer performance between the cross-cutting zigzag heat sink (CCZ-HS) and continuous zigzag heat sink (CZ-HS). Additionally, the particle concentration had a significant impact on the heat transfer, but it did not affect the pressure drop.

Martinez et al. [44] investigated the thermal characterization and stability of water–ZnO nanofluids in a rectangular MCHS with Re ranging from 200 to 1200. The numerical findings revealed that using nanofluids promotes heat transfer at low Re, with the greatest increase in heat transfer coefficient (42.33%). At a concentration of 1 mass%, there was also a decrease in the base temperature of the microchannel, which was more noticeable at a low Re.

In addition, several researchers have conducted comparative investigations on various metallic oxide nanoparticles employed in microchannels. Mohammed et al. [45] investigated the influence of Al2O3, Ag, CuO, diamond, SiO2, and TiO2 nanofluids on the cooling performance of triangular MCHS. They found that the addition of nanoparticles to the coolant resulted in a decrease in the thermal resistance of the triangular MCHS, with diamond-H2O nanofluid showing the greatest improvement at a nanoparticle concentration of 1%.

Salman et al. [46] numerically analyzed the thermal performance of microtubes. Various types of nanofluids, including Al2O3, CuO, SiO2, and ZnO, were utilized in the study. The nanoparticles had sizes of 25, 45, 65, and 80 nm, and the volume fractions ranged from 1 to 4%. Ethylene glycol was chosen as the base fluid for the nanofluids. According to their findings, the SiO2–ethylene glycol nanofluid had the highest Nu. The Nu increased with the volume fraction in all circumstances but decreased as the diameter of the nanoparticles increased.

Table 6 displays a comparison of numerical, experimental, and analytical analyses investigating the impact of Al2O3 and water-based nanofluids on rectangular microchannels. Based on the table's results and study type, it can be inferred that more than half of the research on Al2O3 nanomaterial for nanofluid production is based on numerical analysis. The table also indicates that the use of smaller diameter nanoparticles and increasing the volume percentage of nanoparticles enhances heat transfer. Moreover, higher heat transfer coefficients are observed in flow regimes with higher Re.

(c) Hybrid nanofluids used in various microchannel geometries

Hybrid nanofluids are advanced nanofluids that are created by mixing two distinct nanoparticles in a base fluid. Preparation method of hybrid nanofluid is shown in Fig. 10. Hybrid nanofluids have superior thermal characteristics compared with base fluids and nanofluids. These fluids exhibit better characteristics than ordinary fluids, such as

  • Offer superior thermophysical properties in comparison with mono-nanofluids.

  • High dispersion stability and Brownian motion of particles.

  • A remarkable increase in thermal conductivity with varying particle concentrations.

  • Reduction in pumping power compared with conventional fluid power.

  • High surface area and high heat transfer between fluid and particles.

  • Increased control over thermodynamic and transport properties.

Fig. 10
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Preparation of hybrid nanofluid

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Turcu et al. [133] were the first to use polypyrrole–carbon nanotubes (CNTs) and multiwall carbon nanotubes (MWCNTs) on magnetic Fe3O4 hybrid nanoparticles to create hybrid nanofluids. Jha et al. [134] combined silver nanoparticles with multiwall carbon nanotubes (MWCNTs) to create hybrid nanofluids. The results show the improved thermal properties of the hybrid nanofluids. Suresh et al. [135] created hybrid nanofluids by mixing Al2O3 and Cu and found that there was an enhancement in thermal conductivity of 12.11% with a volume concentration of 2%. To improve the thermal performance of MCHE, an experimental investigation was conducted by Yushuang Huang et al. [136]. They incorporate β-cyclodextrin-ZrO2 as a nanoparticles and ethylene glycol as the base fluid for construction of nanofluids. They found that fabrication of nanofluids utilizing β-cyclodextrin-modified ZrO2 nanoparticles holds significant potential for enhancing heat transfer in microelectronic microchannels. Table 12 shows numerous studies in MCHS and MCHE that use hybrid nanofluids as their working fluid. Table 12 shows that nearly 50% of hybrid nanofluid research relies on numerical analysis. The most commonly used nanomaterials for creating hybrid nanofluids are alumina and copper oxide. Additionally, rectangular cross sections are frequently utilized for constructing microchannels in comparison with other geometries. Based on the outcomes of the above literature survey, it can be concluded that increasing particle concentration and Re leads to improved heat transfer in the microchannel, although this may also result in higher pumping power requirements. Mansouri et al. [137] conducted an experiment to assess the convective heat transfer of a hybrid nanofluid consisting of graphene oxide and gold in a CPU. They varied the concentrations of graphene oxide and gold (ranging from 0.0044 to 0.0114% by mass) as well as the Reynolds number (ranging from 676 to 2185) to optimize the overall performance of the device. The experimental findings indicated that the nanofluid composed of graphene oxide and water, decreased the CPU's surface temperature by 10.6% and 16.2%, respectively, compared to using DI water alone.

Table 12 Hybrid nanofluids used in various microchannel geometries
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Srivastava and Sahoo [138]conducted an experiment to examine the impact of nanoparticles with different shapes on the thermo–hydro performance of a microchannel heat sink (MCHS). The experiment utilized water as the reference fluid. The results showed that the hybrid nanofluid composed of dissimilar-shaped nanoparticles demonstrated improved performance in terms of heat transfer coefficient, Nusselt number, as well as inlet and outlet exergy.

(d) Nanofluids used in various microchannel geometries suspended with carbon nanoparticles

Studies have investigated the cooling performance of MCHS using carbon nanotubes (CNT). Ebrahimi et al. [155] investigated the cooling performance of an MCHS using carbon nanotubes (CNT). It is found that increasing the nanolayer thickness of multiwalled carbon nanotubes (MWCNTs) decreased the MCHS temperature gradient.

Arabpour et al. [156] performed a numerical simulation of an MCHS using kerosene nanofluid/MWCNTs and found that Nu was strongly influenced by the application of an oscillating heat flux at Re values of 10 and 100.

Studying the impacts of porous media on fluid flow and heat transfer in a microchannel filled with MWCNT/Oil Nanofluid. Nojoomizadeh and Karimipour [157] studied the impacts of porous media on fluid flow and heat transfer in a microchannel filled with MWCNT/oil nanofluid and found that a higher Re resulted in a decrease in the heat transfer time between the nanofluids and walls, resulting in a higher local Nusselt number. Table 13 lists various carbonaceous nanofluids used in MCHE and MCHS.

Table 13 Nanofluids used in various MCHS & MCHE suspended with carbon nanoparticles
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Table 13 indicates that over 50% of research on carbon nanomaterial suspended nanofluids relies on numerical analysis. Rectangular cross sections are also commonly used in constructing microchannels, as noted in the table. According to the literature survey, the use of carbon nanoparticles in nanofluid creation leads to a decrease in thermal radiation and improved thermal performance at high temperatures. Furthermore, the enhancement in heat transfer is more prominent at higher Re.

Flow disruption in MCHS

To enhance heat transfer performance, flow disruption is an effective approach that increases mixing and heat transfer by inducing flow instabilities. Turbulent flow is a prevalent method for flow disruption, but in many cases, low velocities or small hydraulic diameters prevent the flow from reaching the crucial Reynolds number. To overcome this, efforts are being made to introduce geometrical alterations to the channel sidewalls, such as grooves, ribbed channels, wavy channels, dimples, and fins, which act as periodic disturbance promoters. These promoters induce self-sustaining oscillations, leading to flow instability and improved mixing. Many studies have investigated the use of different disturbance promoters to enhance flow mixing in conventional channels.

Flow disruption in MCHS introduces various types of wavy channels

Rectangular straight channels are a common choice for MCHS due to their low pumping power and laminar flow resulting in almost straight streamlines. However, these channels have several disadvantages. Firstly, the heat transport is poor due to insufficient fluid mixing caused by the linear streamlines. Secondly, the flow and heat transfer boundary layers thicken throughout the flow path because of the single-direction flow characteristics, leading to a larger temperature difference across the channels, especially at high heat fluxes. Thirdly, these channels are not effective in removing local high heat fluxes in microelectronic devices with hot spots. To overcome these challenges, several theoretical, experimental, and computational methods have been used to enhance the flow and heat transfer performance of MCHS by introducing different channel designs, such as grooves, ribbed channels, wavy channels, dimples, and fins, that promote flow instability and mixing.

When a fluid flows through a curved channel, it experiences not only primary motion but also a secondary motion in the plane of the cross section, which is referred to as the Dean vortex. The Dean vortices arise from centrifugal forces that act on the fluid as it flows through the curved channel. The presence of these vortices induces flow components to stretch and fold, which enhances the fluid mixing and heat transfer. The Dean vortices have been shown to be particularly effective at promoting mixing and heat transfer in microchannels, where the low Reynolds numbers limit the potential for turbulent flow. Consequently, many researchers have investigated the use of curved channels or modified channel geometries that induce Dean vortices as a means of improving the flow and heat transfer performance of microchannels. Sui et al. [165] proposed a new design for an MCHS, which was influenced by Dean vortices. Instead of using straight channels, they suggested using wavy channels that showed superior heat transfer performance when compared to straight microchannels of equivalent cross section. However, the use of wavy channels resulted in a pressure drop penalty, which could be offset by enhancing the heat transfer capability.

In addition, Sui et al. [166] performed experiments to compare the performance of MCHS with wavy channels and those with straight channels. They calculated both the overall Nusselt number and the friction factor to confirm the superior performance of the wavy design. Wavy MCHS with rectangular cross sections and amplitudes ranging from 125 to 500 m were investigated numerically by Mohammed et al. [167]. According to their findings, the friction factor and wall shear stress increased in direct proportion to microchannel amplitude.

Liquid-cooling parallel-flow and counter-flow double-layer wavy MCHS with Re of 50–110 were studied by Xie et al. [168]. The findings show that the counter-flow double-layer wavy MCHS operates better at higher flow rates and provides a more consistent temperature rise. Using nanofluids as coolants, Sakanova et al. [169] investigated for the first time how the wavy walls of an MCHS promote heat transfer. The findings revealed that the wavy MCHS outperformed a regular rectangular MCHS. There is a 5.34–24.1% increase in heat transfer and a pressure drop of around 150–421.7%. Chiam et al. [170] conducted a numerical investigation to study fluid flow in microchannels with secondary branches in the Re range of 50–200. The numerical results show that the heat transfer performance of the system can be improved by adding extra branches. Pandey et al. [171] conducted a numerical analysis to assess the performance of a straight MCHE with wavy channels. Their findings indicate that wavy channels outperform straight channels in terms of thermal performance. On the other hand, when considering pumping power, straight channels exhibit better characteristics. Table 14 represent various studies on wavy channels, along with the most important conclusions from each.

Table 14 Investigation of wavy channels used in microchannels
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Flow disruption in microchannels by introducing various types of ribs

Several experimental and computational studies have been carried out to assess the impact of ribs on the thermohydraulic efficiency of MCHS & MCHE. Ribs are implemented to enhance heat transfer by disrupting the thermal and hydraulic boundary layers. These ribs are frequently referred to as turbulators or roughness components, respectively. Ribs can also promote flow mixing by producing vortices and causing chaos during advection. However, the heat transfer boost provided by the ribs was accompanied by a large pressure loss penalty. Consequently, optimizing the ribs was necessary to ensure a lower pressure drop. Table 15 presents the impact of ribs on the hydrothermal performance of MCHS and MCHE.

Table 15 Flow disturbance of MCHS & MCHE using ribs
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In an experimental and numerical analyses, Wang et al. [177] examined the heat transfer performance of an MCHS with bidirectional ribs (BR) at Reynolds numbers from 100 to 1000. Their findings indicated that the Nusselt number (Nu) of the BR microchannel could reach up to 1.42 times higher than that of a microchannel featuring vertical or spanwise ribs.

Interrupted MCHS in transverse microchambers with various rib forms have been studied by Chai et al. [178]. The transverse microchambers in the interrupted MCHS exhibit a decrease in thermal resistance of 4–31% and a reduction in entropy production rates of 4–26% compared to those of the straight MCHS.

With fan-shaped ribs on the sidewalls, Chai et al. [179] numerically investigated the thermal and hydraulic parameters of laminar flow in an MCHS. The findings indicated that the heat transfer properties were significantly influenced by the height and spacing of the fan-shaped ribs, while the width of the ribs had a lesser impact.

Desrues et al. [180] examined heat transfer and pressure loss in 3D channels featuring alternating opposing ribs. The study found that while the pressure drops increased consistently with Re, heat transfer only improved after Re exceeded a critical value.

Chai et al. [181] utilized numerical simulations to evaluate the thermohydraulic performance of an MCHS that incorporated triangular ribs on the sidewalls. The implementation of these ribs successfully restricted the temperature increase of the heat sink base and prevented a decrease in the local heat transfer coefficient in the flow direction.

The effects of rib shape and fillet radius on the thermal-hydrodynamic performance of an MCHS were investigated numerically by Derakhshanpour et al. [182]. The study revealed that increasing the rib corner curvature resulted in a higher heat transfer coefficient, but also led to an increase in the pressure drop. Using a water–TiO2 nanofluid, Gravndyan et al. [183] conducted investigation on the thermal performance of a rectangular MCHS and studied the impact of rib aspect ratio. The study found that the friction factor was independent of the rib aspect ratio, but dependent on the volume percentage of nanoparticles in the nanofluid.

The use of ribs in microchannels can cause significant pressure loss due to high-flow disruptions, so the optimization of rib geometry is necessary. In order to improve heat transfer while maintaining a minimum pressure drop, grooves or cavities are used to restrict flow and redevelop the thermal boundary layer. Cavities can enhance heat transfer by encouraging mixing between fluid layers near the wall and core through the action of jets and flow throttling. The idea of the ribs and cavities working together has also been used as an effective way to improve heat transfer. Ghani et al. [184] investigated the thermohydraulic characteristics of a microchannel with both sinusoidal cavities and rectangular ribs (MC-SCRR). The results showed that MC-SCRR performed better in terms of thermal performance compared to both rectangular microchannel ribs and sinusoidal microchannel cavities alone. This suggests that the combination of ribs and cavities is an effective way to enhance heat transfer. Xia et al. [185] conducted a numerical analysis to investigate thermal enhancement in an MCHS by using fan-shaped reentrant cavities & internal ribs. They found that the Nu for the fan-shaped reentrant cavities was 1.3–3 times higher than that for a rectangular microchannel. The heat transfer characteristics of an MCHS with triangular cavities and rectangular ribs were numerically studied by Li et al. [186]. Their findings revealed a significant improvement in heat transfer for microchannels equipped with triangular and rectangular cavities.

On the other hand, the combination of ribs and grooves was found to be effective way in enhancing heat transfer and reducing pressure drop. This is due to the high-flow disruption capability of ribs and the lower pressure drop of grooves. To validate this Zhu et al. [187] investigated the thermal and hydraulic performance of an MCHS with rectangular grooves and ribs and found that the combination of grooves and ribs was effective in enhancing heat transfer and reducing pressure drop due to the high-flow disruption capability of ribs and the lower pressure drop of grooves. They concluded that the combination of grooves and ribs can significantly increase the overall performance. Further, the thermal performance of an MCHS with ribs and grooves was experimentally and numerically investigated by Wang et al. [188] for chip cooling applications. Their findings showed that rib-grooved microchannels had a Nu 1.11–1.55 times higher than smooth microchannels.

In addition, the use of porous media is a viable solution to mitigate the high pressure drop associated with the implementation of ribs in MCHS. Porous media are frequently employed in heat transfer applications due to their large surface area that comes in contact with the fluid, which enhances the heat transfer performance. The thermohydraulic characteristics of microchannels with solid and porous ribs have been numerically examined by Li et al. [189]. They found that the thermal performance of microchannels with ribs was better than that without ribs. However, the pressure drops and friction factors were higher for microchannels with solid ribs compared to those with porous ribs. By replacing solid ribs with porous ones in center, symmetrical, and staggered rib arrangements, the pressure drops were reduced by 67%, 57%, and 12%, respectively. Wang et al. [190] conducted a numerical investigation to examine how the combined use of porous/solid fins and nanoparticles affects the cooling efficiency of microchannel heat sinks (MCHS). The results showed that, under constant Reynolds number conditions, heat sinks composed of metallic foam exhibited superior cooling performance. These heat sinks were capable of significantly reducing the surface temperature of the heat sink compared to other configurations. In another research to improve the hydrothermal property of MCHS, an investigation was conducted by Li et al. [191]. They incorporate embedded module ribs and pin with fins. Their outcomes indicate that at a Reynolds number of 458, the heat transfer efficiency increased by 63.91%, resulting in a 67.65% enhancement in temperature uniformity. Furthermore, the pressure drops experienced a modest rise of merely 10.24%.

Polat et al. [192] conducted a numerical investigation to examine the heat transfer performance of a microchannel heat sink (MCHS) equipped with micro pin-fins, focusing on steady laminar flow conditions. Using ANSYS Fluent, simulations were carried out to analyze the heat and fluid flow within MCHSs featuring circular, square, and diamond-shaped micro pin-fins of equal hydraulic diameter. The arrangement of micro pin-fins in each MCHS configuration was optimized for performance using a multi-objective genetic algorithm known as Non-dominated Sorting Genetic Algorithm (NSGA-II). The results of the optimization process revealed that the square-shaped pin–fin configuration exhibited unfavorable performance compared to the other pin–fin shapes. However, among all the configurations tested, the diamond-shaped pin-fins demonstrated a significant improvement in heat transfer while maintaining an acceptable pressure drop ratio. Therefore, the diamond-shaped pin-fins were identified as the most effective choice for enhancing heat transfer within the MCHS.

Zhang and Du [193] proposed a method to enhance the heat dissipation of a MCHS by introducing fins that generate a secondary flow. In their study, they compared the performance of the finned MCHS with a straight microchannel heat sink. The experiments were conducted under a mass flow rate of 3.5 gs−1. The results indicated that the finned MCHS achieved improved heat dissipation compared to the straight microchannel heat sink. Specifically, the maximum temperature of the finned MCHS was reduced by 3.04 K (equivalent to a 6.67% decrease) compared to the straight microchannel heat sink. Additionally, the average temperature of the finned MCHS was reduced by 2.86 K (equivalent to a 6.75% decrease) compared to the straight microchannel heat sink.

Channel material

The literature reviewed shows that the flow channels used in various experimental and numerical investigation were made of various materials such as aluminum (Al), copper (Cu), silicon (Si), stainless steel, & various other metals [186,187,188]. The use of Si is preferred in electronic devices due to its compatibility with micromachining and microfabrication techniques used in semiconductor manufacturing processes, making it well suited for the electronics industry [204]. Generally, water is used as a working fluid which causes corrosion in metallic heat exchangers, owing to thermal property limitations of metallic heat exchangers, these materials become ineffective under extremely high temperatures conditions. To overcome these limitations nowadays numerous researchers have focused on ceramic microchannels.

Ceramic microchannel heat exchangers/heat sink

Metals are commonly used in heat exchangers; however, metallurgical issues or significant deformations are extremely troublesome at high heat-flux settings [190,191,192,193]. Furthermore, some fluids are corrosive, and the use of metals increases the production costs [194,195,196,197]. Ceramics are one of the finest options for high-temperature applications according to various prior studies [213, 214]. These materials exhibit excellent oxidation, sulfidation, and corrosion resistance. Ceramics generally have low thermal conductivities (e.g., zirconia [215], silicon nitride (SiC) [216]). However, a group of ceramics, such as titanium diboride (TiB2) [217], zirconium diboride (ZrB2), hafnium diboride (HfB2) [218], aluminum nitride (AlN) [219], and beryllium oxide (BeO2) [220], have better thermal conductivity.

Alm et al. [221] studied the performance in cross-flow and counter-flow regimes of an Al2O3 MCHE using demineralized water to measure the heat transfer rate. The efficiency varied from 0.10 to 0.22 in cross-flow heat exchangers at mass flow rates of 20–120 kg h−1, while the heat transfer coefficient was 22 kW m−2 K−1. A miniaturized heat exchanger composed of SiC was experimentally investigated by Fend et al. [222] at elevated temperatures. Two SiC heat exchangers with various wall thicknesses and widths were tested and compared at temperatures up to 950 °C. The sample with a larger channel width worked better in these studies because of its lower wall thickness. Furthermore, these heat exchangers were observed to have a large heat transfer surface area to volume ratio of 995 m3 m−2 and a high efficacy of up to 65%.

Villanueva et al. [223] explored the use of ceramics in plate heat exchangers with fins. The findings reveal that the development of vortices in the frontal area of the fins leads to an increase in the heat transfer. Lewinsohn [224] investigated a small SiC ceramic plate MCHE, reported the efficacy, temperature variations, change in pressure, and stresses developed in hot & cold fluid plates. The findings revealed that a microturbine power cycle made of ceramics may achieve greater power cycle efficiency.

Carman et al. [225] examined the impact of an MCHE composed of silicon carbon nitride (SiCN) ceramic material on a microturbine. The study suggests that design optimization can lead to an improvement in the thermal efficiency of the cycle by approximately 9%. Ponyavin et al. [226] investigated small heat exchangers made of SiC employed for hydrogen generation. The high thermal conductivity of silicon carbide helped to eliminate temperature gradients between the channel walls and maintain low stresses, according to the results. Monteiro and de Mello [227] conducted a study on the thermal efficiency and pressure drop of plate heat exchanger with fins made of alumina. Their findings showed that increasing the mass flow rate resulted in a reduction in the pressure drop, which in turn reduced the device's efficiency. Nekahi et al. [228] studied the viability of TiB2–SiC composites doped with a two mass% carbon fiber in an MCHE. They found that the TiB2–SiC and TiB2–SiC–Cf composites improved heat transfer by 15.5 percent as compared to Al2O3. Fattahi et al. [229] conducted a study to investigate the effect of using aluminum nitride (AlN) for constructing MCHE on its heat transfer performance. By replacing Al2O3 with AlN, the heat transfer and efficiency of the MCHE were improved by 59% and 26%, respectively. Vajdi et al. [230] conducted a numerical study on an MCHS made of ZrB2 to analyze pressure drop and heat transfer. The study found that the high thermal conductivity of the ceramics resulted in an effective heat transfer rate. Table 16 summarizes numerous literature studies that use ceramic as a microchannel material.

Table 16 Investigation on various ceramic material used for construction of MCHS & MCHE
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Channel geometry

The flow in the MCHS was predominantly laminar due to the small channel size. Traditional MCHS experiences a growing thermal boundary layer which causes hotter fluids to be collected on the channel wall, while cooler fluids circulate through the core channel. Therefore, early research efforts focused on improving the thermal efficiency of a typical straight rectangular MCHS by modifying the channel length, aspect ratio, and wall thickness. While some researchers have attempted to disrupt the MCHS boundary layer, others have altered the microchannel cross section to enhance its performance, such as circular, triangular, square, and trapezoidal shapes. However, rectangular cross sections were used in most studies, as seen in Tables 5–13, possibly because of their simpler manufacturing process compared to other shapes.

Yogesh and Prajapati [236] conducted a numerical investigation to analyze how altering the fin height of a rectangular MCHS affects its thermo-hydro properties. They explored seven different heat sink designs in the Re range of 100–400 and the heat-flux range of 100–500 kW m−2. Their findings indicate that raising the height of fin enhances the heat transfer from the MCHS, but only until the fin height reaches 0.8 mm. Longer fins obstruct the flow channel more, resulting in increased pressure loss. To see the effect of various crystal structure on the thermomechanical performance of a MCHE, a numerical experiment was conducted by Wu et al. [237]. They employed multi-physics mathematical model using thermal-fluid–structure interaction (TFSI) to investigate the thermomechanical performance of a MCHE. They found that in comparison with simple cubic crystal structure, the thermal–hydraulic performance factors of FCC and BCC corrugated straight plate MCHE were 2.20 and 1.70.

Through numerical simulations, Kumar and Kumar [238] studied the thermo-hydro properties of a rectangular MCHS featuring arc grooves on its surface. The inclusion of grooves has been observed to generate a pseudo-secondary flow, which enhances heat transfer but results in a higher Poiseuille number. Ma et al. [239] presented their findings on thermal characteristics of laminar regime in 3D microchannels having rectangular cross section, where the aspect ratios ranged from 0.1 to 1. They discovered that as the aspect ratio of rectangular microchannels increases and the Reynolds number decreases, the dimensionless thermal entry length also increases uniformly. Lan et al. [240] studied the impact of truncated and offset pin-fins on the thermal behavior and entropy production in a rectangular MCHS that had different flow characteristics. They discovered that increasing the height of the pin-fins generally resulted in higher heat transfer but also increased fluid resistance. Kose et al. [241] conducted a numerical investigation to assess the heat transfer performance of microchannel heat sinks (MCHS) with different shapes. They compared rectangular, triangular, and trapezoidal microchannels under identical design constraints in order to determine efficient MCHS designs. The study revealed that the rectangular microchannel configuration exhibited the highest thermal and hydrodynamic performance among the three shapes. In terms of heat transfer efficiency, the rectangular microchannel required 17% and 40% less pumping power than the trapezoidal and triangular microchannels, respectively, while achieving the same amount of heat transfer. Fani et al. [242] examined how the size of particles affects the thermo-fluidic properties of nanofluids in a MCHS having trapezoidal cross section. The experiment employed CuO nanoparticles with sizes ranging from 100 to 200 nm and volume concentrations of 1% to 4%, using water as the base fluid. The findings revealed that enlarging the nanoparticles led to a decrease in heat transfer and an increase in pressure drop. Furthermore, the base fluid had a more significant effect on the thermal performance than the nanoparticles.

Weilin et al. [243] investigated the fluidic behavior of water in microchannels constructed of silicon having trapezoidal shapes with channel diameters in the range 51–169 μm. They proposed that the equation of motion should include viscosity, specifically eddy viscosity during turbulent flow. Song et al. [244] conducted numerical simulations to investigate the thermo-fluidic properties of trapezoidal microchannel heat sinks (TMCHS). They analyzed six different configurations of TMCHS and compared their thermo-fluidic characteristics. The results showed that among the six designs, the TMCHS with the reverse channel counter-flow large inlet (RCCFLI) design had the most efficient thermal performance. Ahmed et al. [245] studied how different geometrical parameters affect the laminar flow and heat transfer performance in a grooved MCHS. They found that the trapezoidal groove MCHS had the most efficient thermal design among various grooved MCHS, with an increase in the Nu by 51.59 percent and an increase in friction factor by 2.35 percent.

Mohammed et al. [246] used numerical simulations to assess the impact of incorporating nanofluids in a parallel-flow MCHE having square cross section. Their results demonstrated that the use of nanofluids can enhance the thermal characteristics and performance of the heat exchanger, despite causing a slight increase in pressure drop. Zheng et al. [247] conducted a study on the thermo-fluidic performance of circular and annular microchannels with dimples or protrusions. They investigated the impact of various factors, including Re, size of dimples/protrusions, combinations of dimples/protrusions, and positioning patterns. Their research demonstrated that protrusions in circular microchannels are particularly beneficial for energy conservation.

The effect of particle shape on the thermo-fluidic behavior of MCHS with various geometries, i.e., circular, elliptical, triangular, and hexagonal, was studied by Monavari et al. [248]. Their findings showed that the triangular MCHS had the highest heat transfer coefficient value, followed by the elliptical, hexagonal, and circular MCHS in decreasing order. Wang et al. [249] conducted a numerical investigation on the impact of geometric factors on the thermohydraulic performance of microchannel heat sinks (MCHS) with rectangular, trapezoidal, and triangular geometries. Their research showed that the rectangular MCHS had the lowest thermal resistance among the three types, followed by the triangular and trapezoidal MCHS in increasing order of thermal resistance. In another research Tan et al. [250] conducted a numerical investigation to analyze the heat exchange mechanism and optimization of structural parameters for a built-in series combined MCHS. Through an orthogonal test, they investigated the impact of multiple parameters on the heat exchange efficiency. The study revealed that the shape of the microchannel plays a significant role in determining the heat exchange phenomena.

Conclusions

In this study, the focus was on techniques to improve heat transfer in microchannel heat exchangers (MCHE) and microchannel heat sinks (MCHS). Various techniques were reviewed, including the utilization of different working fluids, flow disruptions, different materials for constructing microchannels, and modifications to microchannel geometries. Additionally, statistical factors such as bibliographic analyses were conducted. The main findings of the study are summarized below.

Microchannels have exceptional heat transfer properties that enable them to absorb substantial heat fluxes in very small areas. However, despite their potential, commercially available microchannels have not yet replaced conventional channels due to the high cost of the specialized manufacturing processes required to produce micro- and minichannels. Nearly half of the studies conducted on microchannel heat sinks (MCHS) and microchannel heat exchangers (MCHE) have utilized numerical approaches. In the past decade, there has been a proliferation of numerical studies, with a relative decrease in analytical and experimental research.

Although there have been notable advances in the development of microchannel heat sinks (MCHS) for electronic cooling applications, research in large-scale thermal and energy applications has been limited. In practical thermal applications, heat exchangers often incorporate core components with complex geometric designs. However, most of the studies conducted on microchannel heat exchangers (MCHE) and MCHS have focused on fundamental microchannel geometries, particularly rectangular ones, and working fluids. To accurately represent real-world heat exchangers, extensive research is required on a wide range of configurations, manifold geometries, materials, and working fluids.

Numerous studies have demonstrated that the optimal method for removing high heat flux from small volumes and spaces is by allowing the working fluid to flow through a microchannel. The addition of nanoparticles in small quantities to the base fluid can further enhance the thermal and fluid flow characteristics of the microchannel. Al2O3 is the most commonly used nanofluid due to its low density and high thermal conductivity. However, the use of nanofluid has a negative impact on energy consumption since it requires more electricity for pumping.

The use of flow disruption techniques, such as the wavy microchannel design, has been found to have a significant impact on the heat transfer performance of microchannel heat sinks. The effectiveness of this technique can be attributed to three main mechanisms. Firstly, it increases the surface area available for convective heat transfer. Secondly, it alters the parabolic velocity profile, leading to improved convective heat transfer. Finally, it creates Dean vortices & chaotic advection, resulting in improved convective fluid mixing. While the wavy channel design may cause pressure losses, the benefits to heat transfer outweigh the associated pressure drop penalty.

The researchers evaluated various materials for constructing microchannels, including metals, ceramics, and polymers. Based on their findings, ceramics were determined to be the optimal material for high-temperature applications due to their ability to address metallurgical issues and corrosion problems.

Future scope

Here are some suggestions and recommendations for future work based on the published studies on heat transfer improvement in microchannel heat sinks/heat exchangers:

  1. (i)

    Use of combined active and passive techniques in microchannel heat sinks/heat exchangers for further heat transfer augmentation.

  2. (ii)

    The use of ultra-high-temperature ceramics in place of metals and superalloys in high-temperature power generating applications can solve the metallurgical constraint of the high inlet temperature of microturbines and increase the efficiency of the plant.

  3. (iii)

    The thermal performance of microchannels has to be further enhanced, and other manufacturing processes for miniaturization are needed to cut their cost.

  4. (iv)

    Further research is needed to explore the potential of nanofluids in enhancing the thermal and fluid flow characteristics of microchannels. The effects of different types and concentrations of nanoparticles, as well as their impact on pump power consumption, need to be thoroughly investigated.

  5. (v)

    More studies are needed to investigate the effects of various flow disruption techniques and microchannel geometries on heat transfer enhancement. In particular, further research should focus on exploring the performance of wavy microchannels and their potential applications in large-scale thermal and energy systems.

  6. (vi)

    Future studies should consider a wider range of working fluids to explore their potential in microchannel heat sinks/heat exchangers. Researchers should also focus on investigating the effects of fluid properties, such as viscosity and surface tension, on the thermal and fluid flow characteristics of microchannels.

  7. (vii)

    The development of advanced manufacturing techniques that can produce cost-effective and high-performance microchannel heat sinks/heat exchangers should be prioritized. This could involve exploring the use of 3D printing and other novel fabrication techniques.

  8. (viii)

    Further studies should explore the effects of operational conditions such as flow rate, inlet temperature, and pressure drop on the performance of microchannel heat sinks/heat exchangers.

  9. (ix)

    The potential of microchannel heat sinks/heat exchangers in applications beyond electronic cooling, such as in the fields of energy and biomedicine, should be explored.

  10. (x)

    Finally, researchers should focus on developing accurate numerical models and experimental methods to predict and measure the thermal and fluid flow characteristics of microchannel heat sinks/heat exchangers.