1 Introduction

In the last years, increasing world population led to rise in global energy demand. Therefore, efficient energy conversion systems gain importance because of limited fossil resources and also increasing greenhouse gas emissions. In addition, one of the crucial issues is effective heat transfer that perform by heat exchangers [1, 2]. Several kinds of heat exchangers are utilized in various industries. Moreover, different strategies were utilized to upgrade the thermal performance of the heat exchangers. One of outstanding techniques is mixing nanoparticles into working fluid in order to enhance the thermal performance [3,4,5,6,7]. In this regard metallic or non-metallic nanoparticles is added to a base fluid like water. Nanoparticles has higher thermal conductivity in comparison with the base fluid. Therefore, adding nanoparticles to the base fluid improves thermal conductivity of working fluid by increasing thermal transfer surface area. Also, by using nanoparticles effective thermal capacity of the fluid increases. Additionally, nanoparticles lead to increase in fluid activity and turbulence intensity which is the main parameter that affect thermal conductivity of nanofluid. The physical facts that mentioned above are the main reasons which cause to improve in the heat transfer by using nanofluids [8, 9]. Nanoparticle size, shape and its concentration are the most crucial parameters that influence nanofluid properties [10, 11]. Various nanofluids have been investigated by many researchers in the literature. The utilization of ZnO/water in plate type heat exchanger conducted by Kumar et al. [12]. Their results indicated that corrugation angle of heat exchanger affect the nanofluid thermal performance. Huang et al. analyzed impact of using hybrid type nanofluid including carbon nanotubes and Al2O3 on thermal performance [13]. Their findings indicated that using this hybrid nanofluid had not significant effect on heat transfer enhancement. Sözen et al. tested alumina and fly ash nanofluids performance in concentric tube type heat exchanger in two different mode. The findings illustrated that fly ash nanofluid improved the efficiency of heat exchanger more than alumina nanofluid [14].

The impact of nanoparticle ratio on thermal characteristics of nanofluid have been investigated in different studies. Tiwari et al. examined the impact of particle ratio on performance of four various nanofluids comprising CeO2/water, Al2O3/water, SiO2/water, and TiO2/water. Their findings exhibited that the optimal nanoparticle ratio belongs to nanoparticle material, base fluid, and working conditions [15]. Gürü et al. tested the impact of particle ratio on thermal behavior of bentonite-water nanofluid in the heat pipe. Their findings illustrated a maximum increment of 37% in the heat pipe efficiency [16]. Also, the properties of surfactant is important which is used to avoid sedimentation in the nanofluid solution. Sözen et al. analyzed the influence of utilizing two various surfactants on the performance of TiO2/water in the heat pipe. The experimental findings indicated that using sodium dodecyl benzene sulfonate surfactant was more effective than Triton X-100 [17].

Table 1 summarize some studies that used different nanofluid in various heat exchangers. Generally, in the literature studies metal oxides like TiO2, Fe2O3, Al2O3 and SiO2 were used separately to prepare nanofluid. In this study kaolin nanoparticles has been used to prepare nanofluid solution. The major goal of this study is to predict thermal characteristics of kaolin/deionized water nanofluid in CFCT and PFCT heat exchangers. Unlike conventional nanofluids which are prepared from one metal oxide, kaolin mainly consists of Al2O3, SiO2, TiO2 components together. The most significant feature that separates this work from the available research is that kaolin/deionized water nanofluid has been made by utilizing kaolin nanoparticles in optimal dimension and particle concentration with surface active agent utilization to avoid deposition and sedimentation issues.

Table 1 Some available researches related to nanofluid utilization in different systems
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2 Material and methods

2.1 Experimental apparatus

Schematic of the test apparatus which was utilized in this study is presented in Fig. 1. Two fluid loops are available in the test apparatus consists of hot and cold fluid loops. Hot fluid (nanofluid) is heated by two electrical heaters in the hot fluid tank and is circulated in hot fluid loop. In the experimental setup to monitor the fluid level a bullseye has been placed. Circuit breakers have been placed in the setup to prevent unexpected temperature jump in the hot fluid tank. Also, a pressure regulator has been added against the unexpected jump in pressure. To adjust flow rate in each fluid loop, two flow meters have been mounted to the system. As it is clear in Fig. 1 ten thermocouples have been located in different positions to monitor temperature in cold and hot sides. The technical specifications of the equipment which were utilized in the experimental apparatus are presented in Table 2. In the heat exchanger part of the experimental apparatus, hot fluid (nanofluid) passes over the interior tube, however cold fluid (water) passes over the external tube of heat exchanger. Flexible design of the test setup permits to counter flow and also parallel flow heat exchange. In parallel flow heat exchanger mode, valve number 1–4 were kept open but valve numbers 5–7 were closed. However, among counter flow heat exchanger mode of the experimental apparatus valve numbers 1, 4, 5, 6 and 7 were left open, but valve numbers 2 and 3 were closed (Fig. 1). Hot fuild is heated by electrical heaters in the hot fluid tank and then is transmitted to the heat exchanger part by means of a circulation pump. Hot fluid passes over interior tube of heat exchanger and release its thermal energy and return to the hot fluid tank. However, cold fluid passes over the exterior tube of heat exchanger and absorbs the heat which released by hot fluid and then is drained form the system.

Fig. 1
figure 1

Experimental setup

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Table 2 Technical specifications of the equipment which were utilized in the test apparatus
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2.2 Preparation of the nanofluid

First of all, to reduce kaolin particles’ size and to obtain uniform nanoparticles ball milling process, using Spex-8000, has been applied for 8 h. In the second step, kaolin nanoparticles has been mixed into deionized water as the base fluid. The prepared nanofluid solution has 2% (wt/wt) particle content. Moreover, to prevent flocculation and to enhance stability of the nanofluid, sodium dodecyl benzene sulfonate surface-active agent has been added to the solution. Adding surface-active agent increases wetting capability of the nanofluid solution and reduces surface tension. Finally, nanofluid solution has been subjected to continuous pulsing by utilizing ultrasonic bath with the aim of enhancing stability of the prepared nanofluid.

Many types of nanoparticles do not have toxic effects [25], yet nanoparticles are used in bio applications widely environmentally friendly [26] and some of them have beneficial health effects [27]. Clays are non-toxic nanoparticles. Kaolin is a type of clay found in nature. Kaolin is used in health care products, as a filler and coating in paper industry, as a pesticide in agriculture, an adsorbent and a filter material [28,29,30]. Kaolin waste can also be used as building material. For example, they are used as cement in green concrete mixing [31]. Utilized kaolin in this study is taken from nature. There is no harm to the environment.

Density of kaolin/deionized water nanofluid has been determined by taking weight of a specified volume of nanofluid solution at the ambient temperature by utilizing analytical balance (Radwag AS220 CZ). Viscosity of the prepared nanofluid is other significant characteristics that must be determined. The viscosity of the kaolin/deionized water nanofluid has been determined in different temperatures using viscometer device (AND SV-10). In addition, nanofluid’s heat capacity has been achieved by utilizing Differential Scanning Calorimetry (DSC) technic. In this regard Perkin Elmer Diamond DSC apparatus has been utilized to determine kaolin/deionized water nanofluid’s heat capacity.

2.3 Experimental procedure

In the first step of the experiments in hot loop of the heat exchanger water has been tested as working fluid. Secondly, prepared kaolin/deionized water nanofluid has been utilized in hot fluid loop of experimental setup to determine its thermal performance. The experiments have been performed at 13 various hot fluid flow rates to analyze the effect of using nanofluid. However, flow rate of the cold fluid has been adjusted at 10 g/s. To obtain accurate findings every test has been iterated three times, then mean results have been utilized in the calculations.

2.4 Theoretical analysis

The amount of thermal energy which transferred in a heating system can be expressed as the gained thermal energy by cold side (\( {\dot{Q}}_{cold} \)) or the thermal energy released from the hot side (\( {\dot{Q}}_{hot} \)). The thermal energy which transferred from the hot side could be calculated as:

$$ {\dot{Q}}_{hot}={\dot{m}}_{hot}.{c}_{p, hot}.\left({T}_3-{T}_6\right) $$
(1)

Also, the absorbed thermal energy by the cold side is defined as below:

$$ {\dot{Q}}_{cold}={\dot{m}}_{cold}.{c}_{p, cold}.\left({T}_7-{T}_{10}\right) $$
(2)

It should be indicated that the mean value of specific heat capacity has been used in the calculations.

The highest efficiency can be obtained where the heat transfer in the hot side is the same with heat transfer in the cold side by supposing; the heat transfer from interior pipe (hot side) to exterior pipe (cold side), neglecting thermal resistance of heat transfer surface, and also ignoring measurement errors:

$$ {\dot{Q}}_{hot}={\dot{Q}}_{cold} $$
(3)

The convection heat transfer among pipe surface and working fluid during heat transfer across the pipe could be obtained as:

$$ \dot{Q}=h\ A\ {\Delta T}_{ln} $$
(4)

here h denotes the heat transfer coefficient (HTC) among working fluid and the interior surface of exchanger pipe, A represents interior heat transfer surface area, and ∆Tln illustrate logarithmic mean temperature difference and calculated as:

$$ {\Delta T}_{ln}=\frac{{\Delta T}_{inlet}-{\Delta T}_{outlet}}{\ln \left(\frac{{\Delta T}_{inlet}}{{\Delta T}_{outlet}}\right)} $$
(5)

By rearranging equations above heat removed from the hot side can be calculated as:

$$ {\dot{Q}}_{hot}=\frac{h_{hot}\ {A}_i\ \left[\left({T}_3-{T}_1\right)-\left({T}_6-{T}_2\right)\right]}{\ln \left(\frac{T_3-{T}_1}{T_6-{T}_2}\right)} $$
(6)

HTC in the hot side could be obtained by utilizing Eq. (6). In addition, heat transfer rate in the cold side of system is obtained by using Eq. (7).

$$ {\dot{Q}}_{cold}=\frac{h_{cold}\ {A}_o\ \left[\left({T}_1-{T}_7\right)-\left({T}_2-{T}_{10}\right)\right]}{\ln \left(\frac{T_1-{T}_7}{T_2-{T}_{10}}\right)} $$
(7)

In addition, heat transfer rate can be calculated using Eq. (8). By using Eq. (8) overall heat transfer coefficient (OHTC) in heat exchanger could be obtained [14].

$$ {\dot{Q}}_{hot}={A}_T\ U\ \left[\frac{\left({T}_3-{T}_7\right)-\left({T}_6-10\right)}{\ln \left(\frac{T_3-{T}_7}{T_6-10}\right)}\right] $$
(8)

Heat exchanger effectiveness can be achieved by using the Eq. (9):

$$ \varepsilon =\frac{\dot{Q}}{{\dot{Q}}_{max}}=\frac{C_{hot}\left({T}_{hot,i}-{T}_{hot,o}\right)}{C_{min}\left({T}_{hot,i}-{T}_{cold,i}\right)}=\frac{C_{cold}\left({T}_{cold,o}-{T}_{cold,i}\right)}{C_{min}\left({T}_{hot,i}-{T}_{cold,i}\right)} $$
(9)

where the cold fluid heat capacity rate (Ccold) and the hot fluid heat capacity rate (Chot) are:

$$ {C}_{hot}={\dot{m}}_{hot}\ {c}_{p, hot} $$
(10)
$$ {C}_{cold}={\dot{m}}_{cold}\ {c}_{p, cold} $$
(11)

Exergy could be defined as the maximum quantity of work achieved theoretically at the end of a reversible procedure in that equilibrium with the environment is acquired. In regard to this description, the reference environment conditions should be identified to calculate exergy. In the present work reference environment temperature is 22 °C. In a heat exchanger there are two types of losses including; frictional pressure drop and temperature difference. But in this work, exergy analysis dose not consist pressure drop factor and has been done according to heat transfer irreversibility. Therefore, exergy loss in the heat exchanger is the sum of exergy losses of cold and hot flows [32, 33]:

$$ {\dot{E}}_{loss}={\dot{E}}_{loss, hot}+{\dot{E}}_{loss, cold} $$
(12)

Eq. (12) can be rewritten as:

$$ {\dot{E}}_{loss}={\dot{m}}_{hot}{T}_e\left({s}_{hot,o}-{s}_{hot,i}\right)+{\dot{m}}_{cold}{T}_e\left({s}_{cold,o}-{s}_{cold,i}\right) $$
(13)

By supposing constant thermophysical properties for the fluids, entropy change can be expressed as:

$$ {s}_o-{s}_i={c}_p\ln \frac{T_o}{T_i} $$
(14)

Therefore, Eq. (13) can be rewritten as follow:

$$ {\dot{E}}_{loss}={T}_e{\dot{m}}_{hot}{c}_p\mathit{\ln}\left(\frac{T_{hot,o}}{T_{hot,i}}\right)+{T}_e{\dot{m}}_{cold}{c}_p\mathit{\ln}\left(\frac{T_{cold,o}}{T_{cold,i}}\right) $$
(15)

By considering a base for the fluid with maximum temperature range, i.e. the fluid with minimum heat capacity rate, dimensionless exergy loss for the heat exchanger could be obtained as:

$$ e=\frac{{\dot{E}}_{loss}}{T_e{C}_{min}} $$
(16)

where Cmin denotes minimum heat capacity rate and Te shows ambient temperature.

2.5 Experimental uncertainty

Among the experimental process, temperatures and flow rates have been achieved by utilizing proper measurement. Uncertainty analysis is a useful technique to obtain uncertainties and to appraise the experimental findings. The uncertainties could arise from device selecting, calibration of device, reading, experimental conditions, and connection types of devices [34]. The properties of utilized equipment during experiments and their uncertainty including relative flow rate uncertainty and absolute temperature uncertainty were given in Table 3. The accuracy of wattmeter, flow meter and thermocouples are ±1 W, ±0.01 g/s and 0.5 °C, respectively. Total relative uncertainty in temperature measurement is 2%. Also, uncertainty for heat transfer coefficient obtained as 6.4%. As it is obvious, the obtained results for uncertainty, are in acceptable range by considering sensitivity of utilized devices and available studies in literature [35, 36].

$$ {\mathrm{W}}_{\mathrm{R}}={\left[{\left(\frac{\partial R}{\partial {x}_1}{w}_1\right)}^2+{\left(\frac{\partial R}{\partial {x}_2}{w}_2\right)}^2+\dots +{\left(\frac{\partial R}{\partial {x}_n}{w}_n\right)}^2\right]}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.} $$
(17)
Table. 3 Properties of measurement equipment
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3 Results and discussion

In this part, the experimental findings for both CFCT and PFCT heat exchangers are given and the obtained results are concluded. An average size of 69 nm has been obtained for kaolin nanoparticles at the end of ball milling process. Size distribution for the kaolin nanoparticles is presented in Fig. 2. Chemical composition of kaolin nanoparticles is given in Table 4. Also, the thermophysical specifications of kaolin/deionized nanofluid are given in Table 5.

Fig. 2
figure 2

Kaolin nanoparticles’ size distribution

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Table 4 Kaolin particles chemical content (%) and loss on ignition (LOI)
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Table 5 Thermophysical specifications of kaolin/deionized water nanofluid
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The experiments have been performed at constant cold side flow rate (10 g/s) and 13 different hot fluid (nanofluid) flow rates to clarify the influence of utilizing nanofluid in hot fluid loop.

The variation of OHTC via hot fluid (nanofluid) flow in PFCT heat exchanger for deionized water and nanofluid is presented in Fig. 3. As it is obvious, OHTC of kaolin/deionized water nanofluid is high in comparison with deionized water at each hot fluid flow rate. A peak enhancement of 37% was achieved at 7 lpm flow rate by using kaolin/deionized water. The experimental findings illustrated that utilizing kaolin/deionized water in hot loop of the heat exchanger caused to increment in OHTC.

Fig. 3
figure 3

OHTC variation via hot fluid flow rate in PFCT heat exchanger

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The change in the HTC of hot fluid among the interior surface of tube and the hot fluid via hot fluid flow rate in PFCT heat exchanger is presented in Fig. 4. The obtained HTC of kaolin/deionized water nanofluid is high in comparison with deionized water’s HTC at each flow rate, with a maximum increment of 39%. As it is seen in Fig. 4 hot fluid (nanofluid) heat transfer coefficient improved by rising hot fluid flow rate for nanofluid and deionized water.

Fig. 4
figure 4

HTC of hot side via hot fluid flow rate in PFCT heat exchanger

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The change in HTC of cold fluid with flow rate of hot fluid in PFCT heat exchanger is presented in Fig. 5. The obtained cold side HTC for kaolin/deionized water nanofluid is high in comparison with deionized water at each flow rate, with a peak enhancement of 19%. In addition, it should be stated that HTC of cold fluid improved by rising with hot fluid flow rate for each working fluid.

Fig. 5
figure 5

HTC of cold side via hot fluid flow rate in PFCT heat exchanger

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The change in of OHTC via flow rate of hot fluid (nanofluid) in CFCT heat exchanger for both deionized water and kaolin/deionized water nanofluid is presented in Fig. 6. The OHTC of kaolin/deionized water nanofluid is high in comparison with deionized water for each flow rate of hot fluid. A maximum increment of 12% was obtained for kaolin/deionized water nanofluid at 5.5 lpm flow rate. The achieved findings illustrated that using kaolin/deionized water in hot loop of the CFCT heat exchanger led to enhance in thermal performance of CFCT heat exchanger.

Fig. 6
figure 6

OHTC variation via hot fluid flow rate in CFCT heat exchanger

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The change in the hot fluid HTC among interior surface of exchanger tube and hot fluid via hot fluid flow in CFCT heat exchanger is presented in Fig. 7. The HTC of the kaolin/deionized water nanofluid is high in comparison with deionized water at each flow rate of hot fluid, with a maximum enhancement of 15%. As it can be seen in Fig. 7 HTC in hot side improved by rising hot fluid flow rate for both nanofluid and deionized water.

Fig. 7
figure 7

HTC of hot side via hot fluid flow rate in CFCT heat exchanger

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The variation of HTC in cold side among the exterior area of exchanger tube and cold fluid via hot fluid flow rate in CFCT heat exchanger is presented in Fig. 8. The achieved cold side HTC for kaolin/deionized water is higher than deionized water at each flow rate, with a peak enhancement of 9%. Also, it should be stated that cold fluid heat transfer coefficient improved by rising hot fluid flow rate for both working fluids.

Fig. 8
figure 8

HTC of cold side via hot fluid flow rate in CFCT heat exchanger

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The variation in the OHTC ratio (Unanofluid/Udeionized water) via hot side flow rate for two heat exchangers is shown in Fig. 9. In the PFCT heat exchanger, OHTC ratio varied in a range of 1.17–1.35 in various hot fluid flow rates. Also, OHTC ratio for CFCT heat exchanger changed in a range of 1.07–1.12. Variation of hot side HTC ratio (hh,nanofluid/hh,deionized water) via hot fluid flow rate for two heat exchangers is shown in Fig. 10. In PFCT heat exchanger, hot side HTC ratio varied in a range of 1.25–1.39 in various hot fluid flow rates. However, HTC ratio in hot side for CFCT heat exchanger changed in a range of 1.03–1.15. Variation of heat transfer coefficient ratio in cold side (hc,nanofluid/hc,deionized water) via hot fluid flow rate for both heat exchangers is presented in Fig. 11. In the PFCT heat exchanger, HTC ratio of cold side varied in a range of 1.07–1.19 in various hot fluid flow rates. However, HTC ratio in cold side for CFCT heat exchanger changed in a range of 1.04–1.09. The obtained results indicated that increment in the performance of PFCT heat exchanger is high in comparison with CFCT heat exchanger.

Fig. 9
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OHTC ratio via hot fluid flow rate

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Fig. 10
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Variation of hot fluid HTC ratio via hot fluid flow rate

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Fig. 11
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Variation of cold fluid HTC ratio via hot fluid flow rate

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Moreover, Al2O3/water and fly ash/water nanofluids thermal behavior in concentric type heat exchanger was previously investigated by Sözen et al. [14, 37]. They tested nanofluids performance in ten different flow rates. Figure 12 presents a comparison between OHTC results for PFCT heat exchanger obtained in this study and previous studies. Sözen et al. reported maximum improvements of 62% and 7.2% for OHCT by using fly ash/water and Al2O3/water, respectively. However, in the present study a maximum improvement of 37% for OHTC was obtained by using Kaolin/ deionized water.

Fig. 12
figure 12

Comparison between this study and previous studies

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Variation of the effectiveness with Reynolds number for PFCT and CFCT heat exchangers are presented in Figs. 13 and 14. As it can be seen, utilizing Kaolin/deionized water increased the effectiveness PFCT and CFCT heat exchangers in the range of 7–20%. It is seen that effectiveness was decreased with increasing Reynolds number. As it is known Reynolds number increases with increasing fluid flow rate. Also, it should be stated that fluid heat capacity rate increases with increase in fluid flow rate, therefore temperature change of fluid decreases, which cause to reduce the effectiveness. In high flow rates, temperature variation will be lesser since fluid residual time is limited inside the heat exchanger and cause to decrease in effectiveness. Similar results for effectiveness of concentric type heat exchangers were obtained by Gomaa et al. [38, 39].

Fig. 13
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Variation of effectiveness with Reynolds number for PFCT heat exchanger

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Fig. 14
figure 14

Variation of effectiveness with Reynolds number for CFCT heat exchanger

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Variation of dimensionless exergy loss with Reynolds number for PFCT and CFCT heat exchangers are shown in Figs. 15 and 16. As it can be seen in figures, using Kaolin/deionized water reduced dimensionless exergy loss in PFCT and CFCT heat exchangers in the range of 18–39%. In addition, it should be stated that in CFCT heat exchanger the amount of heat transfer rate is higher in comparison to PFCT heat exchanger. Therefore, exergy loss in CFCT heat exchanger is lower in comparison with PFCT heat exchanger. This fact illustrates once again that, more exergy loss in heat exchanger is not directly related to the heat transfer rate. The significant factor which affect exergy loss is finite temperature difference among cold and hot water in heat exchanger. Therefore, it must be avoided high local temperature difference among hot and cold fluid over the heat exchanger.

Fig. 15
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Variation of dimensionless exergy loss with Reynolds number for PFCT heat exchanger

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Fig. 16
figure 16

Variation of dimensionless exergy loss with Reynolds number for CFCT heat exchanger

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Outcomes of the experiments exhibited that, using kaolin/deionized water in the hot side of the PFCT and CFCT heat exchangers improved heat transfer performance considerably. Also, it should be stated that the viscosity of kaolin/deionized water is relatively close to the viscosity of water (see Table 5). Therefore, utilizing kaolin/deionized water nanofluid don’t cause to pressure drop in the heat exchangers.

4 Conclusions

In the present study the impact of utilizing kaolin/deionized water nanofluid on the thermal efficiency of two different heat exchanger including PFCT and CFCT heat exchangers have been investigated. The experiments have been conducted in different working conditions to determine thermal behavior of kaolin/deionized water. Utilizing kaolin/deionized water nanofluid in the PFCT and CFCT heat exchangers improved heat transfer performance considerably. Maximum improvement in the OHTC in PFCT and CFCT heat exchangers were obtained as 37% and 12%, respectively. The experimental findings exhibited that kaolin/deionized water could be utilized effectively in the concentric tube type heat exchanger. Moreover, kaolin/deionized water’s thermal behavior in various energy conversion systems can be analyzed.