1 Introduction

In the emerging era of the industries and vehicles, heat energy produced due to combustion of fuels play a vital role. The proper maintenance of the components used for the extraction of heat (combustion) is too essential. A high temperature exothermic redox chemical reaction takes place between the fuel and the oxidant is called as Combustion. The reaction becomes self-sustained because of the sufficient energy provided by the heat from a flame produced during combustion. The thermal energy transferred across a well-defined boundary around a thermodynamic system is called as heat [1, 2]. This energy is transferred between systems through three modes called as (a) conduction; (b) convection and (c) radiation. If the heat is transferred through an intervening matter such as solid (e.g. block of metal), then the mode is called as conduction. The basic principle in this mode is that both surfaces of the solid should be at different temperatures (one at a high temperature and other at a lower temperature). If the transfer of heat takes place due to a flowing fluid (gas or liquid), then the mode of transfer is called as Convection. The transmission of heat energy through space (or) vaccum without the intervention of any physical matter is called as Radiation [3, 4].

In most of the automotive sectors, internal combustion (IC) engine is employed to produce the mechanical work from the process of combustion. The basic components/parts of the IC engine have been indicated as shown in Fig. 1.

Fig. 1
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Basic components of internal combustion engine

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The combustion process responsible for the production of work as shown in Fig. 2, produces enormous heat energy. The heat energy is exhibited as a flame with a high temperature of around 850 °C [5]. The flame produced exhibits high temperature to the engine parts. The parts of the engine subjected to high temperature has been indicated in Fig. 2.

Fig. 2
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Parts exposed to flame

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Piston crown, Intake manifold, exhaust manifold, inlet valve and exhaust valve are the engine parts exposed to high temperature flame [4]. Among these parts, piston crown is exposed more to the flame as the combustion takes place on the top of piston crown. Due to the enormous heat energy, piston has been subjected to high heat load, which means the piston crown exposed to variable temperature gradient.

The need to limit the measure of air utilized in liner film cooling and along these lines diminish the yield of carbon monoxide and unburned hydrocarbons has been identified by D.

Ballal et al. [5] to develop familiarity with toxin outflows from gas turbine engines. An examination has been made of trial information acquired on a uniquely structured film-cooling rig which has prompted the improvement of a computation technique for liner divider temperatures within the sight of film cooling the examination of Chen et al. [6] proposes an improved micro combustor with a rectangular rib to improve the temperature level of the combustor divider. In addition, the OH mass portion, temperature conveyance, and external divider temperature of the first and improved combustors of premixed hydrogen/air blazes are numerically examined under different bay speeds and comparability proportions. The results observed by the author shows that the improved micro combustor upgrades heat move between the blend and divider since its distribution zone is bigger than that of the first, along these lines bringing about high divider temperature. On the other hand, warm opposition in the flat heading increments with upstream and downstream advance lengths. Thusly, the external divider temperature diminishes with step length in the improved combustor. A high identicalness proportion (e.g., 0.6) may bring about the pulverization of the combustor in light of the fact that the divider temperature has surpassed the satisfactory temperature of divider material quartz. Therefore, it was finalized from the experiment that the improved micro combustor is suggested for miniaturized scale thermo-photovoltaic frameworks. Namayandeh et al. [7] examined the temperature dispersion of clay boards for a V94.2 gas turbine combustor exposed to sensible activity conditions is introduced utilizing three-dimensional limited contrast technique. A rearranged model of alumina clay is utilized to acquire the temperature appropriation. The outside warm loads comprise of convection and radiation heat moves are viewed as that these heaps are applied to level fragmented board on hot side and constrained convection cooling on the opposite side. First the temperatures of hot and cold sides of earthenware are determined. At that point, the warm limit states of all other fired sides are assessed by the field perceptions. At long last, the temperature disseminations of earthenware boards for a V94.2 gas turbine combustor are registered by MATLAB software. The experimental outcomes show that the gas emissivity for dissemination mode is more than premix thusly the radiation heat motion and temperature will be more. The aftereffects of this work are approved by ANSYS and ABAQUS virtual products. It is demonstrated that there is a decent understanding between all outcomes. Norgren [1] has calculated the liner temperature dependent on a consistent state radiative and convective warmth balance at the liner divider. It was found that the determined liner temperature were around 8 percent higher than test esteems. A radiometer was utilized to tentatively decide estimations of fire temperature and fire emissivity. Film cooling viability was determined from an observational tempestuous blending articulation accepting a fierce blending level of 2 percent. Liner divider temperatures were assessed in a rectangular combustor piece 15 by 30 cm (6 by 12 in.) and attempted at pressures up to 26.7 atm and delta temperatures up to 922 K (1660°R).

The inferences drawn from the above literatures’ states that temperature detection on the surface of the parts of the engine is too essential. The current study deals with the application of special solvent-based paint on the surface to detect the temperature gradient through change in color. A user-friendly and cost-effective method has been projected in this study for thermal profiling of components as the selection of materials for the manufacturing of engine parts depends on the detection of permissible temperature range allowed based on the material of the component.

2 Design and fabrication of specimen

A specimen of varying cross section has been designed and the model has been shown in Fig. 3. The model depicted in Fig. 3. manifests that the area of the model was large at beginning and start to reduce up to a certain point and it starts increasing from that point. The center portion of the sample where it contracts and expands is called as throat. Figure 3a depicts the three-dimensional model of the specimen created using designing software package. The dimensions of the model have been projected in the Fig. 3b.

Fig. 3
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V-notch specimen. a 3D model; b 2D drafting

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The specimen has been fabricated using mild steel (or) low carbon steel and the fabricated specimen has been coated with solvent-based paint as shown in Fig. 4a, b. The holes have been projected to hold the specimen for the experimentation purpose.

Fig. 4
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V-notch specimen. a Fabricated; b painted

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The chemical composition of the carbon steel used for the specimen fabrication has been listed in Table 1. The content of manganese is higher (0.7–0.9%) and it exhibits good thermal properties which indicates that the material chosen for fabrication can withstand high temperature.

Table 1 Chemical composition
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3 Experimentation

The fabricated model after the coating of paint has been exposed to a flame originating from the butane torch. The heating is continued for nearly 30 min. The specimen undergoes a drastic color change as shown in Fig. 5. From the human vison, it was evident that at the center portion of the specimen, a distinct color has been observed. The right and left side of the specimen undergoes different color changes than the color at the center. It was also observed that the color changes on both sides of the center are same.

Fig. 5
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Color contours of V-notch specimen

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From Fig. 5, it was observed that the color at the mid of the specimen is blue, the other colors on both sides are yellow, brown and red based on human vision of color indication. It is mandatory to match these colors with the temperature values in order to get the temperature gradient on the surface of the specimen. This can be achieved by comparing these colors with a calibration data. The following ASTM methods shown in Table 2 has been employed in the current study for evaluation of color obtained and other influencing properties [8].

Table 2 ASTM standards
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3.1 Calibration database

The calibration database is formed through warm up of square specimens of size 1 mm × 2 mm under different temperatures in a furnace. As the temperature increases, the color of the square specimen changes, therefore for each temperature, a distinct color is observed on the specimen.

The observed color contour can be compared with the color of the square specimen shown in Fig. 6 to obtain the value of temperature corresponding to the color as in calibration data.

Fig. 6
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Color change with temperature

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3.2 Color attributes

The colors can also be predicted with the help of its attributes such as RGB and HSV according to ASTM E805 and ASTM E991. Colors arises from the mixing of three primary colors red, green and blue [9]. The method of color identification using the mixing pattern is called as RGB method. The RGB value of each square specimen has been determined using a image processing algorithm.

figure a

The algorithm used for the determination of RGB value has been projected above. The determined RGB values for the square specimens at different temperature and color has been depicted in below Table 3.

Table 3 RGB and color code
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The color codes of the colors existing in calibration data base was also indicated in Table 4. The values of RGB obtained from the algorithm for the square specimens at different colors has been depicted in Fig. 7.

Table 4 RGB to HSL conversion
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Fig. 7
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RGB of square specimens

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3.3 HSV method

The colors can also be identified by using the properties of color such as hue, saturation and lightness [8, 10, 11]. The predominant color existing in the color obtained will be provided by Hue [9]. The saturation of the color indicates the percentage of saturation in the color. The percentage of luminescence is given by the factor lightness. The properties of color have been indicated in Fig. 8.

Fig. 8
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HSL representation

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4 Results and discussion

4.1 RGB vs. temperature

The values of RGB obtained from the algorithm for the square specimens at different colors has been depicted in the form of graph as shown in Fig. 8 in order to observe the variation of the color attributes [10] with respect to temperature.

The graph depicted in Fig. 9 reveals that the value of red (R) was higher at the initial temperature and as the temperature increases, the value of R decreases. It is also observed that the value of green (G) and blue (B) was at low at initial temperature and as the temperature increases, the value of G and B increases. The initial color of the paint was red but with increase in temperature the color changes to green (i.e.) In the RGB value, the value of green becomes higher at temperature of 850 °C.

Fig. 9
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RGB and temperature

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4.2 Detection of Temperature in V-notch specimen

Based on the data provided by Fig. 9, the color obtained on the surface of the specimen can be easily matched with the value of temperature. The combination of RGB values can be predicted from the V-notch specimen surface based on the algorithm provided in the section of color attributes.

Figure 10 depicts the value of RGB at the particular points available on the specimen surface. The data available in Fig. 10 does not exactly matches with the RGB values of the square specimens. The obtained RGB values on the V-notch specimen lies between the range of the RGB values shown in Table 3.

Fig. 10
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RGB of V-notch specimens

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The temperature range over the specimen surface can be detected using the value of RGB obtained as shown in Fig. 11, The exact temperature cannot be visualized due to the inexact value of RGB values,

Fig. 11
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Temperature range

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The temperature range is not acceptable to detect the effect of heat on the engine parts. The exact value of temperature is needed. The exact value may be determined based on the conversion of RGB into HSL. Even though, the observed color is a mixture of several colors, one of the color property Hue depicts the dominant color in it with a prescribed value [8]. The saturation and lightness of the color is also important in the detection of color [8, 10]. Several factors such as object shape, illumination geometry and the properties of reflectance such as hue, value and lightness influence the appearance of color observed on the surface of objects [10].

4.3 RGB to HSL conversion

In order to detect the exact temperature on the heated specimen, the observed RGB values should be converted to HSL. Table 4 lists out the steps and formulae to convert RGB into HSL.

The converted values of HSL and the corresponding RGB and temperature values has been tabulated in Table 5.

Table 5 HSL values
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The calculated HSL values have been plotted with temperature to determine the effect of temperature on Hue. The graph depicted in Fig. 12 shows that the value of Hue increases with increase in Temperature from 3500C up to 8000C. After that, the value of Hue decreases for 850 °.

Fig. 12
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Variation of hue and temperature

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From the trend obtained in graph, it is evident that each temperature value is associated with a distinguished color with separate hue value. Based on the Hue value, the values of temperature has been plotted on the V-notch specimen surface as shown in below Fig. 13. The coating applied to the surface has a peculiar property is that the coating changes its color when it has been elevated to temperature change [12]. It has been clearly evident from [10, 13], the color depends on the property Hue and it has greater influence. The coating molecules were excited to a certain wavelength when it has been exposed to certain temperature [11, 14] due to the property of reflectance. It has been also proven that the wavelength of the respective color and its property Hue has a very close relationship [8, 9, 11, 13]. In this way, temperature is connected to the Hue property and it has been visualized from the graph depicted in Fig. 12.

Fig. 13
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Hue and temperature

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4.4 Isotherm lines on the specimen

Based on the temperature values obtained through the values of hue, the temperature at each point of the surface has been plotted as shown in below Fig. 14.

Fig. 14
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Temperature detection

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The isotherms have been plotted based on the above graph. The area under a certain temperature has been divided from other temperature values and a boundary is created [7]. This sort of boundary creation based on the values of temperature is called as isotherms. The isothermal lines were plotted as shown in below Fig. 15.

Fig. 15
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Isotherm

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5 Conclusion

The experimental recognition of the temperature gradient on the surface of a V-notch specimen has been done. The experimental recognition has been performed by the utilization of a special paint which changes its color as subjected to temperature rise. The colors obtained from the experimental observation has been verified by RGB and HSL method. The color identification methods and the experimental results were hold in good agreement. The temperature on the specimen surface has been identified by the color using Hue property. The isotherms were identified and drawn on the specimen surface. The results observed from the study reveals that the usage of this special paint pays a greater involvement in prediction of temperature variation on the surface of hot specimen. Now a days, selection of suitable materials for the fabrication of gas turbine components has been a challenging task, therefore the color recognition method proposed in the above work has been planned to be implemented in gas turbine industries as a part of future work .