An Experimental Study on the Heat Transfer and Flow Characteristics of Aluminum Heating Elements Coated with Graphene

Abstract

With the rapid development of clean heating, electric heating has received increasing attention. As a common electric heating device, heaters are also widely used in residential and industrial heating, as well as in other fields. In order to improve the reliability and heat transfer flow characteristics of the electric heating element of the heater, a new type of structural electric heating element was designed and developed based on the surface heat transfer enhancement and pit drag reduction characteristics of graphene. The heat transfer and flow characteristics of the electric heating element were analyzed through experiments, and the results showed that the average surface emissivity increases from 0.25 with the non-graphene-coated sample to 0.94 with the graphene-coated one, and the surface temperature of the electric heating element decreased from 289 °C to 237.5 °C. Through wind-tunnel experiments, it was found that the PEC value of the electric heating element with pits was 1.0693, and the Nusselt number increased by 6.22% compared with the smooth surface. Furthermore, after coating with graphene, the Nusselt number increased by 30.3% compared with the smooth surface.

1. Introduction

A heater is a common electric heating device, widely used in medical, vehicle, drying, and aquaculture heating fields [1,2,3,4]. Its main function is to heat cold air to achieve the desired temperature environment. The electric heating element of a heater is its core component [5,6]. However, long-term high-temperature use can easily lead to uneven surface heating, and even fracture, which affects the service life of the heating element. Recent studies have shown that coating graphene on the surface of the heating element of a heater may increase the radiation heat transfer of the heating element and reduce the temperature of the heating element itself, thereby improving the reliability of the heating element.

Gao et al. [7] studied the thermal conductivity of graphene films and found that single-layer graphene films can reduce the device temperature by 13 °C, while multi-layer graphene films can reduce the temperature by 8 °C. Meanwhile, a study by the University of California [8] analyzed the thermal conductivity and influencing factors of graphene films. It was found that the temperature rise in thermal conductive devices with and without graphene films differed by 26 °C. A. A. Jaafar et al. [9] used the chemical spray method to coat graphene on the metal substrate, and tested the performance of the radiator. The results showed that the overall heat dissipation performance was enhanced after adding a graphene coating. Chen et al. [10] proposed a method of combining graphene coatings with deicing components to improve deicing efficiency, based on the heat transfer performance of graphene coatings in the past. R. Saberish et al. [11] experimentally analyzed the heat transfer performance of aluminum fins coated with graphene and uncoated graphene, and the results indicated that the nanocoating had a better heat dissipation effect than the non-coated. Ammar M. Hadi et al. [12] tested the effect of graphene coatings on surface heat dissipation at different Reynolds numbers, and the results showed that samples containing graphene coatings had the best heat dissipation performance. At Re = 12,000, the heat transfer performance of graphene coatings was improved by 74% compared to pure aluminum. Zu et al. [13] carried out a study to observe the effect of the emissivity of a coating containing graphene on the heat transfer performance. The results showed that the high-emissivity coating effectively reduced the temperature of the heat sink, and it was proven that radiation accounted for 38% of the total heat dissipation power.

On the other hand, the study of heat transfer and flow characteristics of electric heating elements is also of great significance [14,15]. Therefore, in the study of more efficient electric heating elements, it is necessary to consider increasing surface heat transfer performance while minimizing or even decreasing the flow-resistance characteristics. It has been found that concave structures have good drag reduction effects; Walsh M [16] was the first to achieve drag reduction by changing the surface structure, which laid the foundation for the surface drag-reduction technology. Ding et al. [17] used numerical simulations to analyze the flow field of different types of pits, and the results showed that at Re = 2000, circular pits can effectively reduce frictional resistance. Hu et al. [18] simulated concave and smooth surfaces, and the results showed that vortices were generated inside the concave, affecting the flow boundary layer of the concave and reducing the total resistance. Afanasyev et al. [19] set up spherical pits on a flat plate and experimentally found that the heat transfer performance can be improved by 30%, with a minimal change in flow resistance. Leontiev et al. [20] experimentally investigated the effects of different pit structures on heat transfer and flow-resistance performance, thereby determining the relationship between heat transfer, flow, and Reynolds number.

Through research, it has been discovered that previous studies primarily centered on the heat transfer research of electro-thermal elements merely coated with fluorine–graphene, and the number of relevant research documents in this regard is relatively scarce. Furthermore, altering the surface structure pattern may lead to a drag-reduction effect and enhance the heat transfer characteristics. A new type of electric heating element has been designed and developed based on the aforementioned factors. Its main structural feature is to add a pit structure on a smooth surface and coat the surface with graphene. We conducted experiments to analyze the effects of factors such as pit structure and graphene on the heat transfer and flow characteristics of electric heating elements, providing a theoretical basis for the design and development of energy-saving heaters.

2. Materials and Methods

2.1. Experimental Measurement System

2.1.1. Surface Emissivity Measurement System

To measure the emissivity of the surface of the electric heating element deposited with graphene, a aluminum plate coated with graphene was used as a preliminary exploration work throughout the investigation [13], as they are made of the same material.

The experimental test setup is shown in Figure 1, and the main parameters of the equipment are shown in

Table 1. Measuring equipment and the main parameters.

Equipment	The Main Parameters
Infrared thermal imager A655SC (Teledyne FLIR, Wilsonville, OR, USA)	−40~650 °C ± 2 °C
Temperature acquisition instrument MODEC (Yokogawa Electric Corporation, Tokyo, Japan)	−100~400 °C ± 0.1 °C
Collection module GX90XA (Yokogawa Electric Corporation, Tokyo, Japan)	5 V, 60 V max,
Constant temperature heating table X3040 (Shenzhen Chengfa Weiye Technology Co., Ltd., Shenzhen, China)	P = 1800 W, U = 220 V
K-type thermocouple TT-K-30 (Omega Engineering Inc., Stamford, CT, USA)	−40~260 °C ± 0.1 °C
Raman Horiba Scientifc LabRAM HREvolution (HORIBA France SAS, Palaiseau, France)	D = 2 mm, P = 1.5 mw, τ = 30 s

.

The emissivity measurement steps are as follows: Select several typical measurement points on the surface of the sample which is composed of aluminum with the dimensions of (10 cm × 10 cm × 1 cm); Figure 2 shows the distribution of the measurement points. Place the calibrated thermocouple tightly against one of the measurement points on the sample surface and use an infrared thermal imager to measure the temperature near the measurement points. Adjust the emissivity in the infrared thermal imaging measurement software to ensure that the temperature measured by the thermocouple and infrared thermal imager is consistent. At this point, the emissivity of the infrared thermal imaging measurement software is the emissivity of this measurement point. Based on the obtained emissivity of each point, the average emissivity is obtained by calculating its arithmetic average.

In this study, a high-temperature-resistant modified graphene coating (suitable for temperatures ranging from −50 to 500 °C), developed by Sichuan Kenye Technology Development Co. (Chengdu, China), Ltd., was selected. The product model is KYCHR-4, and its main components are graphene (with a solid content of 20% to 25%), a diluent, the principal components of which are toluene and xylene, and a co-solvent, the principal components of which are methanol and ethanol. The surface treatment and processing of the samples were carried out in the following main steps: (1) Removal of the surface oxidation and rust from the aluminum plate samples by manual sanding. The surfaces were cleaned with alcohol to remove dust, grease, and other contaminants. After the samples’ surfaces were clean and free of pollutants, they were placed in a well-ventilated environment with a temperature of 15 °C, and a relative humidity of 30% for standby; (2) according to the manufacturer’s product instructions, before spraying, deionized water was added to the graphene coating at a volume ratio of 2:1 for dilution. Then, the diluted coating was placed in a storage tank for backup; (3) the air pump was turned on, and the air-pressure knob of the spray gun was adjusted to an ensure an even and stable airflow. The flow-rate knob and the spray-width knob were adjusted to allow the coating to be sprayed out continuously and evenly. A cross-hatch method was used for spraying; (4) after spraying, the samples were cured in a drying oven at 80 °C for 30 min, then cleaned with distilled water using ultrasonic cleaning, and dried naturally at room temperature.

2.1.2. Wind-Tunnel Experimental System

In order to analyze the heat transfer characteristics, resistance characteristics, and temperature rise characteristics of the electric heating element, a wind-tunnel testing experimental system was established, as shown in Figure 3. The wind-tunnel experimental system consists of air ducts, variable frequency fans, rectifier grilles, adjustable transformers, thermal anemometers, micro pressure differential force meters, thermocouples, and temperature measurement acquisition instruments. As shown in Figure 4, the sample of the new electric heating element is shown. Figure 4a shows the undeposited graphene surface with pits, and Figure 4b shows the surface coated with graphene.

The air duct is made of cylindrical galvanized insulated steel plate with a length of 3000 mm. The air duct has a cross-section diameter of 250 mm and a thickness of 1.5 mm. The distance between the test section in the channel and the intake side should be maintained at 1800 mm to ensure the fully developed flow of air in the test section area. The intake section of the air duct adopts a horn-mouth form, which helps with air intake. In order to reduce heat loss, a layer of insulation material, with a thermal conductivity of 0.034 W/m·K), is wrapped around the outer wall of the air duct. After testing, it has been calculated that the external heat dissipation of the air duct accounts for less than 3% of the total heat generation. The variable frequency fan can continuously change the frequency within the range of 0~50 Hz, so as to achieve a continuous change in wind speed between 0 and 10 m/s in the air duct.

The probe of the thermal anemometer is inserted into the air duct, so that the anemometer can collect and measure the wind speed in the upstream air duct of the electric heating element in real time. The micro differential pressure sensor is connected to the micro differential pressure gauge through a hose, which can detect the inlet and outlet differential pressure of the wind tunnel test section in real time and convert the pressure value into an electrical signal output.

One end of the thermocouple is connected to the recording board of the temperature acquisition instrument to record the collected temperature data in real time. The other end is used to measure the inlet and outlet temperature of the test section, the surface wall temperature of the pipeline, the temperature of the electric heating element specimen, and the ambient temperature. The thermocouple has a measurement range of −50~500 °C and a measurement accuracy of ± 1 °C. Before the experiment, the thermocouples were calibrated one by one using a constant-temperature water bath method.

After fixing the electric heating element, turn on the variable frequency fan, turn on the heating power and observe the anemometer, which is used to measure the wind speed.

2.2. Mathematical Expression

2.2.1. Heat Transfer Characteristics

The heat transfer characteristics of air and electric heating element surfaces are described by the following:

$$Nu={\displaystyle \frac{h\mathrm{a}}{\lambda }}$$

(1)

$$h={\displaystyle \frac{Q}{\left(T_w-T_f\right)A}}$$

(2)

where h is the average heat transfer coefficient on the surface of the component, W/m²·K; a is the equivalent diameter, m, in this experiment, which refers to the thickness of the electro-thermal element, namely a = 25 mm; λ is the thermal conductivity coefficient of the fluid, W/(m·K), in this experiment, which is 3.60 W/(m·K) and represents the arithmetic mean of the thermal conductivities of air corresponding to the room temperature (22.1 °C) and 300 °C; Q is the thermal power, W; A is the heat dissipation surface area of the electric heating element, m²; T_W is the wall temperature of the component, K; T_f is the qualitative temperature of the fluid, K.

2.2.2. Resistance Characteristics

Calculate the flow-resistance coefficient f based on different flow velocities and static pressure differences obtained from experiments [21,22]:

$$f=(2\Delta P-k_e{\rho }_1{\nu }_0^2-k_c{\rho }_2{\nu }_0^2)A^2/{\rho }_{12}{\nu }_0^2{A}_0^2$$

(3)

wherein, ΔP represents the pressure difference measured by the pressure analyzer, with the unit of pascal (pa); K_e denotes the coefficient of sudden contraction resistance; K_c stands for the coefficient of sudden expansion resistance; ρ₁ is the air density passing through the upstream side of the tested heating element, measured in kilograms per cubic meter (kg/m³); ρ₂ is the air density flowing downstream of the tested heating element, also expressed in kilograms per cubic meter (kg/m³); ρ₁₂ is the air density passing through the heating element, likewise quantified in kilograms per cubic meter (kg/m³); A refers to the effective flow area of the fins of the tested heating element, with the unit of square meters (m²); A₀ is the cross-sectional area of the air duct, also measured in square meters (m²); and V₀ is the airflow velocity measured by the anemometer, with the unit of meters per second (m/s).

2.2.3. Comprehensive Heat Transfer Characteristics

The goal of enhancing heat transfer is to increase the heat transfer coefficient while reducing the resistance coefficient, resulting in better overall performance. To comprehensively consider the heat transfer and flow-resistance characteristics, the ratio of Nu value to f value is used as the evaluation index for the comprehensive heat transfer and flow performance. Represented by the performance evaluation criteria (PEC) value [23,24], the formula is as follows:

$$PEC=\left({\displaystyle \frac{Nu}{N{u}_0}}\right){({\displaystyle \frac{f}{f_0}})}^{-1/3}$$

(4)

where Nu and Nu₀ represent the Nusselt numbers of concave elements and smooth elements, respectively; f. f₀ represents the resistance coefficients of concave and smooth components, respectively.

If PEC > 1, it indicates that the performance after structural optimization is better than before, achieving positive optimization, and the larger the PEC value, the better the overall performance.

2.2.4. Radiation Heat Transfer Characteristics

The heat dissipation of electric heating elements is mainly transmitted to the airflow through convection, and a part is transmitted to the wall of the air duct through radiation. The radiative heat transfer is expressed by the formula:

$$Q_r=\sigma ϵA\left(T_{\mathrm{w}}^4-T_{\mathrm{s}}^4\right)$$

(5)

where Q_r is the radiation heat, W; σ is the Stefan–Boltzmann constant, W/(m²·k⁴), approximately 5.67 × 10⁻⁸; $ϵ$ is the emissivity of the surface of the electric heating element; A is the surface area of the electric heating element, m²; T_w is the surface temperature of the electric heating element, °C; and T_s is the wall temperature of the air duct, °C.

3. Results and Discussions

3.1. Graphene Characterization

The JSM-7800F, an electronic scanning electron microscope equipped with a Schottky cathode (magnification 25–10⁴; acceleration voltage 0.01–30 KV), was employed to characterize the surface microstructure and structural dimensions of graphene. Using a 10 KV acceleration voltage and standard current, the sample of graphene-coated electric heating element was cut and sampled, and the surface morphology of the sample was observed, as shown in Figure 5. The thickness of graphene, as measured by a scanning electron microscope, is approximately 24 μm. A three-dimensional graphene structure is composed of numerous small two-dimensional graphene structures randomly stacked, presenting a dark-gray wrinkled shape with irregular shape, rough surface, and porous characteristics. The structure of graphene effectively increases the heat transfer area, while the metallic aluminum substrate mainly conducts heat through free-electron interactions. Graphene heat transfer relies on the diffusion motion of phonons to coat graphene on the surface of the aluminum substrate, and heat is transferred between free electrons and phonons. The two-dimensional sheet-like structure of graphene forms a heat transfer network with the substrate, combined with graphene’s excellent thermal conductivity to further increase surface heat transfer.

For graphene, Raman testing can characterize the quality and number of layers of graphene [25,26]. The characteristic peaks typically observed in the Raman spectrum of graphene include the D peak, G peak, and 2D peak. Through the implementation of Raman tests on the graphene-coated heating elements utilized in the present experiment, a Raman spectrum was acquired, as depicted in Figure 6. Within the spectral range of 1000 cm⁻¹ to 2800 cm⁻¹, three distinct characteristic peaks are discernible: the D peak in the vicinity of 1350 cm⁻¹, the G peak around 1580 cm⁻¹, and the 2D peak near 2680 cm⁻¹. The D peak is indicative of lattice defects and the degree of structural imperfection. The G peak reflects the symmetry and crystallinity of graphene. The 2D peak serves as an important indicator for determining the number of layers of graphene. The intensity ratio of the 2D peak to the G peak (I_2D/I_G) is a commonly employed metric for quantifying the number of graphene layers. A smaller value of this ratio implies a greater number of graphene layers. In the Raman spectrum presented in Figure 6, the intensity ratio (I_2D/I_G) of the 2D peak to the G peak is approximately 0.4, which is characteristic of common multilayer graphene. Additionally, the peak amplitude of the defect peak (D peak) located near 1350 cm⁻¹ in Figure 6 is relatively low, thereby suggesting a relatively high quality of the grown graphene single crystal.

3.2. Surface Emissivity

The average surface emissivity of samples at different heating temperatures (100, 150, 200 °C) was obtained, and its variation with time is shown in Figure 7. As shown in the figure, the surface emissivity of graphene aluminum substrate oscillates over time within 0–20 min, and stabilizes after 20 min. At the same time, the emissivity is highest under heating conditions of 150 °C, second under heating conditions of 200 °C, and lowest under heating conditions of 100 °C. There is a maximum value of the emissivity with temperature variation. The average surface emissivity of the graphene–aluminum substrate, as determined through the experiment, is 0.94. In comparison with the emissivity value of 0.25 for the pure aluminum substrate, it is evident that the average surface emissivity of the aluminum substrate subsequent to the graphene coating has been remarkably enhanced.

3.3. Surface Temperature Characteristics of Graphene Electric Heating Elements

Figure 8 shows the variation curve of the average surface temperature of two new electric heating element samples with time. As can be seen from the figure, in the first five minutes, the temperature of the electric heating element coated with graphene increases more rapidly than that of the smooth surface element. The surface temperature of the graphene-coated electric heating elements reaches equilibrium after about 20 min, while the temperature of smooth-surface electric heating elements tends to stabilize after about 35 min, and the heating rate increases by about 40%. The results indicate that the temperature rise characteristics of the electric heating element are consistent with those of the graphene aluminum substrate samples. The surface temperature of the electric heating elements coated with graphene is lower than that of the smooth electric heating elements, with surface temperatures of 237.5 °C and 289 °C, respectively, and a decrease of about 18% in surface temperature.

Figure 9 shows the infrared thermal-imaging cloud map of the electric heating element. As shown, a graphene coating can reduce the surface temperature of the electric heating element and make the surface more uniformly heated. The results are consistent with the surface temperature characteristics of the graphene aluminum substrate mentioned above. The main reason for this is that the coating of graphene increases the surface emissivity of the electric heating element, which increases the surface-radiation heat dissipation ability, and some of the heat is dissipated into the surrounding environment in the form of radiation.

3.4. Heat Transfer and Flow Characteristics of Graphene Electric Heating Elements

Taking the wind speed of 2 m/s as an example, the inner wall temperatures of three electric heating elements with smooth surfaces, concave structures, and graphene coated surfaces are 24.8 °C, 25.3 °C, and 33.3 °C, respectively. Figure 10 shows the variation in radiative heat dissipation of the electric heating element with Re. It can be seen from the figure that, as Re increases, the radiation of the electric heating element decreases. At Re = 3329, the radiation heat of the three types of electric heating elements with smooth surfaces, concave structures, and graphene-coated surfaces are 110.79 W, 49.1 W, and 40.88 W, respectively. The surface coating of graphene increases the radiation heat of the electric heating element by approximately 171%.

As shown in Figure 11, the surface wall temperatures of the three types of electric heating element samples tested in the experiment vary with Re. It can be seen from the figure that the surface temperatures of all three experimental samples decrease with increasing Re, indicating that the air passing through the electric heating element carries away more convective heat as the turbulence intensity increases. Under the same Re, comparing the three experimental samples, the wall temperature of the smooth surface of the electric heating element is the highest, followed by the electric heating element with a pit structure. The electric heating element with a coupled pit structure and graphene has the lowest wall temperature, indicating that both the pit structure and graphene reduce the surface temperature of the electric heating element. At Re = 3329, the surface temperatures of three types of electric heating elements with smooth surfaces, concave structures, and both the pit structure and graphene-coated surfaces are 264 °C, 252 °C, and 222 °C, respectively.

As shown in Figure 12a–g, the comparison results of the average outlet temperature of three experimental samples under a wind speed of 0.8–2 m/s are presented. From the graph, it can be concluded that, under each operating condition, the average temperature at the outlet of the electric heating element coated with graphene on the surface is the highest. Comparing Figure 12a–d, it can be found that the outlet temperature of the electric heating element with a concave structure is lower than the smooth surface, indicating that the addition of a concave structure reduces the heat dissipation of the electric heating element; however, comparing Figure 12e–g, it can be found that the outlet temperature of the electric heating element with a concave structure is higher compared to smooth surfaces, and the heat dissipation of the concave-structure electric heating element is also higher. In summary, it was found through experiments that the heat dissipation of electric heating elements with concave structures has a critical Reynolds number of Re = 2330.

Figure 13 shows the variation in heat dissipation of electric heating elements with concave structures and smooth surfaces with Re. At Re = 1331–2330, the heat dissipation of electric heating elements with concave structures decreases compared to smooth surfaces. At Re = 2330–3329, adding concave structures increases the heat dissipation of electric heating elements. The reason for this might be that at low speeds, the vortices inside the pits are very small and cannot effectively disrupt the boundary layer, which leads to a reduction in heat transfer. As the air velocity increases, it gradually transitions to facilitate heat dissipation.

The variation pattern of the h value with Re for three different samples of electric heating elements is shown in Figure 14. It can be seen that the h value of the heat transfer performance of all three samples increases with the increase in Re. Under the same Re, the h value of the electric heating element with a concave structure is higher than that of the electric heating element with a smooth surface. As the flow rate increases, there is an optimal air turbulence state for the heat transfer coefficient of the electric heating element. Meanwhile, the h value of the sample further increased after the graphene was uniformly coated on the surface of the concave-structure electric heating element, indicating that the coating of graphene further improved the heat transfer performance of the sample. This is consistent with the experimental results reported in the previous text, which showed that coating graphene on the surface of a metal aluminum substrate significantly improved the average emissivity of the substrate, and thus enhanced the surface radiation heat transfer performance. At Re = 2663, the h value of the concave-structure electric heating element increased by about 6.22% compared to that of the smooth-surface electric heating element, while the h value of the electric heating element with graphene and a concave structure increased by about 30.3%.

The variation pattern of Nu values with Re for three different samples of electric heating elements is shown in Figure 15. From the graph, it can be seen that as Re increases, the heat transfer performance Nu values of the three samples all increase. Compared with the electric heating elements with concave structures and smooth surfaces, under the same Re, the former has a higher Nu value than the latter. Moreover, as Re increases, the optimal air turbulence state exists for the Nu of the electric heating element, which is consistent with the trend of the heat transfer coefficient h of the electric heating element. After uniformly coating graphene on the surface of the electric heating element with a concave structure, the Nu value of the samples further increased, indicating that both graphene coating and a concave structure have a strengthening effect on the heat transfer of the electric heating element.

As shown in Figure 16, the flow-resistance performance of the concave-structure electric heating element and the smooth-surface electric heating element varies with the change in Re. It can be seen from the figure that as Re increases, the flow-resistance coefficient f values of both specimens decrease, indicating that the flow resistance of the electric heating element decreases with the increase in air flow rate crossing the electric heating element. The main reason for this may be that the surface coating of graphene on the electric heating element changes the radiation heat transfer of the electric heating element, which can improve the overall heat transfer performance. However, as the impact on the flow resistance is almost unchanged, only the influence of the pit structure on the flow characteristics of the electric heating element is considered. Compared with electric heating elements with concave structures and smooth surfaces, although the f values of the two do not change significantly, the f value of the former is lower than that of the latter at the same Re, indicating that concave structures have the effect of reducing drag. When Re = 1331–3329, the f value of the concave structure electric heating element is reduced by about 0.93~1.98% compared to the smooth electric heating element.

As shown in Figure 17, the comprehensive heat transfer and flow characteristics of the electric heating element vary with Re. It can be seen from the figure that the PEC value of the electric heating element with an added pit structure and graphene coated on the surface shows a trend of first decreasing, then increasing, and then decreasing. The electric heating element has the best comprehensive heat transfer and flow characteristics. At Re = 2663, the PEC value of the pit-structure electric heating element is 1.0693, higher than that of the smooth-surface electric heating element, and the PEC value of the electric heating element with both graphene and pit structure is 1.3115.

4. Conclusions

In this work, we propose a novel electric heating element on which small pits were constructed and graphene was deposited. The flow and heat transfer characteristics of the electric heating element are analyzed by experiments. The main conclusions are as follows:

(1)

It was found by SEM observation that the “microfin” microstructure formed by the graphene coating can significantly increase the surface area of the coating, thus increasing the contact area with the air, which is one of the reasons for its improved heat dissipation effect.

(2)

The deposition of graphene on the surface of the aluminum plate increased the surface emissivity from 0.25 to 0.94 compared to the surface without a deposition of graphene. In addition, the deposition of graphene can reduce the surface temperature of the electric heating element from 289 °C to 237.5 °C, which is another major reason for increasing the heat transfer performance of the electric heating element.

(3)

The experimental results show that with the increase in air velocity, the wall temperature in the wind tunnel decreases gradually, while the convective heat transfer coefficient increases gradually. This indicates that wind speed can significantly affect the dominant position of radiative heat transfer and convective heat transfer.