Wall Superheat Research Articles

Characterizing effect of particle size on flow boiling in sintered porous-microchannels

Flow boiling in porous microchannels draws extensive attention recently owing to its great potential in high heat flux applications. Subcooled flow boiling of deionized and degassed water was carried out to characterize transport performance. Porous microchannels were sintered from six-sized dendritic-like copper particles: 10, 30, 50, 90, 120 and 150 μm with the same layer thickness of 400 μm. The dimensions of 23 parallel porous microchannels are 600 μm × 1200 μm × 2800 μm (width × depth × length). The inlet subcooling degree and mass flux was maintained at 40 K and 142 kg/m2·s, respectively. Experimental results show that the particle size has great effects on the heat transfer coefficient (HTC) and Critical heat flux (CHF) of flow boiling in porous microchannels. Medium particle size (90 μm or 120 μm) remains higher performance in HTC and CHF. In heat fluxes between 60∼160 W/cm2, average HTCs of both PM-90 and PM-120 could reach almost 120 kW/(m2·K) or so. The visualization study reveals that the underlying heat transfer mechanism in porous microchannels is dominated by the nucleate boiling in low heat flux and then, by the thin film evaporation from moderate to high heat flux. Moreover, two boiling crisis phenomena have been observed: (a) in microchannels sintered from optimum size particles, the rewetting-dryout cycle has been well established and sustained without being interrupted by occasionally explosive boiling events, indicating capillary limits; (b) in non-optimum samples, high wall superheat in the rear part usually leads to explosive boiling persistently, which interrupts periodic oscillation modes with heat flux exceeding to moderate level and results in CHF conditions. An optimal ratio of bottom wall thickness and particle diameter, δ/d, is found in the range of 3∼5 for the flow boiling in sintered porous microchannels.

Experimental investigation on incipient boiling in narrow closed gaps with water

This research deals with characterization of the Onset of Nucleate Boiling (ONB) in an enclosed narrow gap system containing a stationary liquid. In the experimental system, subcooled water at atmospheric pressure is confined between two parallel vertical rectangular copper plates. One of the plates is heated electrically and the opposite one is cooled with an external water-cooling system, maintaining its constant temperature.The experimental gaps explored in this research are: 5 mm, 3 mm, 0.9 mm and 0.5 mm. In each gap, several experiments are performed with a different controlled cold plate temperature. Various heat transfer mechanisms are observed (convection, transition and conduction), depending on the gap size. In the convection regime, the results are in good agreement with a correlation from the literature. In the gap 0.5 mm wide, the heat transfer occurs by conduction through the water, whereas for the gap 0.9 mm wide, single-phase experiments show a “transition” heat transfer regime. The onset of nucleate boiling (ONB) is detected as the point at which the heat transfer coefficient behavior changes dramatically, and is revealed also by visualization of the experimental channel. The wall superheat and the heat flux are evaluated at ONB.An analytical model is developed, based on the [1] correlation and on the experimental values of the heat transfer coefficient. A good agreement for the ONB heat flux prediction is achieved with the model, whereas the ONB superheat is over-estimated, except for the really narrow, 0.5 mm, gap.

Numerical study of a hybrid thermal lattice Boltzmann method for pool boiling heat transfer on a modeled hydrophilic metal foam surface

In this paper, a numerical study is conducted to investigate pool boiling heat transfer on a modeled hydrophilic metal foam surface. A pseudopotential multiple-relaxation-time (MRT) lattice Boltzmann (LB) method is employed to solve the flow field, and a finite-difference (FD) scheme is applied to solve phase change. Thermodynamic nonuniformity is largely avoided by adjusting the forcing term’s corresponding parameters. The metal foam structure is simplified as a staggered square array. Both the bottom wall and solid squares are set as constant temperature boundary. Three kinds of boiling stages on smooth and metal foam surfaces are observed at different wall superheats. The effect of the metal foam porosity and pore size on pool boiling heat transfer is studied. Then boiling curves are obtained to analyze heat transfer performance and bubble behavior under several wall superheats. Finally, numerical simulations of pool boiling under microgravity conditions were performed. The bubble behavior shows great agreement with that of related experimental results, with qualitative proof of the correctness of the model.

Thermal Performance Analysis of Micro Pin Fin Heat Sinks under Different Flow Conditions

Due to microscale effects, the segmented microchannels or micro pin fin heat sinks emerged as a high thermal management solution. In this context, the present work analyzes the influence of different heights of square micro pin fins with an aligned array and investigates their influence on pressure drop and heat transfer behavior. The HFE-7100 is used as the working fluid, and the pressure drop and surface temperature behavior are analyzed for different mass fluxes and inlet subcooling. The single-phase flow was analyzed numerically using the computational fluid dynamics (CFD) software ANSYS FLUENT® for comparing the simulation results with the experimental data, showing that the highest micro pin fins configuration provides a more uniform and lowest wall temperature distribution compared to the lowest configuration. There is a good agreement between the experimental results and the numerical analysis, with a mean absolute error of 6% for all the considered parameters. For the two-phase flow condition, experimental tests were performed, and for the highest subcooling, an increase in mass flux causes an enhancement in the heat transfer for low heat flux; by increasing heat flux, there is a gradual predominance of boiling heat transfer over convection as the heat transfer mechanism. The pressure drop drastically increases with the vapor amount flowing into the system, regardless of the pin fin height; the boiling curves for the higher fin height show a much smaller slope and a smaller wall superheat than the fin with the smallest height, and consequently, a high heat transfer performance. A larger region of the heat sink is filled with vapor for lower inlet subcooling temperatures, degrading the heat transfer performance compared to higher inlet subcooling temperatures.

Dynamics of a Vapor Bubble in Film Boiling and the Superheat Effect

This study aims at developing an improved numerical simulation of the film boiling regime phenomenon to understand and visualize the growth of vapor bubble at a heated surface during low and high superheats. The simulation of the bubble dynamics including the bubble growth, departure, coalescence, rising, and frequency of detachment under different wall superheats is numerically investigated. The continuity, momentum, and energy equations are solved for the two immiscible fluids phases using the finite volume method. The phase change model and the results exhibited a good agreement with the theoretical models. The obtained results show that the velocity of bubble growth and its frequency of emission promotes heat exchange. It is found that the shape of a bubble has been influenced by the wall superheat. It is also found that the high superheat generates a large amount of steam in which the steam bubble takes the shape of a fungus. So, a clear correlation exists between heat transfer and the frequency of detachment. As long as the frequency is greater, the heat transfer increases. Most of the heat transfer is induced by the liquid movements associated with the vapor bubble detachment.

熱力学的自己抑制効果と壁面加熱がノズル内キャビテーションに及ぼす影響

To clarify the thermal aspect of cavitating flow, cavitation experiments were conducted using a heated nozzle with different mainstream temperatures and heating conditions to evaluate two thermal effects that affect the cavity. The decrease in cavity length was observed due to the thermodynamic self-suppression effect by increasing the mainstream temperature, and the cavity length increased by heating from the wall. By comparing the variation of cavity length in the non-heated and heated conditions, the condition balancing the two thermal effect was found. Additionally, the response of each thermal effect to the mainstream temperature was also found. It was also shown that the required wall superheat to cancel out thermodynamic self-suppression effect reaches a maximum value when the mainstream temperature is around 70 °C. The experimental results were compared and discussed with the mathematical model values for the two thermal effects.

Investigation of film boiling over tandem arrangement of elliptical cylinders in vertical cross-flow of saturated liquid

The present study investigates the phenomenon of film boiling heat transfer for the tandem arrangement of elliptical heaters placed in cross-flow of saturated liquid (water) at near critical pressure conditions (P/Pc = 0.99), using the Eulerian approach-based volume of the fluid framework. The effect of vertical spacing between the heaters (pd = 7.5a to 12.5a), their aspect ratio (ψ = 0.4 to 1), degree of wall superheat (Jav/Prv = 0.6 to 1.8) and Reynolds number for liquid flow (Rel = 10 to 40) is investigated on the bubble departure dynamics and heat transfer characteristics. It is observed that the bubble departure size from the bottom heater has remained almost unchanged for all the situations whereas its departure frequency has increased upon an increase of Reynolds number and degree of superheat. The bubble size has varied significantly for the top elliptical heater and its departure frequency has increased with the decrease of aspect ratio and increase of pitch, Reynolds number and degree of superheat. The time-averaged heat transfer coefficient has increased upon an increase in pitch distance, aspect ratio, Reynolds number and degree of superheat, however, the rate of increase has diminished at a high degree of superheat.

Study of low-level pool boiling in vapor chamber of wickless micro-heat pipe based on Lattice Boltzmann pseudopotential method—Effects of superheat and wall wettability

The lattice Boltzmann method, a mesoscopic numerical method with a solid kinetic basis, is used to establish a low-level pool boiling model to exhibit the transport phenomena inside an evaporation chamber of a wickless micro heat pipe using DI water as the working fluid. The effects of wall superheat and wall wettability on bubbles’ nucleation, growth, and detachment in the evaporation chamber were compared and analysed. Moreover, the influence of the wall superheat on the thermal performance of the micro heat pipe was specifically examined. It is found that the low-level pool boiling occurs with a wall superheat that is higher than 0.357T c, indicating a high heat flux for heat removal. Interestingly, when the relatively hydrophobic chamber wall is superheated by 0.46T c, there are more nucleation sites, and the complete bubbles cycle containing nucleation, growth, and detachment is longer. In contrast, on a hydrophilic surface under the same and sufficient superheat condition, the number of bubble nucleation sites is less than that on the hydrophobic surface, and the complete cycle of bubbles is shorter. It is worth mentioning that the model has values for predictions of homogeneous nucleation of bubbles and thermal transport on hydrophobic or hydrophilic surfaces.

Development of bubble dynamic parameter models in subcooled flow boiling of saline solution

Concentrated saline solutions are inevitably used as coolants in the industry heat exchangers. From the aspect of simulation prediction, the closure models for RPI model, including the models of bubble departure diameter (Dw), departure frequency (f) and active nucleation site density (Nw), are of great importance for the overall success of heat transfer calculation. However, the applicability of the existing closure models for the nucleate flow boiling of saline solutions is yet to be answered. In this paper, the experiments of subcooled flow boiling of the NaCl solutions with concentration range of 0%–6% were conducted. The experimental data of the three bubble parameters, Dw, f and Nw, were obtained under different operating conditions, making up for the shortage of the bubble parameter data of saline solution in literature. The impacts of key influencing parameters for each of the three bubble parameters were discussed. For model development, the prediction performance of several existing representative Dw, f and Nw models on saline solution were studied. The results indicate that the available models for Dw, f and Nw show rather large deviations for saline solutions. Therefore, new models for Dw, f and Nw were developed according to the correlation method and the influencing parameter analyses. The parameters and dimensionless groups, which can be used to characterize the variations of Nw, Dw and f, were determined. The dimensionless parameters including Boiling number (Bo), density ratio (ρ⁎), dimensionless surface tension, Prandtl number and a suggested dimensionless temperature (ΛT), can be used to characterize the variation of Dw. And the dimensionless groups of Bo, ρ⁎, ΛT, and the non-dimensional bubble departure diameter, can be used to correlating with the non-dimensional departure frequency. As for Nw, the wall superheat and solution concentration are the most critical factors for model correlating. The new correlations for predicting the Dw, f and Nw during subcooled flow boiling in both the pure water and saline solutions were built and validated.

Effect of orthogonal channel structure on the heat transfer in pool boiling and its heat flux prediction model

Pool boiling has a heat transfer performance far exceeding that of traditional heat transfer methods, so it has broad application prospects. At present, the mainstream means to improve the boiling performance are mainly to change the heat transfer surface and to change the liquid working medium. Previously, a T-shaped microchannel structure has been designed, whose maximum heat flux was 338% of that of a smooth surface, and whose maximum boiling heat transfer coefficient was 184.7% of that of a smooth surface. The structure of this paper is further developed on the basis of this structure. It is found that the orthogonal channel structure can better improve the heat transfer effect. Specifically, within a certain range, the promotion effect of heat transfer is positively correlated with the increase of fin length. When the wall superheat is 12.3 K, the heat flux of the best orthogonal channel structure S4 is 358% of that of the smooth surface. The maximum heat transfer coefficient of S4 is 11.9 W/(cm2·K), which is 281.3% of that of the smooth surface. In addition, a prediction model is proposed for the structure of this paper, and its prediction error is within 25% under medium and high heat flux.

Experimental study of heat transfer in a microchannel with pin fins and sintered coatings

ABSTRACT Increasing processing capacity without modifying the size of electronic devices has made thermal management important in the electronic industry. Commercialized thermal management, such as in a conventional air cooling system, is insufficient for electronic devices with high heat flux dissipation. Pool boiling is a better method for heat transfer because it can dissipate a substantial amount of heat at low wall superheats. This study focused on heat transfer enhancement using passive approaches, including nanostructures and microporous sintered surfaces over open microchannel surfaces and microchannels with pin-fins. In the present work, seven structures were studied in pool boiling, wherein experiments elucidated the effects of microchannels, sintered, and pin fins (micropillar) on boiling heat transfer from a copper chip in a pool of degassed water. Boiling performance is ascertained via critical heat flux (CHF) and heat transfer coefficient (HTC). The best heat transfer performance showed a heat flux of 243.75W/cm2 at 15.46°C on the pin-fins chip, which was 1.9 times the heat flux of the plain chip. The highest HTC was 181.03 kW/(m2 oC) at a heat flux of 172.61 W/cm2 for the microchannel with single pin-fins. The HTC enhancement was 2.8 times greater than the plain surface. It was found experimentally that HTC and CHF improved on all modified surfaces compared to the plain copper chip baseline.

Dimensional Analysis in Error Reduction for Prediction of Nucleate Boiling Heat Flux by Artificial Neural Networks for Limited Dataset

Abstract In this work, artificial neural networks (ANNs) is used to predict nucleate boiling heat flux by learning from a dataset of twelve experimental parameters across 231 independent samples. An approach to reduce the number of parameters involved and to increase model accuracy is proposed and implemented. The proposed approach consists of two steps. In the first step, a feature importance study is performed to determine the most significant parameters. Only important features are used in the second step. In the second step, dimensional analysis is performed on these important parameters. Neural network analysis is then conducted based on dimensionless parameters. The results indicate that the proposed feature importance study and dimensional analysis can significantly improve the ANNs performance. It also show that model errors based on the reduced dataset are considerably lower than those based on the initial dataset. The study based on other machine learning models also shows that the reduced dataset generate better results. The results conclude that ANNs outperform other machine learning algorithms and outperform a well-known boiling correlation equation. Additionally, the feature importance study concludes that wall superheat, gravity and liquid subcooling are the three most significant parameters in the prediction of heat flux for nucleate boiling. Novel results quantifying parameter significance in surface tension dominated (SDB) and buoyancy dominated (BDB) boiling regimes have been reported. The results show that surface tension and liquid subcooling are the most significant parameters in SDB regime with a combined contribution percentage of 60%, while wall superheat and gravity are the most significant parameters in BDB regime with a combined contribution percentage of 70%.

Experimental Study on Combustion Optimization of Supercritical 600MW Quadrangle Tangential Boiler

Given the problems existing in the operation of supercritical 600MW boiler, such as spiral pipe water cooling wall outlet, superheater and reheater wall surface overtemperature easily, the superheater and reheater cooling water amount are too large under high load, and the low load cooling water amount is too small to meet the need of cold reheater heating, the boiler combustion optimization and adjustment are carried out. Through the boiler pulverized coal fineness, air distribution characteristics, swinging Angle of burner and furnace outlet is optimized, such as oxygen level superheater heating surface overtemperature problems are solved, such as the reheater desuperheating water flow under different load can meet the needs of heat supply, at the same time, it also improves the boiler efficiency, effectively improve the safety and economy of the unit.

Chilldown of cryogenic feed lines- An insight into the influence of feed line orientation and mass flux

Chilldown of cryogenic feed lines is the most indispensable part of any process that handles cryogenic fluids. Prediction of chilldown time ‘apriori’, requires a complete understanding of the influencing factors and the phenomenon involved. In the present study, the effect of feed line orientation and mass flux on chilldown performance is experimentally evaluated using liquid nitrogen. Experiments are carried out with a foam insulated stainless steel test section. Tests are performed for two different mass flux conditions with test section held in horizontal position as well as varying upward and downward inclinations. In tests with upward orientation, a significant jump in heat flux at top region is seen when the wall superheat reduces below a critical value and this is attributed to onset of liquid wetting at top region. In tests with downward inclination, a minor increase in the heat flux at top region is seen beyond 30° downward orientation attributable to the flow structure prevailing in the test section. It is noted that heat flux pattern at bottom region is not influenced much by test section orientation. Leidenfrost temperature, minimum wall heat flux, and critical heat flux values obtained from the present study are compared with the predictions made using correlations available in published literature. The findings of the present study would help to understand better the influence of feed line orientation and mass flux on wall heat flux pattern and thereby improve the prediction capability.

Saturated pool boiling heat transfer enhancement of R245fa based on the surface covered by sintered copper powder with and without nanostructure

The pool boiling characteristics on the enhanced surfaces with sintered copper powder coatings with and without nano-treatment were experimentally investigated. Three kinds of enchanted surfaces formed by sintered porous layers with copper powder particles and their further nano-treated surfaces were adopted as the test surfaces. The thermal oxidation process was used to form nanostructures with super hydrophilic characteristic on the surface of sintered copper particles. Refrigerant R245fa was selected as the test fluid due to the potential the application in electronic cooling field. The saturation temperature of boiling experiments was maintained about 20 °C. The result showed that the boiling heat transfer performance was effectively improved by coating sintered copper powder porous layer on the plain copper surface, and the maximum heat transfer coefficient and CHF value enhancement could reach 151% and 160% of the plain one, respectively. The enhanced surfaces no matter with or without nano-treatment could trigger the nucleation process at the wall superheats lower than 12 °C rather than the value as high as 30 °C for plain copper surface. After nano-treatment, the heat transfer enhancement for sintered porous surfaces was further improved at medium and low heat flux, where the bubble nucleation or detachment remained robust. The CHF peaked as 140% of untreated surface, while the heat transfer coefficient increased to 108% at most.

Effects of electric field on pool boiling heat transfer over composite microstructured surfaces with microcavities on micro-pin-fins

In this study, we proposed to achieve synergistically enhanced pool boiling performance by combining the effects of an electric field and composite microstructures. To test the proposed strategy, the pool boiling heat transfer performances of four kinds of microstructured surfaces, including a micro-pin-fined surface (PF-30–60) and three kinds of composite microstructured surfaces with different numbers of microcavities at the top of each micro-pin-fin, were investigated in HFE-7100 under an electric field. The experimental results show that the bubbles on the composite microstructured surfaces are preferential to generate at the top of the micro-pin-fins, which not only effectively reduces the wall superheats at the onset of nucleate boiling on the composite microstructured surfaces, but also prevents a large number of bubbles from generating at the roots of the micro-pin-fins, and thus effectively alleviates the deterioration of boiling heat transfer caused by the field trap effect. Such a trend becomes more obvious with the increase in the number of microcavities. The CHF of the composite microstructured surface can be improved by 50%∼100% by the electric field, but it can only be enhanced by 26%∼78% on PF-30–60. Compared with that of the smooth surface, the CHF of PF-30–60 is increased by 36%∼52%, while the CHF of the composite microstructured surface is increased by 75%∼91% under the electric field.

Coupling Enhancement Effect of the Magnetic Field and Wall Superheat on Boiling Heat Transfer Characteristics of Magnetic Nanofluid (MNF) under Reduced Gravity

Coupling Enhancement Effect of the Magnetic Field and Wall Superheat on Boiling Heat Transfer Characteristics of Magnetic Nanofluid (MNF) under Reduced Gravity

Heat Transfer Deterioration by the Copper Oxide Layer on Horizontal Subcooled Flow Boiling

Water–copper is one of the most common combinations of working fluid and heating surface in high-performance cooling systems. Copper is usually selected for its high thermal conductivity and water for its high heat transfer coefficient, especially in the two-phase regime. However, copper tends to suffer oxidation in the presence of water and thus the heat flux performance is affected. In this research, an experimental investigation was conducted using a cooper bare surface as a heating surface under a constant mass flux of 600 kg·m−2·s−1 of deionized water at a subcooled inlet temperature ΔTsub of 70 K under atmospheric pressure conditions on a closed-loop. To confirm the heat transfer deterioration, the experiment was repeated thirteen times. On the flow boiling region after thirteen experiments, the results show an increase in the wall superheat ΔTsat of approximately 26% and a reduction in the heat flux of approximately 200 kW·m−2. On the other hand, the effect of oxidation on the single phase is almost marginal.

Experimental investigation on the effects of swirling flow on flow boiling heat transfer and instability in a minichannel heat sink

Flow boiling in micro/mini-channel heat sinks provides an effective thermal management solution for high-heat-flux cooling applications. However, the plain micro/mini-channel heat sink has some shortcomings, such as severe boiling instability and insufficiency for more stringent heat dissipation requirements. To overcome these shortcomings, this work designs a minichannel heat sink (MCHS) with twisted tape inducing swirling flow. The cross-section of minichannel is square, as well as the width, height and length of minichannel are 2 mm, 2 mm and 150 mm, respectively. The twisted tape length (Ltt = 50, 100 and 150 mm) and twist ratio (R = 3, 4 and 5) are selected as designed parameters to generate different swirling flow intensities. The flow boiling heat transfer and instability in such MCHS are experimentally examined under two mass fluxes (120.7 and 210.5 kg/(m2·s)), two inlet temperatures (70 and 80°C) and a range of heat flux (42.3–263.0 kW/m2). Results show that the swirling flow can mitigate the rapid growth for bubbles and suppress the coalescence of small bubbles effectively, as well as cause the bubbles to generate helical motion behavior. Especially, the micro-corners between the twisted tape and minichannel wall provide more effective nucleation sites to produce more small bubbles. Thus, the swirling flow induced by twisted tape can not only trigger onset of nucleate boiling (ONB) at low wall superheat and improve heat transfer performance, but also suppress the flow boiling instability. Smaller twist ratio or longer length of twisted tape results in stronger swirling flow, thus bringing greater heat transfer enhancement effect. Also, longer twisted tape presents better stable flow boiling, but the twist ratio has an insignificant effect on boiling instability. Compared to plain MCHS, the MCHS having twisted tape with Ltt = 150 mm and R = 3 achieves a 34.4% decrease in wall superheat of ONB and a maximum of 80.9% increase in heat transfer coefficient. Furthermore, this MCHS reduces the relative standard deviations of inlet pressure by 44.4% at heat flux of 100.5 kW/m2 and mass flux of 120.7 kg/(m2·s). These findings suggest that using twisted tape insert to induce swirling flow can improve overall performance for minichannel heat sinks effectively.

Measurements and identification of supercritical pseudo-boiling heat transfer modes based on fiber optic probes and multiscale entropy

Subcritical boiling includes interfacial evaporation and heat transfer induced by bubble dynamics. However, for supercritical heat transfer, direct experimental evidence of the existence of pseudo-evaporation and pseudo-boiling heat transfer, as well as the conversion between them is lacking. In this work, the experimental study of supercritical carbon dioxide pool heat transfer is conducted. The pressure and temperature of the cell are 8–10 MPa and 15 ℃, respectively. As heating element and temperature sensing element, a nickel-chromium alloy wire with a length of 22 mm and diameter of 70 μm is placed horizontally in the high-pressure cell. The fiber optic probe is placed vertically, with its tip 200 μm above the wire. Four heat transfer modes, i.e. natural convection, pseudo-evaporation, transition of evaporation and boiling, and pseudo-boiling, are found to occur sequentially with the increase of heat flux density or wall superheat. Natural convection occurs when the wall temperature is below the pseudo-critical temperature. This work focuses on pseudo-evaporation and pseudo-boiling heat transfer and the transition between them. In the pseudo-evaporation mode, the heat transfer coefficient decreases slightly with wall superheat increasing. The fiber outputs a high frequency signal with small amplitude, and there is no dominant frequency. The multiscale entropy is large, characterizing random signal fluctuations. In the transition of evaporation and boiling mode, the fiber outputs a large-amplitude/low-frequency periodic signal with a significant dominant frequency and small multiscale entropy, representing an ordered periodic pulsating heat transfer. In the pseudo-boiling mode characterized by bubble-like structure, the fiber signal fluctuation amplitude and multiscale entropy are between the counterparts of the first two modes i.e. natural convection mode and pseudo-evaporation mode. The dominant frequency is not obvious. The multiscale entropies in the specific case are calculated under different key parameters, such as dimensionality, time scale factor, and length of origin data. Optimal parameters are selected based on the best separation of heat transfer modes. Finally, pseudo-boiling can be distinguished from pseudo-evaporation by multiscale entropy of 0.9, and from transition of evaporation and boiling by multiscale entropy of 0.5. In this work, direct experimental evidence of supercritical-like boiling is obtained, which deepens the understanding of the supercritical heat transfer mechanism and provides a basis for theoretical studies and engineering applications in future.