Maximum Heat Transfer Coefficient Research Articles

Experimental comparative study on thermal performance of latent heat storage tanks with pin, perforated, and rectangular fins at different orientations

The low thermal conductivity of phase change materials (PCMs) has limited their widespread use in practical applications. In the present study, different fin structures, namely, rectangular, perforated, and pin were examined to analyze the thermal performance of the melting process in rectangular latent heat storage tanks. Experiments were performed at both horizontal and vertical orientations to evaluate the effectiveness of different fin configurations. Visual observation of the phase change evolution at different time intervals was enabled through a transparent plexiglass shell. Instantaneous heat transfer rate and energy storage were measured using thermocouple readings and melting photographs. The results show that the maximum heat transfer coefficient between the heated wall and PCM is obtained by the pin-finned tank followed by perforated and rectangular-finned tanks. This thermal behavior is justified by the intensification of the upward convection flows through the voids provided by pin fins or perforated fins. Although the rectangular fin structure has the lowest convective heat transfer coefficient, its heat transfer rate is slightly higher than the other structures due to its larger heat transfer area. At a wall temperature of 70℃, the convective heat transfer coefficient and heat transfer rate obtained by the pin fin configuration are respectively 25% higher and 4% less than those of the rectangular fin. It reveals that the pin fin structure provides the most effective heat transfer area compared to its counterparts which have a significantly larger fin volume. In addition, it was found that regardless of the fin configuration, the melting rate in the horizontal tank was significantly higher than in the vertical tank due to the formation of more vortical flow structures within the molten PCM. The melting time in the unfinned horizontal tank was less than those of the vertical finned tanks implying that the tank orientation should be well-chosen to minimize the melting time along with adding fins of various configurations.

On the Aero-Thermal Performance of Rim Film Cooling in the Squealer Tip of a Linear Turbine Cascade

Abstract The present study proposes a novel rim film cooling design, motivated by the fact that the suction-side rim and the cavity floor near the leading edge of a conventional squealer tip with a camber-line film hole array are directly subjected to high-temperature gas. The new design consists of cooling injection for rim-hole or -slot cooling at the leading edge. The parameters of injection geometry and cavity depth are also discussed in this study. The flow physics, leakage flowrate, heat transfer characteristics, and vortices in the cavity are carefully analyzed. The results show that rim film cooling can substantially inhibit tip heat transfer and the amount of hot leakage flow. Compared with the typical design, the maximum and average heat transfer coefficients of the blade tip of the rim slot case are reduced by 12.83% and 5.43%,, respectively. The variation in cavity depth is sensitive to the heat transfer on the cavity floor of the squealer tip blade. With the optimal design, the average and maximum heat transfer coefficients are reduced by 14.42% and 14.21%,, respectively. In addition, the leakage flowrate can be reduced by a maximum of 3.67% by rim injection compared with the conventional squealer tip.

DEVELOPMENT OF HEAT EXCHANGER WITH TWO-PASS DIVERGING MICROCHANNELS FOR EVAPORATION COOLING SYSTEMS

This study developed a two-pass diverging microchannel heat exchanger to avoid flow reversal and partial dry out of HFC-245fa flow boiling in the evaporation cooling systems. It is aimed to improve the diverging microchannel heat exchangers designed by previous studies and to effectively accommodate diverging channels on a limited heating base surface. The test results showed that the maximum heat transfer coefficients in a diverging channel heat exchanger were as much as 20% higher than those in straight channel heat exchanger, while the maximum partial dry-out heat flux is 42% higher in diverging channel heat exchanger. There is no significant difference for the pressure drops in two heat exchangers at low heat fluxes before it reached the partial dry-out condition. For higher flux, the pressure drops in straight channels were higher than those in divergent channels while partial dry out. It is concluded that the two-pass diverging channel design provided higher maximum heat transfer coefficients and higher maximum heat fluxes with lower penalties of pressure drops. It could be a good design to replace the conventional straight channel heat exchanger for evaporation cooling systems.

POOL BOILING HEAT TRANSFER CHARACTERISTICS OF POROUS NICKEL MICROSTRUCTURE SURFACES

Pool boiling is an effective heat dissipation approach in electronic cooling, battery thermal management, etc. This study used the electrochemical deposition method to fabricate one smooth nickel specimen (named Ni-smooth) and three specimens with a porous nickel-stacked structure. The three porous specimens were created with deposition current densities of 0.5 A·cm<sup>-2</sup> (named Ni-0.5), 2.0 A·cm<sup>-2</sup> (names Ni-2.0), and 5.0 A·cm<sup>-2</sup> (named Ni-5.0), respectively. The four samples underwent microstructural characterization via scanning electron microscopy. The increasing current density led to the porous nickel surface exhibiting a more distinct pore structure, and the nickel sphere grains became more refined, developing a loose "mound-like" structure. A marked increase in the nickel film thickness was also observed. Through visual experiments, we evaluated their wettability, and through pool-boiling experiments, we tested their boiling heat-transfer properties. Our findings suggest that samples incorporating a porous nickel structure consistently outperform unmodified samples regarding heat-transfer efficiency. Specifically, sample Ni-0.5A demonstrated the most optimal boiling heat-transfer performance, evidenced by a 32.2% reduction in temperature at the onset of boiling, a 19.9% increase in critical heat flux density, and a 78.6% larger maximum heat-transfer coefficient compared to the smooth nickel sample. These marked improvements are intrinsically linked to the specific characteristics of the porous nickel structure. The higher performance of samples Ni-0.5 can be attributed to the presence of additional nucleation sites within the porous structure and the formation of smaller micro-crystalline dendritic constructs due to the specific current density applied during electrodeposition. Understanding this relationship between surface characteristics and electrodeposition is essential in maximizing heat-transfer efficiency.

Micro/nanostructuring of metal additively manufactured aluminum alloy for enhanced pool boiling of dielectric fluids

Surface micro/nanotextures have the potential to increase bubble nucleation and promote capillary liquid wicking to enhance pool boiling heat transfer significantly. Despite past studies implementing surface micro/nanofabrication strategies to enhance boiling, the role of structure length scale and morphology on boiling heat transfer coefficient (h) and critical heat flux (CHF) of low-surface-tension dielectric fluids remains unclear. In this study, we systematically tune the surface microstructures on additively and conventionally manufactured aluminum alloys (AM AlSi10Mg and Al6061) from 1 µm to 5 µm to study the influence of microstructure length scale on boiling performance. In addition, to further understand the effect of submicron structures on boiling, nanostructures of 300 nm length were generated on a plain AM surface and hierarchically incorporated atop the microstructures. Dielectric fluid, HFE-7100, was used to investigate the effects of surface morphology on pool boiling heat transfer coefficient (h) and critical heat flux (CHF). Our results show that the 5 µm microstructured AM surface, AM-H(400)E(5), attained a CHF of 19.44 W/cm2 and a maximum heat transfer coefficient (hmax) of 2.89 W/cm2·K, which represents a reduction in CHF of 28.5 % and an enhancement in hmax of 103.8 %, as compared to the plain Al6061 surface. In addition, AM-H(400)E(5) achieved a large enhancement ratio of 2.2 as compared to the plain Al6061 at a heat flux of 20 W/cm2, and the highest h value amongst all structured surfaces, indicating its cavity size is optimal for bubble nucleation. Through the systematic tuning of micro/nanostructures length scale and morphology, this study found that the micro/nanostructures of 1 µm size and below are too small to serve as bubble nucleation sites. Additionally, micro/nanostructure wickability plays a negligible role in affecting pool boiling performances of highly wetting dielectric fluids, while surface morphology plays a dominant role in bubble nucleation to enhance boiling. Lastly, high-speed immersion microscopy was performed to understand the effects of micro/nanostructures on bubble characteristics such as bubble departure diameter and growth period. In summary, this work not only reports the first micro/nanostructured AM surfaces utilizing scalable fabrication techniques for enhanced boiling, but it also demonstrates the potential of tunning the micro/nanostructure length scale to optimize the bubble nucleation site density, resulting in significantly improved pool boiling performances.

Experimental investigation on flow boiling heat transfer characteristics of R245fa/oil mixture in horizontal smooth tube

During the operation of the Organic Rankine Cycle (ORC) system, it's inevitable that the lubricating oil in the expander will mix with the working fluid, leading to a noticeable impact on its heat transfer performance. In this investigation, we conducted experimental research on the flow boiling heat transfer characteristics of a mixture of R245fa and POE (Polyolester) in a smooth horizontal tube. We explored the effects of various parameters, including oil concentration (ranging from 0% to 4%), heat flux (6 to 20 kW/m²), mass flux (ranging from 150 to 410 kg/(m²·s)), and vapor quality (ranging from 0 to 0.8). The results revealed that the heat transfer coefficient (HTC) for R245fa/POE increased with higher vapor quality and mass flux for each oil concentration. However, as the oil concentration increased, the rate of increase in HTC gradually diminished. Notably, an optimal oil concentration of 2.3% was identified to achieve a maximum HTC of 6 kW/(m²·K) in our experiments. Furthermore, our investigation into frictional pressure drop (FPD) indicated that higher vapor quality, mass flux, and heat flux all led to an increase in FPD for the R245fa/POE mixture. Additionally, we evaluated the impact of lubricants on the HTC and FPD of R245fa/POE mixtures using enhancement factor (EF) and penalty factor (PF), respectively. The EFs for HTC in the R245fa/POE mix ranged mainly between 0.95 and 1.6, while the PFs for FPD were concentrated between 0.75 and 1.4. The lubricating oil was found to enhance the flow boiling heat transfer performance of R245fa, with a comprehensive evaluation index range of 1.22 to 1.31. In terms of prediction accuracy, the Schlager-R22/150SUS correlation provided the most reliable estimates for the enhancement factor of HTC and the penalty factor of FPD, with mean absolute deviations (MAD) of 9.79% for EF and 14.03% for PF.

Enhancement of pool boiling performance through an asymmetric dual V-groove microchannel structured surface

In pool boiling, microchannels on the heated surface change the bubble dynamics and consequently improve the heat transfer. In this study, two new microchannel configurations, i.e., asymmetric dual V-groove microchannels (VM) and orthogonally intersecting asymmetric dual V-groove microchannels (OIVM) have been introduced to study enhancement in pool boiling. The heat transfer enhancement due to the modified surfaces was established by comparing the heat transfer rates from the reference flat surface. Experiments have been performed using deionized water under saturation temperature at one atmosphere to evaluate the steady state heat transfer characteristics. The bubble dynamics and interface morphology at various heat flux levels have been captured using high-speed camera. The experimental observations have revealed that both the microchannel configurations exhibit a substantial improvement in heat transfer compared to that of flat surfaces. The OIVM surface demonstrates the highest rates of heat transfer and increase of 5.3 times in the maximum heat transfer coefficient (HTC) and 2.4 times in the critical heat flux (CHF) were observed compared to that of flat surfaces. Similarly, the VM surface demonstrates an improvement of 2.2 fold in HTC and 1.7 fold in CHF compared to the flat surface. Moreover, the OIVM surface demonstrates better heat transfer rates than that of modified surfaces investigated in the past studies. The increased heat transfer rate with the OIVM surface can be ascribed to large area for heat exchange, increased bubble initiation sites, bubble motion promoted macroconvection and efficient rewetting mechanism.

Two-phase flow condensation heat transfer characteristics of R-134a inside three-dimensional cylindrical micropillar enhanced tube

This paper presents an experimental investigation for evaluation of heat transfer coefficient, frictional pressure drops and flow regimes with their transitions during the condensation of R-134a inside a smooth, two microfins (MF1 and MF2) and a newly developed micropillar (MP) tubes. The experiments are carried out for a range of mass flux at an average saturation temperature of 35°C. Micropillar tube has produced the maximum heat transfer coefficient, while the microfin (MF1) tube resulted the highest frictional pressure gradient compared to other tubes. The experimental observations of microfin and micropillar tubes are plotted on the mass flux versus vapor quality flow map and Taitel and Duckler flow map to discuss the transitions within different flow patterns achieved. Visual inspection of the flow regimes with varying mass flux and vapor quality showed stratified-wavy, intermittent, and annular flow regimes. The occurrence of annular flow is found to be more in MF1 and MP tube. A performance evaluation factor (PEF) is defined to evaluate the thermal-hydraulic performance of the tubes. Using the same, it has been observed that the newly developed MP tube has a PEF value greater than unity which suggest augmentation of heat transfer coefficient at the expense of lesser pressure drop penalty.

The Effects of Using a Trombe Wall Modified with a Phase Change Material, from the Perspective of a Building’s Life Cycle

The increasing costs related to the use of primary energy carriers, and greater social awareness related to the need for energy saving, necessitate the use of renewable energy sources, including solar radiation. The Trombe wall (thermal storage wall—TSW) is an indirect passive solar energy system solution, aimed at obtaining, storing, and transferring thermal energy into buildings. However, there is no comprehensive information on the impact of the use of such solutions on environmental performance in the life cycle of buildings, especially those located in temperate climates. The aim of this paper is therefore to determine the environmental impact of the construction of barriers using phase change materials (PCM) from the perspective of the life cycle of a model building conforming to the current Polish energy standard (EP < 70 kWh/(m2∙yr)). The subject of the research is the structure of a TSW using phase change materials and a reference wall with a maximum overall heat transfer coefficient of 0.2 W/m2K. A comprehensive computational model of a residential building located in Rzeszów, Poland, was created, taking into account the thermal parameters of the analyzed structure of the wall and its operation under real, specific climatic conditions, as well as the environmental characteristics. High-quality input data (based on real, long-term measurements) were used to conduct a Life Cycle Assessment of the analyzed variants. As a result, the energetic and environmental efficiency of the analyzed thermal storage wall, from the perspective of the whole building’s life cycle, were assessed. According to the analyzed data, a TSW modified with paraffin enables the reduction of the energy requirements for heating by 11.3%, and the payback period of the environmental load does not exceed 1 (GWP) and 5 (IMPACT2002+) years, which were lower than the monitored period of operation.

Heat transfer characteristics and cooling capacity of online quenching equipment for plate

The finite element method was taken to simulate the heat transfer characteristics and capacity of the online cooling equipment of a 3500 mm medium thickness plate production line. The cooling field on the steel plate surface could be divided into the impact zone and the wall jet zone after cooling water was sprayed on the surface of the steel plate. The impact zone had an intense heat transfer capacity because of the static pressure and the high radial velocity of cooling water. The maximum heat transfer coefficient (HTC) in the impact zone of spray and laminar cooling equipment could reach 20,334 and 10,096 W/(m2 °C), respectively. In the wall jet zone, the radial velocity of cooling water was the main factor affecting the heat transfer capacity. With the decrease in velocity, the HTC decreased gradually. There is a linear relationship between the average HTC and the water flow. The average HTCs of spray and laminar cooling equipment in the common range of water flow were 5904–7255 W/(m2 °C) and 3012–3369 W/(m2 °C), respectively. The HTCs were verified in the 3500 mm medium thickness plate production line.

Validation and Application of a Finned Tube Heat Exchanger Model for Rack-Level Cooling

Abstract A thermosyphon-based modular cooling approach offers an energy efficient cooling solution with an increased potential for waste heat recovery. Central to the cooling system is an air-refrigerant finned tube heat exchanger (HX), where air is cooled by evaporating refrigerant. This work builds on a previously published two-dimensional (2D) model for the finned-tube HX by updating and validating the model using in-house experimental data collected from the proposed system using R1233zd(E) as the working fluid. The results show that key system variables such as refrigerant outlet quality, air and refrigerant outlet temperatures, and exchanger duty agree within 20% of their experimental counterparts. The validated model is then used to predict the mean heat transfer coefficient on the refrigerant side for each tube in the direction of airflow, indicating a maximum heat transfer coefficient of nearly 1200 W/(m2 K) for a HX duty of 5.3 kW among the tested cases. The validated model therefore enables accurate predictions of HX performance and provides insights into improving the heat exchange efficiency and the corresponding system performance.

Experimental investigation of heat transfer characteristics of inclined aluminium two phase closed thermosyphon

Abstract A reduction in the size of electronic equipment increases the heat generation rate. Failure of electronic equipment occurs if the heat is not dissipated properly. This paper examines the performance of aluminium two-phase closed thermosyphon for cooling electronic equipment. Acetone charged aluminium two-phase closed thermosyphon was fabricated with an inside diameter of 17.05 mm and 1 mm thickness. A series of experimentations were performed for inclination angles of 10°–90° at selected filling ratios of 30, 60 and 100 %, along with heat inputs of 100, 200 and 300 W. The condenser section flow rate of water was maintained constant. Minimum thermal resistance was obtained at a 30° inclination angle for all filling ratios and heat inputs. The evaporator and condenser sections have a maximum heat transfer coefficient at a 30° inclination angle. Thermosyphon, with a 30 % or 60 % filling ratio, performed better than a 100 % filling ratio for all inclination angles and heat inputs. As the heat input was increased, the heat transfer coefficients of the evaporator and condenser section were increased, whereas total thermal resistance decreased. For 300 W heat input and 30 % filling ratio, the minimum thermal resistance at a 30° inclination angle was 0.158 °C/W. It is found that, the same heat input and filling ratio, the maximum heat transfer coefficient value for the evaporator and condenser section at a 30° inclination angle was 1602 W/m2 °C and 5652 W/m2 °C, respectively.

Optimizing additively manufactured macro-fin structure designs for enhanced pool boiling of dielectric fluids

This paper presents the results of the saturated pool boiling heat transfer performances of macrostructured fins fabricated by Selective Laser Melting, a metal additive manufacturing technique. A plain surface, two-dimensional triangular and rectangular fins, and three-dimensional pin and segregated fin array designs investigated in this study were fabricated using AlSi10Mg. A dielectric fluid, HFE-7100, was used as the cooling medium in a water-cooled thermosyphon to test the structures under atmospheric conditions. A macro-fin design optimization approach was developed by experimentally characterizing the boiling performances and bubble dynamics in fin arrays with different fin parameters such as fin topology, angle, and height. From the results obtained, this study optimized and tuned the macrostructure design to eliminate the detrimental bubble dynamics that were identified through high-speed visualization. Following a systematic study of the boiling characteristics of two-dimensional triangular and rectangular fins, and three-dimensional pin fins, the segregated fin array was developed to tackle the unfavorable bubble dynamics in these conventional fin geometries to enhance both critical heat flux and heat transfer coefficient. Various performance metrics were used to compare and evaluate the boiling performances of different fin topologies. From our studies, the segregated fin specimen (Seg-3) was determined to achieve the best balance between critical heat flux and heat transfer coefficient enhancements, attaining a critical heat flux of 64.19 W/cm2 and a maximum heat transfer coefficient of 2.22 W/cm2·K, enhancing critical heat flux and heat transfer coefficient by up to 97.2 % and 43.1 % as compared to the plain surface, respectively. Furthermore, Seg-3 has achieved boiling enhancements with only 334 mm2 and 50.6 mm3 of surface area and fin volume, respectively, a substantial surface area and fin volume reduction as compared to conventional macro-fin geometries with the same fin height. In all, this work not only provides fundamental insights into the bubble dynamics in various macro-fin structures and their influence on the boiling performance, but it also presents a macro-fin design optimization strategy that can be adopted for other immersion cooling applications.

A new paradigm for the role of disturbance waves on film dryout and wall heat transfer in annular two-phase flow

The entire liquid-film dryout process in vertical annular two-phase boiling flow is characterized experimentally, from inception to completion, in a newly defined flow regime between annular two-phase flow and mist flow, called ‘intermittent dryout’. Experiments are conducted using saturated R245fa flowing vertically through a heated transparent channel. Liquid-film thickness measurements are taken during dryout events. The state (wet or dry) of the heated surface is determined using a laser reflectance measurement, and the signal is used to calculate the time-averaged dry fraction, fdry. The local heat transfer coefficient (HTC) is characterized as a function of fdry. For all the investigated mass fluxes, optimum boiling conditions (OBC), corresponding to the heat flux that results in maximum HTC, consistently occur when the time-averaged dry fraction is fdry≈0.05. Subsequently, the critical boiling transition (CBT), corresponding to the heat flux that results in a significantly lower HTC, can occur for dry fractions of fdry>0.1. Further insight into the liquid film behavior within the intermittent dryout regime is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. In general, the dryout mechanism is dominated by disturbance waves.Mechanistic model frameworks are presented for the prediction of two-phase HTC and heat flux associated with dryout. The model inputs are local flow quantities (pressure, mass flux, vapor quality, and liquid-film thickness) and output a prediction for local heat transfer coefficient. The model framework accurately predicts the local two-phase HTC for a range of mass flux and vapor quality in the annular flow regime. The model framework captures the observed initial increase in the local HTC and subsequent decrease that is associated with liquid-film dryout. The mechanistic approach provides predictions for time-averaged HTC and OBC heat flux that both agree well with experimentally measured values.

Analysis of the influence of deflector shape on heat transfer rate in glass tempering process

To solve the problem of uneven cold air flow during the tempering process, which causes glass self-detonation, distortion, and excessive fan energy consumption. Three deflector-shaped structures are proposed to enhance the cooling efficiency of the glass. Based on the combination of fluent numerical analysis and experiment, this study gives an in-depth examination of the heat transference of glass within a quenching system. The influence of deflector shapes on the heat transfer efficiency of glass is explored. The three deflectors were compared with traditional deflectors in terms of temperature, fluid flow, total heat transfer rate, and heat transfer coefficient. The simulation results agree well with the experimental data, and the established model of the glass quenching system is reliable. The rectangular deflectors reduce the glass base temperature by 3.03 K compared to traditional deflectors. Furthermore, when the rectangular deflectors are utilized, the maximum heat transfer coefficient of the glass can be up to 96.4 W/(m2·K) and the maximum total heat transfer can be up to 2622.82 W. In comparison to the traditional deflector shape, the maximum heat transfer coefficient of the designed rectangular deflectors may be enhanced by 0.58% and the maximum total heat transfer by 0.37%. The proposed design provides a suitable solution for the optimization of the tempering process as well as for the energy savings and efficiency of tempering equipment.

Boiling heat transfer and pressure drop of R290 in a micro-fin tube

The present study focuses on the analysis of heat transfer and pressure drop of R290 in a horizontal micro-fin tube. In this regard, experiments were conducted, and flow patterns, heat transfer coefficient, and friction pressure drop were analyzed under various conditions. A wide range of conditions was studied, including saturation pressures ranging from 0.215 to 0.415 MPa, mass fluxes ranging from 70 to 190 kg m−2 s−1, heat fluxes in the range of 10.6–73.0 kW m−2, and vapor qualities from 0 to 0.96. The performed analyses revealed four flow patterns, including bubbly flow, plug flow, slug flow, and annular flow. It is found that the onset of dry-out occurs when the liquid film thickness is equal to the fin height and occurs earlier at high heat fluxes and low saturation pressures. The maximum heat transfer coefficient and friction pressure drop occur at the onset of dry-out, and the heat transfer coefficient exhibits a significant correlation with the heat flux. The obtained results were verified using the heat transfer correlation of Cavallini and the friction pressure drop correlation of Rollman, where the average relative deviation (ARD) was −24.01% and −8.65%, respectively.

Experimental study for artificial neural network modeling on thermal and flow performances of electric traction motor with oil spray cooling

The oil spray cooling is emerging as advanced thermal management technique for next generation electric traction motor. The performance of oil spray cooling needs to be mapped and predicted based on various influential factors to develop efficient thermal management system for electric traction motors. In the present work, the thermal and flow performances are experimentally investigated and predicted using an artificial neural network (ANN) for electric traction motor with oil spray cooling. The thermal and flow performances are extracted in terms of maximum temperature, heat transfer coefficient, Nusselt number, spray pressure and spray power consumption. The effects of nozzle types, spray heights, volume flow rates and chiller temperatures are analyzed as operating factors. The ANNs are modeled considering Levenberg-Marquardt (LM) training variant, 10 hidden neurons and Tangential-sigmoidal (Tan) and Logarithmic-sigmoidal (Log) as transfer functions. The thermal and flow performances are superior for full cone followed by hollow cone and spiral nozzles in decreasing order. The higher spray height, higher volume flow rate and lower chiller temperature show optimum values of maximum temperature and heat transfer coefficient. The spray power consumption is maximum in case of higher volume flow rate, reported less variation with change in spray height and chiller temperature. The ANN model comprising of LM-Tan algorithm is recommended as the best model with prediction error within ±1% for all thermal and flow performances. The predicted results from the optimum ANN model are validated with experiments with maximum coefficient of determination (R2) and coefficient of variance (COV) as 0.99 and 3.20, respectively. The correlations for thermal and flow performances in terms of all influential factors are proposed using the optimum ANN model.

Numerical study on reaction and heat transfer of supercritical aviation kerosene with pyrolysis and steam reforming in a corrugated cooling channel

Coking and heat transfer deterioration of supercritical aviation kerosene pose grave challenges to regenerative cooling of scramjet engines. In the present study, reaction and heat transfer characteristics of supercritical China aviation kerosene No. 3, RP-3 undergoing simultaneous pyrolysis and steam reforming reactions in a corrugated channel are numerically studied. Compared with reacting flow of RP-3 in the smooth channel, temperature and coke deposition are much lower in majority part of the corrugated channel. The maximum temperature difference between the smooth and corrugated channels is up to 300K. The maximum heat transfer coefficient in the corrugated channel is almost three times of that in the smooth channel under the same condition. However, low-velocity and high-temperature zones are formed near the corrugated micro-structures and cause coke accumulation and heat transfer deterioration along the flow direction of the corrugated channel. With the increase of wall heat flux, both temperature and velocity increase significantly in the corrugated channel. The conversion of RP-3 and formation of coke are also enhanced with increasing wall heat flux. Moreover, the total heat transfer coefficient first increases and then decreases with wall heat flux. The maximum total heat transfer coefficient increases from 4000W/m2∙K to 18,000W/m2∙K with wall heat flux increasing from 0.3MW/m2 to 1.8MW/m2. Periodic wavy structures are found at the interface between the corrugated micro-structure and the bulk flow, which is pronounced when wall heat flux is 1.3MW/m2 or above. With the increase of flow time, local turbulent kinetic energy decreases due to the interaction between the corrugated micro-structures and the wavy structures. The interaction between the corrugated structures and complex reaction and heat transfer leads to intersection of the distributions of temperature and coke in the smooth and the corrugated channels. An empirical heat transfer correlation formula is obtained as Nu=0.1Re0.65Pr1.94 for RP-3 reacting flow in the corrugated channel. The study provides better insight into interaction mechanism between chemically reacting flow and micro corrugated structures.

Enhanced heat transfer using wafer-scale crack-free well-ordered porous structure surface

Boiling heat transfer through porous medium is one of the prevalent cooling strategies in electronic thermal management due to its superior heat transfer capability. However, multiple optimization goals generally have conflicting requirements for the physical design of surface structure during boiling heat transfer. In order to reveal the potential mechanism, the boiling characteristics of a heated surface composed of three-dimensional ordered porous inverse opal (IO) structure were studied. From a numerical perspective, the change in critical heat flux does not always have a linear relationship with the thickness of the structure. When the thickness approaches 4.1 μm, the maximum critical heat flux (CHF) of 122 W/cm2 was obtained at a lower superheat (<15 K). Compared with smooth surfaces, the heat transfer performance of IO structure was largely enhanced with a maximum heat transfer coefficient (HTC) improvement of 945%. For IO structures with the thickness of 3.3 μm, the simulated CHF value is 102 W/cm2 which is in close agreement with the experimental value of 103 W/cm2. Due to its uniform porous structure and high capillary capacity, it can persistently maintain small bubbles before approaching CHF, thereby improving heat transfer performance. This high-performance IO structure provides an alternative thermal dissipation method for electronics.

Enhanced liquid replenishment for pool boiling heat transfer and its CHF mechanism on patterned freeze-casted surfaces

In the current investigation, the pool boiling heat transfer process of patterned porous surfaces, as well as a smooth surface, in ethanol was examined from an academic perspective. The freeze casting (also known as ice-templating) processes was used to fabricate the structured surfaces. It was discovered that freeze-casted coatings significantly improved the performance of boiling heat transfer in comparison to the smooth surface (SMS). The most remarkable increase in Critical Heat Flux (CHF) was achieved by checkerboard porous surface(CHECKERBOARD), with an improvement of 81.6% relative to that of SMS. The maximum Heat Transfer Coefficient (HTC) enhancement was obtained by full porous surface (FULLY-COVERED), with an HTC value 215.8% higher than that of the SMS. For the semicircular porous surface (SEMICIRCULAR), striped porous surface (STRIPED), annular porous surface (ANNULAR) and checkerboard porous surface(CHECKERBOARD) with 50% coating coverage, CHECKERBOARD demonstrated the best heat transfer performance, with CHF and HTC increased by 32.3% and 48.9%, respectively, compared to the SEMICIRCULAR surface. The mechanism of liquid supply at the CHF of the patterned porous surfaces was investigated through phenomenological observations of the boiling phenomena. A modified model, accounting for the coalesced bubble departure frequency and capillary wicking effects, was proposed for CHF prediction. It is noteworthy that the fluid replenishment considers both vertical and lateral replenishment of porous coatings. The CHF data from this study, along with existing literature, were utilized to validate the model, and the predicted results exhibited good agreement with the experimental data, with errors within ±8%.