Maximum Heat Transfer Coefficient Research Articles

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%.

Enhanced capillary-driven thin film boiling on cost-effective gradient wire meshes for high-heat-flux applications

As high-power electronics continue to advance rapidly, the pursuit of efficient thermal management has emerged a critical challenge for their further high-performance and large-scale applications. Capillary-driven thin film boiling phase-change cooling, harnessing the high latent heat of vaporization, presents an effective strategy to efficiently dissipate waste heat and meet the escalating cooling requirements of high-heat-flux applications. In this study, a cost-effective gradient wicking structure was proposed to enhance the thermal performance of the wick by promoting bubble dynamics and optimizing the balance between capillary pressure and permeability in homogeneous wicks. The comprehensive capillary properties of the homogeneous wicks were analyzed through theoretical calculations, and the capillary-driven thin film boiling heat transfer performance of four wick samples was systematically studied experimentally. Additionally, the effects of reduced pressures on the thermal performance of the gradient wick were investigated. Notably, the gradient porous wicking structure demonstrated exceptional thermal performance, with a critical heat flux reaching as high as 202.8 W/cm2 and a maximum heat transfer coefficient of 121.9 kW/m2·K achieved at 88.5 W/cm2. The findings of this study offer valuable design guidelines for the wick structures utilized in phase change equipment, with potential wide-ranging applications in advanced thermal management.

Optimization of heat transfer performance of a micro‐bare‐tube heat exchanger using a genetic algorithm

AbstractThis study presents the optimization of the heat transfer coefficient of a micro bare tube heat exchanger. A physical and mathematical model of a micro bare tube heat exchanger was built in Matlab using the simplified ε‐number of transfer unit method. Using a temperature of minus 30°C and a 0.5–1.0 mm tube outer diameter, a 1.7–8.0 mm longitudinal tube pitch, a 1.7–5 mm transverse tube pitch, and a 1.0–5.0 m/s velocity at minimum free flow area, with carbon dioxide as the refrigerant, a comparative variation analysis was performed to optimize the heat transfer coefficient of a micro‐bare‐tube heat exchanger. The results demonstrate that the heat transfer coefficient increases as the inlet air velocity is increased from 1.0 to 5.0 m/s, with a final gain of 93.17%. The growth rate of the heat transfer coefficient steadily decreases with increasing inlet air velocity and gradually approaches zero. The effect of the gradual decrease of the tube outer diameter from 1.0 to 0.5 mm on the heat transfer coefficient is 22.52% greater than that of the gradual decrease of the transverse tube pitch from 2.2 to 1.7 mm. The study also carried out an optimization analysis on the distribution of four different variables in the heat exchanger. With the use of a genetic algorithm, the study found an optimal distribution to maximize the heat transfer coefficient. The following parameter values resulted in the maximum heat transfer coefficient: a maximum inlet air velocity of 5.0 m/s, a minimum tube outer diameter of 0.5 mm, a 1.7 mm longitudinal tube pitch, and a 1.7 mm transverse tube pitch.

Experimental study on pool boiling enhancement by unique designing of porous media with a wettability gradient

High heat flux dissipation can be achieved in pool boiling heat transfer through regulating the bubble escape and liquid replenishment paths over the heated surfaces. In this work, a unique design of copper foam with a wettability gradient is presented to generate separate liquid–vapor pathways for pool boiling enhancement. The effect of copper foam with a wettability gradient on bubble dynamics during nucleate boiling period is evaluated using bubble visualization experiment. Results demonstrated that the liquid replenishment and bubble escape flow is determined by copper foam with different wettability gradient. Comparing with the super-hydrophilic (SHPi) copper foam and super-hydrophobic (SHPo) copper foam, the super-hydrophilic top with super-hydrophobic bottom (SHPiT-SHPoB) copper foam structure and super-hydrophobic top with super-hydrophilic bottom (SHPoT-SHPiB) copper foam structure can achieve the critical heat flux (CHF) of 113.3 W/cm2 and 108.3 W/cm2, corresponding to the maximum heat transfer coefficient (HTC) of 5.93 W/cm2·K and 5.28 W/cm2·K, respectively. Two bubble escape forms and liquid replenishment models are established to give detailed explanations on the pool boiling enhancement. It is evident that when bubbles escape from sides, the bubble escape path is shorter, resulting in a faster bubble escape rate and a smaller bubble departure diameter. The experimental results also reveal that liquid replenishment rate of the SHPiT-SHPoB copper foam is also the best amongst all samples, providing a trade-off between the bubble escape and liquid replenishment in diverse conditions.

Performance assessment of a parabolic trough solar collector using nanofluid and water based on direct absorption

In this investigation, a parabolic trough solar concentrating collector with an evacuated tube was fabricated with an aperture area, collector length, breadth, and rim angle, inner and outer diameter of absorber as 2.45 m2, 2.1 m, 1.2 m, 90 deg, 0.0286 m, and 0.03058 m, respectively. A copper oxide (CuO) and water-based nanofluid was employed as the working fluid. Different mass fractions (0.05%, 0.075%, and 0.1%) and volume flow rates (70 lit/hr and 140 lit/hr) of nanofluid were taken, and performance of an evacuated tube parabolic trough collector was analysed. When CuO–H2O nanofluid of varying concentrations (0.05%, 0.075%, and 0.1%) is used in place of H2O (DI), it has been observed that the solar collector's efficiency increases. The efficiency of the collector increases in direct proportion to the working fluid's volume flow rate increment. The maximum heat transfer coefficients for CuO–H2O based nanofluid were 280.68 W/m2-K and 466.8 W/m2-K while the maximum convective heat transfer coefficients for water as working fluid were 268.11 W/m2-K and 488.7 W/m2-K at 70 lit/h and 140 lit/h, respectively. An efficiency increase of 45.15%, 51.19% and 53.26% for 70 lit/h and 59.61%, 63.56% and 69.07% for 140 lit/h mass flow rate has been observed for different nanoparticle's concentrations, respectively. Economic studies showed that the cost of the experiment (for 0.05% CuO dispersion) has increased by 1.08% for 70 lit/h mass flow rate when compared to the water based direct cooling method.

Application of novel double-layer micro-jet heat sinks for energy-efficient thermal management of motor inverters in electric vehicles

The relentless advancement in electronics miniaturization and performance has birthed a tremendous amount of heat flux, demanding effective dissipation techniques. Thermal effects pose a significant hurdle in the thermal management of electric vehicles (EVs) and having advanced cooling techniques in place is vital. To tackle this, the present study numerically evaluates the cooling attributes of two novel double-layer micro-jet heat sinks (DLMJHSs) to address the thermal management of insulated-gate bipolar transistors (IGBTs) in the motor inverters of EVs. The numerical analyses are carried out for Reynolds numbers ranging from 5000 to 25,000, IGBT heat fluxes of 100, 200, and 300 W/cm2, and different configurations of DLMJHSs. The potential of using nanofluids is assessed by employing Al2O3–water nanofluid with different volume fractions of 0.8% and 1.7%. The finite volume technique is implemented to solve the equations and the turbulence closure is modeled using the k-ԑ model. Results show that HS #2 provides a 10.2% higher convective heat transfer coefficient leading to a 7.44 ℃ lower baseplate temperature than HS #1. Besides, HS #2 prevents the occurrence of high-temperature spots owing to a more uniform temperature distribution in comparison with HS #1. HS #2 demonstrates the most uniform temperature distribution, with a Coefficient of Variation of only 0.00993 and a maximum heat transfer coefficient of 80.5 kW/m2 ℃. The heat sink’s baseplate temperature drops by increasing either the Reynolds number or the volume fraction. The baseplate temperature declines by up to 6.94 ℃ using a nanofluid volume fraction of 1.7% compared with only the base fluid. A minimum Reynolds number of 15,700 is required to prevent overheating at the peak heat flux. The performance evaluation criterion (PEC) is higher than 1 for Re ≤ 15,000.

Assessment of Newly-Designed Hybrid Nanofluid-Cooled Micro-Channeled Thermal Management System for Li-Ion Battery

Abstract Maintaining both maximum temperature and temperature uniformity within the desirable limit is a crucial issue for high C-rating Li-ion batteries of electric vehicles, which can be achieved by the properly designed battery thermal management system (BTMS). In this research, three new designs of liquid-cooled micro-channeled BTMS are suggested for cylindrical batteries to address the issue of temperature variations and uneven temperature distribution. Using 3D numerical simulation, we investigate the impacts of volume flowrate and the usage of mono/hybrid nanofluids with varying concentrations on the thermal performance of the battery pack at a high C-rate by utilizing a two-phase mixture model. Effects on maximum temperature, temperature uniformity, pumping power, and heat transfer coefficient to pressure drop ratio are investigated. Results demonstrate that the effectiveness of heat transmission and temperature uniformity of the battery pack are positively impacted by an increase in nanoparticle concentration in nanofluid and volume flow rate. Even at high C-rates (5 C), the proposed design can effectively reduce both cell temperature and thermal gradient of the 21700-type cylindrical cell. Design 3 is the most favorable BTMS for Li-ion cylindrical battery in terms of both maximum temperature and temperature uniformity (maximum temperature of 304.72 K and temperature difference of 4.7 K).

Application of imaging techniques for the characterization of lumps behaviour in gas–solid fluidized-bed reactors

Gas-solid fluidized-bed reactors are often used in waste pyrolysis and gasification processes thanks to their excellent mixing properties, which guarantee temperature uniformity. However, this latter property can fail when large objects, such as lumps, are introduced or form in the system. Understanding the motion characteristics and thermal behaviour of lumps in a high temperature fluidized-bed reactor can help determining how the presence of lumps impact reactors’ performance. This was the object of this study. In particular, this work aims to assess how process variables and physical properties impact the segregation behaviour, dispersion coefficients and heat transfer coefficients of these lumps during operation. The system used in this work is a down-scaled pseudo-2D fluidized bed operated at ambient temperature and at fluidization velocities ranging between 1 Umf and 10 Umf. Rutile sand with four different mean particle sizes (60 μm, 100 μm, 153 μm and 215 μm) was used as bed material. Fabricated lumps were introduced in the fluidized bed to reproduce realistic conditions, as when lumps form in a high-temperature fluid bed. The density ratio between the lump and the bed material particle was varied between 0.32 and 0.55 to account for different lump compositions. X-ray digital radiography and infrared thermography were used respectively to track the fabricated lumps and to obtain their temperature time evolution. The lump density was found not to have a significant effect on the lump dispersion coefficients or on the heat transfer coefficient. Optimal values of fluidization velocities that guarantee proper lump mixing and maximum heat transfer coefficient were obtained. This latter increases by up to 10 times if the optimal fluidization velocity is selected. An increase in the bed material particle size was found to cause an increase in the dispersion coefficients and a decrease in the heat transfer coefficient. The trend of the heat transfer coefficient as a function of the fluidization velocity was found to vary significantly between different bed material particle sizes. A new correlation for the Nusselt number as a function of the object Reynolds number and of the size ratio between lump and bed material was obtained. This correlation applies to cases where particle convection is the dominant mechanism of heat transfer. The results of this work provide important knowledge to minimize the impact of lumps on fluidized-bed reactors and to optimize their operation.

Experimental study of heat transfer performance in a mini-corrugated pipe coupling nanofluids and pulsating wave

The combined application of nanofluids and pulsation was adopted to enhance heat transfer in a mini-channel corrugated tube. The corrugated tube provided a higher heat transfer capability than the smooth tube with the maximum Nu enhancement of 21.8% using distilled water (DW) as medium. When the heat transfer medium was replaced by 0.5%Al2O3/DW nanofluids, Nunf/Nubf increased by 16% at Re = 1100. Then, a pulsation wave generator with adjustable frequency and amplitude was designed. It was found there was an optimal St for each Re, and the lower the Re was, the higher the optimum St was. The optimum St for Re = 379, 730 and 1115 were 0.042, 0.03 and 0.024 respectively. When the pulsating frequency and pulsating amplitude (A) matched Re to obtain adequate fluid mixing, the maximum heat transfer coefficient was obtained. In the paper, the best heat transfer enhancement was 147% at St = 0.034, Re = 747 and A = 3.85.

A comprehensive study of electrodeposition by response surface methodology with presenting hybrid bi-conductive surfaces for promoting pool boiling

This study investigated the effect of simultaneously changing influential parameters of the electrodeposition method, with proposing an intensified approach for promoting the pool boiling process by creating hybrid bi-conductive surfaces. With the help of surface response methodology (RSM), current density, time, and the number of electrodeposition steps were varied to experimentally study the wickability, porosity, thickness, and maximum heat transfer coefficient (HTC) of fifteen coated samples. Boiling experiments were performed with deionized water at atmospheric pressure. The characteristics of surfaces were studied by conducting scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and profilometry tests. The results of three-dimensional graphs showed that with increasing the number of steps, the wickability became independent of time and current density of the electrodeposition process, while the thickness of the porous layer increased. Also, when the porosity peaked, the HTC plummeted, which can be due to the water replenishment obstacles. In addition, experimental correlations for the desired parameters were extracted using the regression model. Finally, by combining electrodeposition and bi-conductive methods in the electrodeposited sample with low-conductive channels of 1.5 mm width and 2.5 mm pitch, heat flux and HTC improved by 62% and 260% compared to the plain sample, respectively.

Dropwise Condensate Comb for Enhanced Heat Transfer.

Dropwise condensation on superhydrophobic surfaces could potentially enhance heat transfer by droplet spontaneous departure via coalescence-induced jumping. However, an uncontrolled droplet size could lead to a significant reduction of heat transfer by condensation, due to large droplets that resulted in a flooding phenomenon on the surface. Here, we introduced a dropwise condensate comb, which consisted of U-shaped protruding hydrophilic stripes and hierarchical micro-nanostructured superhydrophobic background, for a better control of condensation droplet size and departure processes. The dropwise condensate comb with a wettability-contrast surface structure induced droplet removal by flank contact rather than three-phase line contact. We showed that dropwise condensation in this structure could be controlled by designing the width of the superhydrophobic region and height of the protruding hydrophilic stripes. In comparison with a superhydrophobic surface, the average droplet radius was decreased to 12 μm, and the maximum droplet departure radius was decreased to 189 μm by a dropwise condensate comb with 500 μm width of a superhydrophobic region and 258 μm height of a protruding hydrophilic stripe. By controlling the droplet size and departure on hierarchical micro-nanostructured superhydrophobic surfaces, it was experimentally demonstrated that both the heat transfer coefficient and heat flux could be enhanced significantly. Moreover, the dropwise condensate comb showed a maximum heat transfer coefficient of 379 kW m-2 K-1 at a low subcooling temperature, which was 85% higher than that of a superhydrophobic surface, and it showed 113% improvement of high heat flux or heat transfer coefficient when it was compared with that of the hierarchical micro-nanostructured superhydrophobic surface at a high subcooling temperature of ∼10.6 K. This work could potentially transform the design and fabrication space for high-performance heat transfer devices by spatial control of condensation droplet size and departure processes.

Effect of electric field, mass flow rate, and subcooling degree on subcooled flow boiling in a micro-slit channel

This study investigated the design and cooling performance of a micro-slit-channel heat sink for electrical device cooling. The subcooled flow-boiling performance of a dielectric liquid, HFE-7000, was experimentally characterized. A flat electrode made from stainless steel with 10 narrow slits, each 1 mm width and 17 mm long, was placed at 600 μm above the heated surface to generate a high electric field. The flow boiling heat transfer performance of an electroplated diamond surface was evaluated. Four electric field intensities from 0 to −5 kV/mm, three mass flow rates from 1.83 to 4.8 g/s, and four inlet subcooling from 5 to 25 K were tested at an atmospheric system pressure. Four flow boiling patterns were categorized using a high-speed camera: single-phase flow, subcooled nucleate boiling, saturated nucleate boiling, and local dry out boiling. The maximum critical heat flux (CHF) and heat transfer coefficient were 95 W/cm2 and 30 kW/m2K, respectively, for mass flow rate of 4.83 g/s, electric field of −5 kV/mm, subcooling degree of 9 K, and mass flow rate of 4.83 g/s. A semi-empirical prediction of the CHF exhibited good agreement with the experimental results within ±18 %.

Hollow fiber polytetrafluoroethylene membrane heat exchanger with anti-corrosion properties

Thin hollow polytetrafluoroethylene (PTFE) fibers (thickness: 200 μm; outer diameter: 1.3 mm; inner diameter: 0.9 mm) were produced by a specially designed die via stretching and sintering, and assembled into a shell-and-tube membrane heat exchanger. The effects of the flow velocities on the heat transfer performance and thermal resistance were studied in a water-water system. The maximum overall heat transfer coefficient (OHTC) was up to 391 W/(m2‧K) (flow velocities: 0.24 m/s for hot water; 0.11 m/s for cold water). The OHTC became larger (increased from 229 to 391 W/(m2‧K)) at a higher cold water flow velocity (increased from 0.02 to 0.11 m/s) on the shell side when maintaining the hot water flows on the tube side. The OHTC increased from 119 to 391 W/(m2‧K) first and then decreased to 298 W/(m2‧K) with the increase of the hot water flow velocities (from 0.08 to 0.40 m/s) on the tube side, while maintaining the cold water flow velocity on the shell side. The thermal resistance results showed that the increased flow velocity (0.11 m/s) on the shell side significantly reduced the thermal resistance to 2.56 × 10−3 K m2/W on the shell side. Furthermore, the flow turbulence on the tube side is favorable for the heat transfer, and the optimum flow velocity on the tube side was 0.24 m/s. The hollow PTFE fibers were extremely stable and durable in corrosive solutions, with less than 1% changes in bursting strength, tensile strength at break, and elongation at break. This work opens new opportunities for the fabrication and application of hollow fiber PTFE membrane heat exchangers to reuse waste heat in high temperature corrosive environments.

Experimental and CFD investigation on heat transfer enhancement in evacuated tube solar collector with coiled wire inserts

ABSTRACT This article presents a novel design of a modified evacuated tube collector fabricated for air heating applications by fixing helical coiled inserts in the vacuum tube. The investigations on modified evacuated tube collector (ETSC-HI) were conducted experimentally and using Computational Fluid Dynamics (CFD) and assessed with the unmodified ETC solar air heater under analogous conditions. The working of the modified evacuated tube collector (ETSC-HI) has been scrutinized under different modes, geometrical parameters, flow parameters and different operating conditions. Experimentally, the influence of these variables on outlet air and thermal efficiency of the modified collector have been studied in detail. The CFD findings have been visualized by plotting graphical contours, vectors and streamlines for various cases. Optimal geometrical parameters for maximum heat transfer coefficient were calculated from the CFD investigations and were considered for further experimental analysis by fixing the coiled wire on the vacuum pipe. It was seen that ETSC-HIperforms better contrasted to the unmodified ETC, registering Nusselt number enhancement of 3.23 times corresponding to P/Dh = 1.5 e/Dh = 0.074 at m = 0.015 kg/s. The highest thermal efficiency for ETSC-HI and unmodified collector are obtained as 70.99% and 64.86% respectively showing significant improvement in the performance. On average, the enhancement of 5.85% in daily average efficiency was seen for the modified ETSC-HI.

Flow boiling of R1336mzz(Z) in tapered microgaps with asymmetric dual-V microchannels

The development of high heat flux cooling technologies has attracted great interest of investigations during the last decades, which motivated a series of investigations regarding flow boiling in microchannels. In a recent investigation, new heat sinks composed of a bottom surface milled with asymmetric Dual-V microchannels combined with a tapered microgap were presented, and the occurrence of boiling inversion was documented for the first time in flow boiling experiments using water as working fluid. It is noteworthy, however, that despite its great thermal properties the use of water as working fluid may not meet some cooling requirements, and the use of refrigerants is necessary. In addition, with the increasing concerns with environmental impacts, novel refrigerants with low environmental impacts have been presented to the market, but limited results are found in the literature for their flow boiling performances. Hence, the present work is focused on flow boiling experiments with the Hydro-Fluoroolefin R1336mzz(Z) in copper heat sinks combined to a tapered manifold. Tests were conducted using three different surfaces, with and without asymmetric Dual-V microchannels, the selected saturation temperature was 41 °C, and inlet mass fluxes varied from 400 to 1000 kg/m2s. Effects of various parameters were evaluated and put in context to the flow patterns observed with a high-speed camera. The best surface promoted the dissipation of up to 93 W/cm2 without reaching the critical heat flux, with wall temperature lower than 74 °C, pressure drop lower than 10 kPa, and maximum heat transfer coefficient reaching 22 kW/m2K.

Experimental and numerical analysis of effect of combined drop-shape pin fins and plate fins type heat sink under natural convection

As the junction temperature of a light-emitting diode (LED) is inversely proportional to its lifespan and efficiency. This study investigates the thermal performance of heat sink-based model with a drop-shaped pin fin for LED cooling under natural convection, experimentally, and numerically. The study used three distinct drop form pin fin arrays with variable vertical fin spacing (Sv = 25, 50, and 75 mm), with the best array being used in subsequent studies with variations in drop-shaped pin fin size (diameter DD and apex length LD ). The heat transfer coefficient and Nusselt number were obtained in the quantitative analysis of thermal performance. A numerical model with Boussinesq approximation for natural convection is used for this study and well-validated with experimental results. A qualitative investigation was also carried out utilizing numerical research to investigate velocity streamlines and their impacts on heat transfer enhancement in various configurations. An experimental test with conventional plate fin, circular pin fins were done for the verification of the test setup results and their results were compared with results calculated from the standard correlations presented in previous well-known literature. The maximum heat transfer coefficient was found to be 12.80 W/m2K for drop form pin fins with vertical fin spacing (Sv ) equal to 75 mm, a diameter of 6 mm, and a length of 7.2 mm, and the corresponding Nusselt number was 94.85. Also, the maximum enhancement in Nusselt number was found to be (Nu/Nu plate fin) 3.11 corresponding to the conventional plate-fin heat sink. This comparison will help the industrialists determine innovative passive cooling technology for electronic devices such as LED street lights.

Effect of particle size distribution on heat transfer in bubbling fluidized beds applied in thermochemical energy storage

The aim of the present work is to characterize the effect of particle size changes from cycling of the thermochemical energy storage material CaO/Ca(OH)2 on fluidizability and wall-to-bed heat transfer coefficients in the fluidized bed. Materials replicated from previous storage cyclization experiments are experimentally investigated in an ambient fluidization test rig. Differential pressure measurement and a horizontally immersed heat transfer probe are used to characterize fluidizability and wall-to-bed heat transfer between immersed cylinder and bed material. Sauter mean diameters of the replicated particle size distributions range from 27µm to 282µm. Particle size distributions with Sauter mean diameters significantly smaller than approximately 50µm were found to be difficult to fluidize due to excessive channeling, resulting in low heat transfer coefficients. In this case, fluidization quality and heat transfer is enhanced for higher gas velocities. Fluidizable particle size distributions ranged from Sauter mean diameters of 48µm to 282µm. Measured heat transfer coefficients show little dependency on the particle size. Observed maximum heat transfer coefficients range from 344Wm−2K−1 to 350Wm−2K−1.

Numerical study on heat transfer characteris tics of supercritical water in straight and helical tubes

The subcritical steam generators in high-temperature gas-cooled reactor pebble-bed modules can be replaced by supercritical steam generators to improve thermal efficiency. The axial and circumferential nonuniform flow and heat transfer characteristics of supercritical water in straight and helical tubes with varying inclinations (0–90°) are simulated to investigate the combined effects of tube structure, flow direction, thermophysical properties, property variations, buoyancy, and centrifugal forces. With heat fluxes of 233–930 kW/m2 and inlet mass fluxes of 225 or 1260 kg/(m2 s) under a working pressure of 24.5 MPa, the friction coefficients, turbulent kinetic energy, secondary flow, temperature, and heat transfer coefficients are studied in the cross section of variable structure tubes. The wall temperature and heat transfer coefficients of four regions are compared in straight and helical tubes; the maximum heat transfer coefficient appeared before the pseudo-critical point. The heat transfer coefficients are the greatest at the bottom of the straight tubes, where fluidity was high, and at the outer of the helical tubes, where the fluid velocity was high under centrifugal force. The secondary flow in the helical tubes causes a nonuniformity in the wall temperature, causing the circumferential maximum wall temperature to move.