查看更多>>摘要：To enhance the heat transfer capacity of the condenser end in the heat pipe heat exchanger, heat transfer and flow characteristics investigations of three densely longitudinal fin-equipped annular heat exchangers have been carried out by using a combined method of experimental testing and numerical simulation. One of the heat exchangers as a baseline is featured by the long straight fins. The other featured folded fins and arc-shaped fins, in which 40 rows of short fins were arranged just like the blade-lattice. The attained results showed that the short fold fins and arc-shaped fins had Nusselt numbers that were 27% and 35% higher than the long straight fins, and had friction factors that were 1088% and 2209% higher than the long straight fins. It indicates that the dense fin arrangement has a relatively limited influence on the wall temperature and overall heat transfer capacity. However, the friction loss is significantly sensitive to the fin arrangement because of the severe flow drag and flow separation induced by the blade-lattice like fins. The thermo-hydraulic performance evaluation suggests that the three densely longitudinal fins are suitable for different application scenarios with different heat transfer and pressure loss requirements. This work serves as a reference for the heat exchanger design in the heat pipe-based thermal power conversion system.
查看更多>>摘要：Among the techniques offered to improve the efficiency of Waste Heat Recovery Shell and Tube Heat Exchangers (WHR-STHX), the porous-filling is reported as an effective technique to improve the heat transfer rate. In this regard, the current study performs a numerical investigation to evaluate the heat transfer performance and pressure loss of several porous-filled STHXs. However, the novelty of this study stems from identifing the mechanisms that manipulated the flow structures and heat transfer to achieve a homogeneous thermal distribution, not from achieving an optimized porous-filling. Furthermore, the industrial feasibility of the porous-filled designs was evaluated by comparing their performance with the conventional type in a wide range of thermofluidic conditions. Based on the results, both porous-filled designs have substantially attenuated the interfacial thermal jumps observed in the conventional STHX; thus, a uniform thermal distribution was achieved. Furthermore, the heat transfer efficiency of the porous-filled cases was improved up to 60% compared with the conventional type; however, they imposed higher pressure drop values. Fortunately, since partially porous-filling provided a lower amount of pressure drop (almost half of full-foam), the noted design was found to be appropriate for low-scale applications in which pressure drop could not be tolerated.
查看更多>>摘要：Reducing the temperature gradient in solid oxide fuel cells could improve their working stability. In this study, we have constructed a 2D-axisymmetric model to investigate the effect of the heat generation management on the temperature gradient reduction in a micro-tubular solid oxide fuel cell. The local heat generation is controlled by placing separators in the fuel channel, which could affect the fuel concentration distribution and the associated exothermic electrochemical reactions in the porous anode. We compared the electrochemical and thermal performance of the solid oxide fuel cells with tubular separators and flanged separators. We demonstrated both separators could effectively reduce the cell temperature gradient, but the flanged separator caused a more uniform temperature profile. A 25 mm long flanged separator could reduce the highest cell temperature gradient from 50 ℃/cm to 18.7 ℃/cm. The spatial temperature gradient diagram was proposed to evaluate the influence of different geometric parameters of the separator on its overall performance. It showed that the radius and length of tubular separators both affected the cell temperature profile while the length of flanged separators was the primary factor affecting the cell performance. This study helps improve our understanding about the thermal management of solid oxide fuel cells through the local heat generation control and build a foundation for the flow channel design.
查看更多>>摘要：In recent years, heat dissipation in high heat flux devices remarkably increased and it is anticipated to reach unprecedented levels in future devices, mainly due to increased power density, compact packaging and high-performance requirements. To address this challenge, in current research, we initially investigate the spray cooling performance and spray residue surface effects of the next generation thermal fluid, called hybrid nanofluid. Subsequently, we investigate the hybrid nanofluid spray cooling potential to address heat dissipation issues in a high heat flux application, that is, the electric vehicle (EV) high power electronics. Our results demonstrate that the critical heat flux (CHF) enhancement up to 126% can be achieved using the hybrid nanofluid spray cooling compared to water spray cooling. The hybrid nanofluid and its spray residue characterization further suggest that high CHF in hybrid nanofluid spray cooling may be due to high latent heat of vaporization and residue wetting and wicking effects. Moreover, the spray cooling efficiency and Nusselt number obtained for hybrid nanofluid spray cooling is more than twice that of water spray cooling. Furthermore, our results indicate that the hybrid nanofluid spray cooling can keep high power electronics of current and future electric vehicles below their failure temperatures, while the same cannot be achieved using water and dielectric fluid spray cooling.
查看更多>>摘要：Although the design methodology of individual heat transfer surfaces in a natural circulation boiler such as evaporator tubes, convective and radiative superheaters are available in the literature, the multiple feasible design solutions of boiler heat exchanger surfaces coupled with drift flux based model of evaporator tube and cost-based optimal solution of the surfaces is not common. The present study proposes a sequential iterative design methodology for all heat transfer surfaces (from the evaporator to air preheater) in a single boiler to bridge the gap between industrial needs and academia. The salient temperature points in different locations are used in the design, alternate solutions are considered, and the overall cost is minimized. The evaporator tubes are designed based on a two-phase flow drift flux model and fixed volume constraint. The radiant and radiative-convective superheaters are modeled using weighted average flue gas temperature. The models of evaporator section and other heat exchangers are validated extensively with data of natural circulation loop and components integrated into real plants, respectively. The feasible and optimal design solutions of all heat transfer surfaces are proposed for a medium-scale (35 MW) Refused-Derived Fuel-fired Boiler (RDFB). The variation of mass flux through the evaporator tubes with the fuel feed rate is observed to be gravity-driven initially followed by friction-dominated. It is observed that for lowering the exit flue gas temperature of 14.93% below 150 °C, the required area of air preheater is 21.83% higher, while for a reduction of temperature of 21.42%, the area increment is 55.8%, which increases the investment cost rapidly. Results also indicate that the radiative heat exchangers require less heat transfer surface for a higher pitch to diameter ratio while the opposite characteristic is observed for the convective heat exchangers.
查看更多>>摘要：The thermal effect-induced problem has become a bottleneck in high power laser systems. The laser crystal, which dissipates a heat flux over 150 W/cm2, is the most challenging component when considering the solution of the thermal problem. In this work, we designed a double-layer microchannel heat sink (DL-MCHS) for the heat release of a small-size laser crystal. The main constraints, including the high power density of the laser crystal, space restriction, and the fabrication method are taking into account. A narrow shape DL-MCHS is proposed. The microchannels are fabricated micro-mechanically, and the layers of the DL-MCHS are welded together using a vacuum brazing method. A systematic experimental study is performed to evaluate the performance of the DL-MCHS. The results show that the narrow shape DL-MCHS, which extends the volume only in its length, shows good heat transfer capability. The overall heat transfer coefficient reaches 42 × 103 W/(m2·K). The surface temperature of the heating source is well controlled below 55 °C at a typical heat power of 234 W. When the same fluid flow rates are adopted in the two microchannel layers, the first microchannel layer (bottom) absorbs two thirds of the total heat, while the rest is absorbed in the second layer (top). In the solid base of the DL-MCHS, thermal resistance in the vertical direction is over three times that in the longitudinal direction. When the fluid flow ratio in the first microchannel layer increases, the overall heat transfer coefficient is enhanced slightly, while the temperature distribution on the heating head is little affected. Comparing the single-layer cooling pattern, the double-layer flow pattern improves the uniformity of temperature distribution on the heat sink when the counter-current flow is used, and saves the pumping power by up to 60%.
查看更多>>摘要：Combining porous media paved on sidewall and microencapsulated phase change material (MPCM) suspension as coolant is presented in double-layered microchannel heat sink (MCHS), in which more surface area of porous copper matrix with high thermal conductivity as well as greater temperature difference between the heating wall and working fluid during phase transition of MPCM will enhance heat transfer. The Forchheimer-Brinkman-Darcy model together with energy equation in local thermal equilibrium are employed to deal with fluid flow and heat transfer in porous ribs, and the equivalent heat capacity method is used to describe the phase change process of microcapsules under laminar flow. The influences of coolants, heat sink designs (including the thickness, height, porosity and pore size of porous medium as well as channel number) and working conditions (involving inlet velocity, heat flux and suspension concentration) on thermal and hydraulic characteristics are numerically investigated. The performance evaluation factor (PEF) is introduced to evaluate the comprehensive thermal–hydraulic performance of double-layered MCHS, and the effective performance evaluation factor (PEFeff) is used to compare the overall performance of porous-wall MCHS between conventional arrangements, different channel aspect ratio and channel height ratio configurations. The thickness and height of porous media paved on sidewall should be adjusted reasonably, especially for the arrangement of non-equal porous media, to obtain better overall performance. Different from the variable porosity configuration, the double-layered MCHS cannot rely on losing part of flow performance to obtain better thermal performance in smaller pore size mode, so a larger pore size can be selected to obtain better thermal–hydraulic performance according to actual working conditions. The PEF at larger flow velocity decreases due to higher pressure drop exceeding the benefit of more convection on heat transfer enhancement, in which case the phase change effect in porous-wall mode with MPCM suspension as coolant is suppressed and higher viscosity amplifies the effect of frictional resistance, resulting in a greater drop ratio of PEF than water. The greater PEF in porous-wall MCHS with suspension concentration increasing from 3% to 20% arises than that in non-porous mode, due to the larger surface area of copper matrix with high thermal conductivity and more MPCM particles available for phase transition. The larger aspect ratio configuration can achieve greater PEFeff rise and better overall performance by optimizing fluid velocity in the current mode due to greater thermal resistance drop and less pressure drop rise at lower flow rates, but the variable height ratio structure makes the comprehensive performance of porous-wall MCHS inferior to conventional arrangement due to larger pressure drop rise outweighing the benefits of heat transfer enhancement at the same total volume flux.
查看更多>>摘要：Metal matrix composite based ultrathin two-phase heat transport devices with excellent thermal conductivity and low thermal expansion can address heat dissipating issues and thermal expansion mismatch-induced mechanical failures in the high-power-density micro-electronic systems. Due to the difficulties in processing metal matrix composites, however, achieving the fabrication of the ultrathin devices and their reliable integration with semiconductor chips has challenges. The precision machining and surface engineering of composite materials address the difficulties in processing metal matrix composite based ultrathin devices and contribute to reliable welding encapsulation of the chip. Here, we generate an ultrathin (≤1 mm) metal matrix composite based two-phase heat transport device with low thermal expansion and integrate such device with the gallium nitride chip. The sandwich-structured molybdenum (Mo) copper (Cu) composite, Cu-MoCu-Cu, is used as the casing material of the hermetically welded device. The Mo-Cu based two-phase heat transport device demonstrates an extremely low thermal resistance, which is 95% lower than that of the Cu plate. This device with superior thermal conductivity of 10200 W?m?1?K?1 enables the stable operation of the high-power-density (7.9 × 102 W/cm2) gallium nitride micro-chip within the safe operating temperature range (20–175 °C). In addition, the Mo-Cu based device also helps reduce the thermal stress generated at the encapsulation interface by 39% compared to Cu cooling plate, and thus mitigates the fatigue risks of the device. This chip-level integration of heat transport system using the metal matrix composite-based ultrathin two-phase heat transport devices offers new opportunities in the integration of high heat dissipation and low-stress encapsulation in compact electronic systems.
查看更多>>摘要：Contemporary high-power LED modules are subjected to severe thermal management challenges due to continuing miniaturization and increased heat flux levels. The thermal loads need to be effectively dissipated to improve module reliability by maintaining the junction temperature below 120 °C. The viability of a Loop Heat Pipe (LHP), with copper as the wick material, as a potential thermal management solution for a 200 W high-power LED module is demonstrated in this work. The investigation is conducted in two stages. Firstly, the thermal performance of the LHP is characterized by using different working fluids, and sink temperatures, to obtain the optimum working conditions, using a heater assembly mimicking the LED module. Subsequently, the LHP is integrated with an actual high-power LED module, and this thermo-mechanical demonstrator is then characterized under different real-time operating conditions. Complimentary 3D computational heat transfer simulations are carried out to estimate the LED junction temperature and visualize the thermal fields generated in the LED module. The results demonstrate the efficacy of the copper-methanol LHP design for successful thermal management of the high-power LED module dissipating over 100 W/cm2 (maintaining junction temperature below 120 °C for the 200 W nominal power LED module).
查看更多>>摘要：Current simulation for dry screw vacuum pumps fails to reflect the influence of heat transfer and component thermal deformation due to the lack of an effective simulation path. This paper proposes a novel simulation method based on the chamber model and thermal resistance network method, which integrally considers the influence of leakage, heat exchange as well as components’ temperature and deformation. First, the working process of dry screw vacuum pumps was analyzed and simplified to introduce the chamber model and thermal resistance network method. In detail, the chamber model was employed to predict the gas pressure and temperature in working chambers, and the thermal resistance network was introduced to simulate the component temperature by assuming rotors and casing only performing axial temperature distribution. Next, the detailed construction processes of the chamber model and thermal resistance network were presented. A simulation procedure was developed based on the iterative solution of the chamber model and thermal resistance network. Further, a specific case with detailed simulation setup, solution process, and experimental verification was given for a test dry screw vacuum pump. The results showed that the proposed simulation method performed a better agreement with the experimental pumping speed curve than the previous isothermal model and could effectively predict the component temperature under different inlet pressures and rotation speeds. Finally, the simulated results were further discussed to reveal the gas temperature, component deformation, and leakage process in dry screw vacuum pumps. The conclusions obtained could effectively guide the design and development of dry screw vacuum pumps.
NSTL
Elsevier