查看更多>>摘要：Comparing with continuous jets inside microchannel, the heat removal capacity is enhanced by introducing protrusions or pulsating jets individually. Therefore, the preferred heat transfer enhancement and satisfactory thermal performance are investigated after taking both advantages of unsteady properties and ellipsoid protrusions simultaneously in this work. Especially, phase difference, delta phi, as a pivotal parameter, is applied in multiple pulsating jets. The total mass flow rate in microchannel, temperature fluctuation on heated wall, thermal and flow resistance are therefore varied. The largest fluctuation of mass flow rate is achieved when the phase difference between each adjacent jet inlets at delta phi = 0.pi, while the smallest at 0.75.pi, and the temperature distribution of the target wall correspondingly fluctuates. The heat transfer enhancement is most prominent at 0.75.pi. The increment in heat transfer is close to that of flow resistance at delta phi = 0.50.pi, 0.75.pi and 1.pi. The thermal performance TP firstly increases and then decreases with delta phi, and reaches the maximum value at 0.75.pi. Furthermore, due to the phase difference, each jet becomes the one with the highest velocity among all jets periodically, weakening adverse effect of cross flow and suppressing the drift phenomenon effectively. This is exactly the mechanism for heat transfer enhancement of unsteady multiple jets with velocity phase difference in ellipsoid protrusion microchannel.
查看更多>>摘要：This study aims to develop a structure combining solar heating and radiative cooling without thermal efficiency reduction. A technical scheme for radiative cooling by reforming a flat-plate collector (FPC) is proposed. A glass cover was attached to a polyethylene terephthalate film, which transformed it into a radiative cooling surface; thus, it was able to cool the air through night sky radiation. A polyethylene film covered the top of the device as a windscreen to reduce unwanted heat loss. The heat transfer model of the device was established in MATLAB, and this model for radiative cooling was verified using experimental data. The experimental results also show that the improved radiative cooling flat-plate collector (RCFC) demonstrated improved solar collecting performance compared to a typical FPC; the maximum temperature rise of the RCFC was increased by 40%. The cooling efficiency of the RCFC was essentially between 48% and 54% in Chongqing. Numerical analysis was used to investigate the effects of ambient temperature, humidity, mass flow rate, selective emissivity, and other factors, on cooling performance. The results indicated that by multiplying the airflow by a factor of 2 or by utilizing the ideal film, the cooling power was approximately increased by 50%.
查看更多>>摘要：To solve the disadvantages of traditional phase-change heat dissipation technologies, such as instability in twophase flow and difficulty in active temperature control, a mechanical-capillary driven hybrid phase-change loop (HPCL) is designed and tested. The experiment results show that the heating power and liquid-vapor pressure difference have a decisive influence on the transformation of heat transfer modes. With the increase of heating power while maintaining the liquid-vapor pressure difference unchanged, the baseplate of evaporator undergoes four heat transfer modes, i.e. flooded, partially flooded, thin film evaporation and overheating. Among them, thin film evaporation has the significant advantages of short start-up time and high heat transfer efficiency. Meanwhile, when the baseplate temperature is maintained below 85 degrees C, the heat dissipation power is enhanced by about 6.5 times if the liquid-vapor pressure difference is increased from 0 kPa to 15 kPa. Therefore, increasing the liquid-vapor pressure difference is an active means in heat transfer enhancement. The heat transfer modes distribution diagram is drawn by taking into account of power and liquid-vapor pressure difference. Transition criterions between different heat transfer modes are given. The diagram suggests that the thin film evaporation region is in the shape of "horn". In engineering application, liquid-vapor pressure difference can be regulated adaptively according to the actual operating characteristics of heat dissipation target to achieve the optimal heat dissipation effect. Furthermore, the accurate and rapid control over baseplate temperature can be realized by controlling liquid-vapor pressure difference. In such way, the accuracy of temperature control is within +/- 0.5 degrees C. Therefore, controlling liquid-vapor pressure difference is a simple and effective means of active temperature control.
Morse, JoshuaPaniagua-Guerra, Luis E.Ramos-Alvarado, Bladimir
14页
查看更多>>摘要：In this contribution, existing gaps in the design process of liquid-cooled heat sinks, i.e., the lack of overall performance assessment metrics and commercial viability analyses, are addressed. This work focused on the design of a cost-effective cooling system for a commercial circuit board with multiple and asymmetric heat sources designed and manufactured by an electronics company. The analysis was performed using 3-D computational fluid dynamics simulations across sixteen different cases, focusing on the effect on performance and manufacturing cost of two primary design features of liquid-cooled heat sinks: the inlet and outlet manifolds and heat transfer enhancing features, where pin fins and minichannels were considered. Specifically, the effect of diameter and spacing of the pin fins and the effect of the distribution and length of the mini channels over an unevenly heated surface were parametrically investigated. The tradeoff between thermal and hydraulic performance of the heat sinks was reconciled using an overall performance parameter, which allowed for a single numerical evaluation of the heat sinks' global performance spanning several operating flow conditions. Likewise, the tradeoff between cost and performance was investigated by comparing the manufacturing cost of the different heat sink designs, including two heat spreader materials. Lastly, a comprehensive assessment of the potential cooling solutions for the circuit board was done by combining the overall performance and manufacturing cost per prototype through a novel parameter proposed herein. The broader liquid-cooled heat sink design approach reported in this work is a missing aspect in similar contributions, as the focus is solely on technical performance parameters, while commercial implementation issues (manufacturability and production cost) are neglected.
查看更多>>摘要：After decades of using reliable synthetic refrigerants, it is now necessary to look for their low-GWP climate friendly safe alternatives. This paper summarizes a search for the best substitutes to commonly used refrigerants and their blends to be used in vapor compression air conditioning systems and temperature conditions typical for a moderate climate. 24 pure refrigerants (including several modern HFOs) were combined (using REFPROP10's mixing functions) in binary and ternary compositions (at 10% mass fraction adjustments) producing 44 915 potential combinations. This broad database was screened on the basis of theoretical COP, specific cooling capacity, and volumetric cooling capacity which were obtained through simulations of an ideal cycle with the temperature of evaporation 0 ?, condensation 38 ? and superheating 10 K. Resulting blends were then further sieved on the basis of Global Warming Potential (GWP), discharge temperature, temperature glide, and flammability. The refrigerants that produced the highest COP under the above screening criteria are: R13I1 and its blends. The COP was 4.94, but R13I1 is hard to acquire in necessary quantities. The other high-COP/low-GWP blends contain R1234ze(Z) and/or R1233zd(E), but the evaporator would work below atmospheric pressure. Further screening returned R152a/R1234ze(E) [0.7/0.3 mass ratio] as the best refrigerant blend for the air conditioning purposes. It is inexpensive, belongs to flammability group 2L, and the ideal refrigeration cycle yields a theoretical COP of 4.861.
查看更多>>摘要：In this study, a crossflow-type heat exchanger coated with aluminophosphate zeolite was tested for desiccant dehumidification with direct-supplied hot water and evaporative cooling using remaining hot water. This evaporative cooling process could accelerate dehumidification by removing the heat of adsorption. Therefore, the influences of the cooling air velocity, regeneration hot water temperature, and adsorption/regeneration cycle time were experimentally investigated. The result showed that the minimum humidity of the adsorption outlet air decreased with an increase in the cooling air velocity and adsorption/desorption time, indicating that the complete regeneration of coated adsorbents is required to significantly decrease humidity. However, the maximum time-averaged amount of dehumidification appeared at an appropriate cycle time, which avoided adsorption saturation during the adsorption period. Further, in any case, increasing the cooling air velocity had a positive impact on both the momentary observed minimum humidity and time-averaged amount of dehumidification.& nbsp;At all cooling air velocity, the temperature of the hot water contributed to the increase in the time-averaged amount of dehumidification. However, when the hot water temperature was 55 ?, the cooling effect of evap-orative was not observed, even when the cooling air velocity reached 1.5 m/s. Considering that the actual temperature of the coated adsorbent was expected to be much lower than that of the supplied hot water, the hot water temperature should have been >=& nbsp;65 ?.
查看更多>>摘要：Vanadium-manganese flow battery is a promising renewable energy storage system due to higher energy density as well as lower cost of Mn(II)/Mn(III) redox couple in the cathode than V(II)/V(III). The non-isothermal 2-D model describing the thermo-electrochemical behaviors in the main reactions of V(II)/V(III) and Mn(II)/Mn (III) along with the side reaction of Mn(II)/MnO2 is established to investigate the energy loss including power consumption and Joule heating loss associated with parameter researches, and the simulations agree with the experimental data. The amount of MnO2 precipitation in the side reaction is inverse proportional to the discharge current density and electrode porosity. While the temperature rises from T = 5 to 45 ?, more power consumption deriving from deposition reaction than dissolution reaction occurs in the discharge, and the Joule heating loss acts as a main factor to influence energy loss as well as it increases with the rise of current density at lower temperatures. The maximum energy density of 32.17Wh/L and voltage efficiency of 93.07% arise respectively with electrode porosities of epsilon = 0.8 and epsilon = 0.7 under the current density of i = 50 mA/cm(2), and the larger voltage efficiency and less power consumption accounting for total energy loss can be obtained with the electrode porosity of epsilon = 0.8, thus, the better performances can be obtained in the vanadium-manganese flow battery with optimized electrode porosity under the applicable operating conditions proposed in this work.
Rouge, ClemenceCarolan, DeclanFergusson, Alexander
14页
查看更多>>摘要：An analytical model to predict the maximum temperature rise in a battery cell or module given a certain cooling regime was developed. The novel analytical model combines a micromechanical model, to compute the effective thermal conductivity of the cells, with a solution of the heat equation. The boundary conditions, geometric parameters and material properties were varied in representative ranges. The temperature fields across the battery were predicted with a very good agreement between the analytical model and the numerical simulations, for a wide variety of configurations under steady-state conditions. The analytical solution can be used as a fast and reliable tool to estimate the heat transfer between the cells and their thermal management system. The results from the analytical model were compared to those estimated by an Artificial Neural Network and found to be equal or better in performance than a data-driven approach. An experimental validation of the analytically predicted temperatures of interest showed a very good agreement. The experimental results reveal the requirement for good thermal management solutions from an efficient energy conversion point of view.
查看更多>>摘要：Heat pumps have always attracted wide interest due to their ability to harvest energy from the environment, thereby alleviating the energy consumption and environmental pollution issues. The recuperative heat pump with refrigerant mixture can realize large temperature lift (over 50 K) to meet such demand and has several advantages over trans-critical CO2 and sorption heat pumps such as low discharge pressure and simple structure. However, the concentration shift results in the recuperative heat pump performance hardly operating under optimal conditions, since working fluid with large temperature glide is used to realize large temperature lift. In this study, a test facility has been built to experimentally examine the influence of charging concentration, throttling degree and water flow on the concentration shift. Besides, modeling studies are conducted to investigate the influence of the phase "hold-up " on the concentration shift. The results show that the concentration shift is remarkable in practical application. The maximum relative deviation is nearly 70% and 85% when the R1270/Hexane and R32/R4310mee refrigerants are utilized, respectively. Besides, the temperature has a significant impact on the concentration shift under most working conditions. In addition, a very high designed concentration of component with a high boiling point may not be realized in real application. The circulating concentration of the component with a high boiling point can be automatically optimized when the charging concentration is higher than the designed value. The modeling results show that the deviation between the charging concentration and the calculated concentration caused by the phase "hold-up " can reach 50%, indicating the influence of the different oil dissolving properties of refrigerants and other factors is remarkable.
Yagnamurthy, SaiRakshit, DibakarJain, SanjeevRocky, Kaiser Ahmed...
13页
查看更多>>摘要：To further the research on R32 refrigerant-based adsorption cooling applications, Maxsorb-III activated carbon composites synthesized with the additives of H25 Graphene nanoplatelets (GNP), 1-Hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide ([HMIM][Tf2N]) ionic liquid with polyvinyl alcohol (PVA) are studied for cooling performance evaluation. Their respective kinetic characteristics and heat of adsorption assessments are carried out for this purpose in the present study. The adsorption kinetics characteristics are studied using gravimetrically for the composite samples. A first-order kinetics model is seen to fit their kinetic uptakes with regression coefficients >= 0.97. The heat of adsorption estimates of the composites for varying uptakes and temperatures are numerically evaluated, incorporating the non-ideal behavior of the refrigerant. An approach for a holistic comparison of the cooling performances of the composites containing their respective heat and mass transfer characteristics for compact heat exchanger designs is further proposed. While the composite with the highest Maxsorb-III mass fraction yields the highest specific and volumetric cooling powers, the composite with the highest thermal conductivity requires the lowest heat exchanger area with a slightly larger adsorbent volume of around 12.1% over the former. A second law thermodynamic analysis is further carried out to evaluate the composite performances for cooling applications.& nbsp;
NSTL
Elsevier