查看更多>>摘要：This paper presents a study on the flow topology in the mixing chamber of a supersonic ejector using Large Eddy Simulation (LES). To this end, a supersonic air ejector of squared crossed-section was modelled using a specialized finite-element code. Comparisons with experimental data showed good agreement, both in terms of the primary jet shock cell structures and wall pressure measurements (mean deviation of 12%). Results have been discussed both in terms of time averaged profiles and instantaneous structures in the mixing layer. The general flow features have been identified by means of instantaneous temperature fields and pressure profiles through the device. Results show that, under the assessed conditions, the mixing layer is laminar at first and transitions towards turbulence in the first quarter of the mixing chamber, where.. vortices have been identified. These evolve into hairpin vortices and finally break down around half of the mixing chamber. Time-averaged velocity profiles show self-similarity in this section. In comparison with an unconfined mixing layer (Fang et al., 2018), the supersonic ejector mixing layer grows slower first but then develops at a similar rate after the transition region. A shock train occurs towards the end of the mixing chamber, which enhances mixing. Given its location, it generates a recirculation bubble in the diffuser which narrows the main flow passage and breaks the flow vertical symmetry. This pioneer study shows the enormous potential that LES offers for the optimization and detailed analysis of supersonic ejectors.
查看更多>>摘要：Since the outbreak of the worldwide COVID-19 pandemic, public transportation networks have faced unprecedented challenges and have looked for practical solutions to address the rising safety concerns. It is deemed that in confined spaces, operating heating units (and cooling) in non-re-circulation mode (i. e., all-fresh air mode) could reduce the airborne transmission of this infectious disease, by reducing the density of the pathogen and exposure time. However, this will expectedly increase the energy demand and reduce the driving range of electric buses. To tackle both the airborne transmission and energy efficiency issues, in this paper a novel recovery heat pump concept, operating in all-fresh air mode, was proposed. The novelty of this concept lies in its potential to be applied to already manufactured/in-service heat pump units as it does not require any additional components or need for redesigning the heating systems. In this concept, the cabin exhaust air is directed to pass through the evaporator of the heat pump system to recover part of the waste heat from the cabin and to improve the efficiency of the system. In this paper, a 0D/1D coupled model of a generic single-deck cabin and a heat pump system was developed in the Simulink environment of MATLAB (R2020b) software. The model was run in two different modes, namely the all-fresh air (as a baseline and a recovery heat pump concepts), and the air recirculation mode (as a conventional heat pump concept with a 50% re-circulation ratio). The performance of these concepts was investigated to evaluate how an all-fresh air policy could affect the performance of the system, as well as the energy-saving potential of the proposed recovery concept. The performance of the system was studied under different ambient temperatures of -5 ?, 0 ?, and 5 ?, and for low and moderate occupancy levels. Results show that implementing the all-fresh air policy in the recovery and baseline concepts significantly improved the ventilation rate per person by at least 102% and at most 125%, compared to the air-re-circulating heat pump. Moreover, adopting the recovery concept reduced the power demand by at least 8% and at most 11%, compared to the baseline all-fresh air heat pump, for the selected fan and blower flow rates. The presented results in this paper along with the applicability of this concept to in-service mobile heat pumps could make it a feasible, practical, and quick trade-off solution to help the bus operators to protect people and improve the energy efficiency of their service.
查看更多>>摘要：The popularity of additive manufacturing has increased interest in the use of triply periodic minimal surfaces (TPMS) in engineering applications due to their potential for superior mechanical, heat and mass transfer properties. Periodic nodal surfaces (PNS) are a class of periodic continuous surfaces that also divide the space into non-intersecting, smooth and continuous domains like TPMS and can potentially have superior mechanical, heat and mass transfer properties. To evaluate the potential for superior performance, in this manuscript we characterize the flow and heat transfer properties of seven TPMS and PNS based structures by numerically computing the friction factors and Nusselt numbers in the laminar flow regime. This is the first study to evaluate the use of PNS as heat exchangers. In addition, it adds to the limited quantitative data available on the flow and thermal properties of TPMS. The friction factors associated with most of the TPMS and PNS based structures in this study are about an order of magnitude higher than that for laminar flow in tubes. These structures also had higher Nusselt numbers compared to tubes, with the enhancement increasing with increase in Reynolds number. Among the TPMS and PNS studied, Schwarz-D had the best heat transfer performance while Schwarz-P was the poorest performing structure. Basic heat exchanger design calculations showed that to remove the same amount of heat and operate under the same pressure conditions, the Schwarz-D based heat exchanger was 3-10 times smaller than a tubular heat exchanger. This shows how TPMS or PNS can be used to design heat exchangers with superior performance, especially in applications where space and weight are at a premium.
查看更多>>摘要：In the process of injection cooling, the selection of working mediums brings different effects. However, most researches still focus on water. Compared with water, the vapor formed by the evaporation of organic cooling mediums such as ethanol can assist combustion in the combustion chamber. The current research on the cooling effect of organic mediums is relatively lacking. Therefore, related tests on the cooling effect of organic working fluids are conducted. The test results show that: as the spray temperature decreases from 45 & DEG;C to 18 & DEG;C, the cooling effect improves (maximum temperature drop reaches 63 & DEG;C). Ethanol solutions of various concentrations can maintain an evaporation rate of more than 75%, and its evaporation and cooling effect are improved with the concentration increase. Not all organic cooling mediums can bring good temperature drop and evaporation effects, including glycol solution. In working conditions, a higher airflow temperature (up to 150 ?) can enhance the cooling effect of the working mediums, and the liquid-gas ratio (6.62-10.76%) has almost no influence on the temperature drop. Finally, the simulation results are calibrated through the test data to verify the accuracy of the simulation and confirm the accuracy and reliability of the test method.
查看更多>>摘要：In the current digital society, more energy-saving and reliable cooling systems are urgently needed for the flourishing electronic industry demanding a very high heat dissipation rate. Through the channel-to-channel heat transfer, a counter flow diverging microchannel heat sink has demonstrated to be a high performance cooling design in our previous studies. This work further integrates such an innovative design with surface modification of microscale cavities with optimal mouth diameter from the nucleation theory and nanoscale coating structures. The results of the present study demonstrate a significant enhancement on boiling heat transfer performance with the corresponding pumping power very close to that of the single-phase flow. Through highly efficient nucleate boiling from well-designed cavities with liquid replenished from the excellent wicking effect of nanostructure and stable two-phase flow, this study achieves a 4.8 kW effective heat dissipation rate on a 3 cm x 4 cm cooling area without sign of reaching the critical heat flux. Remarkably, an unprecedented coefficient of performance, defined as the heat dissipation rate to the pumping power, over 150,000, an order of magnitude higher than that reported in the literature, is accomplished.
查看更多>>摘要：The use of air as heat transfer fluid and a packed bed of rocks as storage medium for a thermal energy system (TES) can be a cost-effective alternative for thermal applications. Here, a porous media turbulent flow (standard k -epsilon) and heat transfer (local thermal non-equilibrium) model is used to simulate the discharge cycle of such system. Temperature fields of corresponding charging cycles are used as initial conditions. Effects of varying mass flow rates (Re number), porosity, permeability (Da number), thermal conductivity ratio and thermal ca-pacity ratio on the effectiveness of the discharge are compared. The examination of these effects indicated that increasing the mass flow rate improved the effectiveness of the discharge, which was not seen for the charging cycle. Also, increasing porosity improved discharge efficiency more significantly than it did in the charging cycle. In both charge and discharge cycles the effect of permeability is significant and reducing Da number improved temperature stratification and efficiencies. The effect of the thermal conductivity ratio was mostly seen on the outlet temperatures, where lower ratios allowed for higher temperature values. Increasing the thermal capacity ratio improved charging effectiveness but, on the discharge cycle, cycle this effect was reduced. Moreover, for lower Re number flows, increasing this ratio reduced efficiency indicating that the mass flow rate should be matched carefully with the thermal capacity of the system. All these effects have important implications which should be taken into consideration when designing an effective thermal energy storage system.
查看更多>>摘要：Numerical simulations have been performed to determine the conduction heat transfer in a sessile droplet for a large range of dynamic contact angle 0 and Biot number Bi. The substrate is set at a constant and uniform temperature, while a convective heat transfer is set at the liquid-vapor interface. In such a configuration, the heat flux is concentrated in the triple line region, so that numerical results can become inaccurate as the Biot number increases. A reference case in which the heat flux can be determined analytically has thus be established to derive an empirical criterion on the local mesh refining needed to obtain accurate numerical results. To consolidate the results obtained with a finite elements code, calculations have been performed with a completely independent tool using Monte Carlo method on a set of cases. A correlation has then been derived from the numerical results data with a maximum deviation of less than 4% in the considered range of 0 and Bi, that covers conditions encountered in all the studies dealing with dropwise condensation of pure vapor. Comparisons with other laws available in literature have then been performed, evidencing some important discrepancies.
Hoe, AlisonBarako, Michael T.Tamraparni, AchuthaZhang, Chen...
18页
查看更多>>摘要：While phase change material based heat sinks have been shown to act as highly efficient transient cooling devices, the effective implementation of these components is prevented by a lack of design guidelines. Here, we develop an analytical framework for optimizing the design of rectangular and cylindrical phase change material composite heat sinks. This is accomplished through the definition of two design objectives: (1) maximize thermal buffering capacity at a given time, and (2) maximize the time the system can achieve a minimum thermal buffering capacity threshold. In this context, thermal buffering capacity can be quantified in terms of heat absorption rate or temperature, depending on the boundary condition applied. We demonstrate that, in finite volumes, there exist two design regimes where the thermal buffering capacity is either limited by the rate at which the system can absorb thermal energy or by the total thermal capacitance of the system. We present analytical expressions describing the optimal volume fraction for each combination of design objectives, form factors, and boundary conditions derived from appropriate analytical solutions for the melting problem. Analytically predicted optimal volume fractions are validated with numerical and experimental results from existing literature and original work. This collective toolbox enables thermal engineers to make rational decisions on architecture to optimize components under specific thermal loads and specific system constraints.
查看更多>>摘要：There is growing interest in operating electronics at low temperatures; however, the complexity of the cryogenic burden imposed by large coolers impedes the widespread usage of low-temperature electronics. In this study, we present a Hampson-type miniature Joule-Thomson (JT) cold stage driven by a linear compressor, and the performance of the cooler operating with a ternary mixture of methane, ethane and neopentane is investigated experimentally and numerically. Gas chromatography analysis shows that the composition of the circulated mixture is different from the charged mixture due to the liquid hold-up of neopentane. The cooler has a cold-end temperature of 143 K in steady state when operated between 0.15 MPa and 1.92 MPa, and a net cooling power of 198 mW at 145 K, 212 mW at 150 K, and 173 mW at 155 K. A dynamic numerical model is developed to predict cooler performance, and the feasibility of the numerical model is validated through the comparison between measured and calculated net cooling powers. The model can be used to estimate cooler performance at different operating conditions. This study is conducive to promote the widespread use of cryogenic electronics in the fields of military, space science, and biomedicine among others.
Liu, Y. Y.Bhaiyat, T. I.Schekman, S. W.Lu, T. J....
12页
查看更多>>摘要：Impingement cooling of an isoflux flat plate by a round jet, generated through a perforated blockage plate, is experimentally investigated at a fixed jet Reynolds number of Re-j = 20,000. With the perforation diameter (D-j) fixed, varying the thickness (t) of blockage plate forms three distinct turbulent jets: (1) orifice jet (e.g., t/D-j <= 0.5), (2) blockage jet (e.g., 0.5 < t/D-j < 8.0), and (3) tube (or nozzle) jet (e.g., t/D-j >= 8.0). Thermofluidic characteristics of these jets in both free exit and impingement were measured and directly compared under consistent conditions. Particular focus was placed upon how these characteristics vary in the intermediate range of jet relative thickness t/D-j, with reference to the two limiting cases, namely, the orifice jet and the tube jet. It was demonstrated that the blockage jet behaves neither as an orifice jet, nor as a tube jet, and its properties cannot be considered interchangeable. Measurements of jet flow properties -exit velocity profile, potential core length and centerline turbulence -highlight the distinguishing features among the three jet types. While the blockage jet exhibits an intermediate thermal performance, the tube jet provides the poorest performance, and the orifice jet performs the best. Since it is not always possible to use an orifice jet in practice (e.g., due to structural constraints), this study provides evidence for the existence of optimal H/D-j for a given jet type (i.e., fixed t/D-j): correspondingly, both primary and secondary thermal peaks shift when t/D-j and H/D-j are varied.
NSTL
Elsevier