查看更多>>摘要：Two-phase cooling is an efficient method for heat removal from electronic systems. One implementation of this approach is based on electrospray atomization, where droplets of the cooling fluid are produced using high electric potentials. Electrospray cooling has several unique advantages, including low power consumption, precise delivery of coolant, and compact design. Past work has mostly focused on spray orientations where the nozzle is directed normal to the high-temperature surface. In this short communication, we examine electrospray cooling at varying spray angles, which can provide easier integration for constrained environments or hard-to-reach surfaces. Water is used as the working fluid at four different flow rates from 60 to 120 mL/hr. The nozzle orientation was varied from normal to the surface (90 degrees) down to an acute spray angle of 25 degrees. We found that any angle larger than 30 degrees achieved an evaporation efficiency exceeding 100% at the critical heat flux. The efficiency dropped to 90% at an angle of 25 degrees for all flow rates. Surface temperatures increased and the critical heat flux decreased with spray angle, with the sharpest change occurring at a spray angle of 25 degrees. This was due to droplet rebound, incomplete droplet breakup at high flow rates, and a reduction in electrohydrodynamically-induced convective cooling. The work reported here will assist in the integration of electrospray atomization for cooling applications.
查看更多>>摘要：A heat pump cycle with refrigerant injection is one of the effective means for enhancing the heating performance in low-temperature climates. The intermediate temperature and injection pressure have been proven to be important parameters influencing the operating performance of a heat pump system. Previous studies on optimal intermediate pressure were mostly based on empirical formulas, and the calculation result greatly depended on the available scope of the empirical formula, especially in low-temperature conditions. In this study, the optimal intermediate temperature of the refrigerant injection cycle, including the vapour injection cycle and two-phase injection cycle, was analysed through theoretical derivations. This method is based on a thermodynamic analysis of the system and the thermodynamic properties of the R134a refrigerant. The aim of the theoretical analysis was to determine the intermediate temperature and injection pressure corresponding to the maximum COP of the system. First, the factors influencing the optimal intermediate temperature of the two injection cycles were studied using the theoretical calculation model. The optimal intermediate temperature was more sensitive to the system subcooling than the suction superheat. Increasing the heat exchange efficiency of the economiser is conducive to improving the heating COP. For the same operating conditions, the optimal injection pressure was higher than the empirical value. Second, an experimental system of refrigerant injection with an economiser was built to verify the accuracy of the method. The results indicated that the optimal injection pressure calculated by the theoretical derivation method was in good agreement with the experimental values for both the vapour and two-phase injection cycles. The maximum error of the injection pressure was within +/- 3%.
查看更多>>摘要：Considering the fact that high energy consumption and low drying temperature of current drying system was not appropriate for the high temperature industrial drying, a closed loop heat pump drying system with high drying temperature was designed, established and explored. As a key operation parameter, the circulating air volume was closely related to the drying temperature and could affect the system performance. The effect of circulating air volume on the circulating air temperature, coefficient of performance (COP), moisture extraction rate (MER) and specific moisture extraction rate (SMER) were all experimentally investigated in this study. The results showed that with the increase of circulating air volume, the circulating air temperature was gradually decreased, while the system COP was oppositely increased. And both the system MER and SMER firstly increased and then decreased with increasing air volume. The drying temperature could exceed 70 ? when the circulating air volume was below 539.8 m(3)/h, and reach its maximum value 71.11 ? when the circulating air volume was 388.5 m(3)/h, meeting the high drying temperature requirement. At the same time, both the system MER and SMER could reach their maximums (i.e., 3.27 kg/h and 1.40 kg/(kW.h)), showing the excellent dehumidification performance of this system. Besides, when the circulating air volume was 736.1 m(3)/h, the system COP could reach its maximum 6.56, indicating high energy efficiency. These results can provide guidance for the popularization and application of heat pump drying technology in the industrial drying field.
查看更多>>摘要：The long-term mission capacity of battery-powered underwater vehicles is quite limited, which necessitates the development of underwater vehicles that utilise ocean thermal engines (OTEng) to capture renewable energy. The literature indicates that OTEng based on solid-liquid phase-change materials (PCM) is the optimum solution; however, the energy capture performance of OTEng strongly affects the motion and power supply of the thermal underwater vehicle, and thus, improving their energy capture performance is critical. In this study, a new OTEng with an enhanced heat transfer structure based on PCM was designed by employing copper foam and filling fluid. An experimental platform for the performance study of OTEng was established, and a corresponding evaluation method was introduced. Comparative experiments on its performance, including the phase-transition time, volume of oil input and output, and amount of energy captured under various scenarios, were carried out. The results were analysed, and it was concluded that (1) the designed OTEng enhanced the heat transfer ability up to 58.3% in the melting process and 21.4% in the solidification process when the temperature difference was 10 degrees C; (2) filling liquid with 0.3 MPa system pressure was the optimal choice during the solidification process; (3) increasing the system pressure or ambient temperature during the melting process could improve the captured energy or average power in one cycle, respectively.
查看更多>>摘要：Recovering the expansion work by replacing throttle valve with expander may improve the performance of a thermodynamic cycle. Based on Kalina Cycle System 11, 34, and 34 g, different redesigned cycles with single screw expander are proposed in this paper. I-redesigned cycle has a single-screw expander which replaces the throttle valve and is located between absorber and regenerator. In II-redesigned cycle, the single-screw expander is located between regenerator and separator, and the original throttle valves is removed. The thermodynamic performance of two redesigned cycles is analyzed and compared with their corresponding original cycle. It can be found that two redesigned cycles perform better than their corresponding original cycle system. II-redesigned cycle is better than I-redesigned cycle. When comparing the redesigned cycle of the same type, the performance ranking from good to bad is Kalina Cycle System 34, 11, and 34 g. II-redesigned cycle for Kalina Cycle System 34, 11 and 34 g can reach the maximum thermal efficiency of 11.44%, 11.14%, and 10.85% respectively at the evaporation pressure of 3.0 MPa. It can be seen that the series connection of regenerator and evaporator is better than the parallel connection and the increase in the number of regenerators can improve the thermodynamic performance of the cycle.
查看更多>>摘要：Heating electrification is a trend for the current carbon neutrality setting. The air-source heat pump (ASHP) has been widely used in residential or commercial buildings to replace household coal-burning stoves due to its higher operating efficiency. Previous studies mainly focused on the heat pump equipment, ignoring the whole system performance in practical application. Among the auxiliary system of the ASHP heating system, the power consumption of water pump cannot be ignored, especially in residential district heating with a large amount of pipeline. This paper proposes the coefficient of performance (COP) improvement of the ASHP heating system through variable temperature difference and flow rate of circulation water loop, showing the feasibility and benefit of large temperature difference with small flow rate. A high-fidelity dynamical model is first built using Dymola, followed by the experiments verification at the environmental laboratory. On all testing conditions, the system COP of large temperature difference with small water flow rate shows average 6% to 8% improvement compared to small temperature difference with large water flow rate. Further, a demonstration heating project of the ASHP system was tested onsite under a realistic operating condition. The recorded hourly operation data showed an average system COP of 2.15 at the condition of large temperature difference, which is about a 7% improvement compared to the small temperature difference condition for the typical heating day.
Beni, Mehdi HashemiEmami, SobhanIsfahani, Amir Homayoon MeghdadiShirneshan, Alireza...
28页
查看更多>>摘要：It is necessary to utilize an accurate control system in D-type boilers under transient loading. In this paper, a saturated D-Type boiler has been modeled and studied under cold start condition. This type of boiler has two kinds of heat transfer surfaces which are radiative zone (RZ) and convective zone (CZ). Since the furnace shape according to the gas path is not symmetric, the RZ section has been modeled via 3D mesh, and the CZ section has been modeled via the zoning method. The presence of D-shaped tubes in the furnace and different heat absorption of other tubes forced the model to have consisted of four circulation circuits of water in RZ and CZ. According to cold start conditions, inverse flow can occur in circulation circuits which this phenomenon has been studied in this paper. The methodology was included: simultaneous modeling of flue gas side and water side via detail modeling of circulation circuits with exergy analysis, and asymmetric 3D modeling of furnace via mathematical models considering cold start constraints. The comparison of model data against experimental data has shown a maximum deviation of 8.6% for drum pressure during start-up. Also, a sensitivity analysis on the initial water level of the drum in cold-start conditions has been performed by increasing from 200 to 500 and 800 mm; and the results indicated a 4.2% and 10.8%, increase in fuel consumption, respectively. Moreover, the boiler's exergy efficiency reaches a maximum of 60% during cold start-up and declines to 38.2% in steady-state conditions.
Zakaria, I. A.Mohamed, W. A. N. W.Azid, N. H. A.Suhaimi, M. A....
13页
查看更多>>摘要：Hybrid nanofluid coolants is a new approach for advanced thermal management of Polymer Electrolyte Membrane fuel cells. Due to the high electrical conductivity of nanofluids, electrical discharge when a nanofluid coolant is used in a fuel cell is a concern and needs to be fundamentally studied. The objective is to obtain experimental correlations between heat transfer and electrical discharge rates of a nanofluid coolant in the form of a novel electro-thermal transfer ratio as a reference for future progress. A hybrid of 1%v TiO2 and SiO2 nanoparticles dispersed in a water and ethylene glycol (40:60) base fluid mixture was tested. The heated surface temperatures of the cooling channel were at 60 degrees C and 70 degrees C while the electrical power was nominally discharged through the test section at 0.7 V and 3 A. Under laminar flow, the concurrent changes to the temperature profile and active current were observed. The cooling was improved for the 40:60 hybrid TiO2:SiO2 nanofluid coolant with an enhancement factor of up to 2 times while the measured electrical current was visibly lower than the nominal current. The electro-thermal transfer ratio reduced exponentially with Reynolds number, indicating that electrical discharge strength into the coolant reduced at higher flow rates compared to the rate of heat transfer. These preliminary findings provide a new improved perspective in the assessment of nanofluid coolants for fuel cell systems and electrically-active systems in general.
查看更多>>摘要：The application of phase change heat storage technology in condensing heat recovery system of air conditioning can effectively solve the non-synchronization of condensate heat discharge and hot water demand, and thus improve the energy utilization efficiency. For this purpose, a novel sodium acetate trihydrate-based phase change material was prepared and tested in our very recent work, which presented high potential to recover condensation heat. In this work, the sodium acetate trihydrate-based phase change material was further optimized for more promising heat storage and release performance. To begin with, in order to improve the heat storage density of sodium acetate trihydrate-based phase change material, deionized water was added into the mixture and their heat storage and release tests were carried out at atmospheric pressure and vacuum degree of 0.09 and 0.10 MPa. The heat storage density of such phase change material reached 8.2581 kWGreek ano teleiah/kg with 7 wt% deionized water added, however, the cycle measurements showed that the heat storage and release performance were unstable under negative pressures. To solve this problem, expanded graphite was used to further stabilize and optimize sodium acetate trihydrate-based phase change material with extra water to improve the cycle stability. The results showed that the addition of 15 wt% expanded graphite in 50 mesh can effectively stabilize heat storage and release performance of these phase change material samples at both atmospheric and negative pressures. In addition, the optimum sodium acetate trihydrate-expanded graphite composite phase change material presents excellent form stability, thermal conductivity (4.566 W/(mGreek ano teleiaK)) and very comparable heat energy density at both atmospheric pressure (7.6301 kWGreek ano teleiah/kg) and vacuum degree of 0.09 MPa (6.3295 kWGreek ano teleiah/kg).
查看更多>>摘要：Based on the high performance and the environmentally friendly characteristics, the transcritical CO2 heat pump has attracted much attention in last few decades. The high-pressure control is one of the essential factors determining the coefficient of performance due to the large pressure changes in the transcritical CO2 heat pump system. In this work, the contribution rates of different factors to the optimal high pressure and the peak performance coefficient were analyzed by the method of the analysis of variance. Then, the influences of compressor frequency, inlet water flow rate and inlet water temperature on the optimal high pressure and the peak performance coefficient were experimentally investigated on the basis of theoretical analysis. Finally, a novel discharge pressure deviation was proposed to calculate the optimal high pressure, and the occurrence regularity of optimal high pressure was investigated. The results indicated that the gas cooler outlet temperature was the main parameter influencing on the optimal high pressure, peak performance coefficient and discharge pressure deviation. The calculated discharge pressure deviation can seek the optimal high pressure with the average error of 0.57% and the performance coefficient loss of 0.34%. Additionally, the optimal discharge pressure of the experiment was likely to appear in the descending stage of the gas cooler outlet temperature.
NSTL
Elsevier