查看更多>>摘要：From the perspective of increasing heat transfer rates in latent thermal energy storage systems (LTESS), less attention has been paid to the thermodynamic characteristics at the physical boundary between phase change material (PCM) and heat transfer fluid (HTF). The current numerical study analyzes a simple method of enhancing the thermal performance of a shell and tube LTESS, wherein the flow direction of HTF is periodically reversed. The periodically reciprocating flow creates higher temperature gradients across the HTF-PCM boundary which leads to higher and more uniform heat transfer rates and thereby lower average temperatures, lower maximum temperatures, and lower temperature differences across the PCM than unidirectional flow. Detailed parametric studies are performed to understand the effects of flow and heat transfer parameters of HTF, thermophysical properties of PCM, and design of PCM container on the relative thermal efficiency of the LTESS with reciprocating flow to unidirectional flow. For certain conditions, the time required for melting could be reduced by over 11.5% by using periodically reciprocating HTF flow instead of unidirectional HTF flow. Increasing the heat transfer rate by increasing the temperature or velocity of the HTF, or enhancing thermal conductivity of PCM, increases the relative effects of reciprocating flow compared to unidirectional flow.
查看更多>>摘要：Flow boiling occurs when a subcooled liquid enters a heating pipe and its temperature near the heating wall exceeds the boiling onset temperature. Bubbles are generated on the heating wall and the more downstream the larger the average bubble size due to progressing evaporation and coalescence. Further downstream, the two-phase flow morphologies may change from bubbly to slug, plug, and annular flow. Since these flow patterns have a great impact on the heat and mass transfer rates, an accurate prediction of them becomes critical. In this work, the recently developed GENeralized-TwO Phase concept (GENTOP) was used for flow patterns transition modelling and their effects on the wall heat transfer during the upward subcooled flow boiling inside a vertical heating pipe. Furthermore, a previously developed mechanistic bubble dynamics model was implemented in the GENTOP framework as a sub-model. This model is based on the force balance on a single growing bubble considering evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble as well as the dynamic inclination and contact angles. It does not require a recalibration of parameters to predict the bubble dynamics. For implementing this model in the Euler-Euler (E-E) framework an extension of the current nucleation site density and heat partitioning model was required. Eventually, for a generic test case, flow boiling regimes of water in a vertical heating pipe were simulated using ANSYS CFX 18.2.
查看更多>>摘要：In this study, we investigated the correlation between the pressure loss and heat transfer coefficient for boiling flows in printed circuit heat exchangers. The effect of channel cross-sectional shapes—semicircle and circle—was evaluated. The hydraulic diameters of the semicircular and circular channels were 1.04 mm and 1.0 mm, respectively. A subcooled liquid with a subcooling temperature of 10 K was heated with hot water, and the saturation temperature was 30 °C. The refrigerant mass flux was set in a range of 100–400 kg/m2·s. Consequently, despite the lower pressure drop, for low outlet quality, the heat transfer coefficient was higher for the semicircular channel than for the circular channel. The heat transfer was enhanced by the promotion of bubble nucleation in the thick liquid film at the corners, and the flow resistance was suppressed by the decrease in the vapor shear stress. However, for high outlet quality, the difference in the heat transfer coefficient decreased with an increase in the outlet quality. The experimental results were compared with the results obtained by correlation equations. The lowest mean absolute error of heat transfer coefficient and pressure drop for the semicircular mini-channel were 15.1 % and 26.7 %, respectively.
查看更多>>摘要：The current paper presents an experimental study on the performance of a novel air gap membrane distillation (AGMD) desalination system where a fan blade is installed within the air gap to improve the system's productivity and energy utilization. The installed blade creates suction inside the air gap behind the membrane and increases the air turbulence, thereby reducing the vapor mass transfer resistance across the membrane and improving the productivity and energy efficiency of the system. Results revealed that the proposed design and process improve the vapor permeate flux by about 250% when compared to the conventional AGMD module. Furthermore, the average improvement in the gained output ratio of the system is about 67%, with a corresponding 77% reduction in the specific thermal energy consumption. Influences of the feed temperature and the motor speed are dominating the performance of the new system.
查看更多>>摘要：For electric heat pump systems (EHP), soft faults such as refrigerant leakages and heat exchangers fouling can lead to significant performance degradations, which can remain unidentified for a long period. This paper, through a digital model, wants to simulate the cooling season behavior of a case study air-source heat pump for residential air-conditioning, analyzing the performance degradation in case of both standalone and simultaneous occurrence of soft faults. The study is carried-out in different scenarios of not planned and ordinary maintenance of the machine and in typical climate conditions of Naples, Miami and Shanghai. The impact of faults on seasonal performances (SCOP) and total equivalent warming impact (TEWI) has also been analyzed. Results show that refrigerant leakages can have an impact on system performance higher than 25%, whereas heat exchangers fouling can downgrade the efficiency of approximately 15%. It is also found that, for each climate condition, a different maintenance strategy can halve the cases with SCOP penalization higher than 10%. Since faults have not the same probability of occurrence, a smart monitoring and a fault detection system can be the solution to guarantee good performances avoiding at the same time excessive maintenance costs.
查看更多>>摘要：Flat Plate Solar Collector (FPSC) has been popularly used in the US and is typically covered on top with a single-pane glass, known as glazed FPSC, which, however, can only be used for heat collection. This paper aims to develop an innovative dual-purpose solar thermal collector that allows heat collection during daytime and cold collection during night. It was developed from a conventional glazed FPSC by using a double-pane cover on the top that allows water or other liquid flowing in the gap between the two panes to accomplish heat removal, while its heat-collection function will still be retained, which is accomplished similarly to a conventional FPSC through the copper tubes embedded in the absorber plate underneath. Specifically, the cooling performance of the developed collector was evaluated in this study through simulations carried out in the environment of COMSOL Multiphysics. The results show that the average cold collection capacity of the developed collector is between 28.5 and 262.4 W/m2 when used in various climates across the US, e.g., from the hot (e.g., Miami) to cold (e.g., Anchorage) climate zones, respectively, as the result of both convective and radiative cooling during summer nights. This type of collector is more suitable for use in commercial buildings that have a large amount of internal heat gains and located in cool/cold climate cities or regions (such as Climate Zone 5, 6, 7, and 8) with high wind and/or rich precipitation (e.g., Kansas City or Chicago) for space cooling.
查看更多>>摘要：As one of the most used cooling technologies, vapour compression refrigeration (VCR) has been widely used in domestic cooling, automobile air conditioning, and industrial cooling. Linear compressor can adjust mass flow rate in response to the system requirements to achieve capacity modulation. A numerical model of variable displacement VCR system driven by a linear compressor is presented in this study to study the characteristic of the system, investigate charge impact, and help to optimize VCR system. The model integrates a heat exchanger model, a refrigerant distribution model, and a detailed compressor model. The proposed model is simulated in MATLAB/Simulink. A prototype linear compressor was studied as a case using R1234yf as working fluid in a variable displacement VCR system. Overall, most of the modelling results fall within ±10% of the experimental data. The predicted compressor stroke, power consumption, and mass flow rate have Mean Absolute Percentage Errors (MAPEs) of 2.95%, 6.18%, and 7.08%, respectively, while predicted cooling capacity, coefficient of performance, and refrigerant charge have MAPEs of 8.26%, 8.66%, and 6.1%, respectively. The numerical model indicates that a larger compressor stroke tends to have a higher optimal refrigerant charge. The numerical model can be used for future works on low-charge vapour compression refrigeration system design using microchannel heat exchangers and control strategy development for various applications.
查看更多>>摘要：Studies about using nanofluids to enhance the Critical Heat Flux (CHF) of In-vessel Retention (IVR) strategy in the third-generation reactor have been conducted extensively and show a significant CHF enhancement effect. However, low carbon steel SA508 used in the reactor vessel is easy to oxidize and the oxidation can lead to changes in the surface properties which may affect the CHF enhancement effect of nanofluids. In this study, pool boiling CHF experiments with low carbon steel SA508 surfaces were conducted in distilled water and nanofluids under different boiling time to investigate the CHF enhancement effect of nanofluids under low carbon steel surface oxidization condition. CHF in distilled water increases rapidly with boiling time due to the rapid surface oxidation during the boiling and the increase ratio can be nearly 2 due to the surface oxidation. CHF in nanofluids is stable and independent of boiling time. The difference between CHF in nanofluids and CHF in distilled water decrease to 17% under the longest boiling time conditions due to the surface oxidation. The deposition layer of nanoparticles on the surface leads to the capillary wicking and decrease in the nucleation site and thus, CHF is enhanced. This study is of great significance for exploring the actual effect of nanofluids on the CHF enhancement of IVR strategy.
查看更多>>摘要：Internal Heat Exchanger (IHX) in vapor compression refrigeration cycle is an alternative to improve the energy performance in refrigeration systems. In this paper, the influence of IHX on thermal behavior of a chest freezer is evaluated experimentally. For the study, R-513A is used as replacement refrigerant of R-134a. Results indicate that the optimal refrigerant mass is less for the R-513A than R-134a under the same operating conditions. When R-513A is used as refrigerant, a reduction of 6.25% in refrigerant mass in comparison with the R-134a refrigerant is achieved. Also, it is found that the stability time in the indoor compartment is reduced by 2 and 4 h when the R-513A was used in comparison with the R-134a with and without IHX, respectively. Finally, energy consumption for R-134a is 8% higher for 24-h energy consumption test, in comparison with R-513A.
查看更多>>摘要：Solar heat for industrial processes is a promising way to meet the high thermal demand required by the industry, while this application becomes an important niche market for solar technology. In this research line, it is proposed a novelty concept based on a rotary Fresnel solar collector to supply heat above 150 °C. This work is focused on the multi-tube receiver for this Fresnel collector, proposing a thermal design based on three criteria that can be generalized for any multi-tube receiver: the fluid flow layout is arranged to meet the symmetry of the solar flux map; the fluid circulates from the lower to the higher flux density zone; and the fluid velocity is modified by modifying the tube diameter, to optimize the heat transfer. Following these criteria, the final configuration of the receiver is chosen based on an exergy optimization, in which both heat loss and pressure drop must be quantified. It has been also accomplished a generalization of the optimization methodology for Fresnel collectors providing heat at different temperatures, showing that, in these cases, the configuration that maximizes the exergy efficiency does not correspond to the one with the highest energy efficiency. This thermal design method can be applied to multi-tube receivers working at higher temperatures in longer Fresnel loops, in which case the optimization will result in more marked differences between the optimal values and the standard ones.
NSTL
Elsevier