查看更多>>摘要：Previous studies have pointed out problems with the dual-temperature air source heat pump. Thus, this paper proposes a novel dual-temperature air source heat pump cycle with a self-defrosting method for simultaneous production of heat sources at different temperatures. An ejector was added to the novel system, which reduced the heat transfer temperature difference of the low-temperature condenser and the utilization of multiple heat sources. In addition, a new type of defrosting, which utilizes the heat from the hot liquid refrigerant to defrost the evaporator using two evaporators and a four-way valve, is used to reduce the energy needed for defrosting and decrease temperature fluctuations. Thermodynamic modeling using the energetic and exergetic analysis method was employed to evaluate the modified cycle performance and compare it with that of the basic heat pump cycle. Eco-friendly refrigerants, such as R134a, R600a, R290, and R1234yf, were adopted as the working fluid. The simulation results show that the heating coefficient and exergy efficiency in the proposed cycle were improved by 29.34% and 43.52%, respectively, compared with those of the standard cycle under typical operating conditions. Among the refrigerants, the eco-friendly refrigerant R600a exhibited the best performance under various operating conditions. Moreover, the COP in the new system was 20.72-44.47% higher than that of a traditional system, and the exergy efficiency improvement was 29.70-49.19% compared to the standard system. In summary, this study confirms the performance enhancement potential of an ejector-based dual-temperature air-source heat-pump cycle and provides theoretical support for its practical implementation.
查看更多>>摘要：Magnetic refrigeration systems are promising cooling solutions that employ the active magnetic regenerator refrigeration cycle to achieve practical temperature spans and environmental benefits. The hydraulic system that ensures a continuous flow of the heat transfer fluid through the system with a reciprocating flow in each regenerator bed is critical to the performance of the refrigeration cycle. Hence, we investigate the characteristics of the parallel flow circuit of a rotary active magnetic regenerator system, which consists of thirteen trapezoid-shaped regenerators, each filled with 295 g of gadolinium spheres. Fluid flow is controlled via electrically actuated solenoid valves (both piloted and direct-acting) connected to the regenerator hot side. By varying the percentage of opening of the control valves, different blow fractions (or fluid flow waveforms) could be investigated. The objective of the study is twofold: (i) assess whether flow imbalances of the heat transfer fluid exist in the cold-to-hot blow (cold blow) and hot-to-cold blow (hot blow) directions, and (ii) determine whether there is an optimal value of the blow fraction both to maximize the cooling performance and realize a rapid temperature pulldown. Flow resistance measurements demonstrate a symmetric flow circuit design and resistances that are similar in the cold and hot blow directions. Moreover, for the studied temperature spans of 6 K and 16 K, the best blow fraction was found to be about 41.6%. For instance, at a 16 K span, a utilization of 0.32, and at 1.4 Hz, increasing the fluid blow fraction from 25.0 to 41.6% enhanced the cooling capacity and second-law efficiency from 70 to 330 W and from 2.6 to 17.4%, respectively. In turn, lower blow fractions favored a more rapid temperature pulldown. The magnetocaloric system was about 30% faster in establishing approximately 14 K temperature span when the blow fraction was reduced from 41.6 to 30.6%. Hence, magnetic refrigeration systems can benefit greatly from solenoid valves, which allow the system to operate either in a time-saving mode or an energy-saving mode.
查看更多>>摘要：Using the lattice Boltzmann method (LBM), a double distribution LB model for annular heat sources based on the variation of thermal conductivity and thermal diffusivity with temperature to study annular laser-material interaction is developed. The model is verified with the enthalpy-based method, and good agreement is obtained. The inner and outer annular radii can regulate convective heat transfer to affect the flow pattern and the depth of the molten pool. The temperature field, velocity vector field and average Nusselt number under different inner and outer annular radii are analyzed. The ratio of Marangoni number to Rayleigh number is proposed for the comparative analysis of natural convection and Marangoni convection intensities in the molten pool. The simulation results reveal that smaller inner and outer annular radii lead to smaller Nu_(avg), and the inhibition of convection inside of molten pool leads to a deeper molten pool. In the scope of present work, the outer annular radius to make the temperature field of the molten pool more uniform is in the range of 2.0-2.5 mm. The inner annular radius to make the molten pool deeper is in the range of 1.5-2.0 mm.
A. Martin-AlcantaraJ. J. Serrano-AguileraL. Parras
13页
查看更多>>摘要：The linear receiver of a Parabolic Trough Collector is the most critical element in the entire system. The Universal Vacuum Air Collector concept is the most extended type of receiver in both experimental and industrial facilities. Besides their considerable cost, their efficiency usually drops as operation time passes. This is mainly due to a partial loss of vacuum in the evacuated annulus between the absorber and the glass cover. An alternative design called HorseShoe receiver is proposed in this work, whose main goal is to maintain the thermal performance throughout its entire lifespan. This innovative receiver is indicated for low-to-medium temperature ranges, which is particularly suitable for solar heat for industrial processes. It consists of a horseshoe-like cavity absorber having its upper border insulated. In addition, two main advantages can be taken by using two symmetric lenses as glass cover: reconcentrate solar radiation into the cavity (improvement of the intercept factor) and protect stratification conditions (reduction of thermal losses). A transient numerical model with customized boundary conditions has been implemented to evaluate both thermal performance and temperature difference in the absorber domain, which is critical for the thermal stress conditions. For that purpose and as a key contribution, not only the Heat Transfer Fluid (HTF) temperature but also the heat transfer coefficient in the duct are set. In particular, HTF temperature ranges from 80℃ to 220℃ and the inner heat transfer coefficient from 600 W/(m~2·K) to 1800 W/(m~2·K). Results show that numerical thermal performance is above 96%, which is mainly due to the reduction of thermal radiation losses, where the absorber active surface emittance is ε_(abs) = 0.3. Since no inclination of the receiver has been considered in this initial study, it is shown that the air inside the cavity is stratified, which reduces convective losses. Moreover, temperature difference between the coldest and the hottest spot of the absorber is no higher than 30 K, which is affordable from a technical point of view. A polynomial heat losses correlation is also provided, whose parameters are on the same order of those by conventional receiver correlations available in the literature.
查看更多>>摘要：Solar evaporation by heat localization has drawn great interest in recent years to ease the global problem of freshwater shortage. Although many evaporators have been proposed, there is still great potential to develop extremely cost-effective solar evaporators with both high evaporation efficiency and salt resistance. Here, we propose a heat-localized solar evaporator based on wasted durian shells. After carbonization and polypyrrole deposition, this durian shell-based evaporator achieved a 99% absorption in the whole solar spectrum, owing to its hierarchical solar absorption structure from macroscale to nanoscale. We further make the bottom surface of the evaporator to be curved to reduce the conductive heat loss, and the structure-optimized durian shell-based evaporator achieved a high evaporation rate of 1.66 kg m~(-2) h~(-1) and evaporation efficiency of 91% under the solar intensity of 1000 W m~(-2). In addition, the pyramid-shaped structures on the surface of durian shells could enable localized salt crystallization, which enables long-term operation of such evaporators in the saline environment. This work presents an attempt with significance in developing extremely cost-effective materials for highly efficient heat-localized solar evaporation.
查看更多>>摘要：The clay directly incorporated with phase change material (PCM-Clay) can prevent core-wall surface from freezing thanks to the latent heat, which is potential to extend the freezing hysteresis time of earth-rock dams in cold regions. The indoor thermal test requires time and effort to prepare different PCM-Clay samples with a tedious procedure to control temperature and fails to completely imitate the actual conditions during the core wall construction process. It is also a challenge to get the utmost out of the latent heat of PCM while prevent the PCM-Clay from freezing. Aiming to solve the above-mentioned problems, this paper proposed a numerical method for simulating the anti-freezing performance of PCM-Clay considering the actual condition of core-wall construction unit during winter construction according to a verified numerical model of indoor thermal test. Based on the proposed method, the temperature profiles of different PCM-Clays in core wall were simulated under the compulsory suspension weather required in specification and extreme weather from local meteorological record, respectively. Results show that: (1) the model efficiencies of temperature for PCM-Clay at different paraffin contents in numerical simulation reach 0.92 and above; (2) 8% PCM-Clay is able to prevent the core wall from freezing under the compulsory suspension weather and extend a freezing hysteresis time of 4.0 h under the extreme weather; (3) freezing hysteresis time of core-wall PCM-Clay decreases with the wind speed under compulsory suspension weather. This paper overcomes the defect of indoor thermal test where the actual environment for core-wall winter construction cannot be completely reproduced and temperature profiles of PCM-Clay at different paraffin contents in core-wall construction unit are analysed, providing a basis for the utilization of PCM-Clay to prevent core wall from freezing during winter construction.
查看更多>>摘要：There is a tremendous waste heat recovery potential for the gas boiler exhaust flue gas. Conventional condensing recovery and state-of-the-art membrane (one-stage membrane more accurately) recovery technologies have upper limit of the total heat recovery efficiency, depending on cooling source temperature. This paper innovatively proposes a two-stage membrane waste heat recovery (2sMWHR) to enlarge the heating capacity of district heating network (DHN). The flue gas exchanges the total heat with the DHN return water and boiler fresh air in two membrane total heat exchangers (MTHX) successively. The system realizes the self-humidification of the boiler fresh air, and therefore the mass transfer potential of the boiler flue gas is elevated, without any additional energy consumption. The 2sMWHR system yields the total heat recovery efficiency of 77.0% and the system thermal efficiency (based on net caloric value of fuel) of 105.4% at the DHN water temperature of 40℃. The waste heat exhausted finally to the ambience accounts for 3.6% only, in comparison with 15% when no heat recovery is employed. Owning to the coupled effect between the two MTHX, the total heat recovery performance is retained at a high level to a certain extent as the DHN water temperature rises from 40℃ to 60℃. The total heat recovery efficiency just decreases from 77.0% to 53.5%. In comparison, both the condensing system and one-stage membrane system perform dramatic decrease from 41.8% to 14.9% and from 45.3% to 15.1%, respectively.
查看更多>>摘要：The excessively high discharge pressure of mixture-based Joule-Thomson (J-T) refrigeration system for low-temperature freezers at the start-up process without active control leads to the reduction of operation reliability of compressors. Start-up characteristics of a J-T refrigeration system with an expansion tank for pressure control were investigated in this study. The binary mixture R1150/R600a was selected as the working fluid, and influences of the control pressure, compressor speed, throttle valve opening, and mixture concentration on the system's dynamic performances were experimentally explored. Results demonstrated that the discharge pressure could be effectively limited below the compressor's safety pressure with an expansion tank. Meanwhile, the close relationship between pull-down rate and time of the pressure control phase was associated with the control pressure, compressor speed, throttle valve opening, and mixture concentration. The pull-down rate can be accelerated by increasing the control pressure and throttle valve opening and reducing the compressor speed and the R1150 concentration at the start-up process. The excessively rich R1150 concentration caused the continuous action of the pressure switch and the malfunction of the low-temperature freezing at an acceptable time range. The build-up of the heat recuperation played a significant role in pressure control time and freezing temperature reduction rate.
Alan S. FungArianna BrambillaAmir Hossein EisapourA. H. Shafaghat...
16页
查看更多>>摘要：The gap between the existing energy demand and supply is of immense significance worldwide, suggesting the need to develop highly effective storage technologies. This study concerns an innovative design for Phase Change Material (PCM) thermal energy storage suitable for solar energy applications. The proposed configuration for the tube benefits from the wavy profiles with different amplitude characteristics and the twisted shape. The impacts of the simultaneous implementation of the wavy and twisted tube are also examined to achieve an optimal model. The best design for the thermal energy storage is then integrated with the ascending and descending conical container shapes to gain the potential advantage of natural convection. Ultimately, the best case is treated with the nanoparticles (1, 3 and 5 vol%) to enhance thermal conductivity in the storage. The results reveal that the descending wave amplitude is the most effective profile when the least charging time is needed. Furthermore, the ascending conical shape container enhances the charging process (68 min for the complete charging process), taking full advantage of the natural convection phenomenon. Overall, the ultimate heat storage design offers nearly 83% faster charging time than conventional straight tube heat storage units.
查看更多>>摘要：A Genetic Algorithm based optimization of spirally wound two-fluid stream exchanger is presented. The proposed method elaborates a design methodology consistent with the user-defined specifications while simultaneously fulfilling a design objective linking optimization of either weight, height or thermo-hydraulic performances, etc., depending on the customer requirement. Optimization variables include tube material, tube size, normalized transverse and longitudinal pitches, number of tubes in the innermost layer, increment in the number of tubes with each successive layer, and the total number of tube layers. The effectiveness-NTU approach is adopted for design calculations, including fluid property variations with temperature. This paper explores the effect of optimization criteria like minimum weight or height and variation in pressure drop constraints on the optimized design. A compromise in exchanger effectiveness can substantially reduce its weight. Among the exchangers of the same effectiveness, the one handling 10 times larger flow could be 35 times heavier (subjected to other constraints). Exchanger designed with the minimum height criterion is nearly two times shorter than the one created on a minimum weight basis, while the shorter unit could be two times heavier. The optimum design is a trade-off between achievable minimum mass and maximum allowable pressure drop.
NSTL
Elsevier