查看更多>>摘要：Cracking in La(Fe,Si)13-based magnetocaloric materials has been observed to predominantly form around La-rich (La2O3) particles and pose a threat to their long-term structural integrity. To understand the formation of these cracks, FIB cross-sectional polishing followed by SEM characterisation was employed to study local microstructural evolution after air exposure. Results suggest that volume expansion and internal degradation associated with a chemical reaction between La2O3 particles and water/moisture can lead to crack nucleation in the 1:13 phase adjacent to La-rich particles. This observation indicates that the formation of La-rich particles should be suppressed, or their size minimised during material processing to ensure the long-term structural integrity of La(Fe,Mn,Si)13 magnetocaloric materials.
查看更多>>摘要：Dimethyl ether (DME), as an environmental-friendly alternative refrigerant, has received broadly attention. Phosphonium-based ionic liquids with good thermophysical properties are regarded as promising absorbents in absorption refrigeration systems. In this work, the measurement of solubility behavior to DME in trihexyltetradecylphosphonium chloride ([P6,6,6,14][Cl]) was carried out from 278.15 K to 348.15 K by means of isochoric saturation method. Non-random two liquid (NRTL) model and Krichevsky-Kasarnovsky (K-K) model were used to correlate the experimental data. The absolute average relative deviation between experimental liquid mole fraction and calculated values was 0.739% and 1.517% for NRTL model and K-K model respectively. Based on the solubility results, the performance of single-effect absorption refrigeration cycle used DME/[P6,6,6,14][Cl] pairs was analyzed, and further compared with other working pairs composed of [P6,6,6,14][Cl].
查看更多>>摘要：In this paper, an apparatus with a 12 L flask according to ASHRAE standards was used to study the influence of trifluoroiodomethane (R13I1) on the flammability of difluoromethane (R32) and propane (R290). The accuracy of this experimental system was verified by the tests of pure R290 and R32. The flammability limits of R32 + R290, R13I1 + R32 and R13I1 + R290 binary mixtures were measured at 60 °C and atmospheric pressure. The experimental data of R32 + R290 can be well predicted by Le Chatelier's formula with maximum absolute deviation of 1.0%. The flammability limit range of R13I1 + R32 reduces with the increase of the molar ratio of R13I1 to R32 and the critical suppression ratio is 0.2. As for R13I1 + R290, the upper flammability limit (UFL) has two inflection points while the lower flammability limit (LFL) always increases. The same phenomenon could be found in the experimental result of R13I1 + R32 + R290 mixture. An extended Le Chatelier model was used to correlate the experimental data of mixtures containing inert gas and the maximum absolute deviation is less than 0.8%. It is expected that the measured values will be useful for the safe application of R13I1/R32/R290 binary and ternary mixtures.
查看更多>>摘要：MnFeP(As, Ge, Si) series compounds are three kinds of MnFe-based magnetocaloric materials, which have giant magnetocaloric effect. In this work, the experimental characteristic curves of new style Mn-Fe-P-Si materials, numbered as 1: Mn1.32Fe0.67P0.52Si0.49, 2: Mn1.37Fe0.63P0.5Si0.5, and 3: Mn1.35Fe0.66P0.5Si0.5 are presented. Based on the experimental data of these component materials and thermodynamic analysis method, a novel composite material is put forward. The optimal molar mass ratios of the composite material are obtained and they are 0.22, 0.33, 0.45, respectively. A regenerative Brayton refrigeration cycle employing the optimal composite material with thermal hysteresis as the working medium is built. By numerical calculation, the influences of thermal hysteresis on the main thermodynamic quantities are evaluated. The results show that the thermal hysteresis of the working medium results in a decrease of 13.6%, 14.6%, 18.8%, and 16.1% of the cooling quantity, net cooling quantity, optimally working temperature range, and coefficient of performance, respectively. These conclusions are beneficial to the optimal parameter design and performance improvement of active magnetic refrigerators.
查看更多>>摘要：Vapor-liquid ejector is able to lift the discharge pressure higher than the primary flow pressure without consuming any external mechanical energy. In this paper, the actual process and thermodynamically ideal process are established, respectively, to reveal the evolution of the pressure lifting ability degradation. Moreover, the internal energy conversions in each component are explored by splitting exergy into temperature-based exergy, pressure-based exergy and kinetic exergy. Seven eco-friendly working fluids are firstly compared at different operating conditions. It is found that there exists an optimum value for the inlet vapor temperature to maximize the actual pressure lift and a different value for the minimum pressure ratio lift. From the exergy analysis, it is obtained that the highest irreversibility occurs in the mixing chamber for all candidates, followed by the shock, vapor nozzle and diffuser. Meanwhile, the pressure lift destructions inside these components have the same order of magnitude. The exemplary study on cyclohexane which has the highest actual pressure lift and the lowest pressure lift ratio indicates that the major energy conversions in the vapor nozzle and the diffuser are almost constant, while they vary considerably in the mixing chamber and shock. For the vapor-liquid ejector working at the studied conditions, only 5.33% – 21.91% of the temperature-based exergy is usefully converted to pressure-based exergy. This study is helpful to identify the key parameters for improving the performance and facilitating the design of the vapor-liquid ejector.
查看更多>>摘要：Traditional refrigerants such as HCFC and HFC have a great influence on global warming. On the other hand, natural refrigerants have little impact on the environment, and low global warming potential (GWP) and zero ozone depletion potential (ODP) are key-characteristics of natural refrigerants. Hydrocarbon refrigerant is one of the natural refrigerants with higher COP than other natural refrigerants. Furthermore, performance improvement is promoted by mixing hydrocarbon refrigerants. Due to their characteristics, refrigerant mixtures have an advantageous heat exchange performance over a single refrigerant. Generally, refrigerant mixtures have temperature glide during phase change. Because of this, temperature difference can be maintained uniform during the heat exchange with secondary fluid. In this study, cooling cycle using isobutane/propane mixtures is researched to maximize the performance when changing mixing ratio by simulation. However, refrigerants with different isobutane ratios can be considered completely different refrigerants. Therefore, a good methodology is needed to compare performances of cycle. In this study, unlike previous studies, various methodologies are used to compare the performance of refrigerant mixtures. Also experiments for isobutane/propane mixtures were conducted. Various temperature conditions are used in the simulation study, and outdoor temperature of 35 °C and indoor temperature of 27 °C were used in the experiment to validate the simulation results.
查看更多>>摘要：Frost deposition phenomena on cryogenic cold surfaces are receiving more and more attention from both researchers and various industries. However, frost formation on fins of cryogenic temperature has been not touched. Therefore, an experimental system of frost deposition on straight cylindrical fins with fin base temperature as low as -190 °C under natural convection conditions was established and a series of frost formation experiments under various conditions were carried out to analyze the characteristics and influence factors of frost formation. The results show that due to the change of frost formation mechanism, the frost formation on the fin is greatly different under different temperature ranges. The distribution of the frost layer thickness on the fins at cryogenic temperature is more uneven along both the fin axial and circumferential directions, and this characteristic is more significant in the early frost stage. The frost formation mechanism changes not only with the base temperature, but also along the fin length when the base temperature is low enough. For low fin base temperatures, say -190 °C, liquid air appears at the fin base, which will lead to no frost in the near-fin-base region for a long time, and lowering the fin base temperature will decrease frost layer thickness. The size of the near-fin-base region increases as the fin base temperature decreases. In addition, increasing air relative humidity or air temperature without changing other conditions will accelerate frost formation. The results also show that fin materials strongly affect the distribution of frost layer on fins.
查看更多>>摘要：This work aimed to establish the main factors that affect an adsorptive air conditioning system applied to a vehicle. They were determined by how they influence the system's performance and continuity, being divided in five groups (input parameters, work pairs, cycle types, reactor's design and output parameters). Analyzing such factors, it is possible to predict how the system will behave and the results it will provide. Some main characteristics of each factor were highlighted, and an attempt was made to determine those that would best fit the system in question. It was noted that the best heat source would be the spark combustion engine exhaust gas, as it has higher temperatures and pressures. The ideal working pairs are silica gel / water, activated carbon / water, activated carbon / R32 and zeolite / water, providing faster cycles and greater adsorption capacity. The best cycle is the Multi-Bed Cycle paired with Recovery Cycles, allowing for greater system continuity without making the cycle extremely complex. The best designs would be the finned tube or plate, as they achieve the best efficiency values, without compromising the reactor size. Finally, the systems needed to present high efficiency parameters, with the least amount of “dead mass”. Going through each factor in this particular order helped to choose the ideal options, while avoiding too problematic crossed influences. Thus, by finally combining the factors, it is possible to develop different conceptions, which may lead to different results, expanding this research and studies for new adsorption refrigeration systems.
查看更多>>摘要：Non-equilibrium phase transition will take place in the pressure oscillation tube if the intake of wave rotor refrigerator (WRR) contains condensable components, which will affect intensity and speed of the wave system in the tube, and further affect the port matching of WRR. In this paper, numerical model of non-equilibrium phase transition is established and the effects on intensity of the wave system and the port matching is studied. The properties of polar gas are described by the cubic-plus-association (CPA) gas state equation, improving the accuracy of computational fluid dynamics (CFD) calculations. By using the model, the design precision of two-phase WRR is improved. Besides, the results of numerical simulation are verified by experiments.
NSTL
Elsevier