R290/CO2复叠式制冷系统的性能实验

通过对R290／C02复叠式制冷系统的性能实验，对低温循环用CO2作为制冷工质，高温循环分别用R22和R290为制冷工质的性能进行比较，结果表明，随着蒸发温度的升高，冷凝温度的降低，R290／CO2复叠式制冷系统的最佳质量流量比增大，COP增加。随着高温循环压缩机入口温度的升高，R290压缩机的功耗略高于R22压缩机的功耗，R290循环的COPh要高于R22循环的COPh。结果表明自然工质R290／CO2复叠式制冷系统具有很好的发展前景。     

机械过冷CO_2跨临界制冷循环性能理论分析

采用蒸气压缩制冷循环(辅助循环)对CO_2跨临界制冷循环气体冷却器出口的CO_2流体进行冷却,可减小节流不可逆损失,提高循环性能。本文对机械过冷CO_2跨临界制冷循环进行热力学循环分析,结果表明:当在最优排气压力和最优过冷度两个参数条件下,循环存在最大COP。环境温度越高、蒸发温度越低,采用机械过冷方法使循环性能提升越显著,相对传统CO_2制冷循环,通过辅助循环可显著提高循环COP,降低CO_2排气压力和温度。相对CO_2压缩机,辅助循环压缩机的功耗较少。分析了辅助循环中采用11种不同制冷剂的性能,可得除R41外,其它10种工质对循环整体COP的提升程度差异不明显。综上所述,机械过冷CO_2跨临界制冷循环更适用于环境温度较高、蒸发温度较低的场合。     

锌基-MOFs对氨吸附制冷性能的分子模拟研究

本文采用巨正则蒙特卡洛(grand canonical Monte Carlo, GCMC)方法,基于UFF(universal force field)和TraPPE(transferable potentials for phase equilibria)力场,对ZIF-8(Zn)吸附NH₃进行了分子模拟研究,并结合分子模拟结果和吸附式制冷热力学循环模型,研究了ZIF-8(Zn)/NH₃工质对的吸附性能和制冷性能。研究表明：在等温条件下,ZIF-8(Zn)对NH₃的吸附量随压力的增大而提高,298 K和398 K下饱和吸附量分别达到0.305 g/g和0.231 g/g;同一温度下的总吸附热也随压力的增大而上升,这主要归因于NH₃分子间相互作用产生吸附热的增加,而ZIF-8(Zn)与NH₃相互作用的吸附热维持在较稳定的状态;NH₃在ZIF-8(Zn)中的吸附密度分布结果表明NH₃在金属位点处被大量吸附,难以通过ZIF-8(Zn)部...     

Research on performance of regenerative room temperature magnetic refrigeration cycle

On the basis of classical Langevin theory along with statistical mechanics, thermodynamics and magnetism, a new expression of magnetocaloric parameters used for room temperature magnetic refrigeration is proposed, which is briefer and more accurate than the existing one, providing a new way for studying performance of regenerative room temperature magnetic Ericsson refrigeration cycle. Influences of temperature of heat reservoirs and magnetic intensity on cycle refrigeration capacity and coefficient of performance are analyzed. The results show that the maximal temperature span of the cycle increases but its increasing rate decreases with the increase of magnetic field strength. In addition, there exists only one maximum value of effective refrigerating capacity. Two cycles with the same COP can reach a same temperature span under a certain magnetic field strength. A large magnetic field strength can improve COP but the increase rate of COP decreases.     

Frontiers in refrigeration and cooling: how to obtain and sustain ultralow temperatures beyond nature''s ambienceAux limites du froid : comment obtenir et maintenir de très basses températures très éloignées de l''ambiance naturelle

The investigation of matter at very low temperatures close to absolute zero is a powerful and important subject as it provides the unique possibility for studying physical properties, e.g. phase transitions, in an environment where most of the unavoidable disturbances present at higher temperatures are almost completely frozen out. In particular, the improvement of the standard cooling techniques for mK- and μK temperatures, i.e. ³He–⁴He dilution refrigeration and adiabatic nuclear refrigeration, respectively, opens up a temperature range for the study of liquid and solid matter several orders of magnitude below the minimum temperature existing in nature (background temperature of the universe, 2.7 K). In this paper, some important contributions obtained at the ultralow temperature facility at the University of Bayreuth will be discussed: (1) the achievement of a minimum electronic temperature of T_el=1.5 μK by adiabatic nuclear refrigeration of platinum, (2) the progress in improving the thermal coupling between different materials at low temperatures, and (3) recent developments in low temperature thermometry.     

跨临界CO2两相流引射制冷系统性能实验研究

对跨临界CO2两相流引射制冷系统性能进行了实验,分析了工况及引射器几何参数对系统性能的影响,结果表明:在实验工况范围内,跨临界CO2两相流引射制冷系统制冷量和COP随气体冷却器压力的升高而升高,随气体冷却器出口温度的升高而降低。对于使用不同喉部直径喷嘴的系统,在相同工况下,引射器喷嘴喉部直径较大的系统的性能较好。对于使用不同直径混合室的系统,随着气体冷却器压力的升高,使用小直径混合室的系统COP变化较大;当气体冷却器压力较低时,使用大直径混合室的系统COP较高,而当气体冷却器压力较高时,使用小混合室直径的系统性能较好。在相同工况下,与传统跨临界CO2循环进行比较,两相流引射制冷循环系统COP最大可提高14%。     

Experimental investigation on the performance of the refrigeration cycle using a two-phase ejector as an expansion device

In the present study, new experimental data on the performance of a never before seen two-phase ejector refrigeration cycle (TPERC) is presented. In this cycle, a two-phase ejector is used as an expansion device. The TPERC enables the evaporator to operate as in a liquid-recirculation system. The results are compared with those of the conventional refrigeration cycle (CRC). The effects of external parameters, i.e., heat sink and heat source temperatures on the system performance are discussed. The results show that the coefficient of performance of the TPERC is higher than that of the CRC over the whole range of experimental conditions. This is due to a higher refrigerant-side heat transfer coefficient in the evaporator, resulting from the higher refrigerant mass flow rate passing through the evaporator. However, the increase becomes relatively smaller as the heat sink temperature increases.     

Electron - phonon effects in heavy fermion systems

A discussion of ultrasonic propagation in normal and superconducting heavy fermion systems is presented. In the normal state the temperature and magnetic field dependence of elastic constants is discussed. The regions of adiabatic and isothermal sound propagation are shown. In the superconducting state attenuation effects and B-T phase diagrams are discussed for different substances. The Grüneisen parameters as the important electron-phonon coupling constants in the different phases are given and discussed.     

Thermal Effects During Tensile Deformation of Ti‐6Al‐4V at Different Strain Rates

An in‐depth analysis of the effect of heat generated by plastic work on the observed tensile behaviour of Ti­6Al­4V at different strain rates is presented. Special emphasis is put on the transition from isothermal to adiabatic conditions and how this transition is affected by several process parameters such as material properties, environmental conditions and sample geometry. Experiments are performed in isothermal conditions at moderate temperatures, from ?10 to 70 °C, as well as at strain rates from quasi‐static speeds to more than 1000 s^?1 using a split Hopkinson tensile bar setup. This experimental data is used in conjunction with numerical simulations to determine the evolution of temperature during the experiments and the temperature and strain rate sensitivity of the material, as well as the Taylor–Quinney coefficient. Finally, a full model of the material behaviour is presented and used to define clear limits for adiabatic and isothermal conditions.     

Optimization of a CO2–C3H8 cascade system for refrigeration and heating

Conventional working fluids (refrigerants) are being phased out worldwide to combat with the twin menace of ozone layer depletion and global warming and natural refrigerants are fast gaining favour lately. Single stage and multi stage refrigeration systems fail to widen the gap between heat source and heat sink temperatures required in many industrial applications requiring simultaneous heating and cooling and cascaded systems appear to be the best alternative. Modest research has been done in cascaded systems based on natural refrigerants thereby offering good potential for research. In this paper, a cascaded system for simultaneous heating and cooling (refrigeration and heat pump system) with a carbon dioxide based HT cycle and propane based LT cycle for simultaneous refrigeration and heating applications has been analyzed. To facilitate prediction of optimum performance parameters, performance trends with variation in the design parameters and operating variables have been presented in this article. Relevant expressions have been developed to serve as guidelines to the user for selecting appropriate design parameters like intermediate temperature so that the system yields optimum performance. Independently developed property codes have been employed for both carbon dioxide and propane for higher accuracy.     

新型吸收制冷工质对CO_2-[emim][Tf_2N]超额吉布斯自由能的预测

针对氨系、水系、氟利昂系等传统吸收式制冷工质对的缺陷,提出以[emim][Tf_2N]为吸收剂的二元混合体系CO_2-[emim][Tf_2N]作为新型吸收制冷工质对。通过将PR状态方程和NRTL活度系数模型结合为一体,并以立方型方程和超额自由能模型结合得到的WS混合规则为纽带,建立GE-EOS热力学模型,得到高温高压下二元混合体系的超额吉布斯自由能,计算结果表明:当温度为453.15 K,压力为13.374 MPa,CO_2液相摩尔分数为0.392时,CO_2-[emim][Tf_2N]二元混合体系的超额吉布斯自由能存在最大值。     

橡胶弹热效应的研究现状与展望

弹热制冷因具有较高的制冷效率和比制冷功率等特点,被认为是最有希望替代传统蒸气压缩技术的新型高效制冷技术之一。本文从橡胶弹热效应的热力学基础、动力学理论、橡胶工质及以橡胶为制冷工质的制冷循环进行分析与讨论,为国内外开展橡胶弹热效应的研究提供参考。根据文献资料可知,天然橡胶的弹热性能最好,在0℃的环境温度下应变为6,可实现的最大温变为12 K。对天然橡胶加捻-解捻过程可以实现更高的温度变化,缩小制冷系统的体积,当应变为3时,实现的最大温变为12.9 K,是未加捻橡胶的5.4倍。目前,对橡胶弹热效应的理论研究已经相对成熟,但原型机的开发有限,需进一步开发推进以天然橡胶为工质的弹热制冷技术的实际应用。因此,本文谨慎的认为天然橡胶在弹热制冷技术应用中存在很大潜能。     

Room-temperature magnetic refrigerator using permanent magnets

A magnetic refrigerator device based on adiabatic magnetic refrigeration is described. The magnetic material is cyclically magnetized and demagnetized by permanent magnets in an adiabatic process. A temperature difference of 1.6 K between the hot and cold regions was obtained under a low magnetic field (0.3 T). Gadolinium was the magnetic material used in experiments at room temperature. The range of working temperatures is between 70 and 300 K for a variety of active magnetic materials. The optimized experimental setup increased the device efficiency by achieving a temperature difference between hot and cold sources up to 5 K     

Investigation on gradient thermal cycle for power and refrigeration cogeneration

In order to improve energy utilization efficiency of low grade heat, a novel gradient thermal cycle for power and refrigeration cogeneration is proposed. The cycle is cascaded with two stages based on different thermal driven temperature. The first stage is pumpless Organic Rankine Cycle (PRC) while the second stage is two-stage sorption refrigerator. R245fa is selected as the working fluid of PRC, whereas CaCl₂-BaCl₂-NH₃ working pair is chosen for two-stage sorption refrigerator. Different heat source temperatures from 80°C to 95°C are adopted for analysis and comparison. Results indicate that the highest average power output and cooling effect are able to reach 204 W and 0.91 kW under the condition of 95°C heat source temperature and 10°C refrigeration temperature. For different heat source temperatures, total energy and exergy efficiency of the gradient thermal cycle for power and refrigeration cogeneration range from 9.49% to 9.9% and 10.9% to 11.8%, respectively. For gradient thermal cycle exergy efficiency of heat utilization ranges from 24% to 18.8% which is 126.5% and 70.9% higher than the PRC and two-stage sorption refrigerator, respectively, when the heat source temperature is 80°C.     

A Comparative Performance Analysis and Optimization of the Irreversible Atkinson Cycle Under Maximum Power Density and Maximum Power Conditions

This article reports the thermodynamic optimization based on the maximum power (MP) and maximum power density (MPD) criteria for an irreversible Atkinson heat-engine model which includes internal irreversibility resulting from the adiabatic processes. The power density, power output, and thermal efficiency are obtained by introducing the isentropic temperature ratio of the compression process, cycle temperature ratio, and the compression and expansion efficiencies. Optimal performance and design parameters of the Atkinson cycle are obtained analytically for the MP conditions and numerically for the MPD conditions. The results at MPD conditions are compared with those results obtained by using the MP and maximum thermal efficiency criteria. The effects of the cycle temperature ratio and irreversibilities on the general and optimal performances are investigated. It is shown that for the Atkinson cycle, a design based on the MPD conditions is more advantageous from the point of view of engine sizes and thermal efficiency.     

Temperature and Time Dependence on Recovery Stress Relaxation in Nickel Titanium Wire during Isothermal Holding at High Temperatures

In this study, the influence of kinematically constrained thermal cycling (heating, isothermal holding and cooling) on the recovery stress in annealed nickel titanium wire was investigated. A 4% pre‐strained nickel titanium wire was heated to temperatures (150, 200, 250 and 300 °C) much higher than the austenite finish temperature. It was observed that the maximum recovery stress obtained at different conditions decreases significantly after the first thermal cycle and reduces gradually with further increasing the number of thermal cycles. It was also seen that the recovery stress increases with time during isothermal holding at 150 °C. During isothermal holding at other temperatures, the recovery stress shows an exponential decrease, and the decrease rate of the recovery stress depends on the isothermal holding temperature. The higher isothermal holding temperature is the more the recovery stress decreases. The decrease rate reduces with increasing the number of thermal cycles.     

二元混合工质HFC32/125绝热指数计算模型分析

绝热指数与定压比热在制冷系统数值分析和计算中极为重要。本文以纯物质的PR状态方程为基础，利用适当的混合法则对绝热指数和定压比热进行理论推导，得出适用于二元混合工质的绝热指数与定压比热方程；对HFC32／125的定压比热分区进行数值拟合，得出定压比热的拟合方程，本文对理论推导和数值拟合的结果进行比较，从而为系统分析和计算提供精确热物理性质方程。     

Ambient pressure colossal magnetocaloric effect tuned by composition in Mn(1-x)Fe(x)As

The magnetocaloric effect (MCE) is the basis for magnetic refrigeration, and can replace conventional gas compression technology due to its superior efficiency and environment friendliness. MCE materials must exhibit a large temperature variation in response to an adiabatic magnetic-field variation and a large isothermal entropic effect is also expected. In this respect, MnAs shows the colossal MCE, but the effect appears under high pressures. In this work, we report on the properties of Mn(1-x)Fe(x)As that exhibit the colossal effect at ambient pressure. The MCE peak varies from 285 K to 310 K depending on the Fe concentration. Although a large thermal hysteresis is observed, the colossal effect at ambient pressure brings layered magnetic regenerators with huge refrigerating power closer to practical applications around room temperature.     

Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling

CO₂ transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO₂ has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO₂ transcritical refrigeration plant, experimentally. It tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO₂ transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO₂ cycle (0 and −10 °C) and three heat rejection temperatures (24, 30 and 40 °C). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7% and 30.3% on COP.