CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

First and second law analysis of pulse tube refrigerator

The first and second law of thermodynamics was used to analyze the orifice type and the double-inlet type of pulse tube refrigerator (PTR). Detailed dynamic characteristics of the thermodynamics, flow and heat transfer processes in the PTR were revealed, including the dynamic pressure variations, transient gas temperature, mass flow rate in the PTR. The exergy loss method was used to analyze each component in the PTR for the first time, and the performance coefficients of all components of PTR have been obtained. It was found that the performance coefficient of the double-inlet PTR was 0.108, 9% higher than that of the orifice PTR. The analysis also showed that the exergy efficiency of the double-inlet PTR was 29.95%, significantly higher than that of the orifice PTR (25.04%). In addition, it was found that the exergy losses in the regenerator and orifice were substantially larger than in other components of the PTR system. The optimal design of these two key components is, therefore, essential for the further improvement of the PTR performance.     

Optimization of an inertance pulse tube refrigerator using the particle swarm optimization algorithm

In this study, a particle swarm optimization method is employed to find the optimal operating parameters and geometrical parameters, which maximize the coefficient of performance (COP) of an inertance pulse tube refrigerator (IPTR). The considered decision variables of the IPTR are the charging pressure, which varies from 15 to 25 bar, operating frequency varying from 20 to 60 Hz, geometrical parameters, such as diameter varying from 15.0 to 25.00 mm, and length varying from 40.0 to 70 mm of the regenerator; diameter varying from 12.0 to 20.00 mm and length varying from 40.0 to 80 mm of the pulse tube; and diameter varying from 2.0 to 6.00 mm and length varying from 1.0 to 3.0 m of the inertance tube. A 1D numerical model, based on the finite volume discretization of governing equations has been selected to build the initial design matrix and solve the governing continuity, momentum, and energy equations. Analysis of variance is performed using the result obtained from the numerical simulation to visualize the variations of COP as a combination of various input parameters. It is observed that after optimizing the input parameters, the COP of the IPTR increases by 15.14%.     

Dynamic Experimental Study of a Multi—bypass Pulse Tube Refrigerator with Two—bypass Tubes

INTRoDUCTIONAfterthebasicpulsetuberefrigeratorwasfirstin-ventedbyGmerdandLongsworthl1],theorilicepulsetuberefrigeratorwasintroducedbyMikulinetal.[2]Double-inletpulsetuberefrigeratorwasreportedbyZhuetal.l3jStillothermodificationswerepresentedbyIshizakietall4landMatsubaraetal.l5]Illterestsinpulsetuberefrigeratorshavegrownrapidlyduetothefactthatithasnomovingpartsatthecoldend,soithasconsiderablesystemadvalltagesovermostoftheothertypesofrefrigeratorsinreliabilitylifetime,vibrationandcostI6-7…     

Three-dimensional numerical simulation of the basic pulse tube refrigerator

A three-dimensional physical and numerical model of the basic pulse tube refrigerator (PTR) was developed. The compressible and oscillating fluid flow and heat transfer phenomenon in the pulse tube were numerically investigated using a self-developed code. Some cross-section average parameter variations such as velocity, temperature and pressure wave during one cycle were revealed. The variations of velocity and temperature distributions in the pulse tube were also analyzed in detail for further understanding of the working process and refrigeration mechanism of PTRs. __________ Translated from Journal of Engineering Thermophysics, 2006, 27(5): 737–740 [译自: 工程热物理学报]     

空气制冷循环特性及一种新型涡流制冷装置研究

通过与蒸汽压缩制冷循环的比较,论述了空气制冷循环的特性,特别是空气制冷循环对解决环保问题的优良特性以及适用温度范围宽广的特点。并且提出一种新型涡流管制冷装置系统。     

Numerical simulation of a two‐stage pulse tube refrigerator based on adiabatic model

Cryocoolers are devices that are capable of achieving and maintaining cryogenic temperatures for a number of applications such as high‐energy physics, cooling of superconducting magnets, sensors, high‐vacuum production, cryotronics, cryonics, and so on. All the above applications need coolers with high reliability, efficiency, low maintenance, and low cost. The absence of moving parts at the cryogenic temperatures makes the pulse tube (PT) coolers quite suitable for the above applications. In spite of considerable developments in the area of PT cryocoolers, many of the fundamental processes responsible for the cold production are not fully understood. In this work, we present the results of numerical simulations of two‐stage pulse tube refrigerators (PTR) using adiabatic flow of gas through the pulse tube system. A two‐stage PTR is the improved version of single‐stage system to achieve temperature close to 4 K. Assuming adiabatic gas flow through PTs, the algebraic equations for pressure, mass flow, and volumes at different locations have been derived and solved by a MATLAB based program. Using the above, the performance of PTR has been optimized for different operational parameters. The cooling powers predicted by the model have been compared with the experimental data, and they are in good agreement with each other.     

混合工质脉管制冷机性能的数值模拟

开发了适用于混合工质的脉管制冷机的计算程序，利用此程序对应用混合工质提高脉管制冷机的制冷性能进行了研究，发现当混合工质采用氦气和氢气，且处于合适的配比下，可以进一步改进脉管制冷机的制冷性能，给出了当混合工质最佳配比时，脉管制冷机内热力参数的瞬时变化及一周期的循环参数，用该计算程序可以获得混合工质脉管制冷机内复杂过程的详细信息，为制冷机设计和改进提供依据。     

回热式变温热源空气制冷循环容积制冷率优化

考虑热阻损失、压缩机与膨胀机的内损失及管路系统的压力损失，研究一个比较接近实际装置的回热式交温热源空气制冷循环，得出了循环容积制冷率制冷系数的解析关系式。由数值计算分析了压比、热导率分配以及工质与热源间的热容率匹配等参数对容积制冷率的影响。     

A steady‐state model for the design and optimization of a centralized cooling system

A simplified steady‐state model has been developed to describe thermodynamically the operation of a centralized cooling system. This model resolves the mass and energy equations simultaneously and uses inputs that are readily available to the design engineer. The model utilizes an empirical relationship for the compressor power as a function of cooling load and key temperatures. The outputs include the chiller coefficient of performance (COP) and the compressor actual power. The model simulation results are validated with a manufacturer performance data and compared with the experimental data collected at Hewlett‐Packard Laboratories site for two chillers: a variable speed and a constant speed chiller. The results of the model are found to closely match the current experimental data with less than 5% average deviation for chiller load over 10% and with a maximum deviation of 18% at 95% chiller load. For the constant speed chiller, the chiller efficiency increases with increasing load and peaks at full load. For the variable speed chiller, the chiller efficiency peaks at part loading between 40 and 80% of the chiller full load depending on the condenser water temperature. This indicates that for variable speed chillers, the chiller design has been optimized for loading less than 100% depending on the ambient conditions, customer specifications and size of the chiller. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental performance of a thermoelectric ceiling cooling panel

An experiment has been performed to investigate the cooling performance of a thermoelectric ceiling cooling panel (TE‐CCP). The TE‐CCP was composed of 36 TE modules. The cold side of the TE modules was fixed to an aluminum ceiling panel to cool a test chamber of 4.5 m³ volume, while a copper heat exchanger with circulating cooling water at the hot side of the TE modules was used for heat release. Tests were conducted using various system parameters. It was found that the cooling performance of the system depended on the electrical supply, cooling water temperature and flow rate through the heat exchanger. A suitable condition occurred at 1.5 A of current flow with a corresponding cooling capacity of 289.4 W which gives the coefficient of performance (COP) of 0.75 with an average indoor temperature of 27°C. Using thermal comfort test data in literature for small air movements under radiant cooling ceilings, results from the experiments show that thermal comfort could be obtained with the TE‐CCP system. Copyright © 2007 John Wiley & Sons, Ltd.     

Cooling load density characteristics of an endoreversible variable‐temperature heat reservoir air refrigerator

The performance optimization of an endoreversible air refrigerator with variable‐temperature heat reservoirs is carried out by taking the cooling load density, i.e. the ratio of cooling load density to the maximum specific volume in the cycle, as the optimization objective in this paper. The analytical relations of cooling load, cooling load density and coefficient of performance are derived with the heat resistance losses in the hot‐ and cold‐side heat exchangers. The maximum cooling load density optimization is performed by searching the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot‐ and cold‐side heat exchangers for the fixed total heat exchanger inventory, and the heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the heat capacitance rate of the working fluid, the inlet temperature ratio of heat reservoirs and the total heat exchanger inventory on the maximum cooling load density, the optimum heat conductance distribution, the optimum pressure ratio and the heat capacity rate matching between the working fluid and the heat reservoirs are provided by numerical examples. The refrigeration plant design with optimization leads to a smaller size including the compressor, expander and the hot‐ and cold‐side heat exchangers. Copyright © 2002 John Wiley & Sons, Ltd.     

Factors influencing the lowest refrigerating temperature of the miniature co‐axial pulse tube refrigerator

Experimental studies of the influencing factors on the lowest refrigerating temperature of a miniature co‐axial pulse tube refrigerator have been carried out in this paper. The results show that with the decrease of the mole fraction of hydrogen in the hydrogen‐helium mixture, the lowest refrigerating temperature decreases, and when the mole fraction of hydrogen is below 20%, the lowest refrigerating temperature is close to that obtained using pure helium. In addition, it is also found that the optimum frequency of the compressor is about 16.7 Hz for different hydrogen‐helium mixtures. When the charge pressure of the compressor increases, the lowest refrigerating temperature decreases; however, the decreasing trend gradually slows down with the increase of the charge pressure. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 219–225, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20065     

Numerical investigations on unsteady flow of a scroll expander for compressed air energy storage

本工作以适应用于微型压缩空气储能（micro-CAES）系统的涡旋膨胀机为研究对象,采用计算流体力学（CFD）的方法对涡旋膨胀机工作过程进行非定常数值模拟,得到膨胀机内部温度场、压力场和速度场的分布,研究了吸气温度对涡旋膨胀机性能的影响规律及工作腔流场分布特点,结果显示：膨胀机吸气温度的升高,能够增加单位质量流量的输出功;随着吸气温度的下降,动涡旋盘所受轴向气体力增大,径、切向气体力减小;膨胀机工作过程中工作腔内的温度分布并不是沿涡旋盘半径方向逐渐下降,两侧背压腔存在较大的机械能损耗,背压腔温度会高于上游排气腔。该研究结果为涡旋膨胀机排气结构的设计提供了理论依据。     

Performance analysis of the vapour compression cycle using ejector as an expander

The purpose of incorporating an ejector into vapour compression cycle is to improve the COP by reducing the throttling loss associated with the expansion device. A computer simulation of the improved cycle is carried out using a one-dimensional model based on mass, momentum and energy balances. Refrigerant characteristics were evaluated using NIST subroutines for equations of state solutions. According to the results of simulation of the improved cycle, it has been shown that the geometric parameters of the ejector design have considerable effects on the system's performance. The maximum COP is obtained for Φ_opt whose value is around 10. Several refrigerants are considered; it has been observed, at Φ_opt and for given operating conditions, that the best performances are obtained with R141b. Compared with the standard cycle the COP of the improved cycle shows an increase of about 22%. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance of a combined organic Rankine cycle and vapor compression cycle for heat activated cooling

Heat activated cooling has the potential of utilizing thermal sources that currently go unused such as engine exhaust heat or industrial waste heat. Using these heat sources can provide enhanced energy utilization and reduced fuel usage in applications where cooling is needed. The concept developed here uses waste heat from stationary and mobile engine cycles to generate cooling for structures and vehicles. It combines an organic Rankine cycle (ORC) with a conventional vapor compression cycle. A nominal 5 kW cooling capacity prototype system was developed based on this concept and tested under laboratory conditions. In order to maintain high system performance while reducing size and weight for portable applications, microchannel based heat transfer components and scroll based expansion and compression were used. Although the system was tested off of its design point, it performed well achieving 4.4 kW of cooling at a measured heat activated COP of 0.48. Both the conversion and 2nd law efficiencies were close to the model results, proving it to be an attractive technology. The measured isentropic efficiency of the scroll expander reached 84%, when the pressure ratio was close to the scroll intrinsic expansion ratio. The reduced cooling capacity was attributed to off design operation.     

Heat transfer effect on the specific cooling load of refrigerators

The maximum possible specific cooling load that can be obtained from two-heart-reservoir refrigerators with a set of high-temperature heat sinks and low-temperature heat sources is analyzed. The refrigerators considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible refrigerators. The irreversibilities are assumed to be caused by heat transfer only. The specific cooling load, defined as the cooling load per unit total heat-exchanger surface area, is adopted as the objective function for the refrigerator performance analysis in this paper.     

Experimental comparison of R245fa and R245fa/R601a for organic Rankine cycle using scroll expander

An experimental test was conducted to compare R245fa with R245fa/R601a on the organic Rankine cycle performance. The major objective of this paper is to ascertain the highest thermal efficiency and the optimal dimensionless volume ratio using the two working fluids. The experimental system consists of an electrically heated boiler, a vapor generator, a scroll expander, a condenser, a working fluid pump, and so on. For the typical weather conditions of May in Tianjin, the experiment results show that the working fluid charge has an important influence on the organic Rankine cycle performances. The optimal isentropic efficiency of the scroll expander corresponds to the design expansion ratio. Underexpanded and overexpanded processes result in the decline of the isentropic efficiency of the scroll expander, with the former playing a major role. R245fa/R601a improves the heat transfer performance in the vapor generator because of the nonisothermal phase change. The highest thermal efficiency for R245fa and R245fa/R601a is 4.38% and 4.45%, thereby illustrating that R245fa/R601a precedes R245fa. The optimal dimensionless volume ratios for R245fa and R245fa/R601a are 0.38 and 0.41, respectively. The experimental test lays foundation of the 500‐kW geothermal plant for demonstration in the next step. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental investigation of a scroll expander for an organic Rankine cycle

This paper presents experimental investigation of the performance of an organic Rankine cycle (ORC) with scroll expander which utilizes renewable, process and waste heats. An ORC test bench is built with a scroll expander‐generator unit modified from a refrigeration compressor‐electrical drive unit. A detailed experimental investigation within the test bench is performed with the organic working fluid R134a. The results show that scroll expander can effectively be used in low‐power ORC to generate mechanical work or electricity from low‐temperature thermal sources (e.g. 80–200 °C, respectively). The experiments are performed under fixed intake conditions into the expander. The pressure ratio and the load connected to the expander‐generator unit were varied. It is found that an optimum pressure ratio and an optimum angular speed co‐exist. When operating optimally, the expander's isentropic efficiency is the highest. The optimum angular speed is around 171 rad/s which corresponds to a generated voltage of 18.6 V. The optimum pressure ratio is about 4. The isentropic efficiency at optimum operation is found in the range of 0.5 to 0.64, depending on the intake conditions. The volumetric efficiency overpasses 0.9 at optimum operation and degrades significantly if the load is increased over the optimum load. A regenerative ORC equipped with the studied expender‐generator unit that operates under 120 °C heat source and has an air cooled condenser generates 920 W net power with efficiencies of 8.5% energetically and 35% exergetically. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of power and cooling cogeneration using ammonia-water mixture

Development of innovative thermodynamic cycles is important for the efficient utilization of low-temperature heat sources such as solar, geothermal and waste heat sources. This paper presents a parametric analysis of a combined power/cooling cycle, which combines the Rankine and absorption refrigeration cycles, uses ammonia-water mixture as the working fluid and produces power and cooling simultaneously. This cycle, also known as the Goswami Cycle, can be used as a bottoming cycle using waste heat from a conventional power cycle or as an independent cycle using solar or geothermal energy. A thermodynamic study of power and cooling cogeneration is presented. The performance of the cycle for a range of boiler pressures, ammonia concentrations and isentropic turbine efficiencies are studied to find out the sensitivities of net work, amount of cooling and effective efficiencies. The roles of rectifier and superheater on the cycle performance are investigated. The cycle heat source temperature is varied between 90-170 °C and the maximum effective first law and exergy efficiencies for an absorber temperature of 30 °C are calculated as 20% and 72%, respectively. The turbine exit quality of the cycle for different boiler exit scenarios shows that turbine exit quality decreases when the absorber temperature decreases.