Measurement of cryogenic regenerator characteristics under oscillating flow and pulsating pressure

This paper describes an experimental apparatus developed to investigate detailed thermal and hydrodynamic characteristics of a regenerator at cryogenic temperature under oscillating flow and pulsating pressure conditions. Cold-end of the regenerator is maintained at approximately 85 K for G-M cryocooler type and 100 K for Stirling cryocooler type operations by means of two cryogenic heat exchangers. At both ends of the regenerator, fine hot wire probes are installed to measure the fast oscillating gas temperature and mass flow rate. The gas temperature sensors installed very close to the ends of the regenerator matrix assure precise gas temperature measurement in the regenerator. In this study, thermal and hydrodynamic behaviors of the well-defined wire-screen regenerator are fully characterized. First, pressure drop characteristics are discussed for different frequencies under room temperature. Second, ineffectiveness of the regenerator is obtained for different cold-end temperatures.     

脉冲管整机条件下回热器交变特性实验研究

搭建脉冲管制冷机动态参数测试实验台,在脉冲管制冷机整机运行条件下对处于实际温度梯度的回热器进行了动态速度以及动态压力测试。为了测量回热器冷端的动态速度,研究针对低温侧的热线探针进行了低温条件下的标定。实验测量了充气压力3 MPa、运行频率40—60 Hz、冷端温度100 K工况下回热器的相位特性以及阻力特性,并得到了整机运行条件下回热器阻力系数经验公式。研究结果表明,随着频率的提升,回热器冷热端速度-压力相位差增大,相同雷诺数下的阻力系数增大;随着温度降低,相同雷诺数下的阻力系数增大,并且在低雷诺数下变化更加明显。该研究结果是在回热器实际工作条件下测量得到,能够为脉冲管制冷机中的回热器模拟计算以及回热器热力学分析提供必要的数据支持。     

Heat transfer characteristics of oscillating flow regenerator filled with circular tubes or parallel plates

Under assumption of small perturbation, linear thermoacoustic theory was applied to analyze heat transfer characteristics of compressible oscillating flow in two kinds of simple regenerators filled with circular tubes or parallel plates. Based on the cross-sectional oscillating velocity and temperature distributions, the exact expressions of Nusselt number were derived in complex notation. The Nusselt number is the function of Prandtl number, kinetic Reynolds number Re_ω and the third dimensionless variable, D. Here, the D is defined as the ratio of heat transfer capability aroused by mean temperature gradient and gas compressibility. Both the gas compressibility and mean temperature gradient effects were discussed and two corresponding Nusselt numbers were given. In particular, simpler expressions for these two Nusselt numbers were deduced for extreme values of Re_ω and D. Finally, combined effect of gas compressibility and non-zero mean temperature gradient on heat transfer characteristics were analyzed via D. The analysis shows that the mean temperature gradient gives predominant contribution to the heat transfer performance of oscillating flow regenerator.     

回热器板叠流动特性研究

回热器内交变流动的研究是回热式制冷机研究的重点,采用数值模拟的方法对回热器内交变流动特性进行了研究.结果表明回热器内压力、速度分布不再呈波动状分布,而是近似的呈直线分布.回热器内的交变流动阻力系数是时间与位置的函数,也具有明显的波动性.     

Novel flow analysis of regenerator under oscillating flow with pulsating pressure

A new model for the oscillating flow combined with pulsating pressure in cryocooler regenerators is developed to overcome inaccuracy of the conventional flow model based on steady flow friction factor. The new model is based on two non-dimensional parameters and one-dimensional governing equations. One of the parameters is the oscillating flow friction factor represented by the amplitudes of the pressure drop and the flow rate. The idea of the other important parameter originates from a volume averaged continuity equation, which includes the characteristic of randomly oriented matrix geometry. This parameter is named as Breathing factor, which implies that the regenerator breathes against the main oscillating flow. Empirical correlations of the parameters are obtained for screen regenerators. It is revealed that the phase angles of the pressure drop and the mass flow rate in regenerators are well predicted by introduction of the Breathing factor.     

Flow characteristics and flow rate prediction of pulverized coal through a regulation valve

In this paper, experiments of dense-phase pneumatic conveying of pulverized coal were carried out in an industrial-scale system to study the control characteristics of the regulation valve and to predict the solid mass flow rate. Firstly, effects of valve sweeping gas on conveying stability and solid mass flow rate were investigated and the optimum valve sweeping gas was determined. Second, effects of valve opening on pressure distribution and solid mass flow rate were investigated by conducting experiments at different conveying pressure drops and different valve openings. A good linear relationship between the valve pressure drop ratio and the valve opening was found, and as the valve opening increased from 13 % to 70 % the solid mass flow rate increased gradually. Limit operating conditions of the regulation valve including flow blockage and control failure were consequently determined and analyzed. Finally, a robust model was established to predict the solid mass flow rate by introducing the valve sensitivity coefficient into the traditional pressure drop ratio model. The model can predict the solid mass flow rate well by providing errors mostly within ± 10 %. This study will provide certain reference for solid mass flow rate regulation in the dry coal gasification process.     

Experimental results for a cryocooler regenerator

In this work some experimental results obtained on a single stage Gifford McMahon cryocooler developed at the Officine Galileo for ir thermal vision applications are presented. The difference in thermodynamic behaviour of three ordinary and easy to find meshed nets used in our cryocoolers as a matrix for the thermal regenerator has been checked. The method used to detect the regenerator performance is indirect but very simple to put into practice and useful for an overall evaluation. A clear indication for the choice of the regenerator material comes from the experimental data.     

Investigation of oscillating flow friction factor for cryocooler regenerator considering cryogenic temperature effect

In this paper, experimental results and correlations on the friction factor of screen regenerators are presented, being focused on the effect of cryogenic temperature. Cryogenic environment of helium gas is simulated by performing a simple room-temperature experiment using a higher density gas. Based on the experimental results, the proper correlations are proposed with several non-dimensional parameters. Numerical simulation results from the model with the proposed friction factor are compared to the experimental data under actual operating condition of cryocoolers. The experimental data confirm the accuracy and the availability of the proposed friction factor.     

新型径轴向混合填充式回热器的流动阻力实验

对新型径轴向混合填充式回热器的流动特性进行了实验研究.实验结果表明:在工质体积流量相同的条件下,新型径轴向混合填充式回热器的稳态流动阻力系数f小于层叠丝网型回热器,而大于丝网卷裹型回热器和平行丝型回热器;在相同体积流量下,以He为工质的稳态流动阻力系数f是以N2为工质时的3.4-3.6倍;回热器稳态流动阻力系数f主要与...     

Flow boiling characteristics and flow pattern visualization of refrigerant/lubricant oil mixtures

A comprehensive review of flow boiling characteristics and flow pattern visualization of refrigerant/lubricant oil mixtures is presented in this paper. First, various parameters influenced by the lubricant oil in convective boiling of refrigerants, such as mass velocity, vapor quality, oil concentration and geometric characteristics of the heat transfer tube are discussed. The effects of the unavoidable introduction of the lubricant oil on the thermodynamics properties of a refrigerant are described. Then, a review of the main experimental studies of flow boiling of refrigerant/lubricant oil mixtures is presented and also describes research with halocarbons, carbon dioxide, hydrocarbons and ammonia. There is no agreement among these studies regarding the effect of the oil in the evaporator, with studies showing an increase or decrease in the heat transfer coefficient. However, in relation to pressure drop, all the results presented the same trend, increasing the pressure drop with increasing oil concentration. Next, the flow patterns of refrigerant/oil mixtures are illustrated together with a selection of video images. It is possible to notice the difference in frothing formation with respect to the particular refrigerant and tube geometry. Some predictions of oil effects on the heat transfer coefficient and pressure drops based on the mixture physical properties are then presented and the trends compared to data. Finally, some suggestions for future work are given.     

A 2D hybrid model of the fluid flow and heat transfer in a reciprocating active magnetic regenerator

This paper evaluates the thermal behavior of a magnetic-Brayton-based parallel plate reciprocating active magnetic regenerator (AMR). A time-dependent, 2D model of the fluid flow and the coupled heat transfer between the working fluid and the solid refrigerant (gadolinium) is proposed. A hybrid calculation method which consists of an analytical solution for the flow and a numerical solution for the thermal field has been adopted. Results for the cooling capacity as a function of the temperature span and mass flow rate agree well with trends observed in experimental data and other theoretical models available in the literature. The volume of fluid displaced through the channels during the isofield processes influences significantly the AMR performance. For a cycle frequency of 1 Hz, the cycle-averaged cooling capacity reaches a maximum when the utilization factor is 0.1 and the displaced fluid volume equals 62% of the fluid volume of the AMR.     

Experimental performance investigation of an active magnetic regenerator subject to different fluid flow waveforms

A flow control mechanism based on cam actuated valves is designed and implemented on an active magnetic regenerator test apparatus. The objective is to overcome the brief low field period of the nested concentric Halbach array by decreasing the fluid blow width, displacing fluid only when the magnetic field is close to the minimum and maximum values. Flow waveforms are simulated to evaluate varying blow durations with the same displaced volume. AMR experiments are performed where the largest Ex_Q of 1.62 W is obtained with V_D = 13.90 cm³ and a diversion ratio of δ = 0.41, demonstrating an 11.2% increase over the sinusoidal waveform.     

斯特林制冷机中蓄冷器的程序化设计

通过对蓄冷器性能的综合研究,详细分析了蓄冷器性能的影响因素及其各种损失计算方法,参考国内外对蓄冷器的研究成果,总结了蓄冷器的设计方法,提出了蓄冷器设计的程序化方法,并运用迭代计算方法找到蓄冷器性能最佳的结构参数,达到了蓄冷器优化设计目的.     

Experimental analysis of a two-material active magnetic regenerator

Experimental results of an active magnetic regenerator (AMR) composed of two equal volumes of gadolinium and Gd_0.85Er_0.15 using 2 T and 5 T are reported. Drive forces and system losses are measured as a function of thermal load and magnetic field. Metrics for coefficient of performance and efficiency are defined and used to distinguish between regenerator and device performance. Results suggest the largest temperature spans are expected to occur when each material is operating with its average temperature near their Curie temperatures. Force measurements indicate that mechanical losses and pumping power are the most significant contributions to network while the net magnetic work is too small to be resolved. COP values for the magnetic cycle are as high as 2.4 while efficiencies are all less than 0.15. A maximum exergetic cooling of 1.94 W is estimated with a corresponding specific exergetic cooling power of 23 W T⁻¹L⁻¹.     

Experimental study on the flow and heat transfer characteristics of a tetra-n-butyl ammonium bromide hydrate slurry (first report: Flow characteristics)

The flow characteristics of a tetra-n-butyl ammonium bromide hydrate slurry were investigated experimentally. Reynolds number, tube diameter and solid fraction were varied as the experimental parameters. For laminar flow, it was found that the ratio of the coefficients of pipe friction increases with solid fraction, and the rate of increase is high in the case of a low Reynolds number. For turbulent flow, the ratio of the coefficients of pipe friction was approximately 1 for all tube diameters at low solids fraction. It then decreased slightly at a particular solid fraction and increased again at high solid fraction. Theoretical analysis was carried out using the experimental results, and it was found that the flow characteristics of the hydrate slurry can be treated as those of a pseudoplastic fluid and clarified using the apparent Reynolds number.     

The effect of tapering on a magnetocaloric regenerator bed

To design a high efficiency magnetocaloric heat pump for the residential sector, we focused on the improvement of the performance of the regenerator bed. In particular, placing the regenerators circumferentially on a plane, we decided to use tapered regenerators instead of the straight channel ones. Therefore, this paper investigates the effect of the tapering of the regenerators, which exhibit better air-gap utilization. Several simulations using a 1D AMR model were run to study the performance of the tapered regenerator, and the results were compared to the case of the straight regenerator bed. Moreover, the temperature span was held fixed at 25 K, and the working temperature of the regenerator was shifted to study the sensitivity to the variation of the working conditions. This paper considers a 10-layer regenerator, with Curie temperature (T_C) spacing of 2.5 K.     

Experimental investigation of a three-material layered active magnetic regenerator

This work reports on experimental studies using an active magnetic regenerative test apparatus (AMRTA) in near room-temperature refrigeration cycles. Experiments using regenerator beds composed of three different magnetocaloric materials combined in a layered configuration with applied fields of 2 T have produced no-load temperature spans in excess of 50 K. The test apparatus uses two active magnetic regenerators each containing approximately 135 g of refrigerant. An overview of the test apparatus, operating parameters, and performance is described. The impacts of operation at varying heat rejection temperatures, applied fields of 1.5 T and frequencies between 0.65 and 1.0 Hz are presented. In addition, the impacts of operating pressure and applied load on temperature spans are discussed.     

Analysis of the flow of a nonlinear viscous fluid for cyclic deformation

A hydrodynamic analysis was carried out for the propagation of shear vibrations in a layer of a nonlinear viscous power-law fluid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 1, pp. 58–62, July, 1982.     

Design concepts for a continuously rotating active magnetic regenerator

Design considerations for a prototype magnetic refrigeration device with a continuously rotating AMR are presented. Building the active magnetic regenerator (AMR) from stacks of elongated plates of the perovskite oxide material La_0.67Ca_0.33−xSr_xMn_1.05O₃, gives both a low pressure drop and allows grading of the Curie temperature along the plates. This may be accomplished by a novel technique where a compositionally-graded material is tape cast in one piece. The magnet assembly is based on a novel design strategy, to create alternating high- and low magnetic field regions within a magnet assembly. Focus is on maximising the magnetic field in the high field regions but also, importantly, minimising the flux in the low field regions. The design is iteratively optimised through 3D finite element magnetostatic modelling.     

A numerical method of regenerator

A numerical method for regenerators is introduced in this paper. It is not only suitable for the regenerators in cryocoolers and Stirling engines, but also suitable for the stacks in acoustic engines and the pulse tubes in pulse tube refrigerators. The numerical model is one dimensional periodic unsteady flow model. The numerical method is based on the control volume concept with the implicitly solve method. The iteration acceleration method, which considers the one-dimensional periodic unsteady problem as the steady two-dimensional problem, is used for decreasing the calculation time. By this method, the regenerator in an inertance tube pulse tube refrigerator was simulated. The result is useful for understanding how the inefficiency of the regenerator changes with the inertance effect.