Simulation of Pulse Tube Cryocooler for Space Applications

Abstract

A cryocooler termed as pulse tube the refrigerator is capable to achieve very low temperature around tens of kelvin. Pulse Tube Cryocooler (PTC) is gaining popularity because of high reliability, and less maintenance requirement. Due to its long life, it has intense application in space technology. It also has its applications in cooling system of infrared sensors, night vision equipment. Most refrigeration systems are working based on VCR cycle but PTC works on Sterling cycle. In case of Stirling cycle desired low temperature can be achieved by successive compression and expansion of gas. Modeling and simulations of the cryocooler helps to understand its performance characteristics under various conditions and in turn that provides useful input at time of its fabrication. One way of this approach is to solve governing equations analytically but with the recent availability of the powerful finite volume based computational fluid dynamics (CFD) tool, one is capable of simulating the near to actual system. In present work CFD analysis of Inertance Tube Pulse Tube Cryocooler (ITPTC) has been carried out to understand heat transfer processes and fluid flow. The working fluid is helium gas, which has the lowest critical temperature of all the gases. It has also high thermal conductivity. Modelling and simulation of the ITPTC is done using ANSYS Design Modeler and Fluent modules. Reciprocating motion of compressor is modeled using layering technique of dynamic meshing. A user defined function (UDF) in form of C code is developed to give sinusoidal pulsating movement to the piston inside the compressor. To make the compact system and achieve proper cooling effect, simulations are started with sufficiently higher initial pressure. Porous modelling method is used for after cooler, regenerator, cold heat exchanger and hot heat exchanger. Thermal boundary conditions such as adiabatic and constant temperature is employed on walls of different components. To achieve steady state temperature, computations are performed for 100 secs. A standard k-ε model is used for turbulence flow simulation. To understand modeling and simulation properly, geometry of ITPTC given in Chaudhary et al. (2021) and Cha et al. (2006) work is used as computation domain and results are matching with their published results within 20%. The variation in results is due to difference in meshing, modeling and simulation methodology. Simulation results shows minimum temperature at Cold Heat Exchanger and that is in tens of Kelvin. Phase lag is found between mass flow rate and pressure. A parametric study was carried out by varying frequency of piston movement in a compressor, wire mesh structure, pulse tube top wall inclination. A study of orientation of ITPTC such as inline and co-axial was also carried out. To have a proper idea of effect of 2-D and 3-D simulations was completed and its results are compared with 2D output. 1-D modelling based on fundamental principles of finite flow, heat transfer and thermodynamics was carried out using SAGE software. As its computational time for determining output is less. The geometrical configuration of 2-D and 3-D CFD based simulations differs substantially with 1-D analysis so the results comparison is not possible however 1-D analysis provided effect of various parameters quickly.