טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShapiro Alexander
SubjectInvestigation of the Turbulent Aspects in a Pulse Tube
Cryogenic System
DepartmentDepartment of Mechanical Engineering
Supervisors Professor David Greenblatt
Professor Emeritus Gershon Grossman
Full Thesis textFull thesis text - English Version


Abstract

A research tool was developed for the investigation of oscillatory turbulent flows for Pulse Tube cryogenic cooler applications. The research was divided into two main phases. The objective of Phase 1 was to identify the most appropriate turbulence modelling approach. This phase involved computations of fully developed incompressible flows using a code developed in-house that incorporated several turbulence models, all of them based on the Mixing Length Hypothesis. The validation of the code was based on the different cases where analytical solutions to unsteady laminar flows exist and the turbulent models were compared to steady empirical laws. A comparison of unsteady turbulent computational results with experimental data showed that an equilibrium version of the well-known Johnson-King model produced the best correspondence. During this phase, transition and relaminarization effects were also explicitly modelled, but this resulted in inferior predictions. In Phase 2, the abovementioned Johnson-King model was implemented in the CFD program Fluent by means of a User-Defined Function package, and the appropriate finite-pipe geometry was created and meshed using the Gambit program. The computations provided with the Johnson-King model were compared with standard turbulent models contained in Fluent that are commonly used in research and industry, namely the k-e model and the Spalart-Allmaras model. Despite being physically and mathematically the simplest model, the equilibrium version of the Johnson-King model provided the best correspondence with the experimental data, relative to the standard models. A discussion about that surprising result is presented in the Conclusions section. The computations were performed for both isothermal incompressible flows and compressible flows with a temperature gradient. The predictions were obtained for velocity, pressure and temperature fields in the finite-length compressible oscillatory and turbulent pipe flow.