טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentNachman Ilan
SubjectCFD Modeling of Pulsating Flow around a Bend with and
without Flow Straighteners as Applicable to Pulse
Tube Cryocooler
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Gershon Grossman
Full Thesis textFull thesis text - English Version


Abstract

            The technology of cryocoolers is prevalent in many aspects of our life. Cryocoolers are in use in almost every field: military, medicine, space, transportation, civil and more. The trend of the market is to increase reliability and reduce size, mass and power in the integrated system. In order to achieve this, more and more systems which involve cryocoolers are designed with miniature Pulse Tube Cryocoolers (PTC). The main disadvantage of the PTC is its low cooling efficiency. The main cause for this disadvantage is the parasitic heat losses in the cold head of the PTC. This work is one of the first attempts to investigate these phenomena in the cold head, and perhaps the first to map out the flow and temperature distributions resulting from the shape of the cold head and their dependence on the dimensionless numbers Valensi and Reynolds, characterizing the pulsating flow.


Coaxial and U-shaped Pulse Tube cryocoolers involve a change in the direction of the flow as it proceeds from the regenerator through the cold heat exchanger to the  pulse tube. This rather sharp U-turn in the flow causes non-uniformities over the cross section which may have an adverse effect particularly in the buffer tube - creating eddies leading to undesirable mixing. Flow straighteners are sometimes introduced in order to deal with this phenomenon.


This study is concerned with CFD modeling of the reciprocating flow around a 180o bend, as applicable to U-shaped and Coaxial Pulse Tube cryocoolers.  Cases with and without flow straighteners (FS) have been considered. Special attention is paid to the role of flow straighteners in terms of convective heat loss and pressure loss. It is shown that the flow characteristic in the cold head of the pulse tube cryocooler is critical, and is of greater significance in miniature cryocoolers. The pressure drop in the flow around a sharp bend vs. the one in a straight tube is studied.  Both the pressure drop and the convective heat transfer are affected by the Valensi and Reynolds numbers characterizing the pulsating flow. The CFD software makes is possible to visualize the flow and map out the temperature and velocity fields as they vary in time, as well as to observe local phenomena such as vortexes and flow reversal.