|M.Sc Thesis||Department of Mechanical Engineering|
|Supervisor:||Prof. Emeritus Grossman Gershon|
|Full Thesis text - in Hebrew|
In several advanced technological applications, the cooling of certain components to cryogenic temperatures is required. Examples of such applications are infrared (IR) detectors, high temperature super conductors (HTSC), cryogenic catheters and cryosurgery. Sensitive electro optical systems require a lightweight cooling source of high reliability and small dimensions. A Pulse Tube cryocooler is used to cool such systems that operate at cryogenic temperatures of 65K-130K and produce a thermal heat load of no more than few watts. The advantages of a Pulse Tube cryocooler over other kinds of cryocoolers are in its compact dimensions, high reliability, high efficiency, less vibrations, less electromagnetic interference and potentially low cost.
A CFD (Computational Fluid Dynamics) solution is presented for the flow and heat transfer in laminar flow in a channel under oscillating flow similar to that found in a pulse tube. An analysis of compressible flow, such as that of Helium, is preceded by that of an incompressible case; the results of the incompressible model have been compared with those of an analytical solution and found to be in excellent agreement. Thus, the analytical model provides a calibration for the numerical one.
The CFD solution helps determine the effects of compressibility on the flow and heat transfer. The velocity profiles created by a reciprocating pressure difference are described. The heat transfer has been studied for a situation of a channel with insulated walls and two extreme temperatures at both ends, simulating the conditions of a Pulse Tube. Temperature profiles are calculated and found to depend on the flow Valensi (Va), Mach (Ma) and Reynolds (Re) numbers, on the fluid’s Prandtl (Pr) number, the Eckert (Ec) number and the ratio of tidal displacement to the channel length. The convective axial heat loss was calculated and found to be non-zero despite the periodic nature of the flow. The geometrical effect of a tapered channel is also considered.