|M.Sc Thesis||Department of Mechanical Engineering|
|Prof. Emeritus Grossman Gershon|
|Dr. May-Tal Ben-Zion|
|Full Thesis text - in Hebrew|
Most cryocoolers based on the Joule - Thomson principle use pure gases and operate at high pressures. Using gas mixtures enables working at sub- critical, low pressures and gains some workability advantage. Operating cryocoolers at sub- critical pressure causes condensation and liquid phase is formed along the heat exchanger. Better understanding and improvement of cryocoolers operated at sub- critical pressures could be achieved by solving the flow and temperature fields at the heat exchanger including the evolution of the liquid layer.
In this work a numerical solution is developed to predict heat and mass transfer at JT cryocoolers that use pure gases at sub- critical pressures. The work is a preliminary step for solving gas mixture problems.
A counter-flow laminar heat exchanger consisting of two concentric pipes is considered. Mass, momentum and energy equations were solved numerically by using the integral method.
A computer program was written that allows to solve the concentric pipe counter flow heat exchanger for any pure substance.
The model ability to analyze performance of JT cryocoolers is demonstrated. Main conclusions obtained by applying the model are as follows:
· There is a minimum of the radial heat flux at gas-liquid boundary in the part of the heat exchanger where the condensation occurs.
· Predictions of the NTU method and the current model for a two phase heat exchanger are compared. The NTU model predicts no condensation at all, even where the current model shows significant liquid layer evolution.
· There is an optimal value of the parameter ξ=Δh/ ΔhT ≤1 (ratio of the enthalpy difference in the heat exchanger to the maximum possible enthalpy difference) for which the length of the heat exchanger is minimal. This value of ξ depends on the fluid used and operating pressures. Minimal heat exchanger length for different cooling capacities is achieved for the same ξ but with different mass flow rates.
· The length of the heat exchanger depends also on pipes geometry. The length is influenced only by the ratio of the pipe radii, ηm=Ro/ Ri ≥1. Varying the pipe radii while keeping the ratio constant does not change the heat exchanger length. For smaller ηm the shorter heat exchanger length is needed.