Experimental and theoretical investigation of operational and survivability issues in thermal radiators for thermionic space nuclear power systems. Final report

Abstract

Heat pipes are a promising candidate for spacecraft radiators. This report describes a program designed to investigate the mass migration phenomenon in heat pipes. The program involved experiments to observe and measure the mass migration rates in both high and low operating temperature heat pipes. The low-temperature experiments were intended to simulate the operation of high-temperature, liquid metal heat pipes. Octadecane was the selected low-temperature working fluid. It is a paraffin and exhibits some of the characteristics of liquid metal working fluids. Sodium was the working fluid used in the high temperature experiment. A one-dimensional compressible flow model was developed for describing the hydrodynamics of rarefied vapor flow in heat pipe condensers. This model was compared with experimental data for the low-temperature octadecane heat pipes and the high-temperature sodium heat pipe. The model was found to satisfactorily predict the temperature profiles and location of freeze-fronts for the low-temperature heat pipes. Mass migration rate predictions using the model were satisfactory for the low-temperature heat pipes as well. However, the mass migration prediction for the high-temperature, sodium heat pipe was not in agreement with experimental data. An analytical model which accounts for property variations in the radial as well as longitudinal directions is recommended. A one-dimensional model was unsatisfactory for predicting mass migration rates in liquid metal heat pipes.