Capillary flow properties of mesh wicks

Abstract

Abstract : Prediction of heat pipe behavior is extremely important in the design of thermal control for all types of spacecraft. Heat pipes are an important element in spacecraft thermal control. Heat pipes may be subjected to pulsed- heat loading or rapid transients. Previous work in this area has pointed out the need for detailed modeling of the liquid flow in the wick. The amount of liquid in any portion of the wick depends not only on the instantaneous heat input and rejection, but on the loading history. Development of a detailed liquid flow model depends upon knowledge of the saturation dependence of the flow properties, because the detailed flow model accounts for the variation in the saturation in the wick with position and time. This report describes a research effort to determine the saturation dependence of the flow properties in a heat pipe wick. The report is divided into three sections covering the x-ray saturation measurement system, the measurement of capillary pressure and the measurement of relative permeability. The objective of the present investigation is to measure the saturation dependence of the capillary flow properties for multilayer, square mech screen wicks. X-ray radiography is used to measure the saturation in the wicks. Results are presented for the capillary pressure and saturation relationship from steady-state tests. The capillary pressure curves are shown to correlate well when cast in terms of the dimensionless Leverett function. Results of transient wicking rise tests are also presented. The transient saturation distributions are used to calculate the relative permeability of the partially saturated wick structures. Keywords: Capillary pressure; Mesh wick; Porous medium; Saturation; Permeability. (kt)