Optimum Cryogenic Heat Pipe Design

Abstract

Candidate working fluids for cryogenic heat pipes [1] exhibit very low values of surface tension and latent heat of vaporization, resulting in low-capacity heat pipes which are very sensitive to acceleration fields. It is important in the design of cryogenic heat pipes to optimize the wick geometry characterized by the pore radius R c,and wick thickness ratio R v/R, w , in order to achieve the maximum throughput Q and length Z for a given pipe ID and a given acceleration field. It is the purpose of this paper to derive expressions for optimum capillary pore radius R co and maximum throughput length product Q(Z + Z a ), to use the expressions in selecting the best cryogenic working fluid and in designing the wick structure, and to show the dramatic effect of acceleration fields on heat pipe performance. To complete the optimum heat pipe design, a heat transfer model based on conduction is presented for both the homogeneous wick structure and the channel wick. Total heat pipe temperature drop comparisons are made between optimized cryogenic heat pipes and the best solid thermal conductor.