Heat pipe development for high temperature recuperator application

Abstract

Heat pipes have been developed for operation in oxidizing atmospheres at temperatures above 1100 K. The heat pipes comprise a metallic liner and wick structure with a protective outer shell of an oxidation resistant material. The working fluids used in the heat pipes are alkali metals. A number of configurations have been evaluated, ranging from pipes using a metallic inner liner of a chemically vapor deposited (CVD) refractory metal applied to ceramic tubing, to one utilizing ferrous materials with an outer layer of a developed oxide. A promising intermediate configuration consisting of free-standing refractory tubing covered with a layered structure of fine grain, equi-axed CVD silicon carbide has also been evaluated. The test heat pipe was fabricated using low-carbon, arc-cast molybdenum tubing and a wick composed of 150 mesh molybdenum screen. Hafnium gettering was used with sodium working fluid. Assembly of the pipe was by electron beam welding. Following closure and capping of the fill tube the assembly was operated in a vacuum for several hours prior to the chemical vapor deposition of the exterior ceramic coating. After coating, the pipe was operated in air and in combusion gases for performance evaluation. The use of iron-chromium-aluminium alloys as container materials for operating in high temperature oxidizing and sulfiding gas streams has been investigated. Alloys of this type develop heavy, protective oxide surface layers when exposed to high temperature oxidizing atmospheres, and are commonly used in electrical heating elements because of their exceptional oxidation resistance. A heat pipe was assembled from Kanthal A-1 seam welded tubing in order to establish limits of operation for these materials. The heat pipe employed a 150 × 150 mesh 316 stainless steel (SS) screen wick and hafnium foils for oxygen gettering with sodium used as the working fluid. This heat pipe has been tested in a fossil fired flame above 1200 K to determine its operating characteristics and the durability of the material.