Measurement of Heat Conduction through Bonded Regenerator Matrix Materials

Abstract

Regenerative heat exchangers have had a significant influence on the development of small refrigerators for cryogenic applications. The optimized design of these regenerators takes into account the axial thermal conduction of the matrix. Until recently this thermal conduction has been unknown even for the commonly used screen or packed sphere matrices. Research at NIST on the thermal conduction through such matrices has shown that the thermal conduction is best represented by a thermal conductivity degradation factor. We have given this factor previously for stacked metal screens of various mesh and porosities and for packed spheres of various metals. This factor is important in optimizing the geometry of the stacked screens or packed spheres. In this paper we discuss the measurements of the thermal conduction in regenerator matrices when they are bonded either by sintering or with the use of thinned epoxy. Such bonded matrices offer some advantages in the fabrication of regen erators. For example, the uniform stacking of large diameter screen matrices with negligible gaps around the circumference can be difficult to achieve. Also, the containment of fine metal powders can be difficult. The bonding of these matrices can solve many of these fabrication problems, but could possibly be a disadvantage because of enhanced thermal conduction. Experimental results with diffusion-bonded 325-mesh stainless steel screen and epoxy-coated lead spheres are presented in this paper. The results show only a small increase in thermal conduction, which does not significantly affect the overall cryocooler performance.