Effects of axial conduction in the fluid on cryogenic regenerator performance

Abstract

Although axial conduction in the matrix has been recognized as a major source of irreversibility in cryogenic regenerators, axial conduction in the fluid phase has largely been neglected. However, in spite of the negligible intrinsic thermal conductivity of most gases the effective conductivity of the gaseous medium in a porous bed may be quite significant, due to eddy diffusion and the consequent mixing of sections of gas at different temperatures. The governing equations of a thermal regenerator have been written in terms of the reduced length, A, reduced period, II, and an axial conduction parameter, λ, which depends only on the void fraction and the bed length to particle diameter ratio for a flow Reynolds number Re > 2. Numerical solutions, using the finite difference technique developed by Willmott and co-workers, have been obtained for several values of the three parameters. It has been established that axial conduction in the fluid phase is important, particularly when the design reduced length Λ > 1 /λ.