On departure from local thermal equilibrium in porous media due to a rapidly changing heat source: the Sparrow number

Local thermal equilibrium is an often-used hypothesis when studying heat transfer in porous media. Examination of non-equilibrium phenomena shows that this hypothesis is usually not valid during rapid heating or cooling. The results from this theoretical study confirm that local thermal equilibrium in a fluidized bed depends on the size of the layer, mean pore size, interstitial heat transfer coefficient, and thermophysical properties. For a porous medium subject to rapid transient heating, the existence of the local thermal equilibrium depends on the magnitude of the Sparrow number and on the rate of change of the heat input. (C) 1999 Elsevier Science Ltd. All rights reserved.