Experiments on flows, boiling and heat transfer in porous media: Emphasis on bottom injection

摘要

This paper deals with basic experiments conducted to analyse the effect of the particles' shape and size distribution on intrinsic properties of porous beds as well as two-phase flow and heat transfer in these porous media. Structural, transport properties, flow laws and heat transfer with phase-change phenomena in several kinds of porous media are presented and discussed. The porosity of stacks constituted by spheres of various sizes is analysed. A variation law of the porosity as a function of the standard deviation of the particle size distribution is proposed. The porosity, tortuosity, permeability and inertial coefficient of the flow law in randomly stacked fibres are established experimentally and theoretically. The porosity of such media is found to vary from 0.35 to 0.92 according to the fibre aspect ratio. Tortuosity and Kozeny-Carman parameters are determined by both electric and hydrodynamic methods. These parameters are found to vary with the porosity of the fibrous bed. New relations of permeability and inertial coefficient are derived from experimental results. Finally, a pressure drop relation is proposed for fibrous beds. Convective boiling phenomena, with emphasis to application on bottom injection, are experimentally determined for fibrous porous media. Temperature field determination evidences the formation of three distinct zones in the porous medium: a liquid zone, a two-phase zone and a superheated zone. For higher heat flux density, a fourth zone is found in which vapour and liquid are in thermal non-equilibrium. A one-dimensional analytical model of pressure drop in two-phase configuration has been performed. Comparisons with experimental data are found in good agreement with the results of this model for moderate heat fluxes. For higher heat flux values, discrepancies are found. These cases correspond to the appearance and the evolution of the thermal non-equilibrium two-phase zone. Heat transfer characteristics at the heated walls are analysed. Formation of vapour in the neighbourhood of the heated walls has a strong influence on the heat transfer coefficient. This behaviour may be related to the critical heat flux phenomenon encountered in usual ducts.