Research On Fluid Flow And Heat Transfer In Porous Media Under Multi Factors

Problems of flow and heat transfer exist widely in building energy efficiency and indoor environment control and exert an important influence. In this paper, influence of porosity distribution and heat conductivity coefficient, etc on flow and heat transfer in porous media is studied, and it can be offered certain reference for the engineering application.Firstly, experimental study of heat transfer in porous media under porosity of uniform distribution and non-uniform distribution is carried out. The results show that:in initial phase, temperature distribution is affected by initial temperature distribution. Influence of temperature distribution in porous media disappears at some point. Temperature of upper position is higher. Temperature of lower position is lower. Heat conduction is obvious when the side of smaller porosity is heated. Heat convection is obvious when the side of larger porosity is heated.Secondly, the influence of porosity distribution, heat conductivity coefficient on flow and heat transfer in porous media is studied using numerical calculation. The results show that:fluid flow velocity in porous media under U-shaped porosity distribution is large near the wall surface. The increase of porosity near the wall surface reduces the low velocity zone and increases the high velocity zone, heat convection gets enhanced. Under exponential distribution, because of the decrease of porosity, the velocity of fluid decreases, and the flow chart shift to the right overall. Average Nusselt number of actual porosity distribution is less than that of evenly porosity distribution. Under uniform, piecewise uniform and continuous distribution porosity, values of stream function is within less than five times, value of continuous distribution is 0.0017 and that of uniform distribution is 0.0001. When ks/kf is large, the temperature gradient and velocity gradient are large near the wall surface. When ks/kf=0.1, heat is transferred mainly by conduction. When ks/kf>1, convection gets enhanced.Thirdly, experiment study and numerical calculation are made on fly ash. When porosity is 0.26, with the increase of moisture content, effective thermal conductivity increases linearly; when moisture content is 0.05, with the increase of porosity, effective thermal conductivity decreases linearly; effective thermal conductivity of sample 5 is 0.19 W·m^-1K^-1; with Ra increases, convection in fly ash gets enhanced and becomes uneven. When Ra<105, Nu is 2.37～3.31; When Ra>105, Nu decreases from low to high, maximum is 30.8, minimum is 1.19.