Study And Application Of Model For Heat And Mass Transfer In Capillary Porous Media During Drying

Based on volume-averaging theory, wet capillary porous media is treated as virtual continuous body in macro-scale. Primary mechanisms of transportation for liquid and gas in drying process of porous media are analyzed. Flow of fluid, including liquid and mixture gas of vapor and air, is presented with Darcy's law. Capillary transportation of liquid and diffusive transportation of vapor and air are considered in heat and mass transfer in porous media during drying.Assume porous media in local thermodynamics equilibrium and based on the mechanisms of fluid flow, interactions of mass, momentum, and energy conservation for three-phase: solid, liquid, and gas in porous media are taken into account. A universal three-equation mathematics model of drying of porous media is established, which is composed of two mass conservation equations of mixture gas and moisture-included liquid and vapor, and one energy conservation equation. Evaporation quantum of liquid is expressed as mass source for vapor in vapor mass conservation equation, while it is dealt with mass sink for itself.Following the universal model, wet and dry regions appearing in the drying process are separated by the criterion, irreducible saturation Str. Drying modelsof wet and dry regions are constituted with different mechanisms of heat and mass transfer in different regions. Two models are coupled via moving boundary of evaporation front. Wet region model is described by three state-variable: saturation of liquid S, temperature T, and pressure of mixture gas pg, whiledry region model is also denoted by three state-variable: pressure of vapor pv, temperature T, and pressure of mixture gas pg.A series of non-linear equation group are obtained through discretization of governing equations, boundary conditions, and evaporation front boundary condition with fully implicit finite difference technique. Predicted values of state-variable are achieved by solving iterative equation group in same time layer, and corrected values of state variable are calculated by solving time advancing equation. Following the circle, numerical results of drying process are realized.Experiment of convective drying of wet quartz sands as porous media, which had adiabatic-impermeable boundary of undersurface and convective boundary of upside, was carried out in laboratory. Averaging moisture content and temperature within the porous media were measured during drying. Comparison between experimental findings and numerical results shows a good coherence, which proves the model reasonable. Moreover, Distributions of drying state-variable were simulated with numerical technique, and intrinsic causes were discussed. In addition, numerical simulation was executed for another drying model, which had isothermal-impermeable boundary of underside and convective boundary of upside. The work as former was done.According to characteristic of operation of paper machine, drying model of paper in dryer section on paper machine is found. Detail behavior of paper drying was forecasted by numerical approach. Numerical results are identical in experimental ones fairly, which approves the drying model of paper sheet adaptable even if the boundaries are changed alternately and periodically. Some drying parameters influencing drying capability of paper machine are farther probed. Seizing knowledge of paper drying on modern paper machine is helpfuland significative to understand the drying parameters affecting drying capacity of paper machine, to evaluate drying purpose and objective, to guide adjustment of drying parameters and operation of paper machine, and to optimize design of paper machine.