Jet impingement drying of a moist porous solid

Abstract

This paper investigates the thermal characteristics of a continuous industrial drying process for semi-porous textile composites. The conservation of mass, momentum and energy are written for a partially saturate porous fiber layer attached to a solid-backing layer. The numerical solution of the one-dimensional and transient conservation equations provides the temperature, volumetric saturation and gas phase pressure distributions in the moist porous solid and the temperature distribution in the solid-backing layer. During the wet region drying period, continuous liquid exists in the pore space, the moisture transport within the solid is described by the Darcy form of the momentum equation. The moisture transport in the sorption region is described by a bound liquid diffusion and gas phase transport. For the jet impingement type dryer, it is assumed that the penetration of the flow field into the porous solid is small (assumed valid due to the presence of the solid backing). The enhanced transport coefficients at the drying surface are estimated with the use of the Kolmogoroff theory of isotropic turbulence. This theory provides correlations for the heat and mass transfer coefficients from the fluid properties and the turbulent energy dissipation rate in the fluid. The model results of the continuous industrial drying process are compared to independent experimental temperature and global moisture content measurements taken in an operational industrial dryer. From the model analysis and experimental data, the heat flux conditions at the drying surface dictate the manner in which the solid is dried. The heat transfer coefficients considered are in the range of 20–130 W m −2K −1 and necessarily affect the manner in which moisture transport occurs within the solid. It is seen that the lower heat transfer coefficients more accurately represent the internal transport phenomena occurring during the drying process and the heating of the solid. The transport coefficients are compared to previously obtained empirical results.