Enhancement of Drying Rate of Moist Porous Media With Electrohydrodynamically Assisted Slot Jet Reattachment Nozzle: A Numerical Study

Drying of moist porous media, such as food or pulp and paper, is an energy-intensive process. Innovative impinging jet nozzles, for instance, radial jet reattachment (RJR) and slot jet reattachment (SJR) nozzles, have been proved to be one of the efficient methods to enhance the drying rate compared to traditional in-line jet and slot jet nozzles. However, the heat and mass transfer in the region immediately underneath these nozzles are relatively inefficient. In this work, the performance of the SJR nozzle is improved by the application of electrical field, specifically, ionic wind generated in the region directly between the nozzle exit and the exposed surface of moist porous material. The numerical model is based on the coupled flow field generated by electrohydrodynamically (EHD) assisted SJR nozzle with the heat and mass transfer taking place within the moist porous media during the drying process. The simulation results show a significant secondary flow induced under the nozzle due to the ionic wind. Up to 40% drying rate enhancement has been achieved. However, the enhancement of drying rate diminishes as the air exit velocity exceeds a certain threshold. The simulation results illustrate the detailed fundamental understanding of this conjugated problem and provide a foundation for the enhancement of convective drying of moist porous materials.

Contact Drying of a Sheet of Moist Fibrous Material

Drying processes consume a significant part of processing time and costs in the manufacture of papers, textiles, ceramics and other porous materials. An analysis was made of the phenomena constituting drying processes for porous materials and analytical expressions developed to describe the internal processes and boundary conditions. The analysis considered the departure from thermodynamic equilibrium associated with evaporation or condensation. The mass and energy conservation equation in one-dimensional form was solved by the finite difference technique. The results of this mathematical drying model were compared with the experimental data of Cowan [2] for the case of the drying of a disc-shaped mat. On the basis of this comparison, it was concluded that the analytical model is satisfactory for predicting the moisture and temperature distribution in moist porous material during drying.

Mass Transfer and Pressure Rise in Moist Porous Material Subjected to Sudden Heating

In this paper the authors treat heat and mass transfer which may occur in a moist porous material when it is subjected to a sudden heating of a prescribed temperature at the surface. The basic equations describing the heat and mass transfer and their dimensionless forms are presented. Thereafter, a procedure to solve the basic equations is mentioned by using their finite difference equations. Referring to the results of the numerical computations, the influences of various parameters including thermal conductivity, heat capacity, void fraction, mobility, and initial water content of the material upon temperature, pressure, and moisture distributions in the material are discussed in detail. As a conclusion of these discussions, the authors present an empirical formula to predict the maximum pressure. The proposed formula is compared with experimental results and it is found that the formula is useful for the prediction of the maximum pressure occurring in the material during heating.