In order to better understand the physics of impulse drying, two numerical models have been developed to predict the transient heat transfer, vapor pressure development, and vapor liquid flow during impulse drying. The first model, MIPPS-I, examines impulse drying as a moving boundary problem in which a sharp front of steam displaces a saturated liquid phase. While several key insights were obtained with this approach, a comparison of predictions with experimental data suggested that the sharp-interface assumption should be abandoned in favor of a two-phase zone between the dry and saturated regions. A new model, MIPPS-II, was then developed which allows a two-phase zone to develop. Both models use finite-difference forms of the mass, momentum, and energy conservation equations adapted for porous media.
Analysis of the numerical results in light of experimental data helps clarify some of the transport processes in impulse drying. In particular, it appears that the impulse drying process depends on the continued boiling of liquid near the hot surface with condensation occurring in the cooler, more saturated regions. The process of boiling and condensation is tied to sheet permeability and pore structure. The liquid for sustained boiling is available in saturated dead-end pores or is supplied by capillary flow.
The numerical results show that the development of an internal vapor zone is critical to several features of the impulse drying process. The pressurized vapor zone enhances water removal through direct displacement and also possibly by reducing or eliminating rewet. Relationships between sheet properties and internal vapor pressure and water removal can now be better understood with the aid of the models.
Several new pieces of experimental information are also presented which have guided recent model developments and, at the same time, can be interpreted in terms of results from the models. The new experimental data include flash x-ray visualization of interface motion in impulse drying and several measurements of thermal processes in both paper and model fibrous porous media.