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Title: A mathematical model of simultaneous heat and mass transfer in a rigid porous material during the falling rate period of drying
Author: Warren, N. J.
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1983
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A mathematical model of the drying process in porous media has been developed. When translated into a computer model written in Fortran code it displayed good agreement with published experimental work. Numerical algorithms were devised to overcome certain problematical features of the model equations. The model proved capable of describing the variation with respect to time of temperature, pressure and moisture content throughout a rigid homogeneous semi-infinite porous slab subjected to identical boundary conditions on each of its two sides. Unlike the simple diffusion model which predicts parabolic moisture content profiles, the model predicts s-shaped moisture content profiles; these are reported in the literature for a wide variety of porous materials. Migration of moisture to the surface has been assumed to take place only in the vapour phase. The model is therefore limited to the falling rate period, by which time liquid phase transport has become extinct. The gaseous phase is treated as binary mixture of two constituents, watervapour and air. The flux of each of these components is described by the dusty gas model which has been the subject of recent improvement by a number of workers in the USA. This work has demonstrated that the dusty gas model may be applicable to porous media when the void fraction is continually changing, though it is recognised that experimental verification is required before this can be stated categorically. An experiment is suggested which would address itself to this question, it would also resolve the controversy surrounding the somewhat ambiguous definition of the porous medium inherent in the dusty gas model itself. iii The suitability of the model as a basis for a computer study in optimal dryer design was evaluated. The most rigorous optimisation method - the classical variational technique - is an iterative procedure wherein the control strategy is improved stepwise over each major iteration forwards and backwards in time. A given control strategy determines a forward trajectory in the real system, a slightly improved strategy is then determined by working backwards in time given the complete history of the forward trajectory. Repetition of this procedure eventually produces an optimal control strategy. The model proved too large, requiring excessive computer storage for a complete history of the forward trajectory. Moreover, the application of the variational technique is complicated enormously by non-linear partial derivatives, and it is clear that its use is limited to fairly simple systems of equations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical engineering Chemical engineering