The questions such as : when to apply microwaves during the drying process; what effects do microwaves have on drying kinetics; what are the benefits that microwaves can provide during the drying process; what proportion of convective energy and microwave energy should be used during the drying process, and is the assumption of thermal equilibrium valid for a microwave enhanced drying environment have generated a great deal of interest over the past decade and most importantly, still remain largely unanswered. To assist with the elucidation of these questions, two models are formulated to describe the drying of a slab of porous material in a combined microwave and convective environment.
The models describe the evolution of temperature, pressure, moisture and power distributions that occur during the drying process, and the microwave internal heat source is calculated from electromagnetic theory. Dielectric properties that vary with moisture content and temperature are an important feature of the developed theory. The inclusion of pressure in the model allows the physical phenomenon of "water pumping", often observed in microwave drying systems, to be accounted for. The influence of sample size on the drying kinetics is examined and found to be a significant parameter during the drying process.
In particular, the effect of microwave resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. as well as analysing the effects that the functional form of the dielectric properties can have on overall drying kinetics.
The complexity and costs involved in measuring the distribution of such quantities as moisture content, temperature, pressure and power generated during a combined microwave and convective drying experiment highlight the need for mathematical modelling as an essential ingredient in the research of the topic. Numerical simulations can aid in the design and understanding of dryers and can indicate the effectiveness of the microwave energy during the drying process. The developed mathematical models can help the end user better understand this most complex heat and mass transfer process at a fundamental level by providing an effective tool for optimising a particular drying process. Simulation results are presented for the combined microwave and convective drying of both hygroscopic and non-hygroscopic materials.