Influence of the Shape Factor on the Heating of an Aqueous Solution by Microwave

In this study, the microwave heating efficiency of a water body is investigated with different shape factors. In particular, the same water volume was deposited in cylindrical containers with different diameters. Here, “shape factor” refers to the ratio between the surface fluid layer, which strongly absorbs microwave energy, and the inner layer, which is heated largely via conduction. For a liquid in a cylindrical container, the shape factor is characterised as the ratio between the depth and diameter of the air/water surface area. The heating efficiency is characterised by relating the energy absorbed in the outer fluid layer with the energy gained in the bulk and monitoring the temperature in the fluid bulk at the point that the outer layer commences boiling. A correlation equation for the uniformity of the sample heating (with stirring) provided a simple link between the physical factors and microwave (MW) parameters. It was found that a depth/diameter ratio approaching 1:1 provided the most uniform heating. The correlations between the fitting parameters and physical conditions provide a simple yet effective method to characterise the thermal homogeneity of microwave heating that can assist with practical parameterisation of the design of microwave reactors.