The Influence of Load Geometry on Temperature Distribution During Microwave Heating

Abstract

Uneven temperature distributions during microwave heating in multi-mode ovens are commonly reported. The influence of uneven field distribution can be partially overcome by using turntables, mode stirrers, and well designed launchers. The next most important influence over temperature distributions is the geometry of the work load. New analytical studies of microwave heating in rectangular, cylindrical, and spherical coordinate systems demonstrate that object geometry and consistency profoundly affect temperature/moisture distributions during microwave heating. These analyses show that the highest temperatures in rectangular blocks usually occur in the corners, just below the surface. For small-diameter cylinders with low dielectric loss, the resulting temperature distribution resembles a dumbbell with two temperature peaks along the longitudinal axis; however, as the loss factor or diameter of the cylinder increases, the temperature profile becomes an annulus of high temperature below the upper and lower circular surfaces. Heating in small-diameter spheres is focused in the center, but like the cylinder, there is a transformation to subsurface heating as the diameter or loss factor of the material increases. Heating time also affects temperature distribution, with the peak in temperature moving towards the core of most objects as heating time increases.