Thermal Runaway in Electromagnetic Heating with Application to the Reheating of Cryopreserved Bio Materials

When the dielectric properties of an electromagnetically heated material are temperature dependent the possibility of thermal runaway exists. Normally, unstable heating can be overcome by heat conduction away from the overheated region, but in cases where the heating rate is very high, thermal relaxation has limited or no effect.We define three stability factors which indicate directly whether or not thermal runaway can occur, irrespective of the heating rate and the thermal properties of the heated material. These factors are dependent on the material’s dielectric properties, as well as the geometry and the orientation with respect to the E-field, of any irregularities. They can be used to provide clear guidelines to choices of source frequency and applicator geometry, which are favorable for the intrinsic control of thermal runaway.Experimental results are presented for a spherical workpiece (representing a cryopreserved rabbit kidney) warmed from-90°C to above 0°C in 30 to 60 seconds, in a 434 MHz resonant cavity applicator. The measured spatial temperature distributions for diffent E-field configurations, clearly indicate the effectiveness of the stability factors in predicting temperature uniformity in practical EM heating situations.