Power window effect in microwave heating

For a long time, power window was thought to be a criterion of nonthermal effects by researchers in microwave chemistry and bioelectromagnetics. In this paper, we prove that the power window is not one of inherent characteristics of nonthermal effects. We support this by calculating the heating process of sodium chloride solution by microwaves in a transverse electromagnetic (TEM) cell with finite element method (FEM). Although nothing except heating is considered in the calculation, we still find the power windows of temperature in the sodium chloride solution. Further calculation shows that the power window is caused by heat convection. Hence the power window cannot be a criterion of nonthermal effects.