Experimental investigation of ohmic heating of solid–liquid mixtures under worst-case heating scenarios

Abstract

Sterilization of solid–liquid mixtures by ohmic heating requires the assurance that all parts of the mixture are treated adequately to ensure inactivation of pathogenic sporeformers. Two previously identified potentially hazardous situations were experimentally investigated, both involving a particle of electrical conductivity significantly different from its immediate surroundings (inclusion particle), one involving a static medium surrounding the solid; and the other involving a mixed fluid. Two sub-cases were examined: (1) where the particle was less conductive than the medium (medium-more-conductive) and (2) where the particle was more conductive than the medium (particle-more-conductive). For medium-more-conductive cases, the temperature of the inclusion particle significantly lagged that of the medium. Heating rates of the inclusion particle were faster under the static than under the mixed (agitated) condition, suggesting that a more conservative result occurred under the agitated condition. For the particle-more-conductive cases, the temperature of the particle led that of the medium, indicating that the mixture cold-spot was in the medium. In experiments involving a small particle (particle to tube area ratio of 0.1), worst-case heating was observed under the mixed condition. When a large particle (area ratio of 0.2) was involved, the worst-case heating occurred under the static condition. Trends in power input and medium heating rates for the large particle suggest that a thermal crossover might be observed after a longer duration of heating. These results are consistent with a separately reported modeling study, and provide verification of model-predicted trends.