Numerical simulation of heat transfer in aseptic processing operations involving non-Newtonian fluids

The effects of blockage due to a large particle suspended in the flow within a tube and rheological characteristics of the carrier fluid on fluid-particle conjugate heat transfer are investigated through computational fluid dynamics simulations. Case studies involving the aseptic processing of a spherical object immersed in pseudoplastic fluids of different power law indices were investigated numerically. Heat transfer rates into the particle placed in a tube of diameter comparable with the particle diameter were found to be higher when compared with those obtained when the particle was immersed in a free i.e. unbounded stream of fluid. Significant effect of the blockage was found on the integrated lethality of the treatment at low Re_p. This indicates that heat transfer correlations developed under conditions where the particle is immersed in a free stream of fluid will predict erroneous integrated lethality of treatment under aseptic processing conditions. The conductive heating patterns inside the sphere, taking into account the local variations in the transient heat flux around the sphere due to the surrounding fluid hydrodynamics, were compared with those estimated by specifying a time averaged as well as surface area averaged heat transfer coefficient at the fluid-solid boundary. Due to the high Biot number, specifying a constant heat transfer coefficient at the fluid-particle interface did not lead to significant deviations in the temperature patterns as well as integrated lethality when compared to the actual situation where local variations in the transient heat fluxes around the sphere was considered.