Simulation of non-Newtonian fluid-food particle heat transfer in the holding tube used in aseptic processing operations

In aseptic food processing, a hot carrier fluid, usually non-Newtonian, is used to thermally sterilize the food particles suspended in a holding tube. The effects of a spherical food particle's diameter relative to the holding tube diameter on the heat transfer rates are investigated using Computational Fluid Dynamics (CFDs) simulations. As the particle to holding tube diameter ratio (blockage-ratio) increases, higher particle heating rates were usually observed when compared to the heating rates of an unconfined particle. Variations in the non-Newtonian fluid viscosities with shear rate and temperature played important roles in affecting the local Nusselt numbers. Significant effect of the blockage ratio was found on the integrated lethality of the thermal treatment at low particle Reynolds numbers (Rep). For such cases, conventional steady state fluid–particle heat transfer coefficient correlations, applicable when the particle is immersed in an unbounded stream of fluid, may lead to erroneous predictions of integrated lethality of treatment inside the holding tube. The thermal processing of the food particle was compared using two approaches. In the first rigorous approach, the transient and spatial fluid–particle heat flux variations around the sphere were accounted while in the second approach, a constant heat transfer coefficient value was specified as the boundary condition. Even at intermediate Biot number values (4–17), considerable differences between the two approaches could be observed in the conductive heating patterns inside the sphere as well as in the integrated lethalities.

► Processing of spherical food particulates in non-Newtonian fluids is analyzed.
► Temperature and shear rate affect fluid viscosity significantly.
► Tube wall affects heat transfer from a confined sphere to the surrounding fluid.
► Variation in heat fluxes around the sphere must be accounted at low Biot numbers.
► Ignoring wall effects may cause over processing of food under some situations.