A dual-solver CFD model for conjugate heat transfer in continuous thermal processing

Abstract

High temperature heat treatment of powders is a common operation in the manufacturing of a range of commercial products. Often, powders are heated or cooled in discrete quantities while being conveyed through a continuous furnace or oven, where a certain temperature–time profile is required for product quality reasons. For powders that are contained and conveyed in oven-ware (saggars), heat-transfer occurs between the surrounding gas phase and the powder material and oven-ware, as well as between oven ware that is vertically stacked.The large physical size and long transit times through typical industrial furnaces (up to 40 m length) means that a full transient CFD solution for both gas and solid phases is computationally very expensive . In this study, a novel CFD simulation model is developed for conjugate heat transfer in the cooling section at the outlet of a continuous furnace. A particular novel feature in this model is the inclusion of heat conduction in multiple solid domains. A dual-solver approach enables the convective and radiant heat-transfer processes in the gas phase are modelled using a steady-state conjugate solver, while the heat conduction processes within the powders and saggars are modelled using a time-dependent conjugate solver that solves for heat conduction only. Using only steady-state solution for the gas phase enables a “snap-shot” of the thermal performance of the system to be generated in a reasonable time frame, which has practical implications for modelling of large-scale industrial equipment. In the “dual-solver” approach, heat fluxes at solid boundaries from the steady-state model are applied as boundary conditions to the transient solver, while the temperature field resulting from the solution of the transient solver is then applied to the steady-state model. Radiative cooling effects are accounted for using a discrete-ordinates model.