Free convection development caused by bed shrinkage in a vacuum-induced smoldering reactor

Abstract

In smoldering reactors, a simple change in the way air is supplied to the system directly impacts the entire combustion process. Buoyancy forces may compete with the airflow entering the reactor, resulting in recirculation zones and stagnating the smoldering front. This paper assesses the competition between free convection and vacuum-induced airflow in a shrinking smoldering bed through dimensionless analysis guided by reliable combustion experiments and numerical heat transfer simulations for 22 scenarios. The dimensional analysis reduces the problem to the knowledge of Grashof (106−108), Darcy (10−10−10−9), Bejan (109−1011), and Prandtl (2.5−5.0) numbers. Parallel streamlines indicate negligible effects of free convection when the shrinkage is meager. From the moment the thermal plume starts to grow, the boundary-layer is squeezed to the reactor wall, gradually quenching the combustion front. The dominance of free convection is established for a critical Grashof number around 108. The intrinsic transient aspect of the problem limits the ratio of free convection velocity relative to the imposed flow velocity. Still, if used, it must be compared with the average velocity taken inside the boundary-layer. A developed correlation for the Nusselt number expresses the energy available in the fluid–porous interface.