Ignition and combustion of a packed bed in a stagnation point flow

A model for the simulation of the combustion of a packed bed in a stagnation point flow is presented. It is one-dimensional and accounts for the coupling between chemical reactions and heat and mass transport in the bed and in the adjacent flow field. The governing equations for mass, momentum, energy, and species are solved using the PISO algorithm (Pressure Implicit with Splitting of Operators). The program was verified by calculating several test cases for which solutions can be found in the literature. To use a well-defined chemistry, the particles are assumed to be of carbon but, so far, only heterogeneous reactions between the particles and the gas have been included in the calculations. By this means, the smoldering combustion of a packed bed of finite and constant length has been simulated. Depending on parameters such as the length of the bed, its effective conductivity, and the oxygen supply, heterogeneous ignition can occur. This becomes evident by a discontinuous increase in the temperature in the bed, in the reaction rate and in the mass burning rate. These ignition effects lead to discontinuities in the steady-state solutions. Hence, the combustion process of a packed bed is often characterized by multiple steady states.