Three-dimensional effects in counterflow heat-recirculating combustors

Three-dimensional (3D) numerical simulations of spiral counterflow Swiss roll heat-recirculating combustors were performed including gas-phase conduction, convection and chemical reaction of propane–air mixtures, solid-phase conduction and surface-to-surface radiation. These simulations showed that in 3D, results are surprisingly similar with or without a turbulence model activated because without turbulence, Dean vortices form in the curved channels which enhance heat transport and thus heat recirculation by nearly the same amount as turbulence does. Turbulence enhances the apparent viscosity to the point that, when the turbulence model is activated, the Dean vortices are not formed at the moderate Reynolds numbers studied in this work. Predictions of both 3D models are in good agreement with experiments with respect to extinction limits and temperature distributions. Comparing 3D to two-dimensional (2D) simulations employing a simple model for the out-of-plane heat losses, the turbulence model is found to be essential for accurate predictions in 2D because Dean vortices cannot form in 2D simulations. Predictions obtained using a 1-step chemical reaction model with the pre-exponential term adjusted to obtain agreement between model and experiments at one test condition are compared to a 4-step reaction model developed for flow reactors. The latter is found to provide good agreement with experiments with no adjustable parameters, which is argued to be plausible because the conditions inside heat-recirculating combustors are closer to those of flow reactors than propagating flames.