Aerodynamic force modifications of a spherical particle with varying temperature: a study of an idealized firebrand

Abstract

Fully resolved direct numerical simulations are used to quantify the effect of evolving heat, due to idealized smoldering processes, on the aerodynamic forces of a spherical particle, representing an idealized fixed-shape firebrand particle. Firebrand particles are small glowing particles that are generated in fires and can be transferred long distances by the wind and create new spot fires. Understanding the transport of firebrands is of great importance in fire science. The simulations are performed at a Reynolds number of 500, relevant for a wide range of firebrand size and wind velocity combinations. The spatiotemporal variation of temperature over the surface of the particle is obtained using a detailed surface energy balance analysis. The firebrand particle is assumed to have the thermal and material properties of pine needles and has a Biot number larger than unity, which means that the particle undergoes notable internal temperature gradients. The results indicate that the buoyancy-induced flow around the particle significantly modifies the trailing vortices and produces two non-interacting tunnel-shaped plumes in the wake of the sphere as the particle’s Richardson number increases. As a result, the particle’s drag and lift coefficients show large deviations from those of a non-heated particle and an isothermal particle. The increased surface temperatures result in an increase in the drag force while inducing a negative lift. The significant variations seen in the aerodynamic forces as a function of the particle’s instantaneous temperature indicate that the influence of the transient thermal conditions of firebrands should be considered in the prediction of the particles’ trajectory and landing spots.