Numerical and experimental observations of spherical diffusion flames

Spherical diffusion flames supported on a porous sphere were studied numerically and experimentally. Experiments were performed in 2.2 s and 5.2 s microgravity facilities. Numerical results were obtained from a Chemkin-based programme. The programme simulates flow from a porous sphere into a quiescent environment, yields both steady state and transient results and accounts for optically thick gas-phase radiation. The low flow velocities and long residence times in these diffusion flames lead to enhanced radiative and diffusive effects. Despite similar adiabatic flame temperatures, the measured and predicted temperatures varied by as much as 700 K. The temperature reduction correlates with flame size but characteristic flow times and Lewis number also influence temperature. The numerical results show that the ambient gas Lewis number would have a strong effect on flame temperature if the flames were steady and nonradiating. For example, a 10% decrease in Lewis number would increase the steady state flame temperature by 200 K. However, for these transient, radiating flames the effect of Lewis number is small. It was also observed that when hydrocarbon fuel is supplied from the ambient the large diffusion distances associated with these flames can lead to unusual steady state compositions near the outer boundary because decomposition products can diffuse to the outer boundary. This results in a loss of chemical enthalpy from the system but the effect on flame temperature is small. Transient predictions of flame sizes are larger than those observed in microgravity experiments. Close agreement could not be obtained without either increasing the model's thermal and mass diffusion properties by 30% or reducing mass flowrate by 25%.