Structure and extinction of spherical diffusion flames in microgravity

Abstract

Spherical diffusion flames in microgravity were studied both numerically and experimentally. The flames are supported on a porous spherical burner which Issues a constant flowrate into a quiescent atmosphere. Experiments were performed in the NASA Glenn 2.2 sand 5.2 s drop facilities. The experiments involved flames burning ethylene and propane at 0.98 bar. The numerical simulations incorporate a one-dimensional flame code with detailed chemistry and transport and an optically thick radiation model. The microgravity environment eliminates the effect of buoyancy resulting In spherically symmetric flames and increased residence times. The increased residence times inflate the Lewis number effect and radiative heat loss In the flames. Comparisons between the model and experiments reveal the model consistently over predicts the flame sizes. One explanation for the over prediction of the flame size in the model could be attributed to the fact that the thermal and mass diffusion in the code is too low. Good agreement between the experiments and model, with respect to flame size, is achieved by increasing the diffusion properties in the code by 30%. The experimental and numerical results show that when pure oxygen is Issued from the burner the flame temperature Is lower than the cases where fuel Is Issued from the burner. The lower flame temperature for the case when diluted fuel Is in the ambient Is expected to be a consequence of a higher than unity Lewis number. In the experiments it is hard to prove this hypothesis since the radiation for each flame Is different. However, in the numerical model the radiation was turned off and the steady state flame temperatures indicate a 200 K reduction in flame temperature from adiabatic for the case with diluted fuel in the ambient. Aside from the Lewis number effect radiative extinction was also investigated. The results indicate that at high flowrates (above 5.1 mg/s) radiative extinction occurs. At high flowrates it was found that extinction time and flame temperature became independent of flowrate.