EFFECTS OF IGNITION AND WIND ON THE TRANSITION TO FLAME SPREAD IN A MICROGRAVITY ENVIRONMENT

A two-dimensional time-dependent model is developed describing ignitio n and the subsequent transition to flame spread over a thermally thin cellulosic sheet heated by external radiation in a microgravity enviro nment. The effects of a slow external wind (0-5 cm/s), and of the flux distribution of the external radiation on the transition are studied mainly in an atmosphere of 30% oxygen concentration. The ignition is i nitiated along the width of a sample strip, giving rise initially to t wo flame fronts spreading in opposite directions. The calculated resul ts are compared with data obtained in the 2.2-s drop tower. Both exper imental and calculated results show that with a slow, imposed wind, th e upstream flame front (opposed mode) is stronger and slightly faster than the quiescent counterpart due to a greater supply of oxygen. Howe ver, the downstream flame front (concurrent mode) tends to die during the transition period. For all calculated cases studied in this work u sing the selected kinetic constants for the global one-step gas phase reaction, the downstream flame front dies out in oxygen concentrations up to 50% and wind velocity up to 5 cm/s. This is caused by the ''oxy gen shadow'' cast by the upstream flame. The ignition delay time depen ds mainly on the peak flux of external radiation, whereas the transiti on time to steady state flame spread depends mainly on the broadness o f the flux distribution. The broader the radiative flux distribution, the greater the transient flame spread rate due to the preheating of t he sample ahead of the flame front by the external radiation and thus the greater the delay to steady state flame spread.