Effects of Ignition and Wind on the Transition to Flame Spread in a Microgravity Environment.
Effects of Ignition and Wind on the Transition to Flame
Spread in a Microgravity Environment.
(976 K)
McGrattan, K. B.; Kashiwagi, T.; Baum, H. R.; Olson, S.
L.
Combustion and Flame, Vol. 106, No. 4, 377-391,
September 1996.
Sponsor:
National Aeronautics and Space Administration, Lewis
Research Center, Cleveland, OH
Keywords:
flame spread; microgravity; spacecraft; ignition; wind
effects; oxygen concentration
Abstract:
A two-dimensional, time-dependent model is developed
describing ignition and the subsequent transition to
flame spread over a thermally thin cellulosic sheet
heated by external radiation in a microgravity
environment. The effects of a slow extermal 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
initiated along the width of a sample strip, giving rise
initially to two flame fronts spreading in opposite
directions. The calculated results are compared with
data obtained in the 2.2-s drop tower. Both
experimental and calculated results show that with a
slow, imposed wind, the upstream flame front (opposed
mode) is stronger and slightly faster than the quiescent
counterpart due to a greater supply of oxygen. However,
the downstream flame front (concurrent mode) tends to
die during the transition period. For all calculated
cases studied in this work using 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 "oxygen shadow" cast by the
upstream flame. The ignition delay time depends mainly
on the peak flux of external radiation, whereas the
transition time to steady state flame spread depends
mainly on the broadness of the flux distribution. The
broader the radiative flux distribution, the greater the
transient flame spread rate due to the preheating of the
sample ahead of the flame front by the external
radiation and thus the greater the delay to steady state
flame spread.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899