  This work presents experimental study on opposed flow flame spread over thin
hollow cylindrical cellulosic fuel of diameters varying from 10 mm to 49 mm in
microgravity environment. To understand the effect of flow and geometry on
flame spread, experiments are conducted in low convective opposed flow
conditions ranging from 10 cm/s to 30 cm/s for both hollow cylindrical and
planar fuels at oxygen concentration of 21% and 1 atm pressure. In the
microgravity environment the flame length and the flame spread rate are seen to
increase with increase in hollow cylindrical fuel diameter over the flow range
studied here. The flame spread rate exhibited a non-monotonic trend with flow
speed, for flow of large diameter whereas a monotonic increasing trend is noted
for small diameters. The flame spread rate over hollow cylindrical fuel is
noted to be higher or at most equal compared to planar fuels over the matrix of
experiments conducted in this study. A simplified analysis is carried out to
arrive at an expression for flame spread rate over thin hollow cylindrical
fuels. The analysis shows that the radiation heat transfer from the hot char to
the inner surface of hollow virgin fuel dictates flame spread rate trend with
fuel diameter of the hollow cylindrical fuels. Higher overall equivalence ratio
in the inner section of the hollow fuels is responsible for higher char length
in hollow fuels and also influence the flame spread rate for smaller fuel
diameters.
