  This paper presents a numerical investigation into the phenomenon of flame
spread over thin circular ducts in normal gravity and microgravity
environments. Flame spread over such geometry is of significant interest due to
its relevance in various practical applications, including tubes for flow
purpose in medical system, fire safety in spacecrafts, ducts as well as wiring
tubes. This study comprises of a comprehensive investigation of key parameters
affecting flame spread rate, including fuel radius and opposed flow speed in
normal gravity and microgravity environments. A 2-D axisymmetric flame spread
model accounted for char and numerical simulations were performed which
revealed valuable insights into the underlying mechanisms governing flame
spread over such geometry. The results computed from the numerical model is
compared with the experimentally observed flame spread rate to validate the
numerical model which can be used to gain a comprehensive understanding of the
underlying physical phenomena. As the radius of circular duct increases the
flame spread rate increases both in normal gravity and microgravity
environments. The conduction heat feedback and radiation heat gain coming from
hot char through gas phase at inner core region are the two major mechanisms
which controls the flame spread phenomena over the circular duct fuels. The
flame spread rate at different flow ranging from quiescent (0 cm/s) to 30 cm/s
is also evaluated and 21 % oxygen and found a non-monotonic increasing
decreasing trend of flame spread rate at different opposed flow speed in both
normal gravity and microgravity environments.
