Flame spread along a fuel rod in the absence of gravity
An analysis is carried out of the steady spread of a flame with infinitely fast kinetics over a thermally thin solid rod in the absence of gravity. Due to the radial convergence on the solid surface of the heat flux heating and vaporizing the fuel, the spread rate is found to be higher than for a flame spreading over a slab, and the region of the gas around the flame which is affected by molecular transport is large compared with the radius of the rod. The effects of radiation, of a buoyancy force opposing flame propagation and of axial heat conduction in the solid are discussed briefly
The effect of surface radiation on flame spread in a quiescent, microgravity environment
Abstract In most theories of laminar flame spread over solid combustibles, radiation is neglected. However, when gas motion is completely absent, which may occur in the quiescent, microgravity environment of a spacecraft, the importance of radiation compared to convection is enhanced. We develop a theoretical model of flame spread over a thin solid fuel into an opposing flow of oxidizer, the opposing flow being present in the quiescent environment in flame-fixed coordinates, including the effects of radiative heat transfer from the fuel surface. Numerical solutions to the conservation equations in the gas and the solid phase that describe the spreading flame are presented for a variety of ambient conditions and surface emittances. When surface radiation is significant the solid surface acts as a heat sink, causing the flame to cool, shrink in size, and spread more slowly. Better agreement with experiment for net heat transfer to the surface is obtained when surface radiation is included. For low ambient oxygen levels, radiative effects lead to flame extinction. A radiation/conduction parameter is identified that adequately describes the importance of surface radiation.
Characterization of flame spread over PMMA using holographic interferometry sample orientation effects
Flame spread over a thermally thick slab of PMMA at several angles of sample orientation from theta = -90/sup 0/ (vertically downward flame spread) to theta = +90/sup 0/ (vertically upward flame spread) in air was investigated by measuring temperature distributions within the PMMA sample in the vicinity of the leading edge of the flame front using holographic interferometry. Samples with widths of 0.32, 0.47, 1.0, and 2.5 cm, a thickness of 2.5 cm, and a length of 30 cm were used. The measured net heat flux from the gas phase to the sample surface at the vaporization front of the sample is about 7 W/cm/sup 2/ for downward flame spread (theta < 0/sup 0/), 6.5 W/cm/sup 2/ at theta = +10/sup 0/, and 2.8 W/cm/sup 2/ at theta = +90/sup 0/. However, the total net heat transfer rate increases with an increase in the angle of sample orientation, because the characteristic heating length, defined as the distance from the adiabatic point on the sample surface to the vaporization point, increases with an increase in the orientation angle of the sample. The total net heat transfer rate into the sample from the gas phase is about 56% of the totalmore » net heat transfer input to the sample at theta = -90/sup 0/, 78% at theta = 0/sup 0/, 87% at theta = +10/sup 0/, and 99% at theta = +90/sup 0/. Therefore heat transfer from the gas phase into the unburnt fuel ahead of the vaporization point is the dominant heat transfer path for all angles of orientation. This was clearly demonstrated by the net heat flow vector patterns within the sample. The streamwise conductive heat transfer rate through the sample decreases with an increase in flame spread rate (increase in the sample orientation angle) due to insufficient time being available for the slow thermal wave to travel through the sample.« less