Pan (1999) and Chen (1999) investigated the steady flame spread characteristics over PMMA in an opposed forced convection environment in a wind tunnel. The variable parameters were the velocity and the temperature of the opposed flow and the thickness of solid fuel. They found that the flame spread rate increases with an increase flow temperature, a drop in the flow velocity or the fuel thickness. Their image results further showed that the thermal boundary layer becomes thicker as the opposed flow temperature increases at a fixed flow velocity or the opposed flow velocity declines at a constant flow temperature. Wu et al.
(2003) developed an unsteady combustion model with mixed convection to explore the flame spread behaviors of a thick PMMA slab of infinite length in an environment with opposed flow. The simulated flame spread rates were compared with the measurements made by Pan (1999). The results were highly consistent except in the low-speed flow regime. The discrepancies can be attributed to the radiation, fuel size and the three dimensional effect are not under the consideration in the model. Fujita et al. (2000) experimentally studied the radiative ignition on paper sheet in microgravity. The results showed that the gas phase temperature becomes higher than that of the solid surface before ignition, and the main mechanism of radiative solid ignition here is due to the gas phase reaction.
Furthermore, the ignition delay time strongly depends on the oxygen concentration and ambient pressure. It decreases with a higher oxygen concentration or ambient pressure. Fujita et al. (2002) experimentally investigated the effect of external flow on flame spread over polyethylene wires in microgravity. The results revealed that the flame spread rate is controlled mainly by preheat length, standoff distance and flame temperature. The flame spread phenomenon can be divided into three regimes based on flow velocity. These are an oxygen transport control
regime, a geometrical effect regime and a chemical-kinetics controlled regime.
Wichman (1983) developed a theoretical model to estimate the rate of flame spread under conditions of heat transfer control with account taken of the fact that the gas velocity was not uniform. The results indicated that the functional dependence of the spread rate on the external gas velocity is modified from the one obtained in the classical study of DeRis (1968).
Thereafter, Wichman (1992) explained two mechanisms for the extinguishment of spreading flames. In the first, the particle residence time in the reaction zone is reduced by the increased flow velocity, giving a blow off extinction. In the second, flow velocity decreases toward flame spread rate and extinction again occurs eventually. Takahashi et al. (2002) analytically and experimentally studied flame spread over a thin PMMA sheet in microgravity. They concluded that reducing the relative flow velocity enlarges the size of preheat zone, increasing radiant loss, and that radiant heat loss reduces the flame spread rate and may also cause extinction. Olson et al. (2001) experimentally investigated the radiative ignition and subsequent three dimensional flame spread over thin cellulose fuels. They found that gas phase residence time over the heated spot is a critical parameter in ignition delay. After ignition, the flame in a fan shaped pattern spreads from the central ignition spot and is toward upstream. The flame spread angle increases with increasing external air flow and oxygen concentration. They also found that due to the oxygen shadow effect, the upstream and downstream flame spread over the fuel plate is not observed simultaneously. The downstream flame only starts to spread after upstream flame spread is complete and extinguished. Ito et al. (2005) experimentally investigated the propagation and extinction mechanisms of opposed-flow flame spread along a thick slab of PMMA.
They showed that as the opposed-flow rate increases or the ambient
oxygen concentration decreases, the Damkohler number decreases.
When the Da falls below a critical value, extinction or no flame spreading may occur. The radiative heat loss has very little effect on the extinction because it is small compared with the other heat transfer rates. The results also demonstrated that the steady flame spread rate is proportional to the net total heat transfer rate to the preheat zone. However, no matter what the enough heat feedback to the preheat zone or not, the flame spread rate decreases rapidly when nearing the extinction limit.
West et al. (1994) studied the surface radiation effects on flame spread over thermally thick fuels in an opposed flow. They concluded that the fuel surface radiation is important for thermally thick fuel at all flow levels, however, and it is important for thermally thin fuel only at low velocity level. Bhattacharjee and Altenkirch (1991) developed a numerical model to study the effect of surface radiation on flame spread in a quiescent microgravity environment by using the oxygen concentration and solid surface emittance as parameters. They found that the flame spread rate and temperature decrease as solid surface emittance increases in any oxygen level, and the flame shrinks in size while moving closer to the surface. In the other hand, the rate of decrease in flame spread rate being more severe at higher values of solid surface emittance and lower oxygen levels. Bhattachariee et al. (2000) experimentally, computationally, and analytically investigated the downward flame spread over a polymethylmethacrylate plate in an oxygen/nitrogen environment at normal gravity. They presented that the flame spread rates in the thermal regime as the fuel thickness was changed from the thin- to the thick-limit.
A simple formula for the transition thickness between the thin and thick fuel regimes was proposed, and it seemed to agree well with experimental measurements. From the computational results, they also concluded that the radiative effects seem not to influence the flame spread rate except at
very high ambient pressures. Rhatigan et al. (1998) examined the effects of gas phase radiation on the burning and extinction of a solid fuel. They plotted the heat fluxes, flame temperature and burning rate as functions of the flow stretch rate. The computed results demonstrated that the gas phase radiative effects are stronger at lower stretch rates.
Fernandez-Pello and Hirano (1982) experimentally studied the controlling mechanism of flame spread over the surface of combustible solids. The heat transfer and gas phase chemical kinetic aspects of the flame spread process were addressed respectively for the flame spread in oxidizing flow.
They indicated that chemical kinetics of gas phase plays a critical role and it must be considered when flame spread in opposed gas flow occurs at near extinction or non-propagating conditions. Son and Ronney (2002) experimentally studied flame spread over thermally thick fuels. They found that the radiative preheating and reabsorption effect are less important in normal gravity, because a substantial flow velocity is caused by buoyancy, reducing the thickness of the flame and thereby reducing the volume of radiating gas. Takahashi et al. (2000) and Ayani et al. (2006) examined flame spread rates over PMMA sheets in normal gravity and in microgravity. They found that the flame spread rate over a thermally thin fuel is inversely proportional to the thickness of the fuel, whereas that over a thermally thick fuel is proportional to the opposed flow velocity, in complete agreement with analyzed research by DiRis (1969). Other investigations, such as Wichman and Williams (1983a), Wichman and Williams (1983b) and Delichatsios (1986), have developed formulas that show identical proportionalities. Tizon et al. (1999) analyzed the wind-aided flame spread process along a solid fuel rod under oblique forced flow. Their results indicated that the effects of gas-phase chemical kinetics were important for large strain rates and the spread rate depended strongly on the strain rate. They also found that the effects of radiation
from the gas phase are negligible because the heat transfer by convection typically dominates at large Reynolds numbers of the transverse velocity.
Zhu and Gore (2005) studied the opposed-flow laminar methane/air diffusion flames by the numerical simulations. They indicated that the peak flame temperature and the soot volume fraction increase with increasing pressure or decreasing injection velocity for all radiation conditions. The soot and gas radiation effects are stronger at the higher pressures or lower velocities. The simulated results also showed that the peak soot volume fraction and soot emission index decrease by 85 and 97%
with an increase in injection velocity from 10 to 100 and 200 cm/s, separately.
Kumar et al. (2003a) used a two dimensional flame spread model with flame radiation to compare the extinction limits and spreading rates in opposed and concurrent spreading flames over thin solids. The varying parameters were oxygen percentage, free stream velocity, and flow entrance length. Numerical results showed that at low free stream velocities with shorter entrance length, the flame spread rates are higher and have a lower oxygen extinction limit, whereas in high free stream velocities, the flame spread rates are lower and have a higher oxygen extinction limit. The flame spread rate in opposed flow varies with free stream velocity in a non-monotonic manner, with a peak rate at an intermediate free stream velocity. The flame spread rate in concurrent flow increases linearly with free stream velocity. Kumar et al. (2003b) also presented a numerical study on flame-surface radiation interaction in flame spread over thin solid fuels in quiescent microgravity and in normal gravity environments. It was observed that the flame in microgravity is very sensitive to the surface radiation properties. The fuel with high solid absorptivity can absorb substantial flame radiation and flame spreads faster than the corresponding adiabatic case irrespective of value of solid
emissivity. Lin and Chen (1999) investigated how the gas-phase radiation, whose model included both the cross-stream and stream-wise gas phase radiation coupled with solid phase one, affected the spreading flame. By comparing the results with the predicted ones of Chen and Cheng (1994), which only considered the radiation effect in cross-stream direction.
They concluded that the stream-wise radiation contributes to reinforce the forward heat transfer rate subsequently increasing the flame spread rate.
Di Blasi (1995a) examined, by numerical simulation, the effects of the thickness on the concurrent spread of flames over thin and thick fuels under forced convection. Three main flame spread regimes were identified. In the kinetic region, the flame spread rate increases with the solid fuel thickness below 0.008 cm. In the thermally thin region, the flame spread rate falls as the solid fuel thickness increases in the range from 0.008 cm to 0.5 cm. Finally, in the thermally thick region, the flame spread rate becomes almost constant when the solid fuel thickness exceeds 0.5 cm. Di Blasi (1995b) also investigated the opposed flame spread over cellulosic fuels in a microgravity environment, using the forced gas flow and the solid thickness as the varied parameters. For very thin fuels, flame spread rate increases with the solid thickness and the solid radiative heat loss controls the flame spread rate. As the fuel thickness becomes thicker, the flame spread rate decreases with the solid thickness and the flame radiative heat transfer playas role of increasing importance. For the thick fuels, flame radiation is reduced whereas surface radiative heat loss is again at a high level. Suzuki et al. (1994) studied the downward flame spread over paper sheets of thickness between 0.4 and 10mm to investigate the mechanisms by which flames spread. They identified four flame spreading behaviors under the conditions in the four regions. The flame spread is stable at the limiting thickness of the paper sheets. They also derived an energy equation for the heat flux through the pyrolytic region
and the solid surface in front of the leading edge.
Nakabe et al. (1994) investigated the ignition and transition to flame spread over a thermally thin fuel in a microgravity environment. A comparison was made between the axis-symmetric configuration and a two-dimensional configuration. The results indicated that ignition is earlier in two-dimensional configuration and the difference between the two configurations is roughly 25% in the same boundary conditions.
Jiang and Fan (1995) made the predictions of flame spread in slow forced flow under gravitational acceleration normal to the fuel surface and flame spread in a quiescent environment in an enclosed chamber under gravitational acceleration parallel to the fuel surface. The results indicated that the effect of oxygen transport on flame spread is greater than that of heat transfer in a microgravity environment. In addition, the microgravity level has a significant effect on the flame spread over a vertical wall in an enclosed chamber under gravitational acceleration parallel to the fuel surface. Mell and Kashiwagi (2000) numerically studied the effects of finite sample width on transition and flame spread in microgravity. They found that the finite width effects are insignificant when the ambient wind is relatively large and the influence of thermal expansion on the net incoming oxygen supply decreased as the ambient wind speed increased. Thus, the flame spread behavior of the three-dimensional flame tended to that of the two-dimensional flame with increasing ambient wind speed.
Nakamura et al. (2002) numerically studied the enclosure effect on the spread of the flame over solid fuel under microgravity. Because the confinement of the flow field and the thermal expansion initiated by heat and mass addition in the chamber, the flame spread rate for the case with enclosure is faster than the one without any enclosure. The predictions also showed that the enclosure effect is stronger at lower flow velocity.
Shih and T’ien (1997) theoretically studied the concurrent flow flame spread over a thin solid in a low speed flow tunnel. They found that the flow is accelerated in the downstream as the tunnel height is decreased.
The flame is pressed to the solid fuel and the heat conduction rate to the solid; the flame length and the spread rate are also increased. However, the conductive heat loss to the wall becomes great which reversed the trend and decreases the flame length as the tunnel height becomes too small.
Shih and T’ien (2000) numerically investigated the concurrent spread of flames over a thin solid in a low-speed flow tunnel in microgravity. The simulated results demonstrated two distinctive flame behaviors. With a high oxygen content or fast flow, the flame was long and far from the quenching limit. With a low oxygen content or slow speed, the flame was short and in the region near the quenching limit. They also found that the three dimensional effect on flame spreading was stronger in the low-speed flow regime. Shih and T’ien (2003) numerically studied the concurrent flame spread over a thin solid in a low-speed flow tunnel in microgravity by using three-dimensional combustion model. Several 3D effects due to the presence of the tunnel walls are examined. The walls change the velocity profiles and accelerate the flow in a direction parallel to the fuel.
The cold walls conduct heat away from the flame, which produces heat loss and a quenched layer. Moreover, the oxygen side diffusion enhances the combustion reaction at the base region and pushes the flame base closer to the solid surface, increasing the flame spread rate. They also concluded that 3D effects are dominated by the heat loss to the side walls in the downstream portion of the flame and the flame spread rate increases with fuel width in higher speed flows.