A computational model of three-dimensional, time-dependent flame spread in microgravity environment is presented. The solid is assumed to be a thermally-thin, pyrolysing cellulosic sheet. The gas phase model includes ...A computational model of three-dimensional, time-dependent flame spread in microgravity environment is presented. The solid is assumed to be a thermally-thin, pyrolysing cellulosic sheet. The gas phase model includes the full Navier-Stokes equations with density and pressure variations and six-flus model of radiation heat transfer. The solid phase model consists of continuity and energy eqllations whose solution provides boundary conditions for the gas phase equations. In the numerical procedure, the gas-and solid-phase equations are solved sepaxately and iteratively at each time step. Predictions have been made of flame spread in slow forced flow under gravitational acceleration normal to fuel surface and flame spread in a quiescent environment in an enclosed chamber under gravitational acceleration parallel to fuel surface. Numerical simulations show that, under microgravity, slow-flow conditions, flame spread process is highly unsteady with the upstream flame spreads faster than the downstream flame after a period of ignition. It has also been shown that the level of microgravity has a significant effect on the name spread process.展开更多
For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere of O2-N2), the conductive heat transfer from the flame to the...For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere of O2-N2), the conductive heat transfer from the flame to the fuel surface dominates the flame spread at lower ambient atmosphere. As the ambient pressure increases, the flame spread rate increases, and the radiant heat transfer from the flame to the fuel surface gradually becomes the dominant driving force for the flame spread. In contrast, if the atmosphere is able to emit and absorb the thermal radiation (e.g. for atmosphere of O2-CO2), at lower pressure, the heat transfer from flame to the fuel surface is enhanced by the radiation reabsorption of the atmosphere at the leading edge of the flame, and both conduction and thermal radiation play important roles in the mechanism of flame spread. With the increase in ambient pressure, the oxygen diffuses more quickly from ambient atmosphere into the flame, the chemical reaction in the flame is enhanced, and the flame spread rate increases. When the ambient pressure is greater than a critical value, the thermal radiation from the flame to the solid surface is hampered by the radiation reabsorption of ambient atmosphere with the further increase in ambient pressure. As a result, with the increase in ambient pressure, the flame spread rate decreases and the heat conduction gradually dominates the flame spread over the fuel surface.展开更多
文摘A computational model of three-dimensional, time-dependent flame spread in microgravity environment is presented. The solid is assumed to be a thermally-thin, pyrolysing cellulosic sheet. The gas phase model includes the full Navier-Stokes equations with density and pressure variations and six-flus model of radiation heat transfer. The solid phase model consists of continuity and energy eqllations whose solution provides boundary conditions for the gas phase equations. In the numerical procedure, the gas-and solid-phase equations are solved sepaxately and iteratively at each time step. Predictions have been made of flame spread in slow forced flow under gravitational acceleration normal to fuel surface and flame spread in a quiescent environment in an enclosed chamber under gravitational acceleration parallel to fuel surface. Numerical simulations show that, under microgravity, slow-flow conditions, flame spread process is highly unsteady with the upstream flame spreads faster than the downstream flame after a period of ignition. It has also been shown that the level of microgravity has a significant effect on the name spread process.
基金This work was supported by the National Natural Science Foundation of China(Grant No.59986004).
文摘For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere of O2-N2), the conductive heat transfer from the flame to the fuel surface dominates the flame spread at lower ambient atmosphere. As the ambient pressure increases, the flame spread rate increases, and the radiant heat transfer from the flame to the fuel surface gradually becomes the dominant driving force for the flame spread. In contrast, if the atmosphere is able to emit and absorb the thermal radiation (e.g. for atmosphere of O2-CO2), at lower pressure, the heat transfer from flame to the fuel surface is enhanced by the radiation reabsorption of the atmosphere at the leading edge of the flame, and both conduction and thermal radiation play important roles in the mechanism of flame spread. With the increase in ambient pressure, the oxygen diffuses more quickly from ambient atmosphere into the flame, the chemical reaction in the flame is enhanced, and the flame spread rate increases. When the ambient pressure is greater than a critical value, the thermal radiation from the flame to the solid surface is hampered by the radiation reabsorption of ambient atmosphere with the further increase in ambient pressure. As a result, with the increase in ambient pressure, the flame spread rate decreases and the heat conduction gradually dominates the flame spread over the fuel surface.
