On the effect of pressure, oxygen concentration, air flow and gravity on simulated pool fires

J. L.Torero, J. M. Most and P. JoulainLaboratiore de Combustion et D6toniqueUPK 9028 au CNRSUniversit6 de Poitiers- ENSMABP 109 -86960 Futuroscope CedexFRANCEIntroductionThe initial development of a fire is characterised by the establishment Of a diffusion flameover the surface of a the condensed fuel and is particularly influenced by gravity, with most of thegaseous flow induced by natural convection Low initial momentum of the fuel vapour, strongbuoyant flows induced by the hot post-combustion gases and con. _sequently low values of theFroude number (inertia-gravity forces ratio) are typical of this kind of scenario. An experimentalstudy is conducted by using a porous burner to simulate the burning of a horizontal combustiblesurface. Ethane is used as fuel and different mixtures of oxygen and nitrogen as oxidiser. Themagnitude of the fuel injection velocities is restricted to values that will keep the Froude numberon the order of 10 -5, when calculated at normal gravity and pressure, which are characteristic ofcondensed fuel b_g. Two different burners are used, a circular burner (62 mm diameter)placed inside a cylindrical chamber (0.3 m diameter and 1.0 m height) and a rectangular burner (50rnm wide by 200 mm long) placed in a wind nmnel (350 mm long) of rectangular cross section(120 mm wide and 90 mm height). The first burner is used to study the effect of pressure andgravity in the absence of a forced flow parallel to the surface. The second burner is used to studythe effect of a forced flow parallel to the ha'her surface as well as the effect of oxygenconcentration in the oxidiser flow. In this case experiments are also conducted at different gravitylevels (micro-gravity, 0.2 go, go and 1.8go) to quantify the relative importance of buoyancy.The Effect of GravityThe gravity level was varied systematically to observe the effect of gravity on the flamecharacteristics. Experiments were performed in reduced and micro gravity conditions in an aircraft(Caravelle) following a parabolic trajectory. The aircraR provides up to 20 sec. of 10-3go perparabola. By performing special manoeuvres, 20 sec. of reduced gravity (0.2go) were alsoobtained. Tests in over gravity conditions are conducted in a centrifuge facility and experimentswere conducted for 3, 6, 9 and 12 normal earth gravity conditions. The maximum value of theratio of the velocities induced by coriolis and centrifugal accelerations and the hot products