Study On Pulsating Flame Spread Over Aviation Kerosene And Characteristics Of Subsurface Flow

Aviation kerosene is a fuel for jet planes. In recent years, with the development of aircraft industry, fires occur frequently because of accidental spills. Compared with alcohol fuel, the greater fire and stronger radiant heat of aviation kerosene usually results in catastrophic losses. Although flame spread is only a short process during a fire, it determines the direction of flame spread and the best time to put out fires. However, the flame spread phenomena and the dominant heat transfer mode during flame spreading over aviation kerosene remains not clear as yet. So it is necessary to investigate the flame spread characteristic and mechanism to provide fundamental and experimental basis to prevent fires caused by spills of aviation kerosene.Aviation kerosene was investigated in this paper and all experiments have been conducted in the State Key Laboratory of Fire Science of the University of Science and Technology of China. Schlieren system, an infrared camera, fine thermocouples, PIV system and a high speed camera were used to study the effect of initial pool temperature and pool dimension on the spread mode and spread rate of the yellow diffusion flame and the blue precursor flame. The structure, length and depth of the subsurface flow and the effect of obstacles which were flush with oil surface and below oil surface on the flame spread phenomena were also investigated. A theoretical model was founded to calculate the velocity of the surface-tension-induced flow, and the calculated values agree with the experimental results. The main work of the present paper is summarized below.The characteristics of the pulsating flame spread over aviation kerosene were investigated and the major findings are as following. For liquid-phase-controlled flame spread, the main flame switches forward and backward alternating with high frequency and low frequency. No clear effect of initial pool temperature on the pulsation amplitude of the main flame, which is about4.4cm-4.8cm, has been found. The jump velocity of the main flame, which is independent of the initial pool temperature, is approximately40-55cm/s. The spread rate of the main flame increases quickly to120cm/s when the initial pool temperature increases from82℃to84℃. The pulsation frequency of the precursor flame increases and then decrease with an increase of the initial pool temperature, and the pulsation amplitude behave direct inversely with the pulsation frequency. The spread mechanism of the precursor flame is different with that of the main flame.The effect of pool width and fuel depth on flame spread was studied and the results reveal that it is shallow pool regime when the depth of the aviation kerosene is less than about8mm; on the contrary, it is deep pool regime. For shallow pool regime, the average rate of the main flame and the pulsation amplitude of the main flame and the precursor increase with an increase of the fuel depth, but the pulsation frequency of the main flame and the precursor decrease when the fuel depth increases. For deep pool regime, the depth of the aviation kerosene has on effect on the flame spread phenomena. Experimental results of the effect of pool width on flame spread shows that when the pool width is less than8cm, the spread rate of the main flame and the subsurface flow increase quickly with an increase of pool width because the viscous drag of the subsurface flow by the side wall of the pool weakens when the pool width increases. The jump temperature rise increases with pool width and fuel depth, and the preheated time behaves inversely.The characteristics of the subsurface flow and the heat transfer mode of the liquid phase were investigated experimentally and theoretically.A vortex is always found in the front of the subsurface flow. When the main flame pulsates with high frequency, a new vortex under the flame front is formed. The length of the subsurface flow decreases with an increase of the initial pool temperature, but initial pool temperature does not affect the thermal and momentum boundary layer depth which are approximately8-1Omm and10mm, respectively. Experimental and theoretical results show that the rate of the subsurface flow, which is a bit larger than the flame spread rate, increase with initial pool temperature. For shallow pool regime, the ratio of the length and depth of the subsurface flow decrease quickly with fuel depth, and there is inverse ratio of the subsurface flow rate and Fr·Ma-0.5. The ratio of the length and depth of the subsurface flow does not vary with fuel depth for deep pool. Flame can be stopped by an obstacle below oil surface for liquid-phase-controlled flame spread. The residence time of the flame decreases with an increase of the initial pool temperature and the distance between the oil surface and the top edge of the obstacle. Experimental results reveal that the region4mm below oil surface plays a key role in heat transfer. It was confirmed that convection is the dominant heat transfer mode in liquid phase by a model founded according to energy balance of the control volume.