Pilot Study On The Injection And Atomization Of Kerosene In Cold Supersonic Flow

In this thesis, experimental and numerical studies were carried out on kerosene injection, mixing and atomization in a supersonic air-stream. The phenomenon is crutially important not only for understanding kerosene-fueled supersonic combustion but also related to spray strategy of a scramjet combustor. In such a topic, complex flow physics are involved among supersonic flow, shock waves, shear layer, turbulence, jet breakup and atomization, phase exchanges of mass, momentum and energy as well as optical diagnostic on spray and atomization. Theoretical consideration on jet breakup in high slip velocity flows and spray measurement on line are especially challenged due to scramjet development and academic interests. Although some experiments have been conducted on jet breakup and atomization in a supersonic air-stream since 1960's, no anticipated progresses have been achieved in jet breakup model and numerical methods. Thus, this thesis focus on experimental studies on liquid spray in a supersonic air-stream by planar laser induced fluorescence (PLIF) and laser scattering. The purpose is to obtain more high resolution images and droplet size distribution on line for deep understanding the kerosene spray in a supersonic flow. The experiments are summarized as follows:(1) A test rig based on supersonic wind tunnel was established for experimental studies on spray in a Mach 2.2 supersonic flow. The test section is almost identical to a single scramjet combustor to avoid the scale effects and satisfy requirements of online measurement. Also, some optic measurements and fuel supply ranged in variety of different injection pressure and orifice diameter are combined to this direct-connected facility.(2) Optical diagnostic including PLIF, Mie scattering and schlieren with high-speed photography were employed to experimentally study the kerosene atomization in a supersonic flow. The quantitative or semi-quantitative data are emphasized in the spray measurement. Different from schiliren images, PLIF ones with high temporal and spatial resolution can supply database on kerosene jet breakup and spreading. The unsteady and three dimensional surface waves appear on PLIF images of kerosene jet. Thus, jet penetration and spreading can be precisely measured from these images by kerosene fluorescence. Meanwhile, droplets' SMD (Sauter mean diameter) distribution was also got in these experiments. (3) By the afro-mentioned laser measurements, the flow features with and without kerosene injection were systemically studied when a cavity embedded in a duct with different geometry and aspect ratio to show whether it can enhance jet mixing in a supersonic flow. The main works are described as following(1) Chapter one presents problem description and related progress on this topic. The thesis is outlined and innovations are listed either.(2) Chapter two describes design and manufacture of this direct-connected test rig for kerosene atomization in a cold supersonic flow, including supplementary system, such as fuel supply and laser instrumentation. The results show that the flow in the test section is quite uniform, the disturbance waves are weak enough to satisfy the requirements of spray experiments.(3) Chapter three shows the results on kerosene injected into a supersonic flow in a duct. The photographs, schiliren and PLIF images demonstrate the jet atomization and spreading in a supersonic flow. The jet penetration, droplets' SMD distribution and spreading were obtained in the experiments. The data imply that jet-to-free stream dynamic pressure ratio is a dominant parameter in determining the jet penetration, breakup and atomization. High dynamic pressure ratio increases the penetration and accelerates the breakup and atomization of the jet column. But it induces strong shock wave and large loss of the total pressure. Three-dimensional unsteady surface waves locate on jet surface and the waves enhance the jet breakup. The surfaces wave emit from column initial curvature and wave altitudes increase while they march along jet downstream. When dynamic pressure ratio is low, the surface waves are suppressed and observed obviously after a certain length of the jet column.(4) Chapter four demonstrates the experiments conducted in a duct embedded a cavity with different geometry and aspect ratio. The experiments emphasize the flow characteristics and kerosene jet mixing to show the effects of different geometries of a closed cavity, such as angles of front and rear wall, leading slot and wavelike rear wall. Some conclusions are yielded as followsThe open cavity (L/D=3) induces weak shock waves, low loss of the total pressure. Moreover, the cavity self-sustained oscillation can be neglected due to its low amplitude. The longitudinal and transverse vortex are strong inside the cavity and they enhance the kerosene mixing both inside and outside the cavity. The distributions of droplets' SMD in a duct with and without a cavity are almost identical but penetration height decreases slightly. The jet shock can be ignored relative to those induced by the cavity.In contrasted to the open cavity, the closed cavity (L/D=15) induces strong shock waves that lead to boundary layer separation on the opposite wall. Meanwhile, loss of total pressure is high and the self-sustained oscillation is observed obviously. Such flow oscillation can be suppressed by reducing the rear wall angle or employing wavelike rear wall. The front wall angle and leading slot have no significant effects on the shear layer and the induced shock near front wall. With kerosene injection, the jet shock can still be ignored relative to the cavity shock waves. At the same time, shear layer and free stream deflects upwards the rear wall and penetration height increases greatly. Shock wave upwards the rear wall and boundary layer separation on the opposite wall both disappear when the wavelike rear wall is employed. Furthermore, jet atomization is not difficult due to the high slip velocity, but the problem is how to weaken the cavity shock and enhance kerosene spreading in a supersonic flow. The cavity flow pattern depends on aspect ratio, the cavity geometry together with jet and incoming flow conditions.The distribution of droplets' SMD is not related to the duct with or without cavity in a supersonic flow. But a closed cavity enhances kerosene spreading and mixing much more than those of an open one.The innovations in thesis are as follows:(1) PLIF and fluorescence (phosphorescence) of kerosene excited by UV laser were employed to show jet spreading and mixing. These approaches have been a new way of studying kerosene atomization in supersonic combustion. PLIF has extensively applied in combustion diagnostics for measuring molar fraction of small radicals or temperature as well as spray detection on gasoline or diesel engine, but it is firstly used to study kerosene atomization of in a supersonic flow. In contrast to shadowgraph and schlieren, PLIF can detect jet mixing zone and surface waves accurately. Therefore, empirical formulas of penetration height of liquid jet based on schlieren images are not so precisely as to compare to PLIF ones. Therefore, updated database are suggested to build up by these fresh PLIF images. It is anticipated PLIF has great potential application in studying kerosene atomization in a supersonic airstream.(2) With or without a cavity, the distribution of droplets' SMD in a duct which is identical to a single scramjet combustor is presented in this thesis. Although LIN got data of droplets' SMD when water is injected into a supersonic flow from an orifice which diameter is 2mm, the flow filed is quite different from that of a scramjet combustor configuration. The results imply that kerosene atomization in a supersonic flow is not a dominant in determining combustion efficiency, but the key is to reduce time delay of ignition and combustion. Cavity contributes much more to kerosene spreading and mixing, but less to decreasing droplets' SMD.(3) Based on obtained PLIF images and droplet's SMD, physical understanding has been improved to know the phenomena of kerosene spreading and mixing in a supersonic airstream, especially on the flow patterns in a duct embedded a cavity with different geometry and aspect ratio. Some knowledge has been updated or revised which are from previous wall pressure measurement or intuition. The methods in this thesis provide a way to study the supersonic combustion of kerosene and development of a scramjet combustor.