Investigation Of Flow, Mixing And Combustion Properties Of Transverse Fuel Injection Upstream Of The Cavity In A Supersonic Combustor

This research studies the flow, mixing and combustion properties of the transverse fuel injection upstream of the cavity in a supersonic combustor by means of experimental investigation and numerical simulation. The main contents of this research include three parts: (1) the fundamental research and application of planar laser-induced fluorescence (PLIF) technique, (2) the research of non-reacting properties of the transverse fuel injection in a supersonic combustor, (3) the research of reacting properties of the transverse fuel injection in a supersonic combustor.In the fundamental research and application of PLIF technique, the expression of PLIF intensity is deduced in detail and the detecting methods for hydroxyl radical (OH) concentration, acetone concentration and density of the flowfield are presented. By designing of the scheduling control system in the PLIF measurement system, we are able to obtain the short pulse width PLIF signal integrally at an exposure time less than 50ns. At the same time we designed the high concentration acetone vapor injection system to solve the problem of the signal-to-noise ratio of the acetone PLIF intensity being too low under the weak exciting laser. Based on these research results, we employed an acetone PLIF technique to visualize and measure the density flowfield of under-expanded free jet satisfactorily.In the research of non-reacting properties of the transverse fuel injection in a supersonic combustor, a helium jet adulterated with acetone vapor was employed to simulate a fuel jet. Using PLIF imaging of the acetone and numerical simulation, we researched the flow and mixing properties of the fuel injected from the wall in the non-cavity combustor and the fuel injected upstream of the cavity in the cavity-carrying combustor. And then the effects of the injection parameters and the cavity geometry parameters on these properties are analyzed.The research results show the mixing degree is enhanced by the large-scale movement and streamwise vortex movement when jets turn sharply toward the airstream, which increases the speed of the mixing of the fuel and the air remarkably. When the mass rate of the fuel is increased, diffusion and mixing can be enhanced more by increasing the injection stagnation pressure of the fuel than by increasing the diameter of the injection port. When the gaseous fuel is injected upstream of the cavity transversely, most of the fuel mass distributes above the cavity and a little fuel mass enters the cavity by going with the spatial evolution of the cavity shear layer and by the turbulent diffusion in the cavity shear layer. Increasing the injection stagnation pressure of the fuel will decrease the fuel mass entering the cavity. Increasing the length-to-depth ratio of the cavity is helpful in causing the fuel to enter the cavity. However, increasing the cavity depth or the aft ramp angle has opposite effects. The low speed flowfield in the cavity is helpful in making the fuel diffuse and mix and extending the spanwise distribution of the fuel near the wall.In the research of reacting properties of the transverse fuel injection in a supersonic combustor, hydrogen and acetylene were employed as the fuel. By means of spontaneous radiation imaging and PLIF imaging of OH and numerical simulation, the properties of lean blowout equivalence ratio, flame distribution and flame structure were researched. And then the effects of the injection parameters and the cavity geometry parameters on these properties are analyzed. By means of high speed photography and spontaneous radiation imaging of the hydrocarbon radical (CH), the ignition properties of the transverse injection of acetylene upstream of the cavity ignited by the pilot hydrogen flame and by its self-ignition and the flame properties of the combustion reaction area are studied. The influences of injection locations of the acetylene and the cavity geometry parameters on these properties are analyzed.The research results show that the cavity installed downstream of the hydrogen port is helpful in hydrogen's self-igniting and lengthening the cavity helps even more. Under equal hydrogen mass rate, the injection under high stagnation pressure makes self-ignition easier and achieves higher combustion efficiency than injection under low stagnation pressure. Most of the hydrogen flame distributes above the cavity and downstream of the cavity. The flame structure is tubelike and the flame lying in the tube wall encloses the hydrogen jets. Increasing the equivalence ratio of the hydrogen causes the flame to move upstream and increase the flame area and the flame intensity. Increasing the length-to-depth ratio of the cavity or the cavity aft ramp angle causes the flame to move upstream and to increase its intensity. The unburned and mixed gas and the high temperature burning gas enter the cavity and keep the cavity flowfield at high temperature and abundant with active molecules. At the same time, the heat and active molecules are transferred continuously from the cavity to the unburned and mixed gas passing the cavity. And then the stable flame is formed. When the acetylene is ignited by the hydrogen pilot flame, the flame extends gently in the ignition process. At the same time, changing the cavity structures or the acetylene injection locations has little influence on the acetylene ignition properties under this condition. The cavity is helpful in self-igniting the acetylene. In the acetylene self-ignition process, the flame extends rapidly. Changing the cavity structures or the acetylene injection locations has great influence on the acetylene self-ignition properties. Increasing the cavity aft ramp angle makes the self-ignition of the acetylene to be easier. Transverse injection from the foreside of the cavity bottom wall is easiest to self-ignite, and transverse injection upstream the cavity or from the middle of the cavity bottom wall is second easiest, and transverse injection from the backside of the cavity bottom wall or reverse injection from the cavity back wall is not easy to self-ignite.