The Study On Self-ignition And Flammability Characteristics In Solid Fuel Scramjet Combustor

Solid fuel scramjet has some advantages such as simple structure, high specific impulse, low cost, safety and short combat reaction time. It is of wide application prospect in hypersonic system in the future. However, the study of solid fuel ramjet is still at a preliminary stage of exploration, and the development of many key technologies is not mature. The combustor is the core component of scramjet, and the ignition, combustion organization and flame holding in it is urgent need to resolve. Some exploring researches on the process, the mechanism and the influence factors of self-ignition and flame-holding were carried out by means of theoretical analysis, numerical simulation and direct-connect test.The solid fuel regression and combustion process of in solid fuel scramjet was analyzed theoretically, and the coupled heat transfer between gas and solid fuel, the injection and combustion of pyrolysis gas, the regression process of solid fuel and the characteristics of supersonic flow were considered, then a regression, pyrolysis and combustion numerical model of solid fuel in supersonic flow was established based on the unsteady moving mesh technology. The turbulence models for supersonic flow were compared to choosing the good one which is employed in this dissertation. The pressure and regression rate obtained from numerical simulation agreed well with the experimental data, therefore the numerical simulation model is suitable for the studies of flow and combustion related issues in solid fuel scramjet.The influence factors on ignition of solid fuel were studies by means of steady numerical simulation, mainly focusing on the inlet parameters and geometry of combustor. The ignition performance of step flame holder usually used in traditional solid fuel ramjet and cavity flame holder usually used in liquid fuel scramjet was compared, and it is found that adding a cavity with proper length and depth based on the step enhances the ignition performance and overall performance of combustor. The inlet poor oxygen limit, inlet rich oxygen limit and inlet total temperature limit of self-ignition of solid fuel were identified in the combustor. The self-ignition of solid fuel is probably established in the combustor with a relatively deep and long cavity, a relatively small cylindrical section diameter, and a relatively large convergence angle.The unsteady ignition and pressurization process in combustor was researched by means of numerical simulation. The variations of pressure, temperature, species fraction and regression rate during the ignition and pressurization process were analyzed. Three stages during the ignition process were identified according to the characteristics of flow and combustion, namely the heat accumulation stage, the ignition of secondary recirculation zone and the flame spread stage in order, the pressure jumps to higher level in the flame spread stage meaning the establishment of pressurization. The ignition delay time increases with the increase of the cavity depth and decreases with the increase of cavity length for the cavity with establishment of self-ignition. A new type cavity was designed to enhance ignition performance and overall performance of combustor according to that the secondary recirculation zone plays the actual role as ignition resource. The mechanism of self-ignition is as follows. The inlet flow rate and total temperature of gas is appropriate to fully heat the solid fuel. The intensity of chemical reaction is strong, and the residual time and recirculation zone area provided by the cavity are enough, which is leading to the accumulation of heat in secondary recirculation zone. After the gas temperature is larger than ignition temperature, the self-ignition is established in secondary zone, and then the rest of solid fuel is on fire.The quasi-steady regression and combustion of solid fuel and the influence factors of flammability were studied numerically based on the parallel burning algorithm. The characteristics of steady combustion and un-sustained combustion were compared and analyzed. During the steady combustion stage in combustor, the temperature in main recirculation zone is higher than that in secondary recirculation zone, the relatively high temperature zone locates at the rear of cavity and near the shear layer above the solid fuel surface and it spreads toward downstream. The parameters are close to the inlet conditions after the extinction. The flammability is better with a relatively high inlet total temperature under the same inlet flow rate. The average regression rate is larger with a higher inlet total temperature at the same working time. The extinction time is shorter with a larger inlet flow rate under the same inlet total temperature. The flame could be maintained in a certain period of time; however the extinction would happen in a relatively long time. Namely, the flammability is not absolute for combustor. It should be properly increasing the area of cavity, deepening the cavity, decreasing the cylindrical diameter, and lengthening the cylindrical section to enhance the flammability under the achievement of working performance and without choking of flow for combustor.The unsteady regression and combustion process in solid fuel scramjet combustor was studied in detail based on the move grid technology. The mechanism of extinction was analyzed numerically. The variation law of solid fuel port contours was obtained and the variations of wall parameters, axial parameters and inner flow field were observed. The largest regression rate locates at joint of cavity and cylindrical section, the regression rate near the inlet and the outlet is relatively small, and the average regression rate of whole fuel surface decreases gradually. The convergence section widens towards the downstream, the cylindrical section would be integrated with the diverging section as a single unite with nearly straight contour. The contour of diverging section near the outlet may be diverging and then converging during the final stage of working. The trend with first increase and then decrease of inner flow temperature was observed. The flame could only be maintained for a short time under some configurations with the same inlet conditions. The reason of extinction is as follows. The fuel port increases, and the main flow velocity increases, however the average of temperature of different zone in combustor decreases during the working time of solid fuel scramjet. The flammability decreases when the residual time provided by the recirculation zone decreases gradually. It results in the fluctuation of flame distribution and the change of flow structure, which is the reason of the loss of flammability; finally the extinction happens under the cooling and blowoff effects of relatively cold main flow.The self-ignition and steady flame were established with inlet flow rate 1000g/s and 500g/s, respectively, simulating the flight Mach number 6 by means of direct-connect experiment. It could also be established using step flame holder, but the wording performance is worse than the cavity with same cylindrical section diameter. The high temperature and highlight area spread from upstream towards downstream, and the brightness first brightens and then darkens during the initial stage of self-ignition. The temperature in secondary recirculation zone is smaller than that in main recirculation zone under the stage of steady combustion, which is same as the numerical results. Flame fluctuation was observed during the process of extinction, and the flame was blown off from downstream towards upstream.