Investigation Of Flow Characteristics In The Dual-mode Scramjet Isolator

The outstanding performance of a dual-mode scramjet in a wide Mach number range makes it to be the most promising propulsion system for the hypersonic vehicle. During hypersonic flight, the isolator has to separate the combustor from the inlet so that the scramjet can keep working stably. Studying on the isolator is a necessary prerequisite for the scramjet development and application. In this research, steady Reynolds averaged Navier-Stokes simulations are carried out to investigate the shock holding capability of the isolator, the interaction between isolator and combustor and the length of pseudo shock under flight condition.Cylindrical isolator and rectangular isolators with different aspect ratios are investigated through three-dimensional simulations. The influences of back pressure, Mach number, total temperature and wall temperature on the pseudo shock and performance of isolator are discussed. Firstly, The flowfield and shock wave structure distinguish with each other in different isolators. The results indicate that the flowfield in a cylindrical isolator is basically symmetrical under back pressure, accompanied with the almost uniform wall pressure distribution. Separation and circumferential vortex are observed just behind the initial bowl shock in the cylindrical isolator. In contrast,separation in rectangular isolators locates near the corner, and the vortex leads to a nonuniform wall pressure distribution. The initial shock is more likely to be a normal shock for a high aspect ratio isolator when the inflow condition and back pressure remain constant. The number of visualizable shock, intensity and spacing of the shocks in shocktrain decrease when the aspect ratio rises from one to five.Secondly, the inflow condition also shows a remarkable effect on flowfield and performance of the isolator. When the Mach number increases, the capability of a isolator to resist back pressure induced by combustion is enhanced, the overall nonuniformity of the exit parameters is significantly improved, and the normal shock train is gradually converted to the oblique shock train. The criticalpressure ratio of the isolator is closer to normal shock under a small Mach number although the pressure ratio rises along with the increase of Mach number. The boundary layer thickness and the pseudo-shock length increase when total temperature rises. The wall temperature shows the same effect on the pseudo-shock length.Mass-weighted average of parameters in the exit of the cylindrical and rectangular isolators are evaluated. The study indicates that the average Mach number at isolator exit decreases linearly with the increase of pressure ratio The linear slope and intercept can be expressed as a power function of inflow Mach number. When pressure ratio increases, the total pressure recovery coefficient decreases linearly while the temperature ratio increases. Slope of these linear relations for different isolator configurations are similar.After studying the characteristics of individual isolators, the isolator and combustor are considered as a whole. Three dimensional simulations of two different scramjet configurations are conducted respectively. Combined with the combustion modes, a preliminary analysis of the shock wave structure and separation characteristics in isolator/combustor interaction is carried out Although different inflow conditions, fuel injection and configurations lead to an obvious difference in the flowfield between cylindrical and rectangular scramjet flowpaths, there are still some common characteristics of the shock waves structure and combustion area in the two scramjets. The results in this paper indicate that a strong barrel shock is produced by the interaction between the primary injectors and the high speed flow in the isolator. Reflection of the barrel shock and shock system induced by the configuration result in a complicated flowfield structure of the combustor during the pure scramjet mode. At the same time, combustion is almost completed just in the combustor and the distributions of hot regions and chemical reaction are relatively concentrated. When the flowpath is in dualmode scramjet operation or dual-mode ramjet operation, the strong shock waves in the combustor will disappear along with the pseudo shock established in the isolator, and the flowfield in the combustor will be more uniform while the pseudo-shock moves upstream. Distribution of the combustion product indicates that some combustion has completed in the isolator. The flowfield of isolator has an evident influence on the shock structure and uniformity level of the combustor for both of the two scramjet configurations.Finally, the flowfield and pseudo-shock structure of the isolator under the effect of inlet and combustor have been simply analyzed through two-dimensional numerical simulation. The lengthes of the pseudo-shock under different flight conditions and back pressure are investigated by data fitting. An empirical correlation for the length of pseudo-shock has been achieved against a rectangular scramjet configuration, which shows the relation between the pseudo-shock length, inflow Mach number, angle of attack and the pressure rise ratio. The correlation provides a reference to establish a pseudo-shock length model to measure the anti-pressure capability of isolator during practical vehicle operation.