Experimental Study Of The Scramjet Engine Isolator Flow And Its Flow Control Mechanism

Hypersonic propulsion is one of the advanced research topics, and the development of dual-mode scramjet has been received vast interests. As the important component of the scramjet engine, the isolator plays a key role in the engine performance. It operates as switching the engine modes, matching the engine gas to the entrance condition of the combustor and preventing the engine unstart.The shock train in the isolator is a kind of fluid phenomenon of intensive interaction between continoues shock waves and boundary layer, of which the mechanism study chllanges the traditional measurement methods. Ever since the concept of hypersonic flight was proposed, progress of corresponding theory, experiments and nummercial simulations has been gained. However, the mechanism which the isolator involves is very complicated and remained to be explored further. Thus, in the present thesis, two sets of isolator wind tunnel were designed which were appropriate to perform NPLS(Nanoparticle-based Planar Laser Scattering). Besides NPLS, high frequency pressure measurement and high speed schlieren were carried out to study the isolator flow features, incoming flow condition impacts, inlet-isolator performance. The three-dimensional structures, unsteadiness, pressure fluctuation, leading edge detection and flow control of shock train flow field was studied by methods of digital image technique, wavelet decomposition, POD(Proper Orthogonal Decomposition) and dimensionless analysis.The transient structures of the isolator shock train were obtained by NPLS, and the results showed that the boundary layer in the isolator was induced to separate and reattach by the shock train, and the spanwise structures of the isolator did not synchronize with the streamwise structures in the isolator with large aspect ratio which indicated highly three-dimensional features. Based on the unsteadiness study, it can be found that the correlation coefficient of shock train structures is exponentially related to the time intervals. When the time interval is less than 20μs, the shock train distorts in small scale structures; but if the time interval is large than 20μs, large scale structures distort, degenerate, disappear and move.The shock train length measuremeted by flow visualization matches with that by pressure measurements. The location of the shock train leading edge was detected by using power spectrum, stand deviation, accumulation and so on. The pressure features of the shock train formation and movement were studied and the results show that the pressure fluctuates greatly at the shock train leading edge while after the shock train, the pressure fluctuations decrease. When the isolator suffers unstart or the shock train retreates to the rear, the fluctuations would become the former state. The choking speed at the rear of the isolator intensively impacts the movement of the isolator, but the reverse pressure is found to be a better parameter to descript the isolator movement. The dimensionless analysis shows a parabola relationship between the reverse pressure and shock train leading edge location. Based on the frequency feature of the pressure fluctuation detected by wavelet decomposition, the shock train can be divied into 3 sub-states namely shock train leading edge arrive, transition state to unstart flow and stable state.To perform flow control study, Delta streamwise vortex generators and spanwise trapezium vortex generators were employed at the entrance of the isolator. The results showed that the shock train structures distorted by the influence of Delta streamwise vortex generators, while the spanwise trapezium vortex generators result in thinner boundary layer and shock train composed with bifurcated normal shock waves. The isolator without any control has the maximal pressure recovery at the starting state while the spanwise trapezium vortex generators can help to obtain higher pressure recovery and push in an isolator running in ramjet state.To study the features of the isolator with asymmetric incoming flow conditions, measurements were carried on in different isolator models with incoming flow Mach number 3.0,3.4,3.8,4.2 and the cowl attack angle changed within the range of 0°~16°. Based on the results, it is found that the isolator suffers innormal starting especially in low Mach number when the cowl angle is 0°~2°which results in large converge ratio. For the 4°~12°cowl angle, the isolator starts under the whole Mach number range.While for the 14°~16°cowl angle, only in high Mach number the isolator performs well. Thus, it is obtained that 4°~12°cowl angle should be recommened as the moderate starting condition. The pressure curves show that the isolator with 8°cowl angle provides the highest pressure endurance. The increasing Mach number improves starting performance of the isolator.Studies on the inlet-isolator performance in the hypersonic wind tunnel were conducted. The results show that the boundary layer on the front-body wall becomes thicker and transits into turbulence ealier with the diameter of the forebody leading edge increases. However, the increase of the whole inlet attack angle mainly results in early transition. When the cowl attack angle increases, the converging ratio and pressure fluctuation increases while the starting performance becomes worse. The OPD distribution of the backward facing step downstream of the isolator was obtained by using NPLS-DT and light tracing method. It can be found that intensive density fluctuation greatly impacts the steadiness of the shock train and flames by the physical explain of POD method.