Numerical Study Of Unsteady Flow In Scramjet

In recent years, with the development of scramjet, the unsteady flowfield inscramjet is getting more and more attention. Of the unsteady phenomena, theoutstanding ones are periodic oscillation and ignition process. To study these unsteadyphenomena, the experiment alone is not enough, because experiment is expensive andthe details of the flowfields can not be achieved. With the development ofComputatinal Fuid Dynamics(CFD) and computer technology, CFD has been one ofthe three fundamental methods to study the scramjet (the other two are ground testand flight experiment), so it is possible to study the unsteady flowfield of scramjetwith CFD. Compared with experiment, CFD is cheaper, can get more details of theflowfield, and has been an unreplaced method for the study of scramjet. This paperstudies the unsteady flowfields of scramjet with Unsteady CFD method.This paper consists of eight chapeters.The first chapter is introduction, in which the background, history, currentresearch situation, and several difficult topics of scramjet are listed, and the work ofthis paper is presented briefly.The second chapter introduces the unsteady CFD methods.The third chapter studies two unsteady combustion processes of premixedcombustible gas, one is the unsteady shock induced combustion first experimentallystudied by Lehr, the other is the ignition process of a ram accelerator. The CFD resultsagree well with the experiments and the relevant CFD results. The work shows thefundamental mechanism of the unsteady combustion, validates and verifies the CFDmethod used in this paper, and is the background of the later works of this paper.The fourth chapter studies the attack angle influence on the ignition and hotflowfield of scramjet. In one way, the work validates and verifies the CFD methodused for scramjet unsteady phenomina, in the other way, the work studies themechanism of the influence of attack angle on the ignition. In experiment we observesthat the attack angle can lead to successful or failed ignition of the scramjet. Becausethe successful igniton must be related with the igniton process, this chaper studies theignition processes of the scramjet using unsteady CFD method under identicalconditions with the experiments. The wall pressure and specific impulse of CFDresults agree well with the experiments. The work shows the mechanism of theinfluence of attack angle on igniton, and from the CFD results we discover that, underthe4.5degree attack angle condition, the wall pressure of hot flowfield has noobvious difference from the cold one, but the combustion dose take place only withthe combustion efficiency being much lower, so it is concluded that in experiment it is not enough to judge the success of igniton only by wall pressure.The fifth chapter studies the oscillation of cold flowfield of scramjet combustor.In experiment the oscillation of cold flowfield of scramjet combustor is observed, inthe oscillation the wall pressure changed periodically with large amplitude, thispressure oscillation must give large influence on the structural strength. So, thischapter studies the oscillation under different conditions, from which the mechanismof the oscillation is achieved, and the method to control the oscillation is given. Thework has important academic and practical meaning.With open cavity scramjets, the sixth chapter studies the mechanism of masschange between the cavity flow and main flow, and finds two important parametersthat can influence the open cavity self-ignition, one is fuel-air ratio and the other iscavity scheme. The work draws three conclusions. First, in a steady flowing cavity,the flow is closed, the mass change between the main flow and cavity is caused by thediffusion of mass concentration but not by convection. Second, the total fuel-air ratioplays an important role on the successful ignition and combustion efficiency of ascramjet. If the fuel-air ratio is too high, the fuel entering the combutor later can crushout the earlier flame, so the higher the fuel-air ratio, the lower the combustionefficiency. Third, the cavity scheme also playes an important role on the successfulignition and combustion efficiency of a scramjet. For a longer cavity, the mixedfuel-air gas entering the cavity has lower fuel-air ratio, which is useful to the flamedevelopment. The work of this chapter shows the orientation of the study of ignitionand is the background of the work of chapter seven.Based on the conclusions of chapter six, the seventh chapter studies the detaildinfluence of fuel-air ratio and cavity scheme on the combustion efficiency, the fuel-airratio and cavity scheme both have their own domain. This chapter concerns a lot ofunstedy numerical simulations of ignition. From the work, a formula relating thecombustion efficiency with the fuel-air ratio and cavity scheme is given, with which ascramjet can be evaluated conveniently, and the fuel rich igniton boundary can bedrawn. The other important work of this chaper is that a rich blowout limit formula ismodified and a rich ignition limit formula is achieved.The eighth chapter is conclusions, including the achievement, the innovativepoints and the outlook of future work.