The Mechanism And Control For Combustion Instabilities Of Aeroengine Afterburner

Oscillation combustion in the afterburner has been one of main concerns in the research and development of modern aero-engine. The relevant mechanism and physical process is neither totally clear for now, nor are the effective ways to suppress the occurrence of instability in the chamber.For the buzz of the aero-engine afterburner, this paper conducted a series of experimental study on thermoacoustic oscillation and combustion instability, by making use of the lean premixed gas combustor experiment for model chamber and thermoacoustic oscillation experiment of Rijke tube. The mechanism of combustion instability and control approaches were discussed, and the thermoacoustic theory method for afterburner of aero-engine was established. Based on the CFD numerical simulation analysis, the aero-engine was studied and analyzed on its thermoacoustic oscillation phenomenon and control mechanism, whose aim is to explore a new way of active and passive combination to control thermoacoustic oscillation.First, the three-dimensional theory method of thermoacoustic oscillation in a tube was developed. The model can be used to describe the sound propagation in a variable cross-section tube with complex acoustic lining structure, such as perforation plate. The method deals with the complex problems of variable cross-section or adding acoustic lining in use of mode matching approach, which includes the influence of heat release and boundary conditions. The model provides theoretical way for the research of the thermoacoustic oscillation in the aero-engine afterburner.Secondly, a series of experiments were conducted on thermoacoustic oscillation of Rijke tube and lean premixed gas combustor. The influence of heat release position on thermoacoustic oscillation character of such factors, open area ratio of perforated plate, acoustic lining length, and bias flow were investigated by means of the Rijke tube. The research results show that the factors of heat release position, open area ratio of perforated plate, acoustic lining length, and bias flow can introduce the variation of effective acoustic pressure, oscillating frequency and unstable region. From the results the control approaches and laws of suppressing thermoacoustic oscillation can be explored. In the experiments of lean premixed gas combustor instability research, the influence of equivalent rate, rotational flow, and expansion structure on oscillation combustion was emphasized. The experiments indicate that self-induced oscillation occurs in the chamber. The oscillation modal is a resonance phenomenon coupled between the chamber and premixed part. The oscillating frequency was determined by the whole system, and oscillation in premixed part was forced oscillation induced by the chamber. Besides, the equivalent rate has an important influence on combustion instability, and expansion with rotational flow together influences combustion instability frequency and oscillation pressure.Thirdly, the numerical simulation was performed on the unstable combustion in lean premixed gas combustor and the afterburner of aero-engine by CFD. The numerical simulation of lean premixed gas combustor explains the phenomenon in the experiments. It is found that the two shear layers of jets with central recirculation zone and corner recirculation zone at the sudden expansion inlet of chamber determine the structure and movement of flame. By applying different numerical models, the numerical simulation on the unstable combustion existing in the afterburner of the aero-engine at the maximum thrust state indicates that: vortex shedding after the flame holders is the important reason of inducing boost buzzing, and the fuel contribution and atomization can influence the configuration of shear layer near the lips of the flame holders as to influence the pulse of flame surface. The numerical simulation results also indicate that the numerical methods have remarkable influence on the structure and configuration of vortex; therefore it is one of difficulty to choose models for CFD research. Besides, the average flow parameters of the afterburner were obtained by CFD simulation, which provide the conditions for the application of thermoacoustic oscillation model of the afterburner.Fourthly, the thermoacoustic oscillation in the afterburner of aero-engine was calculated using the present thermoacoustic model, and the mechanism and control laws of suppressing thermoacoustic oscillation were discussed. The results of the research indicate that the method has applicability in studying such problems and the predicted thermoacoustic frequency has enough accuracy. The calculation results for Rijke tube indicate show the importance of bias flow injection on thermoacoustic oscillation control method of active and passive combination. The review of heat release positions and heat shield structures indicates the influence laws for such structures, which provides the theory evidence for finding new control ways to supress the oscillation in afterburner of aero-engine.