Numerical Study On Ignition Process In Annular Swirling Combustor

Annular combustor is widely adopted in the modern aero-engine,whose ignition reliability influences the work conditions of the aero-engine and the re-ignition at high altitude directly and plays a critical role in combustor design.At present,as the emission standard becomes harsh,many low-emission combustion technologies have been applied,and the aero-engine combustor chamber is trending lighter and shorter.This kind of structure of combustor makes it more difficulty to ignite and to stabilize the flame.Therefore,the studies on the ignition and the flame propagation in the aero-engine combustor and the analysis of the mechanism and the influencing factors are very important to the design of the combustor.In this dissertation,numerical simulations of the annular swirling combustor,the TurboCombo annular combustor of Zhejiang University,have been carried out to investigate the ignition and flame propagation.The primary coverages of this dissertation are listed as follows.1.The velocity field of circular tube of a swirling combustor with single burner has been analyzed carefully with steady RANS(Reynolds-Averaged Navier-Stokes).Then the predicted data of several Reynolds number 4000,5000 and 6000 respectively have been analyzed and compared to the experimental data.The flow field at low speed in the center of the burner is well predicted by RANS.The profiles of time-average axial and radial velocity components show a good agreement with the experimental results.However,the magnitude of the velocity at outer edge of burner is underestimated by RANS.2.On the basis of URANS(Unsteady Reynolds-Averaged Navier-Stokes),along with SAGE detailed chemical dynamics solver,detailed chemical reaction mechanism and AMR method,the simulation of ignition process in the annular swirling combustor has been carried out.After analyzing the structure of the cold flow field,it's found that there exist clockwise circumferential flow along the inner wall and counter-clockwise circumferential flow along the outer wall in the annular swirling combustor.The flame front dynamics,the light-round time and the integral heat release ratio show a good agreement with the experimental results,however,the predicted light-round time is longer than the experimental one.During the early stage of the light-round process,the profile of the predicted integral heat release ratio agrees with the experimental data,however,the discrepancy occurs after the merge of two flame fronts.The asymmetry of the flame propagations in the clockwise and counter-clockwise directions are investigated carefully.The analysis reveals that the clockwise circumferential velocity component along the inner wall is the key factor which leads to a slightly faster velocity of the clockwise flame propagation.3.Simulation of ignition procedure in the annular swirling combustor has been carried out using LES.Compared to the URANS,LES adopts the same condition parameters and shares the same ignition and propagation procedure.Unlike the URANS,the LES uses refined fundamental mesh size which leads to twice computational meshes as much as URANS approximately.The structure of the cold flow field before the ignition has been investigated carefully.Compared to the predicted result with URANS,it's found that LES can capture the recirculation zone at the downstream of the injector more accurately.The analyses of the turbulent fluctuations velocity and the vorticity distribution at differents altitudes above the injector exit show that there exist the procedures of the swirling flow development and the turbulent structure dissipation during the development process of the flow field.The analyses of the flame front topology and the light-round time have been carried out with the predicted results with LES,the experimental data and the predicted results with URANS.It's found that LES can predict the flame surface wrinkles more accurately and the predicted light-round time is closer to the experimental one.On the basis of the analysis of the interactions between the flow field and the flame during the flame propagation,it's found that the pressure difference is the primary cause for the flow counter to the flame front movement.During the flame propagation,the characteristics of flame front shape and location of leading point at different instants showed by the contours of various species are consistent with the ones of temperature.The qualitative analysis of turbulent flame model shows that the flames locate nearby the critical value Ka=1.