Investigation On Single-restricted Swirling Flows In Short Helical Combustors

A novel combustor,named short helical combustors(SHC),is investigated in this thesis.The fundamental feature of the SHC is that the swirler axis is tilted a certain angle along circumferential direction relative to the axis of engine and results in nonaxial air supply,which has the potential to reduce the axial length of the engine,the number of nozzle guide vanes(NGV),the deflection angle of the NGV,aerodynamic losses,and the amount of turbine cooling air demand.However,since the burner of the SHC is tilted at a certain angle,a section of sidewall appears at the junction of adjacent swirlers,and its length is positively correlated with the tilted angle.As a result,the swirling flow is restricted by the sidewall on one side while the other side is unrestricted,leading to single-restricted swirl flow in the SHC.To get deeper insights into the singlerestricted swirling flow in the SHC,the Reynolds average Navier-Stokes(RANS),large eddy simulation(LES)and experiments are performed.Relevant results are summarized as follows:1)the steady RANS is used to analyze the flow field of seven SHCs with different tilted angles.It is found that,with the increase of the tilted angle,the downstream recirculation zone of the swirler presents traditional modes(TM),single-vortex modes(SVM),critical modes(CM),and double-vortices modes(DVM).The shape of vortex on the spanwise plane is a symmetric ring in the TM;In the SVM,the vortex is horseshoe-type;as the flow fields gradually transition to the CM and the DVM,the vortex gradually evolves into a closed ring.The emergence of different vortex modes and corresponding vortex shapes in the flow field is related to the formation and development of an aerodynamic boundary.The aerodynamic boundary is located downstream of the sidewall,which can suppress the circumferential expansion of the swirling flows,suppress the attenuation of the tangential angular momentum in the axial direction,and increase the radial pressure gradient on the non-restricted side of the combustor,leading to different vortex modes in the flow field.The aerodynamic boundary is sensitive to the gas expansion caused by combustion.The constraint of the aerodynamic boundary on the swirling flow is weakened in reacting flows and thus the corresponding tilted angle during vortex-mode transition is larger than that in the nonreacting flow.2)Based on the results of the RANS,four combustors,whose flow field corresponds to the TM,SVM,CM,and DVM are selected for further investigation.The LES,particle image velocity measurement(PIV),proper orthogonal decomposition(POD),and dynamic mode decomposition(DMD)are used to investigate the precessing vortex core(PVC)and its interaction with flames in single-restricted swirl flows.The results show that the PVC and the symmetric coherent structure are found to dominate the fluctuation of the flow field.The PVC causes the recirculation zone to precess around the central axis of the swirler,while the symmetric coherent structure causes deformation.By examining the amplitude of azimuthal disturbance waves caused by the PVC,it is found that the amplitude of the PVC is largest in the TM and the smallest in the SVM.As the flow field transitions to the CM and the DVM,its amplitude gradually increases.Further analysis shows that the presence of the sidewall results in a thicker shear layer thickness in single-restricted swirl flows,which suppresses the absolute instability and thus the PVC.Then the DMD is employed to learn the interaction between the PVC and the flame in the TM and the CM.It is found that the PVC-induced pressure fluctuation in the TM is limited to the region of the swirler,which can cause local heat rate oscillations but global heat rate oscillation is small.However,the fluctuation not only exists inside the swirler,but also propagates to the far field in the CM,and the related global heat rate oscillation is relatively large.3)Based on the insights from the research of single-restricted swirl flows,a preliminary exploration of the overall performance of SHC with different tilted angles is made.The combustion efficiency,total pressure loss,outlet temperature distribution,reversed flow rate and outlet flow-angle are investigated.The combustion efficiency of SHCs is generally higher than that of the conventional combustor.At the same time,its total pressure loss is small;the outlet temperature distribution is relatively uniform.However,its reversed flow rate is smaller than that of the conventional combustor,which may be detrimental to the stabilization of the flame.The outlet flow-angle indicates that the flow direction at the outlet of the SHC will deviate from the engine axis,which is beneficial to reducing the turning angle and the number of NGV.By comprehensively analyzing these parameters,it is suggested that the performance of the SHC with tilted angle 15 degree is superior,but additional optimization should be made to provide foundations for its industrial applications.