Performance,Mechanism And Prediction Of Ignition And LBO For Multi-Swirl Staged Injector

With the development of low emission and high temperature rise combustors,the injector air split can reach as much as 40%-70% of the total combustor air flow,which threats the combustion stability and result in a narrower operation limit.To resolve this problem,the multi-swirl staged injector was put forward to achieve combustion performances at design point,while guarantee operation limits at off-design points.In this study,firstly,a multi-swirl staged injector was developed with the features of a prefilm atomizer coupled with two axial swirlers in the pilot burner and dual counter-swirling swirlers in the main burner.Then,Experiments were conducted to study the effect of pilot and main burner parameters on the flow structures,spray patterns and ignition and lean blow out(LBO)performances.Non-reacting numerical simulations were utilized to quantitatively analyze the ignition and LBO processes,with the intention of bridging the internal relation of lean combustion stability with the flow and fuel distribution patterns,and acquiring an understanding of the dominant mechanisms of ignition physics.Further,according to the acquired ignition mechanisms,an ignition physical model was established and the typical parameters that can describe the sub-phases of ignition were defined and extracted.Finally,a preliminary prediction method of ignition performance was proposed and validated with the ignition experiment data.The research work in this thesis contains four aspects:(1)In the first part,the swirl direction,the swirl angles and the air split ratios of the pilot swirlers are parameterized to study the influence of pilot burner design on the ignition and LBO performances.It is found that changing of the swirl direction from counter-swirl to co-swirl will transform the fuel distribution patterns from a solid cone into a hollow cone,which deteriorates the ignition and LBO performances.Increase the pilot inner swirler angle accelerates the diffusion and mixing of the pilot airstreams and reduces the fuel concentration along the flame propagation route,which is disadvantageous to the spread of flame to the injector.Decreasing the pilot outer swirler angle on the one hand reduces the droplets density along the flame route,on the other hand destroys the flame stabilization environment,so both the ignition and LBO boundaries are narrowed.On the contrary,increase the outer swirler angle causes negligible effects on the flow and fuel distributions,so do the ignition and LBO behaviors.Reduction of the air split of the pilot inner swirler decreases the fuel air ratio(FAR)along the flame spread path and deteriorates the stabilization environment around the flame root,so that the marginal ignition and LBO FAR are increased.Increasing the air split of pilot inner swirler causes that the flow direction of the pilot airstreams changes from negative to positive and the fuel droplets along the flame path are enriched.Under low pressure difference condition,the effect of the fuel concentration is dominant,so the combustion stability is improved.Under high pressure difference condition,the unfavorable effect of the positive flow is dominant,so the ignition and LBO performances are deteriorated.(2)In the second part,the influence of the main burner design parameters on combustion stability are studied by varying the the main stratifier length,the air split ratio and swirl angles of the main swirlers.Conclusions are drawn that the increase of the main stratifier length will enlarge the primary recirculation zone(PRZ),which is beneficial to the ignition and lean combustion limits.Increasing the air split of main inner swirler causes negligible effect on PRZ size,however,the flow instabilities are enhanced which is harmful to the ignition processes.The extinction behavior is improved due to more favorable flame stability environment under this circumstances.Reduction of the swirl angle of main inner swirler causes negligible differences to both the PRZ size and the ignition performance,while the LBO performance is enhanced due to improved stability condition.With the reduction of the outer swirler vane angle,even though the PRZ size is decreased,the enrichment of the kerosene concentration along the flame path dominates the ignition process,which improves the ignition and has little effect on LBO.(3)The dominant mechanisms of flow field and kerosene distribution of the multi-swirl staged combustor are analyzed in this thesis.Designs of the pilot burner mainly affects the local flow structures around the pilot combustion zone,which in turn determine the inner boundary of PRZ and the pilot spray pattern,while designs of the main burner determine the outer boundary of PRZ.The PRZ dimensions is controlled by the two swirling flow of the main burner,which affects the formation of PRZ through three flow modes: the outer swirling flow dominant mode,the comprehensive mode with stronger outer swirling flow,the comprehensive mode with stronger inner swirling flow.The dominant mechanisms of ignition physics can also be summarized into three modes,i.e.the pilot spray pattern dominant mode,the pilot flow structures dominant mode and the PRZ dimensions dominant mode.(4)An ignition physical model coupled with a preliminary prediction method of ignition performance was proposed in this thesis.On the basis of the ignition mechanisms pointed out in this research,an ignition physical model was established,which can be divided into four separated phases,i.e.the generation phase of ignition kernel,the radial propagation of flame,the axial propagation of flame and finally the flame stabilization behind the injector.By quantitatively depicting the separated phases of ignition,a preliminary ignition prediction method was put forward and applied to the calculation of ignition probability of a bluff body flame and the ignition performance of injectors studied in present work.It is demonstrated that the ignition prediction method can reasonably assess the ignition probability contour of the bluff body flame.The model constants for the multi-swirl staged injector are linearly regressed from the ignition data obtained in this work.Compared with the experimental ignition data,the regressed ignition performances are satisfactory considering the variation trend of ignition curves.However,the model accuracy still need to be improved for practical application.The mechanisms,physical model and prediction method proposed in this thesis can be used in guiding the design and optimization of the combustion stability for similar multi-swirl staged injector,as well as the estimation of ignition characteristics of conventional rich dome combustors.All the conclusions drawn from this work lay technology basis for the development of low emission and high temperature rise combustors.