Investigation Of Extending The Combustion Stability Of A Parallel Jet Nautral-gas-fired MILD Burner

The need of higher efficiency and thermal power of gas turbine demands higher combustor outlet temperature and compressor pressure ratio,and under the strict pollutant emission regulation of lower NOx emission and requirement of stronger gas turbine operational flexibility,the advanced gas turbine combustor must be featured with low pollutant emission,efficient flexibility and higher parameter.MILD combustion was considered for its well-known ultra low pollutant emission,low noise,high combustion efficiency,distributed temperature profile and stable combustion.High momentum jet is a feasible way to achieve the MILD combustion,and a high momentum MILD combustor was studied in this research.However,due to the insufficient flame stability of MILD combustor at low load in previous experiment,in this study,the methods to extending the combustion stability were investigated.The premixed jet with low swirl or without swirl can lead to lower NOx emission,and the direct fuel injection is helpful to stabilize the combustion under the lean premixed condition at the cost of higher NOx at base load.It was reported that stratified combustion seemed to be more stable than premixed and non-premixed flame.Therefore,in the present study,firstly the viability of partially premixed combustion in the MILD combustor was demonstrated.Second,as the pilot stage was widely used to strengthen the flame stability,decreasing its contribution to the NO,emission was another part of this study.The investigation of extending the stability limits of the MILD combustor stabilized by high momentum jet was presented as follows.To gain insight into the stabilization mechanism,OH*chemiluminescence imaging,planar laser induced fluorescence measurements of OH,laser doppler velocimetry and numerical simulation were utilized.Firstly,a model MILD combustor featured with the capability to change the air/fuel premixed mode was designed.The effect of the premixed mode to combustion characteristic(stability and pollutant emission)was investigated.It was found that the lean stability limits under partially premixed mode and non-premixed mode was extended largely,and lower premixed degree leaded to higher NOx emission.Changing the mode did not change the rough structure of the center recirculation zone,although shifted the zone's bottom toward the burner.With lower premixed degree,the relatively dispersed reaction zone partly gathered surround the premixed main jet.When decreasing the equivalence ratio,it was the reaction zone surround the fuel jet that stabilize the flame under extreme lean conditions.Secondly,we got insight into the pilot nozzle.At first,by comparing the combustion characteristics of combustor with and without pilot nozzle,we found that the presence of the center pilot made a leaner shift for the lean blow off(LBO),extended the stability range,stretched the recirculation zone in the radial direction and shifted the reaction zone downstream.Then a single pilot nozzle,which was capable of changing fuel/air premixed mode,was investigated.3 distinct configurations of premixed tube outlet were observed(called jet tube,flat tube and recessed tube).The impact of premixed mode to the each configuration was discussed.It was showed that jet configuration had a more stable and symmetric flame structure,while the flat configuration and recessed configuration had greater potential in flame stability.For the jet configuration,at first the NOx emission rose when increasing the air flow rate to the center premixed tube.After reaching the maximum emission at a certain premixed air ratio,the NOx started to drop with further increase.And there was a smaller NOx emission compared to the diffusion mode.Meanwhile,the recessed configuration with a low ratio of premixed air could also had lower emission than diffusion mode,thanks to the presence of recessed premixed section it outperformed the flat configuration.The reaction pattern outside the cone nozzle for each configuration under different mode was quite different.For the jet configuration,with more premixed air,the intensity of the heat release zone along the wall of the cone nozzle rose first and then decreased.And for the flat and recessed configuration,increasing the ratio of premixed air made the relative dispersed reaction zone concentrated around the premixed tube outflow.