Research On Structure And Combustion Characteristics Of Laminar Dual-stage Lean Premixed Flames

As energy crisis and environmental pressure increase,the ground gas turbine used for power generation is beginning to develop towards high efficiency,large power and low emissions.However,the gradual rise of gas turbine has also brought greater challenges to the design of combustor.On the one hand,higher temperature of flue gas from combustor is needed to increase the thermal efficiency of system.However,this will also has a negative influence on suppressing the generation of thermal NOx in combustion zone.So,how to ensure the environmental performance of combustor while pursuing higher efficiency has become an issue to be considered for future gas turbine.Dual-stage lean premixed/Moderate or intense low-oxygen dilution(DLP/MILD)combustion system is conceived by summarizing the exiting combustion technologies in gas turbine combustor,which has combined their strengths.The environmental advantage of lean premixed flames is brought into play here by organizing multi-scale staged combustion.The entire system consists of two parts.In the fore combustion section which located upstream,dual-stage lean premixed(DLP)flame with inhomogeneous equivalence ratio is built.Two types of lean premixed flames with different equivalence ratios are arranged in DLP combustion zone.Depending on the cooperation between primary flame with higher equivalence ratio and secondary flame with lower equivalence ratio,a stable and low NOx-emitted combustion state is obtained.And the flue gas generated here with high temperature and containing low oxygen will mix with reburning fuel before entering aft combustion section located downstream.Then a moderate or intense low-oxygen dilution(MILD)combustion will be built again,which completes the subsequent combustion reaction.This thesis is to investigate the flame structure and combustion characteristics of DLP flame.To judge its feasibility as the combustion organization in fore combustion section,the NOx generation characteristics and ability to adapt to low equivalence ratio are analyzed.The research starts from the DLP flames in laminar condition,which is for the purpose of avoiding influence from too many factors to capture the relationship between primary and secondary flames more accurately.Experimental and numerical methods are used for research.A dual-stage combustion characteristic experimental system is built,which can organize DLP flame in the form of coflow jet.The combustion information of different conditions is measured.Meanwhile,Open FOAM framework combined with laminar SMOKE solver is used to simulate DLP flames.The calculation results obtained are supplemented with experimental content and perform deeper analysis.First,the combustion characteristics of coflow DLP flame is investigated initially through flame images and temperature distributions obtained from experiments.It is confirmed that the secondary premixed gas is ignited under thermal support from primary flame.Flow field in combustion zone and morphological changes of DLP flame are also analyzed.The trail part of secondary flame is found to be a diffusion-controlled premixed flame,because heat and CH4 molecule are transported oppositely from both sides,which determines its combustion position and intensity.The quantitative analyze of ignition effort from primary flame to secondary flame has shown that the secondary equivalence ratio and their contact level are the key factors affecting the amount of fuel in secondary premixed gas to be ignited.Secondly,the extinction limits and combustion temperatures of DLP flames in different conditions are measured.It is found that secondary premixed gas can isolate the primary flame with outside ambient,so the heat dissipation and dilution by air of root flame will be weakened.Then the DLP flame can combust steadily over a wide range of parameters with higher flow velocity and low premixed equivalence ratio.Meanwhile,the heat released by primary flame can be used to ignite peripheral ultra-lean premixed gas to the greatest extent,which makes the DLP flame get a fairly low global equivalence ratio.It has even reached below 0.3 at the lowest.Compared with single jet premixed flame,DLP flame has lower NOx emissions.This is because the hydrocarbon intermediates,which are generated by secondary flame in the low-temperature and low-equivalence ratio condition,can reduce NOx to N2.Finally,the design scheme of actual DLP burner is preliminary discussed.A slit DLP combustion calculation model is built.The effect of flame arrangement and structure size on combustion state is studied with numerical calculation.The ratio of primary and secondary flames is found to be the key factor affecting NOx emissions.And the multi-arrangement of primary flames can enable all secondary premixed gas to be ignited,which will reduce pressure for downstream combustion section to resolve burnout issue.Learning from the idea of micro-nozzle combustion,a honeycomb-like arrangement scheme of DLP nozzles is designed,which can ensure sufficient interaction between the primary and secondary flames to take advantage of DLP combustion.The thermodynamic requirements for MILD combustion are also summarized.And the feasibility of organizing DLP flame in the fore combustion section of DLP/MILD system to provide required gas for downstream combustion is clear.