Numerical And Experimental Study On Premixed Combustion Characteristics Of The Low Swirl Injector

The environmental protection and the increasing demand for fuel flexibility put forward new requirements for the gas turbines combustor technology.In order to achieve the goal of very low NOx emission and fuel diversity,more advanced combustion technologies are needed.The low swirl combustion technology has been successfully applied to small commercial combustion equipment and industrial furnace,and shows good combustion performance.Therefore,it is necessary to systematically study the flame stabilization mechanism of low swirl combustion and the effects of operating parameters and geometric structure parameters under the condition of gas turbine.In this paper,numerical simulation and experimental research were combined to study the effects of parameters on the combustion characteristics and the flame stabilization mechanism of the low swirl injector.Firstly,the appropriate numerical model was selected and the reliability was verified.Secondly,the steady state numerical simulation of low swirl injector under the condition of gas turbine operation was carried out,including comparative analysis of flow field structure and combustion characteristics of low swirl injector and high swirl injector,influence of multiple operating parameters and multiple geometric parameters on the flow and flame characteristics based on orthogonal experimental design method.Then the experiment was carried out under normal temperature and pressure by PIV measurement of the non reacting flow field,the flame shape recording,and measuring the extinction,flashback limits and pollutant emission.The influence rules of important parameters were verified and supplemented.Finally,the interaction between flame and vortex in low swirl combustion flow field was studied by large eddy simulation,and the flame structure and stability mechanism were deeply understood.The main conclusions of this paper are as follows:1.Comparative study of low swirl injector(LSI)and high swirl injector(HSI):Steady state numerical simulations and experimental studies are shown as follows:Under the same mass flow inlet boundary conditions,the central recirculation zone(CRZ)and flame shape of LSI and HSI are different at open atmospheric conditions.The CRZ of HSI is strong and connected with the centrebody,and the flame is stable in the inner shear layer and adheres to the centrebody edge.The center direct flow of LSI makes the center region of the nozzle exit field diverge,after a weak recirculation zone with a W-shaped leading edge.The flame is stable in the inner shear zone and the central divergent zone,and shows a W-shaped lifted flame.In dump liner,the corner recirculation zones of LSI and HSI are similar,both of which add outer flames stabilized in the outer shear layer and attached to the outer edge of the injectors compared with open atmospheric conditions.Inner flame forms of LSI and HSI are different,which is consistent with the differences of the flames at open atmospheric conditions.The blow-out limit and NOx emission of LSI are smaller than those of HSI.2.Effect of operation parameters:Numerical simulation results show that under the gas turbine combustor operation conditions of different air inlet velocity U0=15?45m/s,temperature T=450?650K,pressure P=0.1?0.8MPa and the methane equivalent ratio 0=0.6?0.8,only U0 has a highly significant effect on total pressure loss coefficient of low swirl injector,and the effects of T,P and 0 on NO emission,pressure loss coefficient and outlet temperature distribution are not significant.The dimensionless axial velocity distribution in the center of the LSI exit nearfield has the characteristics of divergence and self-similarity.The flame front is stable at the position where the turbulent flame velocity is equal to the airflow velocity,and the height of the flame lifted is almost unchanged.The experimental data of U0=5?l0m/s under normal temperature and pressure show that the inlet velocity has little influence on the flame shape,the extinction and flashback limits,which is consistent with the numerical simulation analysis.3.Effect of geometric structure parameters:Numerical simulation results show that in the four geometric parameters of LSI including vane angle a=32°?42°,the center channel radius Rc=14.7-22.7mm,porous plate hole diameter d-2.7?3.4mm and the premixed length of L=45?68mm,onlyRc had a significant effect on the total pressure loss coefficient,and the effects of a?d?L are not significant.Rc and d impact the axial velocity distribution greatest,thereby affecting the position,angle,length and shape of the flame.The experimental results show that the effect of a on flame is not significant at normal temperature and pressure.The extinction critical equivalence ratio increases as a increases at open atmospheric conditions while it increases first and then decreases in dump liner,but the effects are small.The effect of a on the flashback limits is greater,and the increase of a can improve the resistance to flashback of low swirl flame.The flame shapes are similar and the scale and height of the flame increase slightly when ?=370,the orifice opening coefficient increases from 0.19 to 0.34 with d,which are consistent with the numerical simulation.But d almost has no effect on quenching and flashback limits.Numerical simulation results show that in the four structure parameters of the liner,including liner entrance divergent angle ? =90°-180°,diameter DL=2D?3D,length LL=3.5D?4.5D and exit shrink angle y=20°-45°,only ? has highly significant effect on NO emission,and the other effects are not significant.The temperature of the corner recirculation zone is high,so NO emissions can be reduced by inhibiting the corner recirculation zone with ? decrease.The experimental results show that under normal temperature and pressure,the corner recirculation zone is compressed,the outer shear layer flame disappears,and the flame length becomes longer as ? decreases.These conclusions are consistent with the numerical simulations.The extinction critical equivalence ratio increases at first and then decreased with the increase of ?,but the change is small.The effect of ?on NOx emission is not clear,which is not consistent with the the result of numerical simulation and still need further study.4.Structure and stabilization mechanism of low swirl flame:The large eddy simulation was carried out to low swirl injector in dump liner.The main frequency of fluctuating pressure at the 12 points in both non reacting and reacting transient flow field are all integral multiples of 39Hz.Combustion makes the minimum main frequency of fluctuating pressure change from 39Hz to 156Hz.Large scale vortex structure and precession vortex core exist in both reacting and non reacting flow fields.The low swirl flame can be divided into outer attached flame and internal uplifted flame.The internal uplifted flame exhibits large scale flame front wrinkling with a particular W-shaped,which is consistent with experimental phenomena.The vortex shedding from the inner shear layer rolls the flame upstream continuely.At the same time,the flame front tends to be pushed off downstream in the low speed inner zone above the burner exit.The competition of the two motions leads to the formation of a characteristic W-shaped flame front.The low swirl flame is essentially stabilized by the vortices in the high speed inner shear layer,while a free propagating flame in the central low speed zone.