Study On Flow And Combustion Characteristics Of Premixed Nozzle For Micro Gas Turbine Based On Biogas Utilization

In the vast rural areas of China,there are abundant biogas resources,but,the utilization ratio is less than 5%and the efficiency is low.The biogas utilization can effectively improve this situation and become an important supplement to the energy structure adjustment of China.Meanwhile,biogas has the problems of unstable composition,dispersive distribution and low energy density,but the distributed energy supply system based on micro gas turbine provides an important way to utilize biogas due to its close proximity to users,high comprehensive efficiency and low emission of pollutants,and can be combined with green agriculture.However,at present,there is still no mature micro gas turbine unit directly utilizing biogas in the market.The biogas utilization in the combustor will enhance the combustion instability,increase the pollutant emission and decrease overall performance.Therefore,improving the fuel adaptability is always the focus of research.In this paper,based on the premixed nozzle for micro gas turbine fed with natural gas,a more efficient premixed nozzle is optimized and designed by means of numerical calculation and experimental analysis.Moreover,the flow and combustion characteristics of the premixed nozzle are deeply investigated,which can provide references for the effective utilization of biogas in the distributed energy supply system.In order to determine the optimization scheme,this paper analyzes the unmixedness degree of premixed nozzle under four equivalence ratios,four fuel components,three fuel holes,three air holes and two air channels.After that,the dynamic characteristics of flow and combustion processes between original nozzle and optimized nozzle are compared using cold PIV experiment and transient numerical calculation.On this basis,this paper designs a combustion test rig for premixed nozzle and observes the turbulent flames under three kinds of gap and four fuel components with an optical measurement system.The three-dimensional structure of flame and signs of chemiluminescence emission are obtained in the combustion experiment.Finally,the combustor space from the regenerator outlet to the turbine inlet is modeled and the coordinated combustion process among six nozzles is fully analyzed.The main conclusions of this paper are as follows.（1）The relationship between unmixedness degree at nozzle outlet and momentum ratio of fuel and air is roughly linear,and the unmixedness degree decreases with the increase of momentum ratio.According to this result,the optimization direction of premixed nozzle can be determined.By changing the air hole axial direction of original premixed nozzle from radial to tangential,low swirl flow is generated in the nozzle.Combined with the T-type air channel,the turbulent kinetic energy of the airflow in the nozzle can be enhanced the region of high turbulent kinetic energy can be expanded.These make the premixed nozzle reach a better mixing performance.（2）The low swirl flow in the nozzle can adjust the flow morphology,rapidly reduce the flow instability generated in the initial mixing area,and effectively decrease the randomness of flow.Because of the sharp edges of the air holes,low swirl premixed nozzle promotes the process of eddy dissipation and improve the mixing performance of fuel and air.Meanwhile,the maximum temperature and overall average temperature in the main combustion zone of combustor reduce.This means a lower NO_xformation.The maximum value and fluctuation amplitude of near-wall temperature also decrease,which effectively weakens the thermal impact on the liner wall.（3）The gap around the nozzle provides a passage for secondary air.With the increase of secondary air,the flame gradually extends the wide side of gap and forms a relatively stable reaction zone near the nozzle outlet.Therefore,the shape instability of flame reduces.Meanwhile,the CO₂ in the fuel will reduce the turbulent burning velocity,so that the biogas flame is much farther away from the nozzle outlet than the natural gas flame and the distance increase with the rise of CO₂.Moreover,as a reaction product,CO₂ in the fuel will enhance the shape instability of flame.（4）For the annular combustor with six premixed nozzle,the gap around the nozzle has an optimal area range.When the area ratio of gap and nozzle outlet is between 0.41 and 0.61,the temperature distribution in the combustor is more balanced and the emission of NO_x is much lower than other gap areas.Compared with using natural gas,when using biogas,the efficiency decrease of combustor equipped with LSPN is less than0.55%,the range of average temperature at the outlet is kept in 40 K,and the increase of volume flow at the outlet is less than 6%.Although the CO₂in the fuel will expand the high temperature area in the combustor and increase the emission of NO_x,the emission will not exceed 9 ppm.