Effect of Dilution hole Geometry on Gas Turbine Combustor Pattern Factors-A CFD Study

The quality of hot gases originating from gas turbine combustor is measured using two parameters viz., circumferential pattern factor and radial pattern factor. These parameters have a direct effect on the life of turbine vanes and blades and ideally they should be as small as possible. A higher total pressure loss in the combustor liner reduces the pattern factors but from the engine overall performance perspective, it has to be a minimum, as every one percentage increase in total pressure loss results in a half percent reduction in thrust and around a quarter of a percent increase in specific fuel consumption. Among all the variables that influence the pattern factors, the dilution hole area significantly alters the pattern factors. In the present work, CFD analyses have been carried out to estimate the effect of dilution hole geometry on gas turbine combustor pattern factors. During this process, it has been ensured that the total pressure remains unaltered. Analyses have been carried on 20 degree sector of combustor geometry using CFD code “Fluent�. The modifications carried out to the dilution hole geometry were categorized into three groups. Turbulent reacting flow analyses have been carried out using steady state RANS model. The liquid fuel has been modelled as a discrete phase. A reduced two step reaction is used to model the chemistry; turbulence has been modelled using realizable k-epsilon model and combustion by the laminar flamelet model (non premixed combustion model). The flow field characteristics within the combustor, pressure loss, mass flow distribution, maximum temperature and pattern factors at the combustor exit were extracted and reported. It is found that even through the dilution hole remains unaltered, the disposition of dilution holes effects the pattern factors significantly.