The design and model simulation of a can combustor has been made for future syngas (mainly H2/CO mixtures) combustion application in a micro gas turbine. In previous modeling studies with methane as the fuel, the analysis indicated the design of the combustor is quite satisfactory for the 60-kW gas turbine; however, the cooling may be the primary concerns as several hot spots were found at the combustor exit. When the combustor is fueled with methane/syngas mixtures, the flames would be pushed to the sides of the combustor with the same fuel injection strategy. In order to sustain the power load, the exit temperature became too high for the turbine blades, which deteriorated the cooling issue of the compact combustor. Therefore, the designs of the fuel injection are modified, and film cooling is employed. Consequently, the simulation of the modified combustor is conducted by the commercial CFD software Fluent. The computational model consists of the three-dimensional, compressible k-ε model for turbulent flows and PPDF (Presumed Probability Density Function) model for combustion process between methane/syngas and air invoking a laminar flamelet assumption. The flamelet is generated by detailed chemical kinetics from GRI 3.0. Thermal and prompt NOx mechanisms are adopted to predict the NO formation.
At the designed operation conditions, the modeling results show that the high temperature flames are stabilized in the center of the primary zone where a recirculation zone is generated for methane combustion. The average exit temperature of the modified can combustor is 1293 K, which is close to the target temperature of 1200 K. Besides, the exit temperatures exhibit a more uniform distribution by coupling film cooling, resulting in a low pattern factor of 0.22. The NO emission is also low with the increased number of the dilution holes. Comparing to the results for the previous combustor, where the chemical equilibrium was assumed for the combustion process, the flame temperatures are predicted lower with laminar flamelet model. The combination of laminar flamelet and detailed chemistry produced more reasonable simulation results. When methane/syngas fuels are applied, the high temperature flames could also be stabilized in the core region of the primary zone by radially injecting the fuel inward instead of outward through the multiple fuel injectors. The cooling issues are also resolved through altering the air holes and the film cooling. The combustion characteristics were then investigated and discussed for future application of methane/syngas fuels in the micro gas turbine. Although further experimental testing is still needed to employ the syngas fuels for the micro gas turbine, the model simulation paves an important step to understand the combustion performance and the satisfactory design of the combustor.
