Computational study of partial load operation of a multi-nozzle burner

In the framework of the European Comission funded project NGT, a FLOX burner with twelve nozzles was developed and tested, for application in micro-gas-turbines. The burner head is cylindrical with the circular nozzles (for partly premixed air and gaseous fuel) arranged around the periphery. For this particular arrangement and relation of area of injection to cross­‑sectional area of the combustor, the global recirculation ratio, as defined by Wünning, was estimated to be K_V,global≈1. A question that cannot be overlooked, when the application is gas turbine engines, is the operation at partial load. Obviously, the most direct way to change the load is to adjust the mass flow rate through the injectors. However, it was found, for the above mentioned burner, that injection velocity, V_j, plays a role on the stability of the combustion process, with extinction occurring for low velocities (37 ms^‑1 for the ensemble of situations tested). An alternative approach for operating at partial load, while maintaining a sufficiently high injection velocity, is to keep active just a part of the total number of nozzles on the burner head. This alters the relation of area of injection to cross­‑sectional area of the combustor and hence the global flow pattern is altered and the global recirculation ratio is expected to increase. The paper presents the results of a computational study in which the aforementioned burner is operated with fewer active nozzles, to assess the feasibility of this approach in addressing partial load operation. The CFD code FLUENT 6.3 is employed and, based on a previous study, turbulence is described with the Realizable variant of the k-ε model. The changes in the flow pattern are identified, especially in what regards the central and peripheral recirculation zones. The global K_V is estimated and compared to the original twelve-nozzle configuration. Results show that K_V can be increased It is shown that, for the isothermal situation, K_V,global does not depend on V_j, at least for the nominal range of operation of this burner. The results show that K_V is greatly influenced by the area ratio α_n=A_chamber/A_injection. The relation presented is valid for other burner configurations based on jet entrainment to achieve dilution of the reactants, and can therefore be used to increase the global K_V of such burners.