Scalar Characteristics of Combusting Flow in a Model Annular Combustor

Temperature and species concentration measurements have been obtained in a model combustor operating at an inlet temperature of 515 K and atmospheric pressure and are reported and discussed. The model comprises two rectangular sectors representing the primary and upper dilution zones of an annular combustor used in small gas-turbine engines. Natural gas (94 percent CH₄) was used as fuel and was delivered through a T-vaporizer at rates that led to air-fuel ratios of 29 and 50, similar to those of take-off and ground-idle conditions, respectively. Temperatures were obtained at the exit of the combustor using fine-wire thermocouples and mean concentrations of major species were obtained in the primary zone and at the exit on a dry basis by gas sampling and analysis. The results show that the 200 K increase in inlet air temperature reduces the pattern factor from 0.55 to 0.3 and increases the combustion efficiency from 69 to 94 percent with the air-fuel ratio of 29. The higher air-fuel ratio improves the combustion efficiency to 97.6 percent but results in a worse pattern factor of 0.48. The results confirm the need for consideration of the rate-controlled CO → CO₂ reaction in the dilution zone if CO emission is to be calculated correctly and temperatures are to be within 150 K. Examination of temperatures obtained from a local enthalpy balance shows that they are higher than measurements obtained with preheat, in contrast to a similar comparison without preheat.