Characteristic time modeling of NOx emissions and combustion efficiency for a staged combustion ramjet

Abstract

The semi-empirical characteristic time model (CTM), in which algebraic equations based on first principles have been validated, primarily for turbine engines, for the correlation of combustor behavior (NOx and CO emissions, combustion efficiency, lean blowoff, and lean lightoff) is examined for its applicability to staged combustion ramjets. The CTM was chosen as some success has been observed in collapsing data from different turbine combustors and in predicting performance for other configurations using a single set of model constants. Its independent parameters are times characteristic of turbulent mixing, chemistry, and fuel spray evaporation. These times are expressed in terms of combustor geometry, inlet conditions, fuel properties, and injector design. Tests of the ramjet combustor were conducted at United Technologies Research Center with varying inlet temperature, pressure, equivalence ratio and combustion length, and the predictions are compared with the measurements. First, however, significant model development was conducted because of a number of design differences between the staged combustion concept and the configurations with which the model was originally validated. A method for computing the fraction of total air flow participating in combustion at the various fuel injection locations is postulated. The results obtained in this study are encouraging, particularly for NOx, and suggest additional tests required for staged combustion CTM development.