Mixture fraction and scalar dissipation rate imaging in turbulent nonpremixed flames near extinction

This dissertation describes the development of a unique diagnostic technique and a data processing routine that was developed to accurately measure all important scalars and the scalar gradient in a turbulent reacting flowfield near extinction. Along with the measurements of a conserved scalar and temperature, which yield mixture fraction and scalar dissipation rate, all relevant species also are determined during post-processing. This processing algorithm then allows for the determination of the reaction rate of the fuel (CO).; Scalar dissipation rate (chi) previously has been identified as one of the most important parameters in the theory and models of turbulent nonpremixed combustion. It characterizes the rate of molecular mixing within the turbulent flow-field; therefore, it is of interest to measure and compare the experimental data to various analytical and computational models. There has been previous evidence that chi plays an important role in the local extinction pattern in turbulent nonpremixed flames. Theories such as flamelet extinction rely on the quantification of a "critical" scalar dissipation rate for determining local and global extinction. Previous techniques for measuring mixture fraction scalar dissipation rate in turbulent nonpremixed flames are inadequate near extinction because of the inherent assumptions required to use the techniques. The present dual-laser technique does not suffer from any such limiting parameters and can be applied in the current turbulent flame near extinction.; A trace amount of nitric oxide (NO) was added to the fuel, and it is demonstrated that the NO mass fraction is a conserved scalar. PLIF detection methods were used to determine the mixture fraction and scalar dissipation rate fields. The use of dry CO as the fuel eliminates the unwanted CH and H radicals that can cause the unwanted destruction of NO. Measurements of scalar dissipation rate are obtained in two turbulent nonpremixed flames having different levels of extinction.