| Abstract: |
The problem of measuring mixture fraction (defined here as the fractional part of mass originating from the exhaust gas jet) during the mixing of an unsteady hot exhaust gas jet impinging into a quiescent premixed hydrogen/air mixture is addressed. The diagnostic method is based on planar Laser-Induced Fluorescence of nitric oxide (NO) that is seeded to the hot jet and used as a tag for mixture fraction. The research work presented in this paper focusses on the analytical and experimental data employed for the estimation of mixture fraction using NO as a tracer. Since the NO LIF-signal does not solely depend on the amount of NO, but also on temperature and overall chemical composition, additional information is normally required to transform LIF-signals into mixture fraction values. To provide this additional information, the concept of reduced state spaces is applied: Correlations between state variables (temperature and species) are exploited to establish relationships between the measured signal and the mixture fraction. Simple numerical model simulations are used in combination with spectroscopic calculations to check for correlations between NO-LIF signal and mixture fraction. The result is that sharp correlations between NO-LIF signals and mixture fraction exist, provided that chemical reactions have not yet significantly influenced the flow. These correlations display only little sensitivity with respect to changes of the jet temperature, amount of NO seeded to the jet, and jet velocity. Measurements of NO-LIF were then performed for a non-reacting case before ignition, and mixture fraction maps were obtained from the measured LIF-signals by exploiting the correlations. The resulting data reveal the different driving forces behind mixing at regions near to the jet tip and at the radial shear layers. |