Flow velocity, static temperature and total density are required to model a transport process. In supersonic combusting flows, turbulence-chemistry interaction plays a crucial role, via mixing, in improving the combustion efficiency, for aerospace applications. The use of intrusive measurement tools results in the formation of shocks in supersonic flow. In a combusting environment, the probe material has to remain inert over high temperatures and should not catalyze the reaction. For such purposes a non-intrusive tool must be used. This paper describes the formalism and experimental-setup of a non-intrusive tool to measure static temperature and number density of species in turbulent, supersonic combusting flows. The total density can be computed from the number density of species. For future work, this measurement tool can be combined with a velocity measurement tool, such as PIV, to obtain all three variables in the transport equation.
The formalism for measurement is based on the principle of Raman scattering. Light incident on a sample volume, under observation, is scattered in all directions. The intensity of the scattered light collected at 900 angle, contains the intensity at wavelength corresponding to the incident light and intensities corresponding to the molecules present in the sample volume. The wavelengths for the molecules are related to the vibration frequency of the molecule. The scattered intensities of the molecules are indicative of the temperature and number density in the sample volume. The probe volume has to be chosen such that there are no gradients in the properties of the flow and for instantaneous measurements, in turbulent flows, time interval over which measurements are made, must be less than the time scale of the smallest eddy (kolmogrov eddy).
The experimental set-up consists of a frequency quadrupled high powered Nd:YAG laser as the incident light source with 150 mJ/pulse, 8.5 ns per pulse with 10 Hz repetition rate. The scattered light is collected by a 750mm focal length monochromator coupled to an Intensified Charge Coupled Device (ICCD) camera. The temperature and number density of the molecules in the sample volume is determined from the integrated intensity of the collected light. Measurements were made in a Mach 2 supersonic plume, with the central core of air, heated to a temperature sufficient for auto-ignition of the fuel, which is a co-flow of hydrogen. Combustion occurs in a shear layer around the central supersonic core. The situation applies to after burning rockets which employ hydrogen as its fuel. Temperature and number density measurements have been made at five downstream locations in the combusting shear layer of the jet. The temperature and number density statistics at a probe volume with time and its deviation from adiabatic/theoretical flame temperature gives the degree of mixing, extent of reaction and combustion efficiency in the probe volume. This will help in future study to implement design modification of the supersonic nozzle, re-circulate hydrogen to improve mixing and combustion efficiency. Another experimental aspect of the future study would be to implement PIV to determine flow velocity with the thermodynamic quantities, to completely describe the supersonic transport process and study finite-rate chemistry effects.