(312b) Temperature and Number Density Measurements Using Raman Spectroscopy in Supersonic, Turbulent, Combusting Flows | AIChE

(312b) Temperature and Number Density Measurements Using Raman Spectroscopy in Supersonic, Turbulent, Combusting Flows

Authors 

Jeyashekar, N. S. - Presenter, National Center for Physical Acoustics, University of Mississippi
Seiner, J. M. - Presenter, National Center for Physical Acoustics, University of Mississippi


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.