The objectives are to obtain temperature, density and velocity profile measurements in the expansion region of low Reynolds number nozzles through the use of optical diagnostics. An LIF system will be used to probe the expansion of a microwave-heated expansion in the Center vacuum facility. The experimental measurements made in this program will be compared to numerical predictions obtained by Drs. Charles Merkle and Lyle Long

Micci, Michael M.

English

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PRESENTATION 2.3.5
N91-28233
OPTICAL DIAGNOSTIC INVESTIGATION
OF LOW REYNOLDS NUMBER NOZZLE FLOWS
725
https://ntrs.nasa.gov/search.jsp?R=19910018919 2020-03-19T17:28:24+00:00Z

OPTICAL DIAGNOSTIC INVESTIGATION OF
LOW REYNOLDS NUMBER NOZZLE FLOWS
Michael M. Micci
Department of Aerospace Engineering
I. Research Objectives and Potential Impact on Propulsion
Small high performance chemical rocket motors and electric propulsion devices
operating at low power levels (<25 kW) suffer from high nozzle losses due to the low
Reynolds number of the exhaust flow. The Reynolds number with the diameter of the
nozzle throat as the characteristic dimension is:
puD
Re- I.t
DPo
To
As chemical and electric thruster research and development strives for higher
chamber temperatures to increase performance, the future trend for nozzles will be for
decreasing Reynolds numbers. Current methods for estimating rocket nozzle
performance assume a boundary layer along the interior wall with an inviscid core.
However at low Reynolds numbers the wall boundary layer fills most if not all of the
nozzle interior producing a large amount of viscous dissipation. Nozzle efficiency, the
ratio of the delivered specific impulse to the ideal specific impulse, has been
measured as low as 60% at a Reynolds number of 200. Computational techniques
are beginning to be able to examine such flows but the need exists for experimental
verification of both numerical results as well as design changes in existing hardware.
The majority of experimental measurements to date have consisted of thrust
and discharge coefficient, both global quantities which give little information about the
detailed physical processes occurring. Different studies looking for the optimal nozzle
wall contour for low Reynolds number flow among cones of various angles, bell-
shaped nozzles and trumpet-shaped nozzles, reached different conclusions. The only
detailed measurements of flow properties were taken by Rothe using an electron
beam diagnostic in a low pressure nozzle flow. Rothe measured density and
temperature as a function of axial and radial location in the nozzle and found that for
low flow Reynolds numbers the static temperature would rise in the nozzle expansion
due to viscous dissipation.
The objectives of this program are to obtain temperature, density and velocity
profile measurements in the expansion region of low Reynolds number nozzles
through the use of optical diagnostics. An LIF system will be used to probe the
expansion of a microwave-heated expansion in the Center vacuum facility. The
experimental measurements made in this program will be compared to numerical
predictions obtained by Drs. Charles Merkle and Lyle Long.
727 PRECEDING PAGE BLANK NOT FILMED
II. Current Status and Results
The majority of the effort expended to date in this program has been involved in
the build-up of the Center vacuum facility. The vacuum chamber has a 1 meter inside
diameter and is 5 feet long. Figure 1 is a diagram of the vacuum chamber with pump,
microwave gas heater and LIF system. The Stokes mechanical pump and the Stokes
diffusion pump were installed and connected with their requisites utilities, electricity,
cooling water and exhaust vents, this past year. To date, the pumps have achieved a
minimum pressure of 1.9 .10 .4 Torr with no mass flow through the system. This
corresponds to an altitude of 101 kin. The diffusion pump has a zero flow rate
specification of 10 .4 Torr. Small leaks through the mounting threads of the two
pressure transducers prevented a lower pressure from being achieved. Since both
these pressure transducers were in temporary positions above the diffusion pump and
are being relocated in the vacuum chamber, an extensive effort to seal these threads
was not undertaken. However, new fittings for these transducers which will provide
better high vacuum sealing are being procured.
With the check-out of the diffusion pump completed, the vacuum chamber has
been mated with the diffusion pump.
A tunable laser system for use as a pump source for an LIF system has been
identified. A Quanta-Ray pulsed Nd:YAG laser will be frequency doubled and used as
the pump for a Quanta-Ray pulsed dye laser. This will provide an output at
wavelengths of 654-647 nm for use in pumping the First Positive system of N2. The
output of the dye laser will be frequency doubled again to reach the 337 nm
wavelength to excite the Second Positive system of N2. Nitrogen has been chosen
because its molecular nature will allow the examination of nozzle frozen flow losses
due to nonequilibrium effects.
The LIF system will be used to obtain simultaneous measurements of
temperature, density and velocity in the expansion flows from low Reynolds number
nozzles. Density will be obtained from the absolute intensity integrated over the entire
fluorescing line. An Optronics calibrated tungsten strip lamp has been procured and
has been used successfully for emission spectroscopy absolute continuum
measurements of the electron temperature in a microwave-heated helium plasma.
Temperature of the heavy particles (neutrals and ions) will be obtained by measuring
the Doppler broadened linewidth of fluorescing lines. Figure 2 shows the amount of
Doppler broadening (including Stark broadening) for the Second Positive system of
N 2 as a function of temperature. An electronically tunable Fabry-Perot etalon has
been procured this past year which when combined with the Spex 0.5 meter
monochromator which was purchased with funding from a prior grant should give a
frequency resolution of 0.004 Angstroms.
The velocity of the gas will be determined from the Doppler shift of the
fluorescing lines. Figure 3 plots the Doppler line shift of the 337 nm line of the Second
Positive system for N2 as a function of velocity. It can be seen by comparing Figures 2
and 3 that the Doppler line shift is much greater than the Doppler broadening, thus the
broadening will not interfere with the velocity measurement.
728
Since the laser system is to be procured during the next fiscal year, the Fabry-
Perot etalon is initially being used to measure the Doppler broadening and line shift of
emitting lines to determine the temperature and velocity of a microwave-heated
plasma expanding to atmospheric pressure and a velocity of Mach 1. Figure 4 is a
drawing of the experimental system being used to make these measurements. The
emitted light from the exhaust plume will be collected in the direction of the velocity
measurement and compared to light collected in a direction where the gas has no
directed velocity to determine the Doppler shift. The Fabry-Perot etalon is used as a
very narrow bandwidth optical filter.
PUMP
VACUUM
CHAMBER
f
I LASER
I I I_LASMA
,-1 2
Fig. 1 Diagram of experimental facility to optically examine nozzle flows.
729
Fig. 2
0.17
°'16t
O.lS- 
_ 0.14 t
0.12t
O.ll
I
0 10000 2O0O0
Temperature (K)
Doppler line broadening for the 337 nm line of N2 as a function of
temperature.
0.3
Fig. 3
A
o
u 0.2o
.,J
o
_¢
,.J
c
_" 0.1
i
L
0
Q
0.0
!
10000
Velocity (m/s)
20000
Doppler line shift for the 337 nm line of N2 as a function of velocity.
780
MICROWAVE
RESONANT
CAVITY
PMT I
0.5 METER 1
MONOCHROMATOR _..,,j-- ETA,ON_--_ _
BEAM MIRROR
SPLITTER
Fig. 4 Experiment to use Fabry-Perot etalon to measure Doppler line
broadening and Doppler line shift of emitting microwave-heated
plasma.
731
III. Proposed Work for Coming Year
The Stokes mechanical and diffusion pumps will be operated with the vacuum
chamber attached and the minimum obtainable pressure as a function of mass flow
rate and gas composition into the chamber will be measured. Early success with no
gas flow through the system indicates that a good vacuum should be obtainable with
experimentally significant mass flows. A fitting for the high vacuum pressure
transducer which seals by the use of two O-rings instead of thread sealant is being
procured and should solve the leakage problems experienced during the pump
testing.
The interim tunable laser system being shared with Dr. Santoro for the LIF
diagnostic should be in place in an adjacent laboratory by July 1, 1990. A system will
be designed and constructed to divert the output beam through an opening in the wall
between the laboratories into the window of the vacuum chamber which is located
approximately nine feet above the laboratory floor. The laser beam will be
intentionally reflected just prior to entering the window in order to aim it at the location
of interest in the chamber. Additional windows are located on the chamber at
positions opposite and perpendicular to the window through which the beam is
entering while the nozzle exhaust is directed along the axis of the chamber.
The operation of the electronically tunable Fabry-Perot etalon as a very narrow
bandwidth optical filter to measure Doppler broadened linewidths and Doppler shifts
will be tested in an emission spectroscopy system. Gas will be heated to temperatures
between 10,000 and 13,000 K in a microwave resonant cavity and accelerated to
sonic velocities by expanding through a converging nozzle to atmospheric pressure.
The principle disadvantage of emission spectroscopy compared to LIF is that emission
spectroscopy is a line of sight measurement whereas LIF is a point measurement.
Thus the initial results with the Fabry-Perot etalon will be somewhat degraded
because of the averaging taking place through the measuring volume.
A circular microwave waveguide capable of applying the full 2.5 kW which is
available to heat a gas is scheduled for delivery early in the grant year. A rectangular
waveguide was used in prior experiments, however a rectangular waveguide
generates a plasma which is attached at two locations to the waveguide walls. In
order to generate a plasma which is not in contact with the waveguide walls, a circular
waveguide was designed which will generate a plasma which is located along the
waveguide axis. It will initially be tested with a choked converging nozzle expansion
to atmospheric pressure and then integrated with the end flange of the vacuum
chamber for expansions through converging-diverging nozzles to low pressures.
Finally, the use of LIF to measure density, temperature and velocity in the
supersonic exhaust region of a microwave-heated gas flow will be evaluated. The
laser will be used to excite the gas and the Fabry-Perot etalon in conjunction with the
0.5 meter monochromator will be used to measure the florescence signal. Spatial
profiles of density, temperature and velocity in the radial direction will be measured.
Initial nozzle geometries will be conical and property variations in the axial direction
will be obtained by testing with nozzles of varying expansion ratios. Eventually,
different nozzle contours such as bell and trumpet shaped will be tested.
J
"/32


Optical diagnostic investigation of low Reynolds number nozzle flows

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