The tasks performed can be divided into the following categories: an analysis of the electric arc phenomena, especially near the electrodes; a parametric study of arcjet performance by means of a computer code (ARCFLO) and verification with experimental data where possible; the development of a data acquisition system to collect the above experimental data using Ames arc-jets; and a study of the critical components (electrodes and constrictor disks) and suggestions of how to improve their performance

Durgapal, Prabha

Shepard, Charles E.

English

NASA Technical Reports Server

NASA CONTRACTOR REPORT
NASA-CR-195821
ARC-HEATER PERFORMANCE RESEARCH
Charles E. Shepard
Prabha Durgapal
Eloret Institute
3788 Fabian Way
Palo Alto, CA 94303
Prepared for
Ames Research Center
under Cooperative Agreement NCC2- 688
NASA
National Aeronautics and
Space Administration
Ames Research Center
Moffett Field, California 94035
(NASA-CR-195821) ARC-HEATER
PERFORMANCE RESEARCH Final
Technical Report, 1 Apr. 1990 - 31
Jan. 1994 (Eloret Corp.) 9 p
N94-32774
Unclas
G3/77 0008442
https://ntrs.nasa.gov/search.jsp?R=19940028268 2020-06-16T12:04:40+00:00Z
NASA CONTRACTOR REPORT
ARC-HEATER PERFORMANCE RESEARCH
Charles E. Shepard
Prabha Durgapal
CONTRACT NAS2-
NASA
1. Report No. 2. Government Accession No.
4. Title and Subtitle
ARC -HEATER PERFORMANCE RESEARCH
7. Author(s)
Charles E. Shepard and Prabha Durgapal
9, Performing Organization Name and Address
Eloret Institute
3?88 Fabian Way
Palo Alto, CA 9^303
12. Sponsoring Agency Name and Address
National Aeronautics and Space
Administration, Washington, B .C . , 20^56
3. Recipient's Catalog No.
6. Report Date
29 April 199^
6. Performing Organization Coda
8. Performing Organization Report No.
10. Work Unit No.
1 1 . Contract or Grant No.
NCC2-688
13. Type of Report and Period Covered
Vl/90 to 1/31/9^
14. Sponsoring Agency Code
15. Supplementary Notes
Point of Contact: Charles E. Shepard and Prabha Durgapal
c/o 229-4 NASA Ames Research Center, Moffett Field, CA 9^ 035
16. Abstract
The tasks performed can be divided into the following categories:
"l>.An analysis of the electric arc phenomena, especially near the electrodes.
2. A parametric study of arc-jet performance by means of a computer code
(ARCFLO) and verification with experimental data where possible.
3. The development of a data acquisition system to collect the above experimental
data using Ames arc-jets;
rv '. 4. A study of the critical components (electrodes and constrictor disks) and
suggestions of how to improve their performance. The investigators have been successful
in completing these tasks as described below.
17. Key Words (Suggested by Author(sl)
arc-heater
constrictor disks
arc- jet
radiative heat flux computation
19. Security Qassil. (of this report)
unclassified
18. Distribution Statement
unclassified, unlimited
20. Security Classif. tot this page) 21. No. o
unclassified
1 Pages 22. Price*
'For sale by the National Technical Information Service, Springfield, Virginia 22161
ARC-HEATER PERFORMANCE RESEARCH
*
* •
-V*-.
:< ••,
H
Final Technical Report
for
Cooperative Agreement NCC2-688
for the period
April 1, 1990-January 31, 1994
S ub mi tied to
*ML{J&'^ / .
National Aeronautics and Space Administration
'-t "' " Ames Research Center
',v ^ %' Moffett Field, California 94035
/ " - :. -f
. _.. •- ^
* "V k^ ' * Thermosciences Division, James O. Arnold, Chief
'
Thermophysics Facilities Branch
Joseph Hartman, Chief and Technical Officer
Prepared by
ELORET INSTITUTE
3788 Fabian Way
Palo Alto, CA 94303
Phone: 415 493-4710
Telefax: 415 424-9876
K. Heinemann, President and Grant Administrator
Charles E. Shepard, Principal Investigator
Prabha Durgapal, Co-Principal Investigator
29 April, 1994
This final report for NASA-Ames/Eloret Institute Cooperative Agreement NCC2-688 is
being submitted in two parts. Part A describes work performed by Principal Investigator
Charles E. Shepard and Research Assistants Thinh M. Ha and Jaswinder S. Taunk,
and Part B focuses on work by Co-Principal Investigator, Dr. Prabha Durgapal.
Part A
Abstract
The tasks performed can be divided into the following categories:
1. An analysis of the electric arc phenomena, especially near the electrodes.
2. A parametric study of arc-jet performance by means of a computer code
(ARCFLO) and verification with experimental data where possible.
3. The development of a data acquisition system to collect the above experimental
data using Ames arc-jets.
4. A study of the critical components (electrodes and constrictor disks) and
suggestions of how to improve their performance. The investigators have been successful
in completing these tasks as described below.
Description of Completed Work
The long-term goal of the Ames Arc Jet Research Group has been the development of a
"next generation" arc jet capable of higher performance (stagnation enthalpy and/or higher
pressure) with a much higher net stream power input. Although the final arc-jet
configuration will depend on the enthalpy - pressure level that is chosen, many problems
that are common to any configuration have been studied. These include the following:
1. The successful design, acquisition, and test of a data acquisition system suitable
for arc-jet research. This system is described in "Analytical and Experimental Analysis of
an Electric Arc Jet" by Jaswinder Singh Taunk.
2. Arc-Jet performance calculations utilizing the NASA/Ames ARC-FLO computer
code (see ref. (1)).
3. The correlation of the above calculations in terms of arc-jet operating
parameters. An example is "A Sonic Flow Equation for Electric Arc Jets6 by Charles
Shepard and Jaswinder Taunk of Eloret and Frank Milos of NASA/Ames.
4. In the course of the above correlation, it was found that the swirl (given to the
air that is injected along the constrictor column) has an effect on the sonic flow
relationship. This effect is quite large (approx. 30 %) for cold-air flow7.
5. An analytic study of the critical downstream multiple electrode (cathode) region.
Results of these studies by Prabha Durgapal are presented in Part B of this report.
6. A preliminary study of the arc column radiation and heat flux was made. The
results of this study are reported in a paper in preparation entitled "Spectral Measurements
in the Arc Column of an Arc-Jet Wind Tunnel"8 by Imelda Terrazas-Salinas, Chul Park,
Tony Strawa of NASA-Ames and Nigel Gopaul of Eloret Institute. This paper will be
presented at the AIAA Aerospace and Testing Conference at Colorado Springs, CO, June
1994.
7. The above experiment required the design and construction of a special
"window" in the arc-jet constrictor. This water-cooled copper disk is described in a
proposed paper entitled "Design and Verification of a Copper Disk for Radiation
Measurements in the constrictor Region of an Arc Jet" by Jaswinder Taunk and Charles
Shepard of Eloret Institute and Armando Carrasco of NASA-Ames. This paper will be
presented at the 40th International Instrumentation Symposium in Baltimore, MD, May
1994.
8. The problem of obtaining equal division of the arc-current for
multiple-electrodes was studied. The results were reported in a memo to Mr. Jerry
Mitvalsky of NASA-Ames10. A series of experiments with the Ames Panel Test Facility
Arc Jet confirmed the finding that the upstream electrode that is furthest from the
constrictor is most sensitive to changes in operating conditions. It was found that the set
of ballast resistors must have a relatively high resistance (greater than about 0.5 Ohm) if
good current balancing is to be maintained.
9. The next-generation arc-jet (capable of much higher power input and higher
performance) will require better water-cooling of the constrictor disks. The present
Maecker cooling design uses a the centripetal force induced by the cusped cooling water
geometry to sweep away and collapse the steam bubbles and thereby avoid burn-out.
Previous heat-conduction analyses indicate that the present working limit of 10 kw/cm2
can be increased to about 20 kw/cm2. A new 3-dimensional polar coordinate Fortran
computer code is being developed to permit more accurate heat flux calculation by
including the effect of a finite constrictor disk thickness.
10. The most critical problem is the improvement of the electrodes that will permit
stable operation at much higher arc current limits. If the pressure is kept below 10
atmospheres, a tungsten cathode could be used. This would be a much simpler electrode.
If the incandescent tungsten were shielded by argon, a single tungsten rod could carry as
much current as the PTF four-electrode package. Multiple rods could carry the required
total current of future arc jets and would be much cheaper and easier to maintain. A small
single tungsten rod and water cooled anode has been set up in Bldg. 238. A brief
discussion of alternative electrodes is included in short paper presented at the October 7,
1992 Arc Jet Research Meeting.' ' Multiple element copper anodes are an alternative
electrode as suggested in an AIAA paper, "Thermal Analysis of an Arc Heater Electrode
with a Rotating Arc Foot" by F.S. Milos and C.E. Shepard.12
PARTB
Abstract
A theoretical model of the electrode phenomena, leading to the computation of the
current density distributions, arc spot temperature, spot size, sheath voltage, ablation
velocity, ablation rate and arcfoot rotation frequency, is put forward. Computations for an
atmospheric arc, at load currents ranging from 800 to 3000 amp, in 1) one segment and 2)
4 segment electrode configurations, are conducted1. In another paper computations are
performed for operating pressures in the range 50-200 atm and load currents in the range
6000-18000 amps2. At high pressures spot size remains reasonably small, even for high
load currents. Smaller spot size may allow us to design thinner electrodes. However
higher load currents lead to high spot temperatures and this can cause ablation of the
electrode material. In order to reduce material ablation at very high currents and prolong
electrode life, the operating pressure must be high and the arc should be rotated.
Radiation and Joule heating in the electrode region of an arc heater are discussed.
Radiative transport equations for a true axisymmetric geometry are used. An expression
for the divergence of radiative heat flux is computed analytically3. A subsonic code is
developed to numerically solve the fluid equations with strongly coupled radiation and
Joule heating, representative of a high pressure arid high current arc heater4. The Joule
heating term is computed using the electromagnetic code. The equilibrium gas model
consists of seven species. The fluxes are differenced using Van Leer flux splitting. Using
this code the effects of radiative cooling on the thermodynamic parameters of the arc core
are discussed.
Objective
The purpose of this work is to study the operation and design of the ring electrodes used
in segmented arc-heaters. The overall objectives of this study are:
(1) to develop a physical understanding of the behavior and dynamics of the arc near an
electrode, and of the arcfoot on the surface of an electrode, in a segmented arc heater;
(2) to develop the computational capability to simulate and predict the arc behavior on
the surface and in the vicinity of an electrode; and
(3) to develop engineering design criteria that will assist in the design of efficient and
long-life electrodes. The results of this study will provide the basis for improved
electrode designs that will support higher power operation.
Description of the problem
Ames Research Center is currently operating a variety of arc-heater facilities. In an arc
heater, as the name suggests, the gas is heated by means of an electric arc. Of interest
here is the segmented arc heater. The main components of a typical facility include a
constricted arc heater, several interchangeable Mach number nozzles, a walk-in test
chamber, and ancillary subsystems consisting of a steam-ejector vacuum system, cooling
water system and automated data acquisition system. The two electrodes are located on
either side of the constrictor. The anode forms the upstream electrode and the cathode
forms the downstream electrode. An electrode package consists of multiple rings that are
the actual electrodes and each active electrode ring is insulated from the others by
insulator rings and is individually ballasted. Each ring electrode has a given current
rating. Electrodes are added to the stock until the number of electrode rings can handle
the total current expected in the arc column. With proper adjustment of the individual
ballasts the electrode rings can be made to share current more or less equally. For the
purpose of this study it is assumed that future high pressure and high enthalpy arc jets
will have similar configurations with variations in electrode number and size.
An electric arc is a discharge of electricity between electrodes in a gas or vapour, with
characteristic cathode or anode voltage drop at the electrodes. An arc at atmospheric
pressure and above is characterised by a small intensely brilliant core surrounded by a
comparatively cooler region of flaming gases. The core is at such high temperatures that
all gases are completely dissociated. The current density at the cathode of an arc is very
large and is practically independent of the arc current. It depends only on the material of
the cathode, specially on its thermionic emission characteristics. The electrode material
most commonly used is copper or copper alloy with high thermal conductivity to assure
an adequate heat transfer rate capability.
Due to the high temperature of the arc spot, the electrode material at that location melts or
sometimes instantneously vaporizes releasing electrode material and contaminating the
flowing gas. Methods to reduce contamination from electric arc include:
(1) arcfoot rotation by vortex generation or magnetic field interaction.
(2) selecting electrode material that can attain high levels of thermionic emission at
relatively lower spot temperatures.
(3) selecting electrode material with high melting point.
The primary limitations to the design of an arc heater, in excess of 60 MW power, have
been 1) arc control, 2) downstream electrode (cathode) erosion.and 3) radiation loss. As
NASA embarks on the design of a 300 MW heater, it is imperative that these phenomena
are well understood and brought under control. In two papers^1'2) the electrical
characteristics in the downstream electrode region (cathode) of an arcjet were
investigated at high temperatures and high pressures. At high temperatures radiative
energy transfer becomes an important mode of heat transfer together with conduction and
convection. For these conditions the energy equation of the flowfield must include the
Joule heating term, together with the radiation term. The final code developed using the
present work will be validated against data obtained from existing arc jets and will later
be used for developing and designing future high pressure, high current arc jets.
Technical Approach and Accomplishments
The physical model of the various processes occuring in an arc heater is being developed
in a step by step procedure. These steps are:
A. F.lectricfll characteristics in the downstream cathode region of the arc iet: The stream
function formulation of the current density in the axisymmetric flow field of the
downstream cathode region, based on Maxwell's equations for given thermal properties,
are derived^1'2). These equations together with the necessary boundary and subsidiary
conditions for the electrode region are solved using Gauss-Seidel successive relaxation
technique. The results of this analysis provide the current density distribution in the
downstream cathode region. It also provides the electric field at the location where the
arcfoot terminates. A simple heat transfer model was also developed for use with this
code^1-2). Thermal properties of the region under consideration are determined using this
model.
B. Arcfoot parameters: A model was developed to compute arcfoot parameters^1'2). These
include arcfoot spot size, spot temperature, cathode voltage drop, ablation velocity, rate
of material loss when the spot temperature is greater than the melting point of the
electrode material, and arcfoot rotation frequency. These are determined by analysing the
ionization and thermionic emission processes in the electrode region. Arcfoot rotation
frequency is determined by balancing the magnetic force against the aerodynamic drag.
Arcfoot rotation frequency is compared with recently acquired experimental data. Results
are provided for an atmospheric arc with load current in the range 800 to 3000 A in Ref.
1, and for high pressure (50-200 atm) and high current (6000-18000 A) arcs in Ref. 2.
C. Radiative Heat transfer: An expression for the divergence of the radiative heat flux is
computed analytically in axisymmetric geometry3. The radiation term is strongly
coupled4. For this the Jacobians with respect to the species density and temperature are
needed. The derivative of the divergence of radiative heat flux with respect to the density
was also computed analytically. In reference 3, the expression for the radiative heat flux
is rendered analytically differentiable/integrable by using appropriate techniques to
remove the singularities. This makes it possible to obtain the expression for the
divergence of radiative heat flux analytically. This leads to considerable reduction in
computation time.
D. Joule Heating: At high temperatures radiative energy transfer becomes an important
mode of heat transfer together with conduction and convection. For these conditions the
enegy equation of the flowfield must include the Joule heating term together with the
radiation term. A subsonic code is developed to numerically solve the fluid equations
with strongly coupled radiation and Joule heating terms, representative of a high pressure
and high current arc heater. The Joule heating term is computed using the previously
developed electromagnetic code^1'2). The equilibrium gas model consists of seven
species. The fluxes are differenced using Van Leer flux splitting. Using this code4 the
effects of radiative cooling on the thermodynamic parameters of the arc core are
discussed.
PUBLICATIONS and PRESENTATIONS
1. Prabha Durgapal, "Current Distribution in the cathode Area of an Arc Heater," AIAA
Journal of Thermophysics and Heat Transfer, 7, 241, 1993; also AIAA-Paper 91-1385,
1991.
2. Prabha Durgapal, "Electrode. Phenomena in High Cu rrent, High Pressure Arc
Heaters," AIAA Journal of Thermophysics and Heat Transfer, 7, 412, 1993; also
AIAA-Paper 92-0810, 1992.
3. Prabha Durgapal, Radiative Transfer in the Electrode Region of an Arc Heater,"
AIAA-Paper 92-2872; presented at the 27th Thermophysics Conference, Nashville, TN,
1992.
4. Prabha Durgapal and G.E. Palmer, Strongly Coupled Radiative Transfer and Joule
Heating in the Electrode Region of an Arc Jet," AIAA-Paper 93-2801; presented at the
28th Thermophysics Conference, Orlando, 1993.
5. Jaswinder S. Tauf\k,"Analyrical and Experimental Analysis of an Electric Arc Jet," MS
Project Report, San Jose State University.
6. Charles E. Shepard, Frank S. Milos, and Jaswinder S. Taunk,"/! Sonic Flow Equation
for Electric Arc Jets," AIAA-Paper 93-3183.
7. Charles E. Shepard, Internal memorandum to John Balboni, NASA-Ames, Oct.6,1993.
8. Imelda Terrazas-Salinas, Chul Park, and Tony Strawa, "Spectral Measurements in the
Arc Column of an Arc Jet Wind Tunnel," AIAA paper, Colorado Springs, 1994, in
preparation.
9. Jaswinder S. Taunk, Charles E. Shepard, and Armando Carrasco, "Design and
Verification of a Copper Disk for Radiation Measurements in the Constrictor Region of an
Arc Jet," 40th International Instrumentation Symposium, Baltimore, MD, 1994, in
preparation.
10. Charles E. Shepard, Internal Memorandum to Jerry Mitvalsky and Wayne Harry,
NASA-Ames, Nov. 12,1993.
11. Charles E. Shepard, "New Electrodes for Arc Jets," Internal paper for ArcJet Research
Group, Oct. 7, 1992.
12. Frank S.Milos and Charles E. Shepard," 77? ermal Analysis of an Arc Heater Electrode
with a Rotating Foot," AIAA-Paper 93-2855, Orlando, FL 1993.


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