The primary objective is to understand how buoyancy affects the structure of the shear layer, the development of fluid dynamic instabilities, and formation of the coherent structures in the near-nozzle regions of gas jets. The secondary objectives are to study the role of buoyancy in lifting and reattachment process of diffusion flames, to evaluate the scaling behavior of diffusion flames, and to aid development and/or validation of theoretical models by providing quantitative data in the absence of buoyancy. Fast reacting hydrogen or hydrogen-inert fuels are used to isolate the effects of buoyancy on fluid dynamics without masking the flame behavior by soot and radiative heat transfer. This choice of fuel also permits an evaluation of simulating low gravity in low pressure ground experiments because the similarity constraints are relaxed for the fast reacting, nonsooting diffusion flames. The diagnostics consists primarily of a color schlieren system coupled with computer generated rainbow filters, video recording, and image analysis. The project involves (1) drop tower experiments, (2) ground experiments, and (3) theoretical analysis

Agrawal, Ajay K.

Gollahalli, Subramanyam R.

Griffin, Devon

English

NASA Technical Reports Server

N96- 15600
EFFECTS OF ENERGY RELEASE ON
NEAR FIELD FLOW STRUCTURE OF GAS JETS l
Ajay K. Agrawal and Subramanyam R. Gollahalli
School of Aerospace and Mechanical Engineering
University of Oklahoma, Norman, OK
DeVon Griffin
NASA Lewis Research Center, Cleveland, Ohio
Introduction
The interaction of flames and flow structures in combustion systems has been the subject of several investigations
in recent years [1-6]. The phenomena occurring in the near-field of a gas jet determine the burning characteristics
and pollutant generation from its diffusion flame. The diffusion of the oxidizer near the nozzle exit governs the
flame stability and flame lift-orE Faminmem of the oxidizer into the fuel jet is controlled by the Kelvin-Helmholtz
instabilities in the shear layer between the fuel jet and ambient fluid, and by the buoyancy driven instabilities
outside the flame. Thejet growth and fuel-air mixing rates depend upon these near-field fluid dynamics which could
be modified to alter the energy release and pollutant emission characteristics of diffusion flames. Thus, an
understanding of the near-field structures of jet flames and physico-chemical factors affecting them is important
for impro .,4.rigthe combustion characteristics of cliff.ion flames.
Figure 1. taken from Savas and Gollaimlli [3] shows the schlieren photographs of nonburning and burning propane
jets at the same nozzle exit Reynolds number of approximately 13,000. These photographs clearly show that the
flame inhibits coalescence of small scale coherent structures and hence, suppresses the radial growth of the shear
layer. The lengthening of the coherent structures has been attributed to the changes in the jet fluid viscosity [1],
volummetric expansion [7], and the stretching effect of buoyancy [8]. The buoyancy associated with ground
experiments has prevented a clear identification of the mechanisms responsible for this effect. The buoyancy also
generates vortical structures outside the flame resulting in the flame flicker [9].
The primmy objective of this research is to understand how buoyancy affects the structure of the shear layer, the
development of fluid dynamic instabilities, and formation of the coherent structures in the near-nozzle regions of
gas jets. The secondary objectives are to study the role of buoyancy in lif_g and reattachment process of diffusion
flames, to evahtate the scaling behavior of diffusion flames, and to aid development and/or validation of theoretical
models by providing quanu'tative data in the absence of buoyancy. Fast reacting hydrogen or hydrogen-inert fuels
are used in this study to isolate the effects of buoyancy on fluid dynamics without masking the flame behavior by
soot and radiative heat transfer. This choice of fuel also permits an evaluation of simulating low gravity in low
pressure ground experiments because the similarity constraints are relaxed for the fast reacting, nonsooting
diffusion flames. The diagnostics consists primarily of a color schlieren system coupled with computer generated
rainbow filters, video recording and image analysis.
The project involves (i) drop tower experiments (ii) ground experiments, and (iii) theoretical analysis. The
following sections describe the work accomplished to-date and the plans for future work of this project.
This project was started in June 1994
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https://ntrs.nasa.gov/search.jsp?R=19960008434 2020-06-16T05:20:51+00:00Z
Drop Tower Experiments
The apparatus for this series of experiments, shown in Figure 2, was developed by C.aiffin and Greenberg [10]. This
_msim of a color schlieren system mounted on a drop module for studies in the 2.2 second drop-tower at NASA
Lewis Research Center. The schlieren system is based on 75 mm diameter, off-axis parabolic mirror optics. The
white fight is provided by a fiber-optics cable connected to the light source either within the module or outside at
the control platform. The light merging fiom a 50 micron slit is collimated by a 300 mm focal length lens and
decollimated by a 3000 ram focal length lens combination. A computer generated rainbow filter is used at the light
source image to color code the ray deflections. The schlieren image is captured by a CCD color camera and
transmitted fiber optically to a S-VHS video recorder and a TV monitor at the control platform.
The fuel supply system consists of a 75 ml fuel tank at 5 bar, a flow meter, a solenoid activated flow control valve,
associated tubing, and an interchangeable fuel-tube. The fuel was ignited prior to the drop by a retractable hot-wire.
The apparatus was controlled either manually or by an inboard computer. Prior to the drops a pre-btma was done
to ensure proper functioning of the system. The schlieren images were recorded during the main run at both the
normal and low gravity conditions.
Ground Experiments
The ground tests consist of experiments at atmospheric and sub-atmospheric pressures. Penner [11] found that no
practical combustion process can be exactly modeled in a strict sense of similarity. This prompted Spalding [12]
to suggest partial modeling where only a select few of the similarity restrictions are followed. Mortzavi et al [13]
used scaling approach to obtain weakly-buoyant flames simulating low-gravity by exploiting the fact that the
buoyancy scales with p2g in laminar flames where p=pressure and g=gravitational acceleration. The finite rate
reactions, soot formation and radiative heat transfer in hydrocarbon flames studied by these authors impose severe
similarity constraints. Because of the fast reactions, the absence of soot and the lack of significant radiation in
hydrogen flames, similarity constraints are considerably relaxed in the present study.
Figures 3 and 4 show, respectively, the schematic and a photograph of the test chamber. This 0.75m high stainless
steel combustion chamber has a square cross-section (0.3m x 0.3m). The hydrogen or hydrogen/inert mixture is
supplied to a fuel-tube at the center of the chamber. Because of the large chamber volume, the influence of side
walls on the jet flow characteristics is expected to be negligible. Optical access is provided by glass windows (0.2m
x 0.60m and 10 ram thick) on two parallel side walls. An access door to facilitate changing of the burner and a
retraOzble spank-ignitgr are provided on other side walls. Upstream and downstream ends of the chamber connect,
respectively, to a diffuser and a nozzle with flow smoothening honeycombs. The downstream end is connected to
a vacuum pump rated to provide 0.055 m3/s (117 cfm) at up to 0.0033 bar absolute pressure. The vacuum pump
is driven by a 3-phase 7.5 HP electric motor. The entire chamber is mounted on a swiveling bracket to facilitate
orienting the flow direction at any angle with the gravity vector.
The burner is mounted in the collimated optical path of a color schlieren system set up on an optical rail. The
schlieren system consists of a halogen light source with fiber-optics, a 50 micron aperture, and 63 mm diameter,
490ram focal length collimating and decollimating lenses. A computer generated rainbow filter is used at the light
source image. The schlieren images are captured by a 3-chip CCD color video-camera connected to a
microcomputer through a frame grabber. The real lime schlieren images can be viewed on the computer screen and
the t_ame sequences can be digitized and stored for post processing.
Theoretical Analysis
The analysis follows the recent investigations to simulates effects of heat release on flow structures [6,8,14-15]
312
by solvingtime-dependent conservation equations of the mixture mass, species mass fraction, momentum and
energy. These cquatious in cylindrical polar coordinates have the following general form:
(1)
where 4_stands for the dependent variable. P, and SI are, respectively, the diffusion coefficient and generation
rate of the variable 0- P is the density and¢ is the velocity vector. The dependent variables are pressure, radial
and axial velocities, sensible enthalpy, and species mass fractions. The transport equations are solved by a
comn_rcial congx_tional fluid dymmics code, PHOENICS modified to include chemical kinetics and trausport
properties computed by the chemical kinetics code CHEMKIN.
Results and Discussion on the Work Completed
The first series of drop tower _ were conduOed at low fuel flow rates because of the safety concerns with
releasing hydrogen in an unconfined environment. Figures 5 and 6 show schlieren pictures obtained from video
recordings at a cold jet Reynolds number of 90 for two different fuel-tubes. Figure 5 pertains to the tube inside
diameter of 0.58 mm, tube wall thickness of 0.15ram and the average exit velocity of 16m/s while the
ccxrespcazling values in figure 6 are 1.19ram, 0.23mm, and Sm/s. The rainbow filter was one-half of the symmetric
filter shown in figure 7. Figures 5 and 6 indicate that the flame widens in the absence of gravity. Video recordings
also indicated low gravity flames were more stable compared to their normal gravity counterparts. These results
agree with previous experiments with diffusion flames, for example, by Bahadori et al [16]. Figures 5 and 6 also
show that the diffezence in flame width at normal and low gravity depen&ed upnn the fuel flow rate.. At the same
cold jet Reynolds number, the normal gravity flame was 12% wider at the higher fuel flow rate while the
corresponding low gravity flame was 55% wider. The flame anchored upstream of the fuel tube exit at both normal
and low gravity. This upstream distance of 20D at normal gravity increased to approximately 60D at low gravity.
Similar observations were made by Haggard and Cochran [17] who used black and white infrared film to
photograph the low gravity flames.
The ground experiments were conducted at atmospheric pressure for various fuel tubes and flow rates. Figure 8
shows schlieren pictures at different Reynolds numbers for a 0.58mm ID tube. These pictures were obtained using
a synm3etric rainbow filter shown in figure 7. The flame appears laminar at a Reynolds number of 540 (figure 8a).
At higher Reynolds manbers the laminar and turbulent zones are distinguished by the flame necking. The necking
distance deereases with increasing Reynolds number as the flame becomes fully turbulent, lifts-off and eventually
blows-off.
The experiments at low pressures will be conducted after the validation tests on the vacuum chamber facility are
completed. The analytical procedure has been used for a different project to predict ignition of natural gas [18].
This procedure is currently undergoing validation for use in the present configuration.
Future Plans
Future experiments in the drop tower will be conducted at higher Reynolds numbers. The ground experiments will
be conducted for cold jets and flames at various pressures and Reynolds numbers. A high power strobing white
fight source will be used to capture details of the flow sUuctures. The schlieren images will be processed to yield
refractive index and tempcrattae distributions. The analytical effort will focus on numerical simulations of the cold
jets and flames studied experimentally.
313
Acknowledgments
This project is sponsored by the NASA Microgravity Science and Application Division grant NAG3-1594. We
wish to acknowledge P.S. Grcenberg and F. Miller for the helpful discussions on the rainbow schlieren system. We
wish to thank R. Tudey for his meticulous and hard work in fabricating and setting up the vacuum chamber facility.
References
.
2.
.
4.
5.
6.
.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Yule, A.J., Chigier, N.A., Ralph, S., Boulderstone, IL, and Ventura, J., 1981, "Combustion Transition
Interaction in a Jet Flame," A/AA Journal, vol. 19, pp. 752-760.
Eickoff, H., Lenze, B., and Leuckel, W., 1986, "Experimental Investigation on the Stabilization
Mechanism of Jet Diffusion Flame, 20th Sym. (Int.) on Combustion, pp. 311-318, The Combustion
Institute, Pittsburgh, PA.
Mungal, M.G., Dimotakis, P.E., and Broadwell, J.E., 1984, "Turbulent Mixing and Combustion in a
Reacting Shear Layer," A/AA Journal, vol. 22, pp. 797-800.
Savas, O., and Gollahalli, S.IL, 1986, "Flow Structure in the Near-Nozzle Region of Gas Jet Flames,"
AIAAJournai, voI. 24, pp. 1i37-1140.
Cnen, L.-D., Seaba, J.P., Roquemor¢, W.M., and Goss, L.P., I988, "Buoyant Diffusion Flames," 22nd
Sym. (Int.) on Combustion, pp. 677-684, The Combustion Institute, Pittsburgh, PA.
Davis, ILW., Moore, E.F., Roquemore, W.M., Chen, L.-D., Vilimpoc, V., and Goss, L.P., 1991,
"PreliminaryResultsofaNumerical-ExperimentalStudyoftheDynamic StructureofaBuoyancy Jet
DiffusionFlame,"Combustion andFlame, vol.83,pp.263-270.
Chen, L.-D.,Roquemore, W.M., Goss, L.P.,and Vilimpoc,V., 1991,"VoticityGenerationin Jet
DiffusionFlames,"Combustion Scienceand Technology,vol.77,pp.41-57.
Katta,V.IL,and Roquemore,W.M., 1993,"Roleof Innerand Outer StructuresinTransitionalJet
DiffusionFlame,"Combustion and Flame,vol.92,pp.274-282.
Buckmaster,J.,and Peters,N.,1986,"TheInfiniteCandleand ItsStability-AParadigmforFlickering
DiffusionFlames,"21st Sym. (int.)on Combustion, pp. 1829-1836,The Combustion Institute,
Pittsburgh,PA.
Griffin,D.W.,andGreenbcrg,P.S.,1992,"SelectedMicrogravityCombustion DiagnosticTechniques,"
Proc.ofthe2nd Int.MicrogravityComb. Workshop,pp. 14I-148,NASA CP- I0113.
Penner,S.S.,1955,"SimilarityAnalysisand theScalingforLiquidFuelRocketEngines,"Combustion
Researchesand Reviews(AGARD), Chap. 12,Butterworths.
Spalding,D.B.,1963,"TheArtofPartialModeling,"9thSym. (Int.)on Combustion,pp.833-843,The
CombustionInstitute,Pittsburgh,PA.
Mortazavi,S.M_,Sumicrland,P.B.,Jumg, J.,and Faeth,G.M., 1992,"Structureand SootPropertiesof
BuoyantandNon-BuoyantLaminarRound-JetDiffusionFlames,"Proc.ofthe2nd Int.Microgra_ty
Comb. Workshop,pp. 101-I14,NASA CP-10113.
Ellzey,J.L.,and Oran,E.S.,1990,"Effectsof Heat Releaseand Gravityon an Uns_ Diffusion
Flame,"23rdSym. (Int.)on Combustion,pp. 1635-1640,The Combustion Institute,Pittsburgh,PA.
Katta,V.R.,Goss,L.P.,andRoquemore,W.M., 1994,'_NumericalInvestigationsofTransitionalI-L2/N2
JetDiffusionFlames,"A/AA Journal,vol.32,pp. 84-94.
Bahadori,M.Y.,Stockcr,D.P.,Vanghan,D.F.,Zhou,L.,andFtlelman R.B., 1992,"EllcctsofBuoyancy
on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames," Prec. of the 2rid Int. Microgravity
Combustion Workshop, pp. 91-106, NASA CP-10113.
Haggard, J.B., and C,ochran, T.H., 1973, "Hydrogen and Hydrocarbon Diffusion Flames in a Weightless
Environment," NASA TATD- 7165, 29pp.
Bi, H., and Agrawal, A.K., 1995, "Influence of Fluid Dynamics on Ignition of Natural Gas at Diesel
Environments," Paper No. 95-S-092, Joint Technical Meeting, San Antonio,TX.
314
Fig. 1 Schlieren Photographs of propane cold jet 
and flame at Re13000 
Fig. 3 Schematic of the Combustion Chamber 
Fig. 2 The Drop Tower Test Rig 
Fig. 4 A view of the Combustion Chamber 
315 
Fig. 5 Schlieren Photographs of Fig. 6 
Hydrogen Flame at R d O ,  
0.58mm ID Fuel Tube; 
top) Normal Gravity, 
bottom) Low Gravity 
Schlieren Photographs of Fig. '7 Symmetric Rainbow 
Hydrogen Flame at Re=90, Filter 
1.19mm ID Fuel Tube, 
top) Normal Gravity, 
bottom) Low Gravity 
a) Re=540 b) Re=1200 c) Re=1800 d) Re=2400 
Fig. 8 Schlieren Photographs of Hydrogen Flame at Normal Gravity, 0.58mm ID Fuel Tube 
ORIGINAL PAGE 
COLOR PHOTOGRAPH 


Effects of energy release on near field flow structure of gas jets

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