The objectives of the present research are to improve design capabilities for low thrust rocket engines through understanding of the detailed mixing and combustions processes. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an on-going experimental program at NASA LeRC. Detailed computational modeling requires the application of the full three-dimensional Navier Stokes equations, coupled with species diffusion equations. The numerical procedure is performed on both time-marching and time-accurate algorithms and using an LU approximate factorization in time, flux split upwinding differencing in space. The emphasis in this paper is focused on using numerical analysis to understand detailed combustor flowfields, including the shear layer dynamics created between fuel film cooling and the core gas in the vicinity on the nearby combustor wall; the integrity and effectiveness of the coolant film; three-dimensional fuel jets injection/mixing/combustion characteristics; and their impacts on global engine performance

Merkle, Charles L.

Tsuei, Hsin-Hua

English

NASA Technical Reports Server

N95- 70863
MULTI-DIMENSIONAL COMBUSTOR FLOWFIELD ANALYSES IN GAS-GAS ROCKET ENGINE
Hsin-Hua Tsuei'and Charles L. Merkle t
Propulsion Engineering Research Center
Department of Mechanical Engineering
The Pennsylvania State University
SUMMARY
The objectives of the present research are to improve design capabilities for low thrust rocket engines
through understanding of the detailed mixing and combustion processes. Of particular interest is a small
gaseous hydrogen-oxygen thruster which is considered as a coordinated part of a on-going experimental
program at NASA LeRC. Detailed computational modeling requires the application of the full three-
dimensional Navier Stokes equations, coupled with species diffusion equations. The numerical procedure is
performed on both time-marching and time-accurate algorithms and using an LU approximate factorization
in time, flux split upwinding differencing in space. The emphasis in this paper is focused on using
numerical analysis to understand detailed combustor flowiields, including the shear layer dynamics created
between fuel film cooling and the core gas in the vicinity on the nearby combustor waft; the integrity and
effectiveness of the coolant film; three-dimensional fuel jets injection/mixing/combustion characteristics;
and their impacts on global engine performance.
TECHNICAL DISCUSSION
The first part of the current research stems from interest in the mixing layers that arise from film cooling in
the combustor of a small chemical rocket engine [ 1-5]. This engine is a small gaseous hydrogen-oxygen
engine originally designed to provide auxiliary propulsion and attitude conu'ol for Space Station Freedom.
It provides about 110 N (25 lbf) of thrust and uses the entire hydrogen gas flow for regenerative cooling
after Which about two-thirds of it is split off for wall film cooling while the remaining one-third is mixed
with the oxidizer and used for primary combustion. An unknown fraction of the hydrogen cooling film
mixes with the oxidizer-rich core gas and burns between the injection plane and the throat. This combustion
reduces the thickness of the coolant layer and weakens its effect, but also improves the overal efficiency
of the thruster. The resulting shear region that is created between this coolant layer and the hot combustion
gases in the core region takes on much of the character of a classical shear layer. This shear layer, however,
incorporates not only a velocity difference, but also includes large molecular weight differences, chemical
reactions and strong heat release. Classical shear layer instability, unsteady vortex roll-up and related
shear layer dynamics can therefore be expected in the mixing layer. The vortex roll-up, however, will be
confined by the presence of the adjacent wall and the subsequent roll-up will be diminished although the
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unsteadiness remains. The issues of interest in the analysis are to document the role that these various
mechanisms have on unsteady mixing in the shear layer and the integrity of the film of coolant adjacent to
the wall.
Axisymmetric computational results have previously been compared with their e_ntal counter-
part [3,4]. Comparisons have shown qualitative agreement, however, additional physics must be included
to provide more accurate combustor flowfield predictions. Discrepancies arise mostly in the detailed local
flowfield comparisons, including number density of major chemical species, the local temperature field
and the exit velocity profile. In order to better understand where the disagreement stems from, an optical-
access chamber was built in LeRC [3] to provide physical insight of the combustion chamber. Major
chemical species and local temperature in the combustion chamber can therefore be detected by means of
Raman scattering technique. Because the hydrogen fuel is injected through 12 ports around the chamber
perimeter, detailed numerical modeling requires full three dimensional consideration to simulate the fuel
jets and the mixing/ignition/combustion processes. The second pan of this paper is thus centered on these
issues. Preliminary three dimensional computational results for cold hydrogen fuel injection as well as the
reacting flow calculations are presented. Efforts towards further comparisons with Rarnan measurement
data are currently underway.
The numerical algorithm is based on extending earlier supersonic reacting flow calculations [6,7] to
subsonic combustion problem [1.2,8]. The analysis uses an unsteady, three-dimensional, finite volume
Navier-Stokes procedure that include chemical non-equilibrium effects. The governing equations can be
written in a generalized coordinate system as :
w
m
0 0 0
+ _-_(E- E_,) -4- -ff_(F- F,,) -4- -_(C-C,,) - H
where Q = (p, pu,/m,/m,, e, p_)T is the vector of primary dependent variables, and E, F, and G are the
inviscid flux vectors, and E,,, F,,, and G,_ the viscous flux vectors in the _, 17and _ directions, respectively.
The vector H represents the source terms associated with chemical reactions or axisymmetry.
Numerical computation for steady flow is achieved by an implicit time-marching algorithm using an
LU approximate factorization in time and flux split upwinding differencing in space. The time-accurate
calculation for unsteady flow is conducted by a dual time stepping procedure [7]. The finite rate chemical
reaction model used in the present work for gaseous hydrogen-oxygen combustion [6], involves nine
chemical species and eighteen elementary reactions.
RF_ULTS
To demonstrate the dynamics of unforced hydrogen-oxygen reacting shear layers in the gas-gas combustor,
we begin by investigating the effects of the wall location. The upper wall is moved radially outward while
the_ g'_'p__nd _ewi_lth of the spli_=p]a_=re_ unchanged. Figure 1 shows results of
calculations for four different outer wall locations. The location of the splitter plate (radius of 10.3 mm)
and the width of its base (1.3 mm) remain unchanged. The flow velocities are chosen to match the engine
baseline operating condition: the hydrogen fuel (top portion) is injected at a speed of 125 m/s while the
oxygen core flow 0ower portion) enters at 185 m/s, giving a nominal velocity ratio of 0.67 and a density
ratio of 16. The plots show contours of the OH concentration, a reasonable indicator of the instantaneous
diffusion flame location. In Fig. la, the outer wall is sufficiently far away from the shear layer (radius ffi
30 mm) that it has fit-de effect on the vortex dynamics. The hydrogen-oxygen interface undergoes strong
distortion and roll-up in the classical sense. The small bulge near the splitter plate represents the nucleus
of the next succeeding roll-up. At an intermediate wall location (radius = 22 ram) some wall interaction
is observed (Fig. lb), but strong distortion is still present. The presence of the wall appears to cause
the vortex to sweep forward more strongly than in Fig. 2a when the wall is further away. It is clear that
the presence of the nearby wall affects the shape of the distorted shear layer. Figure Ic shows results for
when the outer wall is brought still closer to the shear layer (radius = 17 mm). At this location the wall
begins to prevent roll-up and diminish the unsteadiness. The distortion hits the outer wall before the entire
roll-up process is completed, and the subsequent roll-up is forced to decay. Figure ld shows results for
the case when the outer wall is very close to the splitter plate (radius = 12.7 mm); the geometry of interest
for the gas-gas combustor. A quick visual inspection shows the dominant influence of the adjacent wall.
For this case, the wall is too close to the shear layer to allow the strong distortions seen in the previous
two plots, and consequently the ensuing vortex roll-up does not occur. The shear layer does, however,
maintain a substantial amount of unsteady oscillation. Performance computations [I-3] show that this
unsteadiness causes a small decrease in the combustion rate of the fuel film with a corresponding decrease
in performance.
In the engine combustor, hydrogen fuel used for primary combustion is injected radially through 12
annual ports, mixes with core oxygen then ignited by a spark plug. To accurately model the fuel jets mixing
and combustion characteristics, three-dimensional consideration is required in the analyses. A cylindrical
combustor without the spark plug is first used to demonstrate three-dimensional capability of the present
theoretical model. Preheated core oxygen enters the combustor at 2000 K, I00 m/s while the hydrogen
fuel is injected at 600 K, 600 m/s. For clarity, only one-quarter of the combustor, which contains three
hydrogen injection ports, is plotted. The OH radical concentration contours and temperature contours
are shown in Fig. 2a and 2b, respectively. In Fig. 2a, the dark contours represent higher OH radical
concentration. It is observed that the shape of the diffusion flame is totally three-dimensional. The flame
surface starts to wrinkle downstream of the hydrogen injectors because a recirculation region is formed
behind each injection port that induces a vortex between the jet and the combustor wall. The hydrogen jets
no longer maintain their near-cylindrical cross-sectional shape and deform in response to the vortical flow
above them. The core oxygen gas therefore rolls up and wraps around each hydrogen jet causing further
mixing and reaction near the combustor wall, resulting in a very high OH concentration region.
Figure 2b shows the temperature contours. Darkened contours indicate higher temperature, with a
maximum temperature in the flame zone about 3600 K. It is also noted that the hydrogen jet is heated up
due to heat transferred from the three-dimensional diffusion flame as it travels along the chamber.
To simulate the operating ignition condition, preliminary computation of cold hydrogen fuel injection
into a cold oxygen core environment is investigated to provide perspective for the mixing processes under
the designed operating condition. The spark plug is included in this calculation represented by the shaded
region, located just upstream of the 12 hydrogen injection ports. A quarter of the combustor which involves
three injection holes is again used to show the three_nsional effects. The operating condition is chosen
to match the experiment setup, with cold hydrogen injected at 325 m/s while the room temperature oxygen
core enters at 13 m/s. Figure 3 shows the hydrogen mass fraction contours at two different time levels
during the hydrogen injection process. Figure 3a shows an early stage shortly after the hydrogen injection
is initiated. The mushroom-like mixing front is clearly observed. In Fig. 3b, after a certain amount of
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hydrogen is pumped into the chamber, the mixture front keeps growing larger in size and turns toward
the streamwise direction because of interactions with the core oxygen stream. A recirculadon bubble is
expected downstream of the spark plug, where the mixture is ignited. The computation is still currently
underway while the ignition process will be initiated when a high enough mixture r-ado is maintained in
the recirculadon region to sustain the flame after ignition. Further comparisons with experiment data will
be made to validate the accmacy of the current analyses.
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ACKNOWLEDGEMENTS
This work has been supported by NASA Grant NAG 3-1020 and NAGW-1356. Partial computational
support has been provided by the Center of Academic Computing at The Pennsylvania State University
and the NAS Program at NASA Ames.
REFERENCES
1. Tsuei, H.-H. and Merlde, C., AIAA Paper 94-0553, Reno, NV, 1994.
2. Tsuei, H.-H. and Merkle, C., Proc. 14th Intl. Conf. on Numerical Methods in Fluid Dynamics.,
Bangalore,India,1994.
3. de Groo_ W. and Tsuei, I-L-K, AIAA Paper 94-0220, Reno, NV, 1994.
4. de Groot, W. and Weiss, J., AIAA Paper 92-3353, Nashville, TN, 1992.
5. Weiss, J. and Merkle, C., AIAA Paper 93-0237, Reno, NV, 1993.
6. Yu, S.-T., Tsai, Y.-L. P. and Shuen, J.-S., AIAA Paper 89-0391, Reno, NV, 1989.
7. Molvik, G. and Merkle, C., AIAA Paper 89-0199, Reno, NV, 1989.
8. Venkateswaran, S., Weiss, J., Merkle, C and ChoL Y.-H., AIAA Paper 92-3437, Nashville, TN, 1992.
b. Outer wall radlu -- _ mm
_. Out_ wd tmiim - tO mm
r. Omter win radim - 17mm d. Outer wall radiu ---12.7 mm
Figure 1. The OH radical concentration contours for four different outer wall locations.
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Multi-dimensional combustor flowfield analyses in gas-gas rocket engine

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