TITAN Aerocapture entry has been studied in collaboration with the Hypersonics Centre of the University Queensland (UQ), Australia The simulation of the experimental conditions and also the flight conditions are made using CFD coupled with chemistry libraries of which CHEMKIN. This can be compared to in-code implementation for the Earth reentry. Reduced models based on combustion data bases are taken for the reactions data set for Titan’s entry

Leyland, P.

Sobbia, R.

Vos, J. B.

University of Queensland eSpace

16th Australasian Fluid Mechanics Conference
Crown Plaza, Gold Coast, Australia
2-7 December 2007
Chemical kinetic and radiating species studies of Titan aerocapture entry
Pe´ne´lope Leyland1, Raffaello Sobbia2, and Jan B. Vos 3
1 STI-ISE-LIN, Station 9, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
2 CRPP - Centre of Research on Plasma Physics, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne,
Switzerland
3 CFS Engineering, CH-1015 Lausanne, Switzerland
Abstract
TITAN Aerocapture entry has been studied in collaboration
with the Hypersonics Centre of the University Queensland
(UQ), Australia The simulation of the experimental conditions
and also the flight conditions are made using CFD coupled with
chemistry libraries of which CHEMKIN. This can be compared
to in-code implementation for the Earth reentry. Reduced mod-
els based on combustion data bases are taken for the reactions
data set for Titan’s entry.
Introduction
To improve our knowledge of (re-)entry phenomenon around
space vehicles it is necessary to understand the chemical kinetic
mechanisms present in the flowfield between the strong bow
shock and the vehicle surface. Indeed the strong shock triggers
off chemical reactions of the species of the atmosphere, with
dissociation, recombination and creation of derived species. In
particular the hot gas can become radiating, dependent on the
composition. The use of Computational Fluid Dynamics (CFD)
as a tool to simulate the external aerothermodynamics over ve-
hicles, requires significant physical modelisation in order to
correctly represent the phenomena taking place within entry.
Backed up by analytical techniques for margin estimations and
scaling parameters, experimental means are necessary in order
to achieve ground measurements that either cannot or were not
part of the on-board flight experimentation. To improve the
modelisation, a large amount of effort should be put into the
development of a chemical model adapted to the chemistry of
the entry phase. For Earth reentry, several well developed mod-
els exist for hypersonics, such as Park’s model and its exten-
sions, [1]. The radiating component is difficult to achieve in
ground facilities due to binary scaling, [3]. For other planetary
atmospheres, the gaseous composition of the atmosphere can
be challenging. One of these is given by the Titan’s aerocapture
flight where the outer atmosphere is composed of methane, ni-
trogen (approximately 5 and 88 % respectively) and argon (0 to
10 %). Here the chemical reactions are close to hydrocarbure
ones, with many derived species, in particular the highly radi-
ating CN, and ionisation products. The kinetic model is hence
complex, and includes a high number of species. Work of Park,
Nelson and co-authors have derived models, that are resumed
and analysed by T. Go¨kc¸en, who has presented a specific re-
duced model, [4]. The interest of the Titan aerocapture con-
ditions is that radiation is not annulled by binary scaling and
remains present in the ground facilities.
To simulate with CFD the complete aerothermodynamic flow-
field over the Titan entry capsule means that a high number of
species equations must be added to the reactive Navier Stokes
equations. This results in a very large system. For convergence
requirements, the use of implicit techniques lead to severe mem-
ory problems even on the largest systems available. This is even
true for the reduced model proposed by T. Go¨kc¸en. Also, the
coupling of the chemical kinetic modeling and the flow solver is
highly complex. For this reason the use of coupling the Navier-
Stokes real gas solver with a chemical kinetic mechanism li-
brary of CHEMKIN has been adopted here. In order to have a
chance to compute, a further reduced model has been devised
which takes into account the main features. This mechanism is
tested over equivalent cylinders to the flight and experimental
models. Particular attention is made on the relative importance
of ionised species and electrons.
Validation of the Reduced Chemical Model
The Titan atmosphere gas is simplified by taking an initial
composition of 95% N2 and 5% CH4, as in the UQ experi-
ments. To obtain a reduced model the following analysis was
made: first a sensitivity analysis of rate-limiting steps in produc-
tion/consumption species using CHEMKIN, second, the CFD
results on small cylinder geometry in the conditions of flight
and the ground experiments in X3 were analysed to see which
species were negligible. It was seen that the argon components
can be ignored, as well as the ionisation species of C+, CH+,
H+ and N2+.
Validation of the present model has been made against the re-
duced Go¨kc¸en model [4] using the SENKIN and SHOCK pro-
grams in the CHEMKIN package [5] for a shock tube without
boundary layer correction with incident shock velocity of 6.3
km/s, pressure of 0.1 Torr, and temperature before the shock of
300 K. The reaction set is presented in table 2, which consists
of 13 neutral chemical species (C2, C, CH4, CH3, CH2, CH,
CN, H2, H, N2, N, HCN, NH), two positive ions (CN+, N+),
and electrons (E). The fits to C0p/R, H
0/RT and S0/R consist of
seven coefficients for the temperature range from 300 to 20000
K. The downstream model equations assume that the flow is adi-
abatic and that the transport phenomena associated with mass
diffusion thermal conduction and viscous effects are negligible
[5]. Figure 1 and figure 2 show the variation of representative
species mole fractions with respect to time.
A very good agreement between the present reduced model and
the Go¨kc¸en’s reduced model is achieved for the computed neu-
tral species considered in this work, 1. However, this is not the
case for the ionised species. CN+ shows a longer relaxation
time and relaxes to a higher value in the present reduced model
than in Go¨kc¸en’s one. For N+, both models show the same
trend, but Go¨kc¸en’s model gives higher values. For electrons,
the behaviour is quite different. Go¨kc¸en’s model give a strong
rise, whereas our model first shows a local maximum and then
decreases towards a lower value.
CFD Results with the Chemical Model
The model is tested by considering the flow over a cylinder in
the same flight and ground facilities of the X3 expansion tube
at UQ. The flow conditions are as follows, see table 1.
The results over the cylinder show complete and immediate dis-
sociation of CH4 behind the shock in both flight and X3 flow
fields, with creation of highly radiating CN in the shock layer,
497
Flight X3 -UQ 1:40.7 Model
V km/s 5.76 5.83
ρ (kg/m3) 1.49×10−4 2.43×10−3
ρ L (kg/m2) 5.2×10−4 5.2×10−4
p Pa 6.9 1130
α AOA 16o 16o
Table 1: Flowfield conditions - Flight and X3 for Titan aerocap-
ture entry
see figure 3. A significant quantity of N+ is also produced, see
figure 4. The difference with Go¨kc¸en’s reduced model is a loss
of species away from the stagnation centreline, (seen by diffu-
sion near the corners). The electronic component is evident near
the surface in flight conditions, and also is present in X3 con-
ditions, figure 4. This plasma “creation” could be of significant
importance also for measurement techniques. Coupled calcu-
lations on the Titan capsule were made for X3 conditions and
compared to measurements of B. Capra [3]. The flow field is
shown in figure 5. The comparison of the heatfluxes between
the CFD and the X3 results were reasonable: 0.78KW/cm2
for the heat flux in the stagnation region for the CFD against
0.7kW/cm2 for the X3 experiment, and 0.6kW/cm2 at an exte-
rior point for the CFD, against 0.5kW/cm2 at the same position
for the experiments.
References
[1] Park, C. “Non equilibrium hypersonic aerothermodynam-
ics”. John Wiley and Sons. 1990
[2] Capra, B. R., Morgan, R. G., and Leyland, P., in Aerother-
modynamics for Space Vehicles, Cologne, ESA Publica-
tions 2005.
[3] B. Capra. Ph D Thesis. University Queensland, AUS.
2006.
[4] Go¨kc¸en, T., “N2-CH4-Ar chemical kinetic model for sim-
ulations of atmospheric entry to Titan ”, AIAA, 2004-2469.
[5] The CHEMKIN collection, Release 4.1, Reaction Design,
Inc., San Diego, CA, USA.
498
Figure 1: Time histories of computed neutral mole fractions using the reduced model of Go¨kc¸en, labeled as Gokcen, and the present
reduced model, labeled as reduced.
Figure 2: Time histories of computed charged particle mole fractions using the reduced model of Go¨kc¸en, labeled as Gokcen, and the
present reduced model, labeled as reduced.
499
Figure 3: CH4 dissociation (left) and one of the products, CN, (right) in Titan Flight flowfield conditions
Figure 4: N+ population in the shock layer (left), max. 0.25; electron population (right), max. 10−5
500
Figure 5: Titan flowfield in X3 conditions: temperature contours shows the shock structure (top), and the pressure field on the capsule
surface (bottom).
501
Reaction & Forward reaction rate coefficients Ta = (T Tv)0.5
k f = AT n e
−Ta
T [cm3/mole− s] A(cc/mol/s) n Ta(K)
Dissociation Reactions of N2
N2+N2→ N+N+N2 7.00x1021 −1.6 −113200
N2+CH4→ N+N+CH4 7.00x1021 −1.6 −113200
N2+CH3→ N+N+CH3 7.00x1021 −1.6 −113200
N2+CH2→ N+N+CH2 7.00x1021 −1.6 −113200
N2+CH → N+N+CH 7.00x1021 −1.6 −113200
N2+C2→ N+N+C2 7.00x1021 −1.6 −113200
N2+H2→ N+N+H2 7.00x1021 −1.6 −113200
N2+CN → N+N+CN 7.00x1021 −1.6 −113200
N2+NH → N+N+NH 7.00x1021 −1.6 −113200
N2+HCN → N+N+HCN 7.00x1021 −1.6 −113200
N2+N → N+N+N 3.0x1022 −1.6 −113200
N2+C → N+N+C 3.0x1022 −1.6 −113200
N2+H → N+N+H 3.0x1022 −1.6 −113200
N2+CN+→ N+N+CN+ 7.00x1021 −1.6 −113200
N2+N+→ N+N+N+ 7.00x1021 −1.6 −113200
N2+E → N+N+E 3.0x1024 −1.6 −1132000
Dissociation Reactions of CH4, CH3, CH2, CH
CH4+M →CH3+H +M 4.70x1047 −8.2 −59200
CH3+M →CH2+H +M 1.02x1016 0 −45600
CH3+M →CH +H2+M 5.00x1015 0 −42800
CH2+M →CH +H +M 9.68x1015 0 −41800
CH2+M →C+H2+M 9.68x1014 0 −29700
CH +M →C+H +M 9.68x1014 0 −33700
Dissociation Reactions of C2
C2+M →C+C+M 1.50x1016 0 −71600
Dissociation Reactions of H2
H2+M → H +H +M 2.23x1014 0 −48350
Dissociation Reactions of CN
CN+M →C+N+M 2.53x1014 0 −71000
Dissociation Reactions of NH
NH +M → N+H +M 1.8x1014 0 −37600
Dissociation Reactions of HCN
HCN+M →CN+H +M 3.57x1026 −2.6 −62845
CH3 Radical Reactions
CH3+N → HCN+H +H 7.0x1013 0 0.0
CH3+H →CH2+H2 6.03x1013 0 −7600
CH2 Radical Reactions
CH2+N2→ HCN+NH 4.82x1012 0 −18000
CH2+N → HCN+H 5.00x1013 0 0.0
CH2+H →CH +H2 6.03x1012 0 +900
CH Radical Reactions
CH +N2→ HCN+N 4.40x1012 0 −11060
CH +C →C2+H 2.00x1014 0 0.0
C2 Radical Reactions
C2+N2→CN+CN 1.50x1013 0 −21000
CN Radical Reactions
CN+H2→ HCN+H 2.95x1005 0 −1130
CN+C →C2+N 5.00x1013 0 −13000
N-Atom Radical Reactions
N+H2→ NH +H 1.60x1014 0 −12650
C-Atom Radical Reactions
C+N2→CN+N 5.24x1013 0 −22600
C+H2→CH +H 9.68x1022 0 −11700
H-Atom Radical Reactions
H +N2→ NH +N 3.00x1012 0.5 −71400
H +CH4→CH3+H2 1.32x1004 3 −4045
Associative Ionization Reactions
C+N →CN++E 1.00x1015 1.5 −164400
Ionization Reactions (Electron-impact)
N+E → N++E +E 2.50x1034 −3.82 −168600
Charge Exchange Reactions
CN++N →CN+N+ 9.8x1012 0 −40700
Table 2: Forward reaction rates where M represents cyclically all other mixture components.
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