Development of an ethanol/air reduced mechanism and its application
to two-phase detonation
T. Shimada1, M. Koshi2, Y. Tatsumi1, A. K. Hayashi1, E. Yamada1 and N. Tsuboi3
Abstract/
The ethanol-air reduced mechanism is developed from its detailed mechanism and
studied its properties such as flame speed and ignition delay time behind a reflected
shock wave for the simulation of application. These calculated properties agree with
the experiments and simulations of other mechanisms.
Then liquid ethanol-air
two-phase detonation is simulated using two-phase compressible Euler equations with a
one-step overall ethanol-air reaction mechanism to study its detonation characteristics.
At this time the developed ethanol-air mechanism was too large to simulate detonation.
The ethanol-air detonation structure with an evaporation of the initial mixture is
simulated to see the liquid phase effects on its structure.
1. Introduction
In order to overcome the global warming by CO2, environmental pollution by NOx, and
energy problem by draining of oil, methane, ethane, dimethyl ether as gaseous fuels and
methanol, ethanol produced from biomass as liquid fuels are studied extensively
recently. Especially ethanol as one of bio fuels is used for a long time for an automobile
and aerospace fuel by being mixed in gasoline. This mixture is an alternative fuel
which minimizes CO2 and nitrogen oxides emissions at the exhaust period. Ethanol is
expected in the future as various thermal engine fuels. However although ethanol
combustion study has been conducted for a long time such as an ignition delay time
experiments by shock tubes, a premixed and non-premixed flame velocity by burners,
these physical data in high pressure are not obtained enough for studies under high
pressure conditions such as detonation.
In order to apply ethanol combustion to high pressure and high temperature aerospace
problems, ethanol-air two-phase detonation engine is picked up for the study. To this
end, the mixture of ethanol-air is used to calculate liquid-gas two-phase detonation
properties using two-phase compressible Euler equations.
This paper deals with an investigation of a property of a developed reaction model for
ethanol combustion in high pressure and high temperature phenomena such as
detonation. Then ethanol-air two-phase detonation is simulated first using a simple
ethanol one-step reaction mechanism to study properties and structure of liquid-gas
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(ethanol-air) two-phase detonation. In this case the developed ethanol-air reaction
mechanism is still large to provide a costly calculation, then a simple one-step model is
used presently.
2. Numerical system
First of all, in order to study the combustion mechanism of ethanol-air, a detailed,
but reduced ethanol-air reaction mechanism is investigated comparing with other
reaction mechanisms.
The calculation is performed using a commercial code of
CHEMKIN-PRO [1] for flame speed and ignition delay time as representative reaction
properties. The detonation property such as C-J detonation velocity is calculated using
a STANJAN code developed by Reynolds [2]. The Koshi model developed by Koshi is
reduced from the ethanol oxidation model developed by Ogra et al. [3].
Table.1 Recation step and species in various reaction model compared
in the present study.
reaction
step
Pressure
species
dependent
or not
Pressure Temperature
resion
resion
Koshi
295
50
Yes
-
-
Marinov [4]
378
56
Yes
1-3.4
1300-1700
Leplat et al. [5]
252
38
Yes
-
-
142
32
No
1
1300-1700
1
5
No
-
-
Norton and Dryer
[6]
Westbrook and
Dryer [7]
2.1 Property of Koshi model
The properties of Koshi model are studied in terms of flame speed and ignition
delay time by comparing with other models such as Marinov model [4], Leplat et al.
model [5], Norton and Dryer model [6], and Westbrook and Dryer model [7]. Figure 1
shows the comparison of flame speed between Koshi model and the experimental data of
Gulder [8] and Egolfopoulos et al. [9], where Fig.1-a is at the atmospheric pressure and
room temperature and Fig.1-b is at the pressure of 2 bar and room temperature. Both
cases show a good agreement between the simulation and experimental data and
especially the Koshi model well predicts the flame speed at the higher pressure case.
2
Exp(Gujder,300K,0.2MPa)
Koshi model
50
45
40
35
30
25
20
15
10
0.4
0.6
0.8
1
1.2
1.4
1.6
Laminar flame speed [cm/s]
Laminar flame speed [cm/s]
Exp(Egolfopolous et al.,298K,1atm)
Exp (Gujder,300K,0.1MPa)
Koshi model
Equivalence ratio
45
40
35
30
25
20
15
10
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
Equivalence ratio
50
45
40
35
30
25
20
15
10
(a) 1bar
Exp(Egolfopolous et al.,298K,1.013bar)
Exp (Gujder,300K,1bar)
Koshi model
0.4
0.6
0.8
1
1.2 1.4
Equivalence ratio
Exp(Gujder,300K,2bar)
Laminar flame speed [cm/s]
Laminar flame speed [cm/s]
(b) 1bar
1.6
45
Koshi model
40
35
30
25
20
15
10
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
Equivalence ratio
Fig.1 Ethanol flame velocity: the comparison between Koshi model analysis and the
experimental data of Gulder and Egolfopoulos (a) at the atmospheric pressure and room
temperature and (b) at the pressure of 2 bar and room temperature.
Figure 2 shows the comparison of ignition delay time behind reflected shock wave
between the analyses by Koshi, Marinov [5], Leplat et al. [6], Norton and Dryer [7] and
Westbrook and Dryer’s one-step model [8] at the pressure of 10 bar and the equivalence
ratio of unity of ethanol-air mixture and the experimental data by Cancino et al.[10].
Koshi model shows a better agreement at the higher temperature as Marinov model and
Leplat et al. model, but a less agreement at the lower temperature since these three
models do not have the low temperature reactions such as RO2 related reactions.
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Three reaction models; Koshi, Marinov, and Leplat et al., are developed at high pressure
and high temperature conditions, however Norton and Dryer model was developed at
the condition of low pressure; atmospheric pressure, and relatively low temperature,
then it has RO2 reactions.
Cancino et al. also developed their detailed reaction model with 136 species and
1349 reactions and showed a good agreement with their experimental data [11]. The
detonation calculation needs a cost performance for the detailed space precision, then at
least a reduced reaction model such as Koshi model should be developed. But this time
Koshi model is still large to spend a lot of computational time.
100000
Experiment results(Cancino et al.,10bar)
Koshi model
Marinov model
Norton and Dryer model
Leplat et al. model
Westbrook and Dryer model
Ignition delay time τ [μs]
10000
1000
100
10
0.8
0.9
1
1.1
1000/T [1/K]
Fig. 2 Comparison of ignition delay time for the ethanol-air mixture of
Stoichiometric condition at the pressure of 10 bar and the room temperature.
3. Detonation application of liquid ethanol-air mixtures
In order to apply ethanol fuel to aerospace propulsion, pulse or rotating detonation
engine is one of such propulsion systems. Then a study of detonation in ethanol-air
mixture will be presented numerically.
3.1 C-J detonation velocity
Ethanol has a similar property as C2 hydrocarbons, but has a weak bond-strength
of OH radical like alcohol, then it is easy to proceed thermal decomposition reaction and
radical exchange reaction. Due to this weak OH radical bond, its heat of reaction is
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also low among C2 hydrocarbons. First of all the CJ detonation velocity of ethanol-air
mixture is calculated using a STANJAN code to compare with that of other hydrocarbon
fuels and of hydrogen. The calculation is performed with the initial condition of the
pressure of 1.013 bar and the temperature of 300 K for the mixture at the equivalence of
unity. This calculation is the equilibrium calculation; i.e., the reaction mechanism free
calculation, but Fig. 3 shows such comparison that hydrogen-air mixture provides the
similar C-J detonation velocity curve except hydrogen-air mixture. This result expects
that ethanol detonation engine can provide the similar thrust as other fuels.
2200
Detonation Velocity [m/s]
2100
2000
1900
1800
1700
H2-Air
C3H8-Air
JP10-Air
CH4-Air
Ethanol-Air
1600
1500
1400
1300
0.4
0.6
0.8 1.0 1.2 1.4
Equivalence Ratio
1.6
1.8
2.0
Fig.3 Comparison of C-J detonation velocity in various fuels.
3.2 Ethanol-air two-phase detonation structure
Ethanol-air two-phase detonation is simulated using a liquid-gas compressible
Euler equation system with a one-step overall reaction mechanism developed by
Westbrook and Dryer [7]. Liquid ethanol evaporates quickly depending on the d2-law
of evaporation and reacts with oxygen based on the one-step reaction model. The
two-phase Euler equations has a structure of liquid-phase and gas-phase mass,
momentum, and energy conservations. Figure 4 shows the case of liquid ethanol – air
detonation propagation. The size of ethanol droplets is 3 μm in diameter and the
pre-evaporation is 30 % of liquid ethanol. The figure shows the pressure profiles at the
different periods and shows the incident shock wave (IS), reflected shock wave (RS),
transverse shock wave (TW), and transverse detonation wave (TD) (Fig.4-a). As the
detonation head propagates forward (toward right hand side of the figure), TD moves
upward together with bifurcating since non-reacted ethanol flows down. This type of
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burning occurs often for the liquid fuel detonation case while the gaseous detonation
barely happens except for the limit situation. The one-step reaction model is used in
the present simulation, then the results may show the faster reaction than that using
the detailed mechanisms.
TW
RS
a
d
IS
TD
b
e
c
f
Fig.4 Pressure distributions at droplet diameter of 3μm and of the initial
evaporation ratio of 30%
4.
Conclusions
Ethanol-air reduced reaction mechanism is developed and is investigated whether it is relevant
for simulation use or not. Then the Koshi mechanism is compared in terms of the flame speed and
ignition delay time behind reflected shock wave with that of other reaction mechanisms. Since it
does not have low temperature mechanism of RO2, it does not predict well at the low temperature
region of ignition delay time.
Liquid ethanol-air two-phase detonation was simulated using one-step reaction mechanism.
The typical liquid-gas detonation propagation structure was obtained using such simple mechanism.
The results show the importance of pre-evaporation condition for detonation propagation.
Acknowledgments
This research was supported by KAKENHI (21360420) and collaborated with the JSS Systems in
JAXA supercomputer system.
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その他
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