Advanced Combustion Theory and
Modeling
April 8, 2011
Microgravity Droplet Combustion:
Space-Based Experiments and Detailed
Numerical Modeling
 i Hi
 qR
Anthony J. Marchese, Ph.D.
Associate Professor
Dept. of Mechanical Engineering
Colorado State University
Overview
Microgravity Combustion and Heat Transfer
Why study liquid fuel combustion?
Why study liquid fuel combustion in
microgravity?
Spherically symmetric, time-dependent
numerical modeling
Ground based microgravity experiments
2.2 second drop tower
5 second Zero Gravity Facility
Space based microgravity experiments
FSDC-1, DCE and FSDC-2
Continuing Research
Microgravity flame spread through layered
gas mixtures
Microgravity boiling heat transfer
Why are we still studying fossil fuel
combustion?
Reliability on fossil fuels continues...
85% of all energy consumed in the U.S. is
derived from the combustion of fossil fuels.
39% of all energy consumed in the U.S. is
derived from the combustion of liquid fossil
fuels.
97% of all energy consumed in the
transportation sector is derived from the
combustion of liquid fossil fuels.
Meanwhile, emissions standards continue to
tighten…
California NOx Standards for Gasoline-powered*
light duty vehicles:
1971
4.0 g/mile
1993
0.4 g/mile
2003
0.2 g/mile
* Note: Diesel-powered light-duty vehicles no longer for sale in
California.
Why are we still studying fossil fuel
combustion?
Developing accurate models of the combustion
process is the key to designing more efficient,
cleaner burning engines...
The physical phenomena occurring in an internal
combustion engine includes:
vaporization,
mass transfer,
heat transfer,
turbulent fluid mechanics, and
complex chemical kinetics (~ 100 species).
The problem is three dimensional and timedependent…and impossible to solve with even the
most powerful computers!
What fossil fuel combustion problem
can we solve?
The spherically symmetric combustion of a
single liquid droplet in an infinite oxidizing
medium can be solved numerically in full detail:
r
Multi-component
liquid fuel droplet
Flame
n-C7H16
C2H5
CH3
C8H18
O2
H
OH
• Real vapor/liquid equilibrium,
• multi-component gas-phase transport,
• liquid-phase heat and mass transport,
• radiative heat transfer,
• detailed gas-phase chemistry (~100 species),
• time-dependent.
Spherically Symmetric
Droplet Combustion
By creating and igniting a single liquid droplet in
microgravity it is possible to achieve spherically
symmetric combustion ...
r
1g
10-6 g
...of droplets large enough to permit accurate
photographic analysis.
The experimental results are compared directly
with detailed numerical modeling.
Time-Dependent, Spherically Symmetric,
Bi-component Droplet Combustion Model
(Cho, et al., 1992; Marchese and Dryer, 1996)
Gas Phase:
• Multicomponent molecular diffusion
• Complex chemical kinetics
(e.g. 50 species, 250 reactions)
• Non-luminous thermal radiation
• UV flame emission
Mass Conservation:

1 
(  g )  2 ( r 2 g v r )  0
t
r r
Species Equations:
g
Yg ,i
t
 g vr
Yg ,i
1 
  2 ( r 2gYg ,iVr ,i )   g ,i
r
r r
Energy Conservation:
Droplet Surface:
• Surface regression
• Evaporation of fuel
• Condensation of products
• Radiative heat addition
Tg
1 
 2 ( r2 g
 qR )
t
r
r
r r
n
n
Tg
  g  ( Yg ,iVr ,iC p ,g ,i )
   g ,i H g ,i
r i 1
i 1
 g C p ,g
Tg
  g C p.g v r
Tg
Net Radiative Heat Flux
m v
m p
drs
dt
qR
Dl ,i
Yl ,i
r
l

T
r
T
r
Droplet Interior:
lC p ,l
Yl ,i
Tl
1   2 Tl 
 2
 r l

t
r 
r r 
Yl ,i 
1 
 2  r 2 Dl ,i

t
r 
r r 
Gas Phase Chemical Kinetic Mechanism
N-Alkane Droplet Combustion
Goals:
• Generate test matrix, and analyze results of
DCE n-heptane experiments using detailed,
transient numerical model.
Existing Chemical Kinetic Mechanisms:
• Too large:
Chakir (1992) - 72 species
Lindstedt (1995) - 109 species
• or too empirical:
Warnatz (1984) - 32 species, 96 reactions
for detailed, time-dependent, one-dimensional
diffusion flame modeling.
Result
A new compact semi-empirical n-heptane
mechanism* has been developed that includes:
Fuel thermal decomposition
Site-specific H-atom abstraction
37 species, 241 reactions
*Held, Marchese and Dryer (1997).
N-Heptane Droplet Combustion
Fuel Consumption Path
• For typical DCE conditions (He/O2)during quasisteady combustion:
C7H16
 products (~ 50%)
C7H16+ H  products (~ 49%)
• Decomposition generally dominates over
isomerization for n-alkyl radicals.
C2H5
CH 3
1-C5H11
1-C6H13
C3H7
24%
12%
+
H
+H
7%
1-C7H15
14%
+
H
C7H16
+H
16%
17%
10%
2-C7H15
C4H9
19%
9%
3-C7H15
4-C7H15
22%
7%
100 %
C2H4
90 %
C3H6
43 %
15 %
1-C4H8 1-C6H12
93 %
1-C5H10
How do we perform experiments in
microgravity?
Parabolic Flight Aircraft
“The Vomit Comet”
Orbiting Spacecraft
Drop Towers
Earth-Based Microgravity Facilities
NASA 5 Second Zero Gravity Facility
Earth-Based Microgravity Facilities
NASA Lewis 2.2 second drop tower
2.2 Second Drop Tower Experiments
Experimental Apparatus
User Interface
Microprocessor
Video Cameras
Optical Access Ports
High Speed Camera
Back Light
Test Chamber
Power Supplies
Oxidizer/Inert Inlet Ports
Earth-Based Microgravity Facilities
Rowan 1.1 Second Drop Tower
Deceleration System
100 ft3 welded steel cage
22-oz nylon coated polyester
airbag (100 ft3)
12-inch polyurethane foam mat
Four 6-inch PVC Check Valves
1.5 HP, 127 CFM radial blower
Earth-Based Microgravity Facilities
Rowan 1.1 Second Drop Tower
2.2 Second Data Analysis System
• Back-lit, high-speed movie camera
• Video “set-up” camera
• Xybion ISG-250 CCD video camera
- UV Transmissive Lens
- Narrow band interference filter centered
at 310 nm; full-width, half-max = 10nm
- Data acquired at 30 fps
Igniters
Droplet
Milliken Camera
Xybion Camera
Set Up Camera
Flame
Needles
SVHS
07:28:90
SVHS
07:28:90
Video Cassette Recorders
Drop Tower Experiments
Methanol Droplet Combustion
Visual Video Image
Ultraviolet Flame Image
Spherically
Symmetric
Diameter-Squared History
Pure Methanol Droplets
For 1 mm droplets, the numerical model accurately
reproduces the measured burning rate for pure
methanol droplets in various O2/N2 oxidizing
environments.
1
.
0
1
8
%
O
2
2
1
%
O
2
2
4
%
O
2
0
.
8
3
0
%
O
2
2 2
d
/
d
o
0
.
6
0
.
4
0
.
2
0
.
0
0
.
0
0
.
4
0
.
8
1
.
2
2
2
t
i
m
e
/
d
[
s
e
c
/
m
m
]
o
1
.
6
OH* Chemiluminescence
Data Analysis
Relationship Between Measured
Signal and Actual OH* Emission Intensity
y
F(r) : Actual OH* emission
intensity field
r
x
P( r): Lineintegral
of sight integral
P(r) : Line-of-sight
projection
as measured by
projection
as measured
the Xybion Camera
by Xybion Camera
r
Recover actual OH* intensity field, F( r), using
the Inverse Abel Transform (Dasch, 1992):

1
dP / d
F( r )    2
d
 (   r 2 )1 / 2
r
OH* Chemiluminescence
Numerical Modeling (Marchese, et al., 1996)
Numerical Modeling Technique:
•Incorporate OH* submechanism into gas phase
chemical kinetic mechanism.
Possible Production Routes of Electronically
Excited OH:
k
1
CH  O2 
 CO  OH *( 2  )
k
2
H  OH  OH 
 H 2O  OH *( 2  )
Collisional De - excitation:
k
d
OH *( 2  )  Mi 
 OH( 2 )  Mi
Emission:
k
em
OH *( 2  ) 

 OH( 2 )  h
•Calculated OH* Emission [W/cm3]:
iOH * ( r ,t )  N a k em hCOH * ( r , t )
OH* Chemiluminescence
Methanol Flame Results
Flame Structure, t = 0.90 sec
Methanol/35% O2/65% N2, 1.0 Atm
0
.
0
1
6
2
1
.
0
T
e
m
p
e
r
a
t
u
r
e
C
H
O
H
3
0
.
0
1
2
i
O
H
*
0
.
6
0
.
0
0
8
0
.
4
O
H
0
.
2
O
2
MasFractionOH
MNaosrmFarlaizcetdionOHO*EmisonItensity 2,CH 3OH,CO,CO
0
.
8
0
.
0
0
4
C
O
2
C
O
0
.
0
0
0
.
0
0
0
3
6
r
/
r
s
9
1
2
Instantaneous Flame Position
Pure Methanol Droplets
Predicted location of maximum OH* emission
agrees with experiment to within 1 normalized radii
6
CalcutedFlamePosDitiamnetr[d OH* /d s]
5
4
3
2
1
%
O
2
2
3
0
%
O
2
3
5
%
O
2
2
1
%
O
E
x
p
.
2
1
3
0
%
O
E
x
p
.
2
3
5
%
O
E
x
p
.
2
0
0
.
00
.
10
.
20
.
30
.
40
.
50
.
6
2
2
t
i
m
e
/
d
[
s
e
c
/
m
m
]
o
Space Shuttle Experiments
Fiber Supported Droplet Combustion
Investigation - 1 (FSDC-1)
• Completed experiment aboard Space Shuttle
Columbia flight STS-73, November 1995.
• Droplet diameters: 3 to 5 mm
• Fuels:
Methanol
Methanol/Water
Heptane
Heptane/Hexadecane
Droplet Combustion Experiment (DCE)
• Isolated droplet experiments,
up to 5 mm
• First flew aboard Columbia
flight STS-83 and STS- 94 in
April and July 1997.
• Heptane in O2/He environments
Fiber Supported Droplet Combustion
Investigation - 2 (FSDC-2)
• Also flew aboard STS-83 and STS-94
Fiber Supported Droplet Combustion
FSDC-1
  i H i
  qR
• First ever space-based droplet combustion
experiment:
•Single and multicomponent droplets
•2 to 5 mm initial diameter
•suspended on silicon carbide fiber.
• Conducted aboard Space Shuttle Columbia as part
of the Second United States Microgravity Laboratory
(USML-2), October 1995.
FSDC-1 Results
Pure Methanol Droplets (Dietrich, et al., 1996)
• Measured burning rate decreases with increasing
initial diameter.
• Neglecting radiation, numerical modeling does not
reproduce this phenomenon.
1
.
0
0
d
=
4
.
6
m
m
o
d
=
2
.
9
m
m
o
d
=
1
.
3
m
m
o
N
u
m
e
r
i
c
a
lM
o
d
e
lN
o
R
a
d
i
a
t
i
o
n
0
.
7
5
d2/d o2
2
K
=
0
.
4
2
m
m
/
s
b
0
.
5
0
2
K
=
0
.
5
9
m
m
/
s
b
2
=
0
.
5
6
m
m
/
s
b
0
.
2
5 K
0
.
0
0
0
.
0
0
.
4
0
.
8
1
.
2
1
.
6
2
2
t
i
m
e
/
d
[
s
e
c
/
m
m
]
o
2
.
0
2
.
4
FSDC-1 Results
Pure Methanol Droplets (Dietrich, et al., 1996)
•Neglecting radiation, the numerical modeling predicts a
linear increase in extinction diameter with increasing
initial diameter.
•Modeling under-predicts extinction diameter
measurements.
• Measured extinction diameter appears to increase nonlinearly with increasing initial diameter.
E
x
t
i
n
c
t
i
o
n
D
i
a
m
e
t
e
r
v
s
.
I
n
i
t
i
a
l
D
i
a
m
e
t
e
r
4
.
0
3
.
5
N
u
m
e
r
i
c
a
l
M
o
d
e
l
N
o
R
a
d
i
a
t
i
o
n
F
S
D
C
1
D
a
t
a
3
.
0
2
.
5
2
.
0
ExtincoDiametr[m]
1
.
5
1
.
0
0
.
5
0
.
0
0
2
4
6
I
n
i
t
i
a
l
D
i
a
m
e
t
e
r
[
m
m
]
8
The Effect of Radiative Heat Loss
in Microgravity Droplet Combustion
At increased initial droplet diameters, gas phase
radiative heat loss can no longer be ignored!
In droplet combustion, the vaporization rate is limited by
the rates of diffusion of heat and mass, resulting in:
d 2
( d )  Cons tan t
dt s
Thus, the mass burning rate and overall instantaneous
heat release rate in the flame is directly proportional to the
droplet radius:
d 4
1
d 2

 m
 f H c    rs3l  H c   l rs
Q
d H  rs
c

dt  3
2
dt s c
 
Meanwhile, the radiative heat loss varies as the radius
cubed:
2
 rf  2
4
  A   T    r   T 4  r2  r3
Q
R
f g B f
s g B f
s g
s
 rs 
 g  1  exp(  k g PL )  k g PL  rs
Non-Luminous Gas Phase Radiation
Model Results
Calculated gas phase species and temperature for 1, 3,
and 5 mm methanol droplets at t = 0.4
FSDC-1 Results
Comparison with Radiation Model *
Diameter-squared vs. time for 1, 3, and 5 mm
droplets in air.
1
.
0
0
d
=
4
.
6
m
m
o
d
=
2
.
9
m
m
o
d
=
1
.
3
m
m
o
0
.
7
5
2 2
d
/
d
o
0
.
5
0
0
.
2
5
0
.
0
0
0
.
0 0
.
5 1
.
0 1
.
5 2
.
0 2
.
5
2
2
t
i
m
e
/
d
[
s
e
c
/
m
m
]
o
* Marchese and Dryer, 1997
Comparison with FSDC-1 Experiments
The Effect of Initial Water Addition
Diameter-squared vs. time for methanol/water
mixtures with 0, 10 and 20% initial water content.
1
.
0
2
0
%
H
O
d
=
4
.
6
m
m
2
o
1
0
%
H
O
d
=
4
.
8
m
m
2
o
0
%
H
O
d
=
4
.
6
m
m
2
o
0
.
8
0
.
6
R
a
d
i
a
t
i
o
n
22
d
/
d
o
0
.
4
0
.
2N
o
R
a
d
i
a
t
i
o
n
0
.
0
0
.
0 0
.
5 1
.
0 1
.
5 2
.
0 2
.
5
2
2
t
/
d
[
s
e
c
/
m
m
]
o
Comparison with FSDC-1 Experiments
The Effect of Initial Water Addition
Instantaneous burning rate for methanol/water
mixtures with 0, 10 and 20% initial water content.
2/s]
0
.
8
2
0
%
H
O
d
=
4
.
6
m
m
2
o
1
0
%
H
O
d
=
4
.
8
m
m
2
o
0
%
H
O
d
=
4
.
6
m
m
2
o
0
.
6
N
o
R
a
d
i
a
t
i
o
n
R
a
d
i
a
t
i
o
n
GasifcatonRate[m
0
.
4
0
.
2
0
.
0
0
.
0
0
.
5
1
.
0
1
.
5
2
2
t
/
d
[
s
e
c
/
m
m
]
o
2
.
0
2
.
5
Comparison with FSDC-1 Results
Extinction Diameter vs. Initial Diameter
• Model quantitatively predicts radiative extinction
predicted asymptotically by Chao, et al. (1990).
• For methanol in air, flames surrounding droplets
greater than about 6 mm rapidly self-extinguish.
• Results may have potential impact on spacecraft fire
safety.
4
3
0
%
W
a
t
e
r
1
0
%
W
a
t
e
r
2
0
%
W
a
t
e
r
0
%
W
a
t
e
r
1
0
%
W
a
t
e
r
2
0
%
W
a
t
e
r
ExtinconDiametr[m]
2
1
0
0
1
2
3
4
5
I
n
i
t
i
a
l
D
i
a
m
e
t
e
r
[
m
m
]
6
Droplet Combustion Experiment
DCE
• First isolated space-based droplet combustion
experiment:
• n-heptane in O2/He mixtures
• 2 to 5 mm initial diameter
• no suspension fiber.
• Conducted aboard Space Shuttle Columbia as part
of the Microgravity Science Laboratory (MSL-1), April
and July, 1997.
•Entire range of droplet combustion phenomena have
been observed:
•Radiative flame extinction
•Diffusive flame extinction
•Complete burn out.
Droplet Combustion Experiment
DCE
Space Shuttle Experiments
Results
DCE Space Shuttle Experiments
Results
Complete burnout of droplet (do = 3.5
mm)
Radiative Extinction of flame (do = 5 mm)
Model Predictions
Quasi-steady Burning Rate
For n-heptane/air:
• Model accurately reproduces measured burning
rate and variation with initial diameter.
For n-heptane/O2/He:
• Model appears to over-predict the burning rate.
• Gas-phase transport properties?
Ongoing Work
Combustion of Mars-Based Metallized Rocket Propellants
The combination of spherically symmetric combustion
modeling and microgravity experiments can be applied to a
host of problems, such as…
Gas Phase:
• Mass, Species, Energy Conservation
• Multicomponent molecular diffusion
• Complex chemical kinetics:
Mg(g) + CO2 -> MgO + CO
• Thermal radiation
• Condensed phase species agglomeration
and thermophoresis.
MgO(g)
CO2
MgO(s)
CO
MgO(g)
C(s)
MgO(s)
Mg(L)
Mg(g)
qR

Particle Interior:
• Energy transport
• Species transport
CO
T
r
Magnesium Particle Surface:
• Surface regression
• Evaporation of fuel
• Condensation of products
• Radiative heat transfer
• Surface chemistry:
Mg(s) + CO -> MgO + C
Ongoing Work
Microgravity Boiling Heat Transfer
T < 300ºF
As computers become faster, they
generate more heat. Is it possible
to use boiling heat transfer to cool
computer chips in space-based
applications?
Experimental Apparatus
Immersion
Heater
Lexan Fluid
Box
High-Speed
Digital
Camera
HP Function
Generator
HP Data
Acquisition Unit
Laptop
Speaker
36V/30A Power
Supply
½ Inch Thick
Lexan Fluid
Box
Platinum Wire
Copper
Electrodes
(w/ backup)
Speaker
Controls
Thermocouples
Ongoing Work
Microgravity Boiling Heat Transfer
Rowan Students Conducting Experiment
on NASA KC-135
Boiling Heat Transfer Curv e (.008" Platinum in water)
1200000
2
Heat Flux (W /m)
1000000
800000
600000
400000
Normal Gravity
KC-135
200000
0
1
10
100
T - T sat (C)
1000
Summary and Conclusions
Experimental techniques have been developed to
generate spherically symmetric combustion of large
droplets.
Data analysis techniques have been developed to
accurately determine burning rates and flame position.
Numerical model accurately reproduces measured
burning rates and flame position for 1 mm size droplets
neglecting radiation.
For larger droplets, gas phase radiation loss can not be
neglected.
Radiation model predicts that methanol droplets of > 6
mm will radiatively extinguish.
•Result has now been verified in FSDC-2 and DCE.
•Potential significance for spacecraft fire safety
issues.
Transport, chemistry, vapor/liquid equilibrium and
radiation (non-luminous and UV emission)sub-models
are applicable to more detailed flow situations.
Ongoing work in microgravity heat transfer and
combustion in support of future manned space
activities.