Transcript PowerPoint Presentation - Thermal transpiration based

Thermal Transpiration-Based
Microscale Combined Propulsion &
Power Generation Devices
Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul Ronney
Dept. of Aerospace & Mechanical Engineering
Univ. of Southern California, Los Angeles, CA
http://carambola.usc.edu/
Bruce Dunn
Department of Materials Science and Engineering
University of California, Los Angeles, CA
Motivation - fuel-driven micro-propulsion systems
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Hydrocarbon fuels have numerous advantages over
batteries for energy storage
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≈ 100 X higher energy density
Much higher power / weight & power / volume of engine
Nearly infinite shelf life
More constant voltage, no memory effect, instant recharge
Environmentally superior to disposable batteries
The challenge of micropropulsion
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… but converting fuel energy to thrust and/or
electricity with a small device has been challenging
Many approaches use scaled-down macroscopic
combustion engines, but may have problems with
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Heat losses - flame quenching, unburned fuel & CO emissions
Friction losses
Sealing, tolerances, manufacturing, assembly
Etc…
Thermal transpiration for propulsion systems
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Q: How to produce gas
pressurization (thus thrust) without
mechanical compression (i.e.
moving parts)?
A: Thermal transpiration - occurs
in narrow channels or pores with
applied temperature gradient when
Knudsen number ≈ 1
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Kn  [mean free path (≈ 50 nm for air at
STP)] / [channel or pore diameter (d)]
First studied by Reynolds (1879)
using porous stucco plates
Kinetic theory analysis &
supporting experiments by
Knudsen (1901)
Reynolds (1879)
Modeling of thermal transpiration
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Net flow is the difference
between thermal creep at wall
and pressure-driven return flow
Analysis by Vargo et al. (1999):
Pno flow
A
T

;A 
f1 (Kn)
P1
1 A /2
T
1 d 
P T 
M 
1


 f 2 (Kn)


2 L  Pno flow  T 
Zero-flow pressure rise (Pno flow)
increases with Kn but Mach # (M)
decreases as Kn increases
Max. pumping power ~ MP at
Kn ≈ 1
Length of channel (L) affects M
but not Pmax
1
f(Kn)
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f (Kn)
0.1
1
f (Kn)
2
0.01
0.1
1
10
Knudsen number
100
Aerogels for thermal transpiration
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Q: How to reduce thermal power requirement for transpiration?
A: Vargo et al. (1999): aerogels - very low thermal conductivity
Gold film electrical heater
Behavior similar to theoretical prediction for straight tubes
whose length (L) is 1/10 of aerogel thickness!
Can stage pumps for higher compression ratios
Aerogels
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Typical pore size 20 nm
Low density (typ. 0.1 g/cm3)
Thermal tolerance 500˚C
Thermal conductivity can be
lower than interstitial gas!
Typically made by supercritical
drying of silica gel using CO2
solvent
Jet or rocket engine with no moving parts
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Q: How to provide thermal power without electric heating as in
Vargo et al.?
Answer: catalytic combustion!
Can combine with nanoporous bismuth (thermoelectric material,
Dunn et al., 2000) for combined power generation & propulsion
Low-temperature
thermal guard
(non-catalytic)
High-temperature
thermal guard
(catalytic)
Medium-temperature
thermal guard
(electrically conductive,
non-catalytic)
High-temperature
thermal guard
(catalytic)
Low-temperature
thermal guard
Products (electrically conductive,
out (high T,
non-catalytic)
Reactants
in (low T,
low P)
high P)
Nanoporous Bi
membrane
Subsonic
nozzle
(converging
section only)
Reactants in
(ambient T, P)
Electrical
pow er out
Aerogel
membrane
Si aerogel
membrane
Products out
(higher T,
ambient P)
Theoretical performance of aerogel rocket or jet engine
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Can use usual propulsion relations to predict performance
based on Vargo et al. model of thermal transpiration in aerogels
Non-dimensional TFSC of silica aerogel (k ≈ 0.0171 W/mK) only
2x - 4x worse than theoretical performance predictions for
commercial gas turbine engines
10
10
Specific thrust x 10
Specific thrust x 10
Thrust specific
fuel consumption
8
8
6
Specific impulse / 1000
(seconds)
6
Specific impulse / 1000
(seconds)
Pumping
efficiency (%)
4
2
Thrust specific
fuel consumption
4
Pumping
efficiency (%)
2
Membrane exit
velocity / 10 (cm/s)
0
0
Membrane exit
velocity / 10 (cm/s)
0.05
0.1
0.15
0.2
0.25
Pressure rise / ambient pressure ( P/P )
1
0
0
50
100
150
Mean pore diameter (d) (nm)
Except as noted: Hydrocarbon-air, T1 = 300K,
T2 = 600K, P1 = 1 atm, L = 100 µm, d = 100 nm
200
Theoretical performance of aerogel rocket or jet engine
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Membrane thickness affects thrust but not pressure rise, specific
thrust or efficiency
Performance (both power & fuel economy) increases with
temperature
10
10
Thrust specific
fuel consumption
8
8
Specific thrust x 10
Specific thrust x 10
6
4
Specific impulse / 1000
(seconds)
4
Pumping
efficiency (%)
2
0
6
Thrust specific
fuel consumption
2
Specific impulse/1000
Membrane exit
(seconds)
velocity (cm/s)
0
1.5
1
0.5
Membrane thickness (L) (mm)
Pumping
efficiency (%)
Membrane exit
velocity / 10 (cm/s)
2
0
300
400
500
600
700
800
Hot-side temperature (T ) (K)
2
Except as noted: Hydrocarbon-air, T1 = 300K,
T2 = 600K, P1 = 1 atm, L = 100 µm, d = 100 nm
900
Multi-stage pressurization
Multi-stage pressurization (much better propulsion
performance) possible by integrating with “Swiss roll” heat
exchanger / combustor
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Combustion
volume
1600
1400
700
600
500
Products
Reactants
1600
1200
500
400
300 K
Sw is s - roll
w alls
R eac tant
inlet
Mic roc apillaries
H igh P
flo w ou t
Low P
flo w in
C ombus tion
z one
C ombus tion
z one
Aer ogel
membr anes
C old H ot
flo w flo w
C old
flo w
H ot
flo w
Pr oduc ts out
Pr oduc ts out
R eac tants in
H igh pres s ure in
Transpirat ion stages
C ooling stages
Feasibility testing
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Simple (“crude”?) test fixture built
Electrical heating to date; catalytic
combustion testing starting
Conventionally machined
commercial aerogel (L = 4 mm)
O-ring seals
Inlet plenum
Thermal guard
Catalyst support /
outlet plenum
Thermal guard
Pt catalyst
Aerogel
Feasibility testing
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Performance ≈ 50% of theoretical predictions in
terms of both flow and pressure (even with thick
membrane & no sealing of sides)
70
4 mm thick x 12 mm diameter
aerogel m embrane
T = 150ÞC
10
8
Pressure differential (Torr)
Flow rate (ml/min)
12
Experiment
Theory /2
6
4
2
0
50
40
30
20
10
20
30
40
50
Differential pressure (Torr)
60
70
Experim ent
Theory /2
10
0
0
4 mm thick x 12 mm diamet er
aerogel membrane
No f low (maximum P)
60
0
20
40
60
80
100
120
Temperature differential (ÞC)
140
160
Really really preliminary ideal design
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Airbreathing, single stage, TL = 300K, TH = 600K, P = 0.042 atm, 5.1
W thermal power
Hydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, ISP = 2750 sec
With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈
100 mW of power, but with ≈ 30% less ISP & 2x weight
Metalization of hotside of membrane
w ith Pt
Mg alloy low temper ature
therma l gua rd
Subsonic
nozzle
(Ti alloy)
Reactan ts in
(ambi ent T, P)
Si a erogel
membrane
Pro ducts out
(hi gher T,
amb ient P)
Really really preliminary ideal design
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Components
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Nanoporous membrane: 1 cm2 area, 100 µm thick, 100 nm
mean pore diameter, weight 0.00098 mN
Catalyst: Pt, deposited directly on high-T side of membrane
(no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN
Low-temperature thermal guard: Magnesium ZK60A-T5
alloy, 50 µm thick for 4x stress safety factor, weight 0.089
mN (less if honeycomb; limited by strength, not
conductivity), k = 120 W/mK
Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK,
weight 0.114 mN for 4x stress safety factor; hot-side
radiative loss 4% even for aerogel = 1
Ideal performance
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Total weight 0.22 mN, Thrust/weight = 14
Hover time of vehicle (engine + fuel + Ti alloy fuel tank, no
payload) = 2 hours; flight time (lifting body, L/D = 5) = 10
hours
Other potential applications
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Could eliminate need for
pressurized rocket
propellant tanks - mass
savings
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ISP with N2H4 ≈ 100 sec
Non-reacting gas in
(low T, low P)
Non-reacting gas out
(high T, high P)
Aerogel
membrane
(no catalyst)
Reactants in
Combined pump & valve
Products out
(low T, low P)
(high T, high P)
(no T, no flow)
Required low Required highPropellant pumping for
temperature thermal
temperature thermal
guard (non-catalytic)
guard (non-catalytic)
other micropropulsion
technologies
Aerogel
Catalyst
membrane
(no catalyst)
Microscale pumping for
gas analysis, pneumatic
Concept for co-pumping
accumulators, cooling of
of non-reactive gas
dense microelectronics, …
Conclusions
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Nanoporous materials have many potential
applications for microthermochemical systems
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Thermal transpiration
Insulation
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Best non-vacuum insulation available
Probably best insulation per unit weight for atmospheric
pressure applications
Thermoelectric power generation (nanoporous Bi)
Catalyst supports
Could form the basis of a micro/mesoscale
jet/rocket engine with no moving parts
Aerogel MEMS fabrication development
at UCLA
Etching mask
Alumina aeroge l
Sacrificial si licon
Silicon
Qu i c k T i m e ™ a n d a Gra p h i c s d e c o m p re s s o r a re n e e d e d t o s e e t h i s p i c t u re .