Thermal Transpiration-Based Microscale Thermal Transpiration-Based Microscale Combined Propulsion & Power Combined Propulsion & Power Generation Devices Generation Devices Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul Ronney Ronney Dept. of Aerospace & Mechanical Engineering Dept. of Aerospace & Mechanical Engineering Univ. of Southern California, Los Angeles, CA Univ. of Southern California, Los Angeles, CA http://carambola.usc.edu/ Bruce Dunn Bruce Dunn Department of Materials Science and Engineering Department of Materials Science and Engineering University of California, Los Angeles, CA University of California, Los Angeles, CA
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Thermal Transpiration-Based Microscale Combined Propulsion & Power Generation Devices Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul Ronney Dept.
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Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul RonneyFrancisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul RonneyDept. of Aerospace & Mechanical EngineeringDept. of Aerospace & Mechanical EngineeringUniv. of Southern California, Los Angeles, CAUniv. of Southern California, Los Angeles, CA
http://carambola.usc.edu/
Bruce DunnBruce DunnDepartment of Materials Science and EngineeringDepartment of Materials Science and Engineering
University of California, Los Angeles, CAUniversity of California, Los Angeles, CA
Motivation - fuel-driven micro-propulsion systems Hydrocarbon fuels have numerous advantages over
batteries for energy storage ≈ 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 … 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 Heat losses - flame quenching, unburned fuel & CO emissions Friction losses Sealing, tolerances, manufacturing, assembly Etc…
Thermal transpiration for propulsion systems Q: Q: How to produce gas How to produce gas
pressurization (thus thrust) without pressurization (thus thrust) without mechanical compression (i.e. mechanical compression (i.e. moving parts)?moving parts)?
A: A: Thermal transpirationThermal transpiration - - occurs in narrow channels or pores with applied temperature gradient when Knudsen number ≈ 1 Kn [mean free path (≈ 50 nm for air at
STP)] / [channel or pore diameter (d)] First studied by Reynolds (1879) First studied by Reynolds (1879)
using porous stucco platesusing porous stucco plates Kinetic theory analysis & Kinetic theory analysis &
supporting experiments by supporting experiments by Knudsen (1901)Knudsen (1901)
Reynolds (1879)
Modeling of thermal transpiration Net flow is the difference Net flow is the difference
between thermal creep at wall between thermal creep at wall and pressure-driven return flowand pressure-driven return flow
Analysis by Vargo et al. (1999):
Zero-flow pressure rise (Pno flow) increases with Kn but Mach # (M) decreases as Kn increases
Max. pumping power ~ MP at Kn ≈ 1
Length of channel (L) affects M but not Pmax
€
Pno flow
P1
=A
1− A /2;A ≡
ΔT
T f1(Kn)
€
M =1
2γ
d
L1−
ΔP
ΔPno flow
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟ΔT
T
⎛
⎝ ⎜
⎞
⎠ ⎟f2(Kn)
0.01
0.1
1
0.1 1 10 100
f1(Kn)
f2(Kn)
Knudsen number
Aerogels for thermal transpiration 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 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 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
(electrically conductive,
non-catalytic)
Nanoporous Bi
membrane
Reactants in
(ambient T, P)
Products out
(higher T,
ambient P)
High-temperature
thermal guard
(catalytic)
Medium-temperature
thermal guard
(electrically conductive,
non-catalytic)
Si aerogel
membrane
Electrical
power out
Subsonic
nozzle
(converging
section only)
Low-temperature
thermal guard
(non-catalytic)
High-temperature
thermal guard
(catalytic)
Aerogel
membrane
Reactants
in (low T,
low P)
Products
out (high T,
high P)
Theoretical performance of aerogel rocket or jet engine
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
Except as noted: Hydrocarbon-air, T1 = 300K, T2 = 600K, P1 = 1 atm, L = 100 µm, d = 100 nm
4 mm thick x 12 mm diameter aerogel membraneNo flow (maximum )P
Performance ≈ 50% of theoretical predictions in terms of both flow and pressure (even with thick membrane & no sealing of sides)
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70
ExperimentTheory/2
Differential pressure (Torr)
4 mm thick x 12 mm diameter aerogel membrane = 150˚T C
Really really preliminary ideal design Airbreathing, single stage, TAirbreathing, single stage, TLL = 300K, T = 300K, THH = 600K, = 600K, P = 0.042 atm, 5.1 P = 0.042 atm, 5.1
W thermal powerW thermal power Hydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, IHydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, ISPSP = 2750 sec = 2750 sec With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈ With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈
100 mW of power, but with ≈ 30% less I100 mW of power, but with ≈ 30% less ISPSP & 2x weight & 2x weight
Catalyst: Pt, deposited directly on high-T side of membrane Catalyst: Pt, deposited directly on high-T side of membrane (no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN(no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN
Low-temperature thermal guard: Magnesium Low-temperature thermal guard: Magnesium ZK60A-T5 alloy, 50 µm thick for 4x stress safety factor, weight 0.089 mN (less weight 0.089 mN (less if honeycomb; limited by strength, not conductivity), k = 120 if honeycomb; limited by strength, not conductivity), k = 120 W/mKW/mK
Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK, Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK, weight 0.114 mN weight 0.114 mN for 4x stress safety factor; hot-side radiative hot-side radiative loss 4% even for loss 4% even for aerogelaerogel = 1 = 1
Ideal performance Total weight 0.22 mN, Thrust/weight = 14 Hover time of vehicle (engine + fuel + Ti alloy fuel tank, no