1. Francisco Ochoa, Craig Eastwood,
Thermal Transpiration-Based Mesoscale / Microscale Combined Propulsion & Power Generation Devices
Francisco Ochoa, Craig Eastwood,
Jeongmin Ahn, Lars Sitzki, Paul Ronney Dept. of Aerospace & Mechanical Engineering Univ. of Southern California, Los Angeles, CA

2. 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

3. 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…

4. Thermal transpiration for propulsion systems
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
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)

5. Modeling of thermal transpiration
Net flow is the difference between thermal creep at wall and 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 ~ MP at Kn ≈ 1
Length of channel (L) affects M but not Pmax

6. 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

7. 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

8. Fuel-driven jet 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

9. Theoretical performance of aerogel 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 ≈ 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

10. Theoretical performance of aerogel jet engine
Membrane thickness affects thrust but not pressure rise, specific thrust or efficiency
Performance (both power & fuel economy) increases with temperature
Except as noted: Hydrocarbon-air, T1 = 300K,
T2 = 600K, P1 = 1 atm, L = 100 µm, d = 100 nm

11. Multi-stage pressurization
Multi-stage pressurization (much better propulsion performance) possible by integrating with “Swiss roll” heat exchanger / combustor P r o d u c t s R e a n C m b i v l 1 6 2 4 3 K 5 7

12. Feasibility testing Simple (“crude”?) test fixture built
Electrical heating to date; catalytic combustion testing starting
Conventionally machined commercial aerogel (L = 4 mm)

13. Feasibility testing
Performance ≈ 50% of theoretical predictions in terms of both flow and pressure (even with thick membrane & no sealing of sides)

14. Really really preliminary ideal design
Airbreathing, single stage, TL = 300K, TH = 600K, P = 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

15. Really really preliminary ideal design
Components
Nanoporous membrane: 1 cm2 area, 100 µm thick, 100 nm mean pore diameter, weight 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 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 mN for 4x stress safety factor; hot-side radiative loss 4% even for aerogel = 1
Ideal performance
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

16. Other potential applications
Could eliminate need for pressurized propellant tanks - mass savings
ISP with N2H4 ≈ 100 sec
Combined pump & valve (no T, no flow)
Propellant pumping for other micropropulsion technologies
Microscale pumping for gas analysis, pneumatic accumulators, cooling of dense microelectronics, …
Concept for co-pumping of non-reactive gas

17. USC contributions to microthermochemical systems
Identified flameless combustion in broad reaction zones in heat-recirculating burners
Stability of gas-phase & catalytic modes
Tradeoffs between gas-phase & catalytic combustion
Effect of equivalence ratio (independent of flame temperature) on catalytic combustion
Effect of wall thermal conductivity
Effect of heat losses in 3rd dimension
Importance of radiation in scale-down
Designs
Fin design for thermoelectric power generation
Use of SOFC in a Swiss roll
Catalytic combustion based thermal transpiration propulsion
Multi-stage thermal transpiration pumping using Swiss roll

18. Conclusions
Nanoporous materials have many potential applications for microthermochemical systems
Thermal transpiration
Insulation
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
NASA-sponsored joint USC/UCLA program
to start 10/1/03