Thermal Transpiration-Based Microscale Combined Propulsion & Power Generation Devices

1. Thermal Transpiration-Based Microscale Combined Propulsion & Power Generation Devices Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul RonneyDept. of Aerospace & Mechanical EngineeringUniv. of Southern California, Los Angeles, CA http://carambola.usc.edu/ Bruce Dunn Department of Materials Science and Engineering University of 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. 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

9. 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

10. Theoretical performance of aerogel rocket or 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. C C o o m m b b u u s s t t i i o o n n v v o o l l u u m m e e 1 1 6 6 0 0 0 0 1 1 4 4 0 0 0 0 7 7 0 0 0 0 6 6 0 0 0 0 5 5 0 0 0 0 P P r r o o d d u u c c t t s s R R e e a a c c t t a a n n t t s s 1 1 6 6 0 0 0 0 1 1 2 2 0 0 0 0 5 5 0 0 0 0 4 4 0 0 0 0 3 3 0 0 0 0 K K Multi-stage pressurization • Multi-stage pressurization (much better propulsion performance) possible by integrating with “Swiss roll” heat exchanger / combustor

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 = 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

15. Really really preliminary ideal design • Components • 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 • 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 rocket 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. 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