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 http://carambola.usc.edu/

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: 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)

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

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

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

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 ≈ 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

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

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

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

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

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

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

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

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

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