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 Bruce Dunn Department of Materials Science and Engineering University of California, Los Angeles, CA

Motivation - fuel-driven micro-propulsion systems n Hydrocarbon fuels have numerous advantages over batteries for energy storage l ≈ 100 X higher energy density l Much higher power / weight & power / volume of engine l Nearly infinite shelf life l More constant voltage, no memory effect, instant recharge l Environmentally superior to disposable batteries

The challenge of micropropulsion n … but converting fuel energy to thrust and/or electricity with a small device has been challenging n Many approaches use scaled-down macroscopic combustion engines, but may have problems with l Heat losses - flame quenching, unburned fuel & CO emissions l Friction losses l Sealing, tolerances, manufacturing, assembly l Etc…

Thermal transpiration for propulsion systems n Q: How to produce gas pressurization (thus thrust) without mechanical compression (i.e. moving parts)? n A: Thermal transpiration - n A: Thermal transpiration - occurs in narrow channels or pores with applied temperature gradient when Knudsen number ≈ 1 l Kn  [mean free path (≈ 50 nm for air at STP)] / [channel or pore diameter (d)] n First studied by Reynolds (1879) using porous stucco plates n Kinetic theory analysis & supporting experiments by Knudsen (1901) Reynolds (1879)

Modeling of thermal transpiration n Net flow is the difference between thermal creep at wall and pressure-driven return flow n Analysis by Vargo et al. (1999): n Zero-flow pressure rise (  P no flow ) increases with Kn but Mach # (M) decreases as Kn increases n Max. pumping power ~ M  P at Kn ≈ 1 n Length of channel (L) affects M but not  P max

Aerogels for thermal transpiration n Q: How to reduce thermal power requirement for transpiration? n A: Vargo et al. (1999): aerogels - very low thermal conductivity n Gold film electrical heater n Behavior similar to theoretical prediction for straight tubes whose length (L) is 1/10 of aerogel thickness! n Can stage pumps for higher compression ratios

Aerogels n Typical pore size 20 nm n Low density (typ. 0.1 g/cm 3 ) n Thermal tolerance 500˚C n Thermal conductivity can be lower than interstitial gas! n Typically made by supercritical drying of silica gel using CO 2 solvent

Jet or rocket engine with no moving parts n Q: How to provide thermal power without electric heating as in Vargo et al.? n Answer: catalytic combustion! n Can combine with nanoporous bismuth (thermoelectric material, Dunn et al., 2000) for combined power generation & propulsion

Theoretical performance of aerogel rocket or jet engine n Can use usual propulsion relations to predict performance based on Vargo et al. model of thermal transpiration in aerogels n 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, T 1 = 300K, T 2 = 600K, P 1 = 1 atm, L = 100 µm, d = 100 nm

Theoretical performance of aerogel rocket or jet engine n Membrane thickness affects thrust but not pressure rise, specific thrust or efficiency n Performance (both power & fuel economy) increases with temperature Except as noted: Hydrocarbon-air, T 1 = 300K, T 2 = 600K, P 1 = 1 atm, L = 100 µm, d = 100 nm

Multi-stage pressurization n Multi-stage pressurization (much better propulsion performance) possible by integrating with “Swiss roll” heat exchanger / combustor

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

Feasibility testing n 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 n Airbreathing, single stage, T L = 300K, T H = 600K,  P = atm, 5.1 W thermal power n Hydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, I SP = 2750 sec n With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈ 100 mW of power, but with ≈ 30% less I SP & 2x weight

Really really preliminary ideal design n Components l Nanoporous membrane: 1 cm 2 area, 100 µm thick, 100 nm mean pore diameter, weight mN l Catalyst: Pt, deposited directly on high-T side of membrane (no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN l Low-temperature thermal guard: Magnesium weight mN (less if honeycomb; limited by strength, not conductivity), k = 120 W/mK l 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 l Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK, weight mN hot-side radiative loss 4% even for  aerogel = 1 l 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 n Ideal performance l Total weight 0.22 mN, Thrust/weight = 14 l 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 n Could eliminate need for pressurized rocket propellant tanks - mass savings l I SP with N 2 H 4 ≈ 100 sec n Combined pump & valve (no  T, no flow) n Propellant pumping for other micropropulsion technologies n Microscale pumping for gas analysis, pneumatic accumulators, cooling of dense microelectronics, … Concept for co-pumping of non-reactive gas

Conclusions n Nanoporous materials have many potential applications for microthermochemical systems l Thermal transpiration l Insulation u Best non-vacuum insulation available u Probably best insulation per unit weight for atmospheric pressure applications l Thermoelectric power generation (nanoporous Bi) l Catalyst supports n Could form the basis of a micro/mesoscale jet/rocket engine with no moving parts n Aerogel MEMS fabrication development at UCLA