Thermal Transpiration-Based Microscale Combined Propulsion

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

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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

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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

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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)
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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

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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

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Aerogels

• Typical pore size 20 nm
• Low density (typ. 0.1 g/cm3)
• Thermal tolerance 500C
• Thermal conductivity can be lower than
  interstitial gas!
• Typically made by supercritical drying of silica
  gel using CO2 solvent

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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

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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
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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
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Multi-stage pressurization

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

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Feasibility testing

• Simple (crude?) test fixture built
• Electrical heating to date catalytic combustion
  testing starting
• Conventionally machined commercial aerogel (L 4
  mm)

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Feasibility testing

• Performance 50 of theoretical predictions in
  terms of both flow and pressure (even with thick
  membrane no sealing of sides)

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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 lt T lt 400K)
  could generate 100 mW of power, but with 30
  less ISP 2x weight

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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

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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
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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