Thermal Transpiration-Based Thermal Transpiration-Based Microscale Combined Propulsion & Microscale Combined Propulsion &

Power Generation DevicesPower Generation Devices

Francisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul RonneyFrancisco Ochoa, Jeongmin Ahn, Craig Eastwood, Paul RonneyDept. of Aerospace & Mechanical EngineeringDept. of Aerospace & Mechanical EngineeringUniv. of Southern California, Los Angeles, CAUniv. of Southern California, Los Angeles, CA

http://carambola.usc.edu/

Bruce DunnBruce DunnDepartment of Materials Science and EngineeringDepartment of Materials Science and Engineering

University of California, Los Angeles, CAUniversity of California, Los Angeles, CA

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: Q: How to produce gas How to produce gas

pressurization (thus thrust) without pressurization (thus thrust) without mechanical compression (i.e. mechanical compression (i.e. moving parts)?moving parts)?

A: A: Thermal transpirationThermal 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) First studied by Reynolds (1879)

using porous stucco platesusing porous stucco plates Kinetic theory analysis & Kinetic theory analysis &

supporting experiments by supporting experiments by Knudsen (1901)Knudsen (1901)

Reynolds (1879)

Modeling of thermal transpiration Net flow is the difference Net flow is the difference

between thermal creep at wall between thermal creep at wall and pressure-driven return flowand 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 ~ MP at Kn ≈ 1

Length of channel (L) affects M but not Pmax

€

Pno flow

P1

= A1− A /2

;A ≡ ΔTT

f1(Kn)

€

M = 12γ

dL

1− ΔPΔPno flow

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ΔTT

⎛ ⎝ ⎜

⎞ ⎠ ⎟f2(Kn)

0.01

0.1

1

0.1 1 10 100

f1(Kn)

f2(Kn)

Knudsen number

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

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

Low-temperature

thermal guard

(electrically conductive,

non-catalytic)

Nanoporous Bi

membrane

Reactants in

(ambient T, P)

Products out

(higher T,

ambient P)

High-temperature

thermal guard

(catalytic)

Medium-temperature

thermal guard

(electrically conductive,

non-catalytic)

Si aerogel

membrane

Electrical

power out

Subsonic

nozzle

(converging

section only)

Low-temperature

thermal guard

(non-catalytic)

High-temperature

thermal guard

(catalytic)

Aerogel

membrane

Reactants

in (low T,

low P)

Products

out (high T,

high P)

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

0

2

4

6

8

10

0 0.05 0.1 0.15 0.2 0.25Pressure rise / ambient pressure (P/P

1)

Thrust specific fuel consumption

Pumpingefficiency (%)

Specific impulse / 1000(seconds)

Membrane exitvelocity / 10 (cm/s)

Specific thrust x 10

0

2

4

6

8

10

0 50 100 150 200Mean pore diameter (d) (nm)

Thrust specific fuel consumption

Pumpingefficiency (%)

Membrane exitvelocity / 10 (cm/s)

Specific thrust x 10

Specific impulse / 1000(seconds)

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

0

2

4

6

8

10

0 0.5 1 1.5 2Membrane thickness (L) (mm)

Thrust specific fuel consumption

Pumpingefficiency (%)

Membrane exitvelocity (cm/s)

Specific thrust x 10

Specific impulse/1000(seconds)

0

2

4

6

8

10

300 400 500 600 700 800 900Hot-side temperature (T

2) (K)

Thrust specific fuel consumption

Pumpingefficiency (%)

Membrane exitvelocity / 10 (cm/s)

Specific thrust x 10

Specific impulse / 1000(seconds)

Multi-stage pressurization Multi-stage pressurization (much better propulsion

performance) possible by integrating with “Swiss roll” heat exchanger / combustor

Cold

flow

Hot

flow

Cold

flow

Hot

flow

Low P

flow in

High P

flow out

Swiss-roll

walls

Micro-

capillaries

Aerogel

membranes

Reactants in

Products out

Combustion

zone

Products out

Combustion

zone

Reactant

inlet

Transpiration stages Cooling stages

High pressure in

Products

Reactants

Combustionvolume

1600 1200 400 300 K500

1400 600 5007001600

Products

Reactants

Combustionvolume

1600 1200 400 300 K500

1400 600 5007001600

Feasibility testing Simple (“crude”?) test fixture built Electrical heating to date; catalytic

combustion testing starting Conventionally machined

commercial aerogel (L = 4 mm)

T h e r m a l g u a r d

T h e r m a l g u a r d

P t c a t a l y s t

A e r o g e l

C a t a l y s t s u p p o r t /

o u t l e t p l e n u m

I n l e t p l e n u m

O - r i n g s e a l s

Feasibility testing

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

ExperimentTheory/2

Temperature differential (˚C)

4 mm thick x 12 mm diameter aerogel membraneNo flow (maximum P)

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

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70

ExperimentTheory/2

Differential pressure (Torr)

4 mm thick x 12 mm diameter aerogel membraneT = 150˚C

Really really preliminary ideal design Airbreathing, single stage, TAirbreathing, single stage, TLL = 300K, T = 300K, THH = 600K, = 600K, P = 0.042 atm, 5.1 P = 0.042 atm, 5.1

W thermal powerW thermal power Hydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, IHydrocarbon fuel, thrust 3.1 mN, specific thrust 0.36, ISPSP = 2750 sec = 2750 sec With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈ With nanoporous Bi (ZT ≈ 0.39; 300K < T < 400K) could generate ≈

100 mW of power, but with ≈ 30% less I100 mW of power, but with ≈ 30% less ISPSP & 2x weight & 2x weight

Mg alloy low-temperature

thermal guard

Reactants in (ambient T, P)

Products out (higher T, ambient P)

Metalization of hot-side of membrane

with Pt

Si aerogelmembrane

Subsonic nozzle

(Ti alloy)

Really really preliminary ideal design ComponentsComponents

Nanoporous membrane: 1 cmNanoporous membrane: 1 cm22 area, 100 µm thick, 100 nm area, 100 µm thick, 100 nm mean pore diameter, weight 0.00098 mNmean pore diameter, weight 0.00098 mN

Catalyst: Pt, deposited directly on high-T side of membrane Catalyst: Pt, deposited directly on high-T side of membrane (no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN(no need for hi-T thermal guard), 1 µm thick, weight 0.02 mN

Low-temperature thermal guard: Magnesium Low-temperature thermal guard: Magnesium ZK60A-T5 alloy, 50 µm thick for 4x stress safety factor, weight 0.089 mN (less if weight 0.089 mN (less if honeycomb; limited by strength, not conductivity), k = 120 honeycomb; limited by strength, not conductivity), k = 120 W/mKW/mK

Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK, Case & nozzle: 5 mm long, titanium 811 alloy, k = 6 W/mK, weight 0.114 mN weight 0.114 mN for 4x stress safety factor; hot-side radiative hot-side radiative loss 4% even for loss 4% even for eeaerogelaerogel = 1 = 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 Could eliminate need for

pressurized rocket pressurized rocket propellant tanks - mass propellant tanks - mass savingssavings IISPSP with N with N22HH44 ≈ 100 sec ≈ 100 sec

Combined pump & valve (no Combined pump & valve (no T, no flow)T, no flow)

Propellant pumping for Propellant pumping for other micropropulsion other micropropulsion technologiestechnologies

Microscale pumping for gas analysis, pneumatic accumulators, cooling of dense microelectronics, …

Required low-temperature thermal guard (non-catalytic)

Aerogel membrane

(no catalyst)

Reactants in (low T, low P)

Products out(high T, high P)

Required high-temperature thermal guard (non-catalytic)

Catalyst

Non-reacting gas in (low T, low P)

Non-reacting gas out (high T, high P)

Aerogel membrane

(no catalyst)

Concept for co-pumping of non-reactive gas

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 developmentat UCLA

QuickTime™ and aGraphics decompressorare needed to see this picture.Etching maskAlumina aerogelSacrificial siliconSilicon