pennstate introduction to micro-scale combustion and micro scale propulsion... · department of...

Department of Mechanical & Nuclear Engineering

PENNPENNSSTATETATE Introduction to MicroIntroduction to Micro--Scale CombustionScale Combustion

Pictures are from http://www.onera.fr/conferences/micropropulsion/

For “Power MEMS” devices, typically in applications where batteries are currently used.

- high power density

For “Power MEMS” devices, typically in applications where batteries are currently used.

- high power density

Micro spacecraft

Micro turbine

Primary propulsion and attitude control of micro spacecraft.

Precise positioning control of spacecraft constellations for interferometry missions.

Potential gain in thrust-to-weight ratio

Primary propulsion and attitude control of micro spacecraft.

Precise positioning control of spacecraft constellations for interferometry missions.

Potential gain in thrust-to-weight ratio1c

Thrust LWeight

−∝2

3

c t c

c

Thrust P A L

Weight L

∝ ∝

∝


PENNPENNSSTATETATE Characteristics and Challenges of Characteristics and Challenges of MicroMicro--Combustion SystemCombustion System

• Power-generation devices currently developed are those that aim to generate power in the range of a few watts to milliwatts. The corresponding combustion devices are of the order of one centimeter in size.

• The characteristic length of micro combustiors being developed to date, even in MEMS-sized systems, is sufficiently larger than the molecular mean-free paths of air and other gases flowing through the systems in which the physiochemical behavior of fluids is fundamentally the same as their macro-scale counterparts.

• As combustion volumes are reduced in size, issues of residence time, fluid mixing, thermal management, and wall quenching of gas-phase reactions become increasingly important.

• Surface-induced catalytic reactions is an attractive alternative in micro-systems.

• Power-generation devices currently developed are those that aim to generate power in the range of a few watts to milliwatts. The corresponding combustion devices are of the order of one centimeter in size.

• The characteristic length of micro combustiors being developed to date, even in MEMS-sized systems, is sufficiently larger than the molecular mean-free paths of air and other gases flowing through the systems in which the physiochemical behavior of fluids is fundamentally the same as their macro-scale counterparts.

• As combustion volumes are reduced in size, issues of residence time, fluid mixing, thermal management, and wall quenching of gas-phase reactions become increasingly important.

• Surface-induced catalytic reactions is an attractive alternative in micro-systems.


PENNPENNSSTATETATE ThermalThermal--Fluid IssuesFluid Issues

• For micro-devices with small characteristic lengths and consequently small Reynolds and Peclet numbers, the flow is primarily laminar, viscous effects and diffusive transport of mass and heat become increasingly important.

• Low Reynolds number makes mixing of reactants a potential problem in micro-systems. • For diffusion flames, molecular diffusion is the rate-controlling process. • Since turbulence mixing is weak, species mixing is primarily through diffusion. Based on

scaling analysis, the diffusion time and corresponding flame length is given by

• Complete and rapid mixing of adjacent laminar streams is desired, as is required for the initiation of a chemical reaction.

• As the device scale is reduced, the increased surface-to-volume ratio results in a large heat loss to the chamber wall. Further, the temperature gradient within the solid wall decreases due to the reduced Biot number.

• For micro-devices with small characteristic lengths and consequently small Reynolds and Peclet numbers, the flow is primarily laminar, viscous effects and diffusive transport of mass and heat become increasingly important.

• Low Reynolds number makes mixing of reactants a potential problem in micro-systems. • For diffusion flames, molecular diffusion is the rate-controlling process. • Since turbulence mixing is weak, species mixing is primarily through diffusion. Based on

scaling analysis, the diffusion time and corresponding flame length is given by

• Complete and rapid mixing of adjacent laminar streams is desired, as is required for the initiation of a chemical reaction.

• As the device scale is reduced, the increased surface-to-volume ratio results in a large heat loss to the chamber wall. Further, the temperature gradient within the solid wall decreases due to the reduced Biot number.

2

~ indiff

dτD

τ2

00~ ~ in

f diffU dL UD


PENNPENNSSTATETATE Combustion IssuesCombustion Issues

• For complete combustion, the flow residence time must be larger than the time required for chemical reactions. For non-premixed combustion, extra time and volume are needed for complete mixing.

• Flame quenching occurs if the total power generated inside the combustor is less than the loss to the wall

• A higher chamber pressure and mass flow rate help prevent flames from extinction. An exceedingly high flow velocity, however, may lead to blowoff.

• For complete combustion, the flow residence time must be larger than the time required for chemical reactions. For non-premixed combustion, extra time and volume are needed for complete mixing.

• Flame quenching occurs if the total power generated inside the combustor is less than the loss to the wall

• A higher chamber pressure and mass flow rate help prevent flames from extinction. An exceedingly high flow velocity, however, may lead to blowoff.

2~ρ π ρ= ∆ ∆tot g r g g r gW H U DL H U L

transtot WW <

1/ 2

1/3 3/ 2 1/ 2Pr ( )µ

−

−

∼ g gtrans g f g g w

g g

p UW k L T T T

R

1/3 1/ 2 1/ 21/ 2 1/ 2 1/ 2

1/ 2

Pr ( )g g f f f wg g

g r

R k T T Tp U L

Hµ−

<∆


PENNPENNSSTATETATE Development of Micro Power Generation Using CombustionDevelopment of Micro Power Generation Using Combustion

• Micro-scale power generation using combustion:

- micro-combustors/reactors- micro turbines/engines- micro-rockets

• Micro-scale power generation using combustion:

- micro-combustors/reactors- micro turbines/engines- micro-rockets

3-D Swiss-roll-type combustor-thermoelectric generator

developed at USC

3-D Swiss-roll-type combustor-thermoelectric generator

developed at USC

MEMS-based gas turbine power generator develop at MIT

MEMS-based gas turbine power generator develop at MIT

Meso- and micro- scale combustors developed at Penn State.

Meso- and micro- scale combustors developed at Penn State.


PENNPENNSSTATETATE Whirl Combustor Developed at Penn StateWhirl Combustor Developed at Penn State

10mm3

25mm3

49mm3

85mm3

108mm3fuel

oxidizer

exhaust

sapphire window

combustion volume

• A scaled down version of a macroscopic whirl combustor (Yetter, Glassman & Gabler, 2000).

• Made of Inconel with electro-discharge machining (EDM).• Combustor volume ranging from 10 to 108 mm3.• Fuel injected perpendicularly to the tangentially injected oxidizer, and the

flow exits the combustor tangentially.• Approximate flow residence time on the order of 0.1 to 1 ms for a total

mass flow rate at around 0.02 g/s (evaluated at 1500K).

• A scaled down version of a macroscopic whirl combustor (Yetter, Glassman & Gabler, 2000).

• Made of Inconel with electro-discharge machining (EDM).• Combustor volume ranging from 10 to 108 mm3.• Fuel injected perpendicularly to the tangentially injected oxidizer, and the

flow exits the combustor tangentially.• Approximate flow residence time on the order of 0.1 to 1 ms for a total

mass flow rate at around 0.02 g/s (evaluated at 1500K).


PENNPENNSSTATETATE Theoretical FormulationTheoretical Formulation

Full conservation equationsFull conservation equations

0=∂∂

+∂∂

i

i

xu

tρρ

j

ij

ij

jii

xxp

xuu

tu

∂

∂+

∂∂

−=∂

∂+

∂∂ τρρ )(

[ ]j

iji

i

i

i

i

xu

xq

xupE

tE

∂

∂+

∂∂

−=∂+∂

+∂∂ )()( τρρ

NkxUY

xuY

tY

j

jkkk

j

jkk , ... ,1 , , =∂

∂−=

∂∂

+∂

∂ ρω

ρρ

L-step reaction with N speciesL-step reaction with N species

∑ ∑⇔= =

=′′′N

k

N

kkki

k

kkki Lifor

fi

bi1 1

, ... ,2 ,1 χνχν

)/exp()( TRETATk uib

ii −=

( ) [ ] [ ]∑ ∏∏=

′′

=

′

=

=

−′−′′=

L

i

N

kkbi

N

kkfikikikk NkforkkMW

kiki

1 11

, ... ,2 ,1 νν

χχννω


PENNPENNSSTATETATE Numerical MethodNumerical Method

Preconditioning method (Hsieh et al. 1997)Preconditioning method (Hsieh et al. 1997)

gppp += 0

0=∂−∂

+∂−∂

+∂−∂

+∂∂

+∂∂

ΓzGG

yFF

xEE

tQQ vvv )()()(ˆ

τ

1 2 1ˆ , , , , , , , ,

T

g NQ p u v w T Y Y Y − = …

( ) ( ) ( ) HzGG

yFF

xEE

tQ vv =

∂−∂

+∂−∂

+∂−∂

+∂∂ ν

Finite volume approachFinite volume approach

∫∫∫∫∫∫∫∫∫ =•+•+•+∂∂

+∂∂

ΓVSSSVdVdSnWdSnWdSnWdV

tQQ H)

ˆ(

ζηξζζηηξξτ

y

x

z

i,j,k

i+1,j,ki+1/2,j,k

i+1/2,j,k+1/2

i+1/2,j-1/2,k

i+1/2,j,k-1/2

i+1/2,j+1/2,k

Sξ

Sη

Sζ1

2

34

5

6

7

8

Schematic of three-dimensional adjacent cells

Schematic of three-dimensional adjacent cells


PENNPENNSSTATETATE Behavior of Central Recirculation Zone (1/6)Behavior of Central Recirculation Zone (1/6)

Geometry of cylindrical combustorGeometry of cylindrical combustor Pseudo streamlinesPseudo streamlines

Air1.0 mm

1.0 mm

H2

0.5 mm

5.95 mm

5.16 mm

0.1 mm

One-step reversible reactionOne-step reversible reactionOHOH 222 22 ⇔+

( )

2

2 2 2

2 2

19

[ ]2 [ ] [ ] [ ]

1.102 10 exp 8025

Hf H O b H O

f

d ck c c k c

dtk T

= − −

= × ⋅ −

Basic flame structureBasic flame structure



, 20 /in airU m s= , 40 /in airU m s= , 80 /in airU m s=

non-slip upstream endnon-slip upstream end

slip upstream endslip upstream endAir

H2


• generation of recirculation zone is caused by centrifugal effect.

• viscous effect at head end reduces the size of flow recirculation.

• generation of recirculation zone is caused by centrifugal effect.

• viscous effect at head end reduces the size of flow recirculation.



0 1P atm=



0 2P atm=

Air

H2

• Both higher injection velocity and chamber pressure facilitate generation of central recirculation zone.

• Both higher injection velocity and chamber pressure facilitate generation of central recirculation zone.



0

0.5 11.5 2

0

0.511.5

0

4

81216

28

0

4

812

0

48

x=0.001mm

0

0.51 2 3

x=0.003mm x=0.005mm

Tangential velocityTangential velocity vθ

Mass flux Mass flux xvρ

x=0.001mm x=0.003mm x=0.005mm

• Fluid is transported downstream mainly in outer region.

• Tangential velocity is much higher in outer region.

• Fluid is transported downstream mainly in outer region.

• Tangential velocity is much higher in outer region.

x



Iso-surface of temperatureIso-surface of temperature1300T K=

Iso-surface of mass fractionIso-surface of mass fraction

2.02=HY 2.0

2=OY

Air

H2

• Flame structure is determined by the injection directions of fuel and air.

• Flame front is located where the fuel and oxidizer meet in stoichiometricproportions.

• Flame structure is determined by the injection directions of fuel and air.

• Flame front is located where the fuel and oxidizer meet in stoichiometricproportions.




0 1P atm=

0 2P atm=


Air

H2

• The flame length increases with increasing injection velocity.

• The flame length decreases with increasing chamber pressure.

• The flame length increases with increasing injection velocity.

• The flame length decreases with increasing chamber pressure.


PENNPENNSSTATETATE Combustion in Whirl Combustor (1/4)Combustion in Whirl Combustor (1/4)

0.22 mm H2

Air0.36 mm

2.38 mm

Products0.52 mm

Products0.52 mm0.22 mm H2

3.18 mm

Air0.36 mm

H2

airupstream

flow recirculation

downstream flow

recirculation

main flow

• Combustion products are exhausted through a tangential square port.

•Uin,air = 100 m/s

•Pc = 1atm, Tw = 800 K

•Φ = 1.0

• Combustion products are exhausted through a tangential square port.

•Uin,air = 100 m/s

•Pc = 1atm, Tw = 800 K

•Φ = 1.0

Geometry of cylindrical combustorGeometry of cylindrical combustor

• The flow injected into the combustor is divided into three parts: main flow, upstream and downstream recirculating flows.

• The flow injected into the combustor is divided into three parts: main flow, upstream and downstream recirculating flows.

Basic flow structureBasic flow structure



H2

air

H2

air

H2

air

3-D flow evolution3-D flow evolution 3-D flow traces with velocity magnitude3-D flow traces with velocity magnitude

3-D structure of flow reversal zone3-D structure of flow reversal zone• The small flow velocity in

the recirculation region helps stabilize the flame in the upstream regime.

• The small flow velocity in the recirculation region helps stabilize the flame in the upstream regime.



Distribution of temperatureDistribution of temperature

x = 0.2 mm x = 1.0 mm x = 2.0 mm x = 3.0 mm

z = 0 mm y = 0 mm



x = 0.2 mm x = 1.0 mm x = 2.0 mm x = 3.0 mm

z = 0 mm y = 0 mm

Distribution of water mass fractionDistribution of water mass fraction

• Reactions occur in a limited regime near the head end.

• Reactions occur in a limited regime near the head end.

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