Transcript Integrated Micropower Generator

Extinction Limits of Catalytic
Combustion in Microchannels
Kaoru Maruta, Koichi Takeda
Akita Prefectural University, Honjyo, Akita, Japan
Jeongmin Ahn, Kevin Borer, Lars Sitzki, Paul D. Ronney
University of Southern California, Los Angeles, CA USA
Olaf Deutschmann
University of Heidelberg, D-69120 Heidelberg Germany
SIAM Conference on Numerical Combustion
Sorrento, Italy, April 8-10, 2002
Supported by U.S. Defense Advanced Research Projects
Agency (DARPA)
Motivation
• Advances in portable electronic devices require power sources
with higher energy/mass than batteries
• Micro-combustors are a possible solution - hydrocarbon fuel
energy/mass ≈ 100x lithium-ion batteries
• Development
of
micro-scale
combustors
challenging,
especially due to heat losses
• Catalysis may help generally can sustain catalytic
combustion at lower temperatures than gas-phase combustion
- reduces heat loss and thermal stress problems
• Higher surface area to volume ratio at small scales beneficial
to catalytic combustion
Motivation
• Experiments in scaled-up “Swiss-roll” heat recirculating
burners with and without Pt catalyst in center show
Equivalence ratio at lean limit
– Dual limits - Low velocity heat loss, high velocity “blow off”
– Catalyst vs. non-catalyst (reversal of limits)
– Lean limits richer than stoichiometric (!) (catalytic only)
3
1
0.8
0.6
0.4
0.2
Ceramic (no cat)
Ceramic (cat)
Inconel (no cat)
Inconel (cat)
Weinberg 4.5 turn CH4
Conventional
lean limit
Pr opane
10
100
Reynolds number
1000
Motivation
• Limit temperatures for catalytic combustion are lower than
non-catalytic combustion, even when limit fuel
concentration is higher with catalytic combustion!
• Fuel % and temperature at limit indicates Pt catalyst
inhibits combustion under some conditions!
Center temp. at limit (ÞC)
1200
Inconel (no cat)
1000
Pr opane
Inconel (cat)
Ceramic (no cat)
800
Ceramic (cat)
600
400
200
10
100
Reynolds number
1000
Objectives
• Model interactions of chemical reaction, heat loss,
fluid flow in simple geometry at small scales
• Examine effects of
– Heat loss coefficient (H)
– Flow velocity or Reynolds number (2.4 - 60)
– Fuel/air AND fuel/O2 ratio - conventional experiments
using fuel/air mixtures might be misleading because
both fuel/O2 ratio and adiabatic flame temperatures are
changed simultaneously!
Model
• Cylindrical tube reactor, 1 mm dia. x 10 mm length
• FLUENT + detailed catalytic combustion model
(Deutchmann et al.)
• Gas-phase reaction neglected - not expected under
these conditions (Ohadi & Buckley, 2001)
• Thermal conduction along wall neglected
• Pt catalyst, CH4-air and CH4-O2-N2 mixtures
Wall boundary condition
H = 0, 5 or10 W/m 2ÞC
1 mm
diameter
Fuel/air
inlet
1 mm
non-catalytic wall
9 mm
catalytic wall
Results - fuel/air mixtures
• “Dual-limit” behavior similar to experiments
observed when heat loss is present
a
b
c
Results - fuel/O2/N2 mixtures
• Ratio of heat loss to
heat generation ≈ 1 at
low-velocity extinction
limits
Results - fuel/air mixtures
• Surface temperature profiles show effects of
– Heat loss at low flow velocities
– Axial diffusion (broader profile) at low flow velocities
Results - fuel/air mixtures
•
Heat release inhibited by high O(s) coverage (slow O(s) desorption) at
low temperatures - need Pt(s) sites for fuel adsorption / oxidation
a
b
Heat release rates and gas-phase CH4 mole fraction
Surface coverage
Results - fuel/O2/N2 mixtures
• Computations with fuel:O2 fixed, N2 (not air) dilution
• Minimum fuel concentration and flame temperatures needed to
sustain combustion much lower for even slightly rich mixtures!
Results - fuel/O2/N2 mixtures
• Combustion
sustained at
much smaller
total heat
release rate for
even slightly
rich mixtures
Results - fuel/O2/N2 mixtures
• Behavior due to
transition from O(s)
coverage for lean
mixtures (excess O2)
to CO(s) coverage
for rich mixtures
(excess fuel)
Lean
Rich
Experiments
• Predictions qualitatively consistent with experiments (propaneO2-N2) in 2D Swiss roll (not straight tube) at low Re: sharp
decrease in % fuel at limit upon crossing stoichiometric fuel:O2
ratio
• Lean mixtures: % fuel at limit lower with no catalyst
• Rich mixtures: opposite!
• Temperatures at limit always lower with catalyst
• Similar results found with methane, but minimum flame
temperatures for lean mixtures exceed materials limitation of
our burner!
• No analogous behavior seen without catalyst - only
conventional rapid increase in % fuel at limit for rich fuel:O2
ratios
900
T
max
3
(non-cat)
800
T
max
2.5
(cat)
700
2
600
Fuel % (cat)
500
1.5
Fuel % (non-cat)
1
400
0.4
0.6
0.8
1
Equivalence ratio
1.2
1.4
Mole % propane at limit
Maximum temperature at limit (K)
Experiments
Experiments
Re = 35
3
680
2.5
660
T
max
(cat)
2
640
Fuel % (cat)
1.5
620
1
600
0.4
0.6
0.8
1
Equivalence ratio
1.2
1.4
Mole % propane at limit
Maximum temperature at limit (K)
700
Conclusions
• Computations of catalytic combustion in a 1
mm diameter channel with heat losses reveal
– Dual limit behavior - low-speed heat loss limit &
high-speed blow-off limit
– Behavior dependent on surface coverage - Pt(s)
promotes reaction, O(s) inhibits reaction
– Effect of equivalence ratio very important transition to CO(s) coverage for rich mixtures, less
inhibition than O(s)
• Behavior of catalytic combustion in microchannels VERY
different from “conventional” flames
• Results qualitatively consistent with experiments, even
in a different geometry (Swiss roll vs. straight tube)
with different fuel (propane vs. methane)
Conclusions
• Pt catalyst actually inhibits combustion at low
temperature, but only for lean mixtures
• Typical strategy to reduce flame temperature: dilute
with excess air, but for catalytic combustion at low
temperature, slightly rich mixtures with N2 or exhaust
gas dilution to reduce temperature is a much better
operating strategy!