12-10-2012, 04:23 PM
DEVELOPMENT OF A CATALYTIC SILICON MICRO-COMBUSTOR FOR
HYDROCARBON-FUELED POWER MEMS
MEMS.pdf (Size: 1.74 MB / Downloads: 43)
ABSTRACT
This paper reports the development of a high efficiency,
hydrocarbon-fueled micro-combustion system for a microscale
gas turbine engine for power generation and micro-propulsion
applications. A three-wafer catalytic combustor was
fabricated and tested. Efficiencies in excess of 40% were
achieved for ethylene-air and propane-air combustion. A
fabrication process for a six-wafer catalytic combustor was
developed and this device was successfully constructed.
INTRODUCTION
MIT’s Gas Turbine Laboratory and Microsystems Technology
Laboratory have been developing a micro-gas turbine
engine capable of producing 10-50 W of power in a volume
less than 1 cm3 while consuming 7 grams of fuel per hour
[1]. Applications include power generation, propulsion systems
for small air vehicles, and a variety of blowers, compressors,
and heat pumps. Power densities for these devices
are estimated to be an order of magnitude greater than those
for current battery technology.
As part of this program, several micro-fabricated, silicon,
gas phase combustion systems have been designed,
fabricated, packaged, and tested [2,3,4]. The development
of the micro-engine combustion system has been pursued
through a combined program of experiments, numerical simulations,
and analysis. With hydrogen fuel, these systems
reached gas temperatures exceeding 1800 K, combustor efficiencies
greater than 90%, and power densities above 1100
MW/m3, well within the requirements of a micro-gas turbine
thermodynamic cycle. However, for hydrocarbon fuels such
as propane, the power densities drop to approximately onetenth
of that for hydrogen. These fuels are more challenging
to micro-combustor development due to the 5-50 fold
increase in reaction time when compared to hydrogen. However,
they are required for practical devices due to their
increased energy density, availability, and ease of storage.
This has motivated the current study of catalytic combustors
as a means of developing a high efficiency, high power density,
hydrocarbon-fueled micro-combustor.
CHALLENGES AND CONSTRAINTS
The functional requirements of a micro-combustor are
similar to those of a conventional gas turbine combustor.
These include the efficient conversion of chemical energy to
fluid thermal and kinetic energy with low total pressure loss,
reliable ignition, and wide flammability limits. However
the obstacles to satisfying these requirements are different
for a micro-scale device. As first described by Waitz et al.
[5] a micro-scale combustor is more highly constrained by
inadequate residence time for complete combustion and high
rates of heat transfer from the combustor. Micro-combustor
development also faces unique challenges due to material
and thermodynamic cycle constraints.
FABRICATION AND TEST RESULTS
Initial feasibility tests were carried out on the threewafer
level combustor shown in Figure 1a. Nickel foam
was selected as a convenient catalyst support material and
was conventionally machined to fit in the combustion chamber.
An electron-beam deposition process was used to coat
the substrate with platinum, which serves as the active catalytic
surface. The combustor’s packaging scheme consists
of placing the silicon between two conventionally machined
invar plates. As a result, inserting catalyst material into the
combustion chamber can be done outside the clean room
environment. Figure 2 shows an exploded view of the threewafer
combustor, the surrounding invar plates, and the catalytic
insert. Figure 3 shows a photograph of the nickel foam
support as it is fit into a three-wafer device.
CONCLUSION AND FUTURE WORK
Three-wafer combustors were fitted with platinumcoated
nickel foam inserts and used as an initial catalytic
combustor test-bed. Both ethylene-air and propane-air reactions
were sustained at overall efficiencies in excess of 40%
when previously it was not possible to burn hydrocarbon
fuels in this device. This resulted in power densities approximately
half of that obtained with hydrogen rather than the
5-50 fold decrease expected for hydrocarbon-air reactions.
A method for fabricating a six-wafer combustor fitted with
platinum-coated nickel foam has been developed. Fabrication
of these devices was completed. Pressure/mass flow characteristics
and chemical and overall efficiency will be measured,
and operating space of the catalytic device will be mapped.