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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

# Design and Characterization of a Liquid-Fueled Microcombustor

[+] Author and Article Information
Jay Peck1

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139jpeck@aerodyne.com

Stuart A. Jacobson2

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

Ian A. Waitz

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

According to Dodds and Bahr (17), of the typical 5–8 ms combustor residence time in a conventional gas turbine combustor, approximately 60% (3–5 ms) is devoted to fuel vaporization and mixing and about 40% (2–3 ms) to mixing of dilution air. The chemical reaction time is fairly negligible.

Values of $Cp$ were evaluated as if the working fluid is pure air, which is in fact not the case. This assumption generally underpredicts $Cp$ by about 15% compared with the more rigorously estimated $Cp$ of the combustion reactants and products.

$T¯=(Tinlet+Texit)/2$. The validity of using the average was demonstrated by Mehra (11).

After inspecting the trend of the Peclet number distribution, a form of $Pe=(a/ηthermal)+(b/ηchemical)$ was used, then coefficients $a$ and $b$ were determined by the least square method. The map shows regions of both interpolation and extrapolation.

$Err=1/n∑k=1n[1−(a/ηthermal,k+b/ηchemical,k)/Pek]2$

1

2

Present address: Joule Biotechnologies, Inc., 83 Rogers Street, Cambridge, MA 02142.

J. Eng. Gas Turbines Power 133(7), 072301 (Mar 16, 2011) (10 pages) doi:10.1115/1.4002621 History: Received April 20, 2010; Revised April 26, 2010; Published March 16, 2011; Online March 16, 2011

## Abstract

As part of an effort to develop a microscale gas turbine engine, this paper presents the design and experimental characterization of a microcombustor that catalytically burns JP8 fuel. Due to the high energy densities of hydrocarbon fuels, microscale heat engines based on them may enable compact power sources with specific energies higher than those of current battery systems. In addition, utilizing a commonly available logistics fuel would provide advantages for military applications. Thus, a microscale engine burning JP8 fuel is attractive as a portable power source. A liquid-fueled microcombustor with a combustion chamber volume of $1.4 cm3$ and an overall die size of $36.4×36.4×6.5 mm3$ was designed, microfabricated, and experimentally characterized. Two configurations were tested and compared, one with the combustion chamber entirely filled with a catalyst and the other with the combustion chamber partially filled with a catalyst. In the configuration filled with a catalyst, JP8 combustion was sustained at mass flow rates up to 0.1 g/s and an exit gas temperature of 780 K; an overall combustor efficiency of 19% and a power density of $43 MW/m3$ were achieved. The primary limitation on increasing the mass flow rates and temperature further was the structural failure of the device due to thermal stresses. With the partially filled configuration, a mass flow rate of 0.2 g/s and a corresponding power density of $54 MW/m3$ were obtained. The exit gas temperature for the partially filled configuration was as high as 720 K, and the maximum overall efficiency was over 22%. Although the reduced amount of catalyst led to incomplete combustion, smaller thermal losses resulted in an increase in the overall combustor efficiency and power density. A nondimensional operating map was constructed based on the experiment, and it suggests that improving the thermal efficiency would be necessary to achieve higher efficiencies in the device.

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## Figures

Figure 1

Cross-sectional view of the MIT microengine

Figure 2

Typical result of the catalytic combustion model for various tube diameters (ϕ=1.0 and P=2.0 atm)

Figure 3

Final design of the liquid-fueled microcombustor test rig

Figure 4

Front side of the completed silicon piece (showing the front side of layer 3)

Figure 5

Photograph of the JP8 combustor in operation

Figure 6

JP8 combustion result: exit gas temperature versus equivalence ratio for different mass flow rates

Figure 7

JP8 combustion result: (a) exit gas temperature and (b) structural temperature versus mass flow rate for different equivalence ratios

Figure 8

JP8 combustion result: efficiency breakdown for ϕ=1.1

Figure 9

JP8 combustion result: Peclet number versus mass flow rate

Figure 10

JP8 combustion result: (a) thermal efficiency and (b) chemical efficiency versus Peclet number

Figure 11

Relevant parameters divided into nondimensional parameters

Figure 12

JP8 combustion result: lines of constant Peclet numbers on a chemical efficiency and thermal efficiency plane

Figure 13

Partially filled catalytic microcombustor result: temperatures versus mass flow rate for ϕ=0.9

Figure 14

Partially filled catalytic microcombustor result: efficiencies versus mass flow rate for ϕ=0.9

Figure 15

Device comparison: (a) exit gas temperatures and (b) structural temperatures versus mass flow rate for ϕ=0.9

Figure 16

Efficiency breakdown comparison between the filled and partially filled catalytic combustors (ϕ=0.9)

Figure 17

Comparison of chemical efficiencies versus Peclet number between the filled and the partially filled (with corrected Peclet number) catalytic combustors

Figure 18

Device comparison on a nondimensional operating space

Figure 19

Nondimensional operating map of liquid-fueled microcombustor

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