Microchannel Combustor/Evaporator Development

M. Kevin Drost
Rich Peterson, Oregon State University
Kriston P. Brooks
Peter M. Martin

Project Description

The U.S. Department of Defense is investigating ways to improve the "soldier system." A soldier of the future may require lightweight and compact sources of electric power and microclimate control. If such is the case, thermal energy could drive the electrical generation and provide the heating or cooling. Combustion of hydrocarbon fuels produces high-density thermal energy. These fuels have an energy density (W/kg or W/m³) a factor of 100 greater than the most advanced batteries.

Battelle has developed a microchannel combustor/evaporator through the funding of the DOD's Defense Advanced Research Projects Agency (DARPA). The objective of this project was to characterize this combustor/heat exchanger in terms of combustion efficiency and emissions and find ways to further improve its performance.

This project has completed the first two phases of work and has recently been approved for its third phase. During the first phase, high heat transfer efficiencies were demonstrated. Using methane as a fuel, this project developed a microchannel combustor/evaporator that had the capacity to transfer 25 W/cm² to a microchannel evaporator. The second phase focused on achieving low air-toxic emissions during operation and developing an approach to mass production. Emissions of both NO_x and CO were reduced to below regulatory limits. Using diffusion bonding of laminates, a low cost fabrication method was developed and tested.

The first two project phases have been completed successfully. The third phase will be performed during FY 1999 and FY 2000. Previous work was done with gas phase combustion using methane in a meso-scale device (it fits in the palm of your hand). The third phase will focus on investigating the fundamental size limitations of combustion. This phase will use liquid hydrocarbon fuels such as diesel or JP-8 rather than methane. It will study both gas phase and catalytic combustion in an effort to develop a millimeter or sub-millimeter scale combustor. The fundamental limits of combustion are believed to be associated with minimizing external thermal losses and the ability to make and vaporize very small fuel droplets.

Technical Accomplishments

high flux microchannel combustor/evaporator unit

Successfully completed Phase 1: Prototype Unit Development - This task developed and demonstrated a high flux microchannel combustor/evaporator unit with a heat transfer rate of 25 W/cm² and a thermal efficiency of > 80% (based on the fuel higher heating value). These results were achieved using a methane/air flame. The unit had a thermal response time of less than a minute.
Successfully completed Phase 2: Low Emissions Unit Development - This task demonstrated the ability to use microscale emissions control concepts to minimize the emissions of CO and NO_x. Using spatially varying stoichiometry, platinum catalyst in the microchannels, and improved combustion conditions, CO and NO_x emissions below California standards were obtained.
Developed and tested a prototype damper system to be placed under the combustion zone. These micro-scale dampers will allow higher turn-down ratios than available by varying the gas flow rate alone by restricting the flow to a controlled region of the flame holder. The system is designed with 50 micro-scale dampers in a region of less than 9 cm².
Designed and tested a method of low cost combustor/evaporator fabrication using diffusion bonding of metal laminates. The combustor fabricated in this way compared favorably to a machined combustor in terms of thermal efficiency, emissions, and heat flux.

Publications and Presentations

Brooks, K. P., C. J. Call and M. K. Drost. "Integrated Microchannel Combustor/Evaporator Development," AIChE 1998 Spring National Meeting, New Orleans, LA, March 1998.

Call, C. J., M. K. Drost, and R. S. Wegeng, "Combustion and Partial Oxidation in Compact Microchannel Reactors," AIChE 1996 Spring National Meeting, New Orleans, LA, February, 1996

Drost, M.K., C.J. Call, J.M. Cuta, and R.S. Wegeng. 1997. "Microchannel Integrated Evaporator/Combustor Thermal Processes." Accepted for publication in Journal of Microscale Thermophysics Engineering.

Drost, M.K. C.J. Call, J.M. Cuta and R.S. Wegeng ,1997, "Microchannel Integrated Evaporator/Combustor Thermal Processes", Journal of Microscale Thermophysics Engineering. Vol 1, No 4, pp 321-333

Drost, M.K., C.J. Call, J.M. Cuta, and R.S. Wegeng. 1996. "Microchannel Integrated Evaporator/Combustor Thermal Processes." Presented at 2nd U.S Japan Seminar in Molecular and Microscale Transport Phenomena, Santa Barbara, California, August 8-10.