A Study of Ignition and Flame Propagation in a Small, High Surface-to-Volume Ratio Combustor

Combustion-driven micro-generators, to be fabricated using MEMS (micro-electromechanical systems) technology, are required to have combustion chambers that are small and have high surface-tovolume (S-V) ratios. Since little is known about the ignition and flame propagation in closed-volume combustors of such limiting dimensions, these phenomena are the subjects of this study. The shape and magnitude of the combustor pressure history is a strong f%nction of flame-wall interaction. Decreasing the S-V ratio of the combustion chamber or delaying the time at which flames come into contact with chamber walls tends to increase the peak combustor pressure. Precompression of reactants results in a higher peak combustor pressure as well. The combustor pressure history, in a single-shot case, is insensitive to the thermal conductivity of chamber walls. In addition, increasing the spark energy above the minimum value required for ignition has no effect on the combustor pressure history. Flammability limits have also been established for combustors of different S-V ratios. INTRODUCTION The evolution of micro-technology demands that we extend our conventional understanding of many physical phenomena to increasingly smaller scales. For example, the advent of MEMS technology has increased interest in the research and development of combustiondriven ‘micro-generators” for electric power generation. Electrical generators will be required to power various military and civilian electronic devices. Currently batteries are used to power such electronics, but are undesirable because of their relatively short life and low power density. The momentum behind the development of micro-generators is primarily the military’s demand for higher power density electric generators that are minimally intrusive to the soldier in the field and are cheap to fabricate. In addition, such systems will have to be self sufficient, so that the energy required for ignition will have to be minimized. ’ Graduate Research Assistant and AIAA Member ’ Research Engineer * Professor, Associate Chair and Senior AlAA Member Copyright 1999 by J. C. Faulkner, D. E. Scarborough, and J. I. Jagoda. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. A micro-generator of this type is being developed under this study. It will consist of a tiee piston that oscillates between two pulse combustors with small chamber volume. The piston will be coupled with a linear electrical power generator capable of producing 20 Watts of power at a piston frequency of 25-50 Hz. The micro-generator will be cheaply fabricated by means of MEMS processing techniques, which impose limitations to the size and shape of the combustion chambers and piston. Namely, the piston must be flat and the thickness of the combustion chambers must be no more than 10 mm; this thickness limit is state-of-the-art for current MEMS processing techniques. Because not much is known about ignition and combustion in such narrow chambers with a high SV ratio, it was first of all necessary to prove the possibility of ignition, to study flame propagation, and to evaluate losses under these constraints. This study is the topic of this paper. BACKGROUND A premixed flame propagates by preheating reactants to their ignition temperature, causing a chemical reaction which releases energy and thus maintains the flame front. Flame quenching occurs when heat losses, for example, to a wall, establish an enthalpy sink that lowers the flame temperature and thus the reaction rate to the point at which the flame is extinguished. Several studies in the literature have characterized flame quenching at a cold wa11.14 Of particular interest to this study are some of the results obtained by Clarke et al.’ They conducted tests in a spherical constant volume combustion chamber with central ignition and investigated the effect of gravity on the combustion process. Their results showed that, in gravity, buoyancy caused the center of the flame to rise; therefore, the upper flame front reached the wall earlier than it did under microgravity. As a result, heat transfer to the chamber walls occurred earlier in a gravitational field. Delaying the interaction of the flame with the chamber walls in microgravity resulted in a higher peak combustor pressure, as shown in Fig. 1. The implications of this result will be shown to correspond with the trends discussed in this paper for the behavior of flame propagation in small, high S-V ratio combustors. In the interest of combustion efficiency and HC emission reduction, conventional ICE cylinders are designed such that the S-V ratio is minimized. This 1 American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring organization. helps assure that most of the combustion takes place away Corn the influence of walls that essentially rob energy from the combustion process. The microgenerators under development here, however, have small, flat rectangular combustion chambers. In fact, their minimum thickness is controlled by the quenching distance for the fuel used. Thus, the flame is in contact with the chamber walls for a significant portion of the burning process.