Secondary air injection in miniature liquid fuel film combustors Roberto Mattioli a , Trinh K. Pham b , Derek Dunn-Rankin c, * a Department of Energetic Engineering, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy b Department of Mechanical Engineering, California State University, Los Angeles, CA 90032, USA c Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697-3975, USA Abstract Previous investigations on fuel film combustors have shown the capability to burn liquid fuel in a min- iaturized system, but challenges remain with respect to reaching a higher miniaturization level coupled with fully burning the fuel inside the combustion chamber. In order to achieve higher performance, this paper explores the idea of secondary air injection applied to a one centimeter diameter fuel film burner operating at ambient pressure on liquid heptane. An investigation of operating limits was performed to map the sta- bility and instability regions in the operating plane of air flow-fuel flow. The combustion and flame con- finement was studied by varying the overall equivalence ratio, the length of the chamber and the distribution of the air flow into tangential air injectors. Visualization of the flow in cold and hot conditions were accomplished respectively with laser sheet and schlieren techniques accompanied by relative pressure measurements inside the combustion chamber in cold flow and by a comparison with CFD using a three- dimensional turbulent nonreacting model. An external wall temperature analysis provided an estimate of the heat loss and predicted the vaporization point of the fuel film. These properties affect the burning effi- ciency through the available residence time and the fuel/air mixing behavior. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Mesoscale combustor; Portable power; Flame stability; Flame confinement; tubular flame 1. Introduction Miniaturization, flame confinement, and high efficiency are necessary for any personal power system (PPS) device based on combustion phe- nomena. The purpose of a PPS is to utilize stored high energy density in a manner compatible with a generic personal power application. Typical power levels needed for a combustion-based PPS is about 100 W, so to keep that generic system working for a period of 1–10 h, total energy stor- age of 100 W h to 1 kW h is needed. Despite improvements in electrochemical power devices, the current state of technology fails to achieve this goal. As reported previously, several promising configurations of fuel film combustors have been developed [1,2] since the concept of the fuel film was introduced [3] as an alternative to spray com- bustion systems at small scales. In particular, prior work has shown the possibility of creating combustors with diameters of 1 cm and the capa- bility to burn liquid fuel from the inside wall of a cylindrical combustion chamber. Despite these 1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.06.174 * Corresponding author. Fax: +1 949 824 8585. E-mail address: ddunnran@uci.edu (D. Dunn- Rankin). Available online at www.sciencedirect.com Proceedings of the Combustion Institute 32 (2009) 3091–3098 www.elsevier.com/locate/proci Proceedings of the Combustion Institute