Fundamental characteristics of hydrocarbon combustion within
spaces with extremely small dimensions
Physical processes that transfer mass and energy by diffusion or convection occur in gaseous combustion. The management of internal features of a cavity structure for channels offers the possibility of inherently effective operation within the flammable limits of a combustible fluid stream while preserving high stability for the flame. However, the precise mechanism by which the cavity method generally provides increased flame stability for millimeter-scale systems remains unclear. The combustion characteristics of methane-air mixtures in millimeter-scale systems with a cavity structure are investigated experimentally and numerically to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into the characteristics of combustion within spaces with extremely small dimensions. Stable temperature profiles are obtained from thermographic measurements using infrared radiation. The measurements are compared with the model predictions. Subsequent model calculations demonstrate the effects of variations in wall thermal conductivity, heat losses, and equivalence ratio. Methods of applying a cavity structure to channel walls are developed, which may be utilized with presently existing designs of micro-combustion systems. The factors affecting flame stability and combustion characteristics are determined for the systems. The results indicate that the thermal conductivity of the burner walls plays a vital role in flame stability. The design with anisotropic thermal conductivity has significant performance advantages. Improvements in flame stability are achievable by using walls with anisotropic thermal conductivity. Heat-insulating materials are favored to minimize external heat losses. There are issues of efficiency loss for fuel-rich cases. Burners with large dimensions lead to a delay in flame ignition and may cause blowout. The combustion is stabilized by recirculation of hot combustion products induced by the cavity structure.
Keywords: Thermographic measurements; Infrared radiation; Internal features; Flammable limits; Fluid streams; Effective operation