Micro-burners are increasingly studied for the catalytic and non-catalytic portable production of heat and energy [1, 2]. The energy produced can be utilized by thermos-electrics to generate electric power or via endothermic reactions, such as ammonia decomposition and steam reforming, to produce hydrogen for fuel cells [3, 4]. Hydrocarbons contain a significantly larger energy density than traditional lithium batteries, rendering possible the production of long-lasting and lighter devices for a number of applications such as telecommunications, unmanned compact military aircraft, cell phones, and laptops [5, 6]. Furthermore, devices that are powered by hydrocarbons can be easily refilled by adding fuel, whereas batteries can require a lengthy time and specialized equipment for recharging [7, 8]. This consideration is especially important for military applications.
One approach for creating micro-combustors is to simply miniaturize conventional large-scale combustion devices [9, 10]. Gas-phase combustion provides a high volumetric heat release as compared to catalytic combustion, which is slowed by mass-transfer limitations. However, flames are typically extinguished when confined in gaps of less than one millimeter because of thermal and radical quenching at walls [11, 12]. These quenching mechanisms become more pronounced as the surface-to-volume ratio increases in devices of one millimeter gap size or below.
Because of the importance of power generation at the microscale [1, 2], several groups have recently revisited the development of homogeneous micro-combustion devices [3, 4]. Micro-burners have been developed to allow self-sustained homogeneous combustion in channels with gaps of less than one millimeter [13, 14]. This important achievement was accomplished by using components fabricated from alumina that were modified to reduce radical adsorption and insulated to reduce thermal losses [15, 16]. Specifically, radical trapping sites were eliminated by annealing at high temperatures and cleaning the surface to remove heavy metals. These burners have been shown to successfully combust methane-oxygen mixtures.
In parallel with experimental efforts, simulations have been performed using detailed gas and surface chemistry under the boundary-layer approximation or one-step chemistry in full, two-dimensional computational fluid dynamics models to study the role of different fuels, materials of construction, heat losses, and radical quenching in the flame stability of homogeneous micro-burners [17, 18]. These studies have shown that the reactor walls provide upstream heat transfer that preheats the cold, incoming feed, as well as transverse heat losses to the surroundings [17, 18]. This mechanistic tradeoff leads to a narrow range of construction materials that conduct sufficient heat to ignite the feed, yet are insulating enough to minimize external heat losses. Even when the wall materials are optimized, the allowable heat losses are generally very low, so that the flow is restricted to a relatively narrow window of operation [19, 20]. Finally, wall temperatures in gaseous micro-burners often exceed 1500 K and can reach adiabatic flame temperatures. Such high temperatures greatly limit the available materials of construction, result in nitrogen oxides production, and require considerable device insulation and packaging [21, 22]. Although temperature reduction is possible by shrinking the device size, self-sustained oscillations in temperature and pressure emerge, leading eventually to mechanical device failure [23, 24]. If gaseous micro-burners are to be used for the production of energy in direct contact with reactors containing endothermic reactions, the stability of the micro-flames must be robust enough to withstand substantial heat harvesting [25, 26]. This currently appears to be a challenging undertaking. A fundamental understanding of the stabilization mechanisms of a flame within very small spaces by the cavity method is of both fundamental and practical significance. However, the precise mechanism by which the cavity method generally provides increased flame stability remains unclear and warrants further study.
This study relates to the combustion characteristics of a micro-structured cavity-stabilized burner. Numerical simulations are conducted to gain insights into burner performance such as temperatures, reaction rates, and flames. The effects of wall thermal conductivity, inlet velocity, and heat transfer coefficient on flame stability are investigated. The factors affecting combustion characteristics are determined for the cavity-stabilized burner. Particular focus is placed on determining essential factors that affect the performance of the burner.