Scale and geometry effects on heat-recirculating combustors

2022

Preprint

Flame stabilization is a common problem in these combustion systems, and a flame will propagate through a fuel-air mixture only when certain conditions prevail. However, the precise mechanism underlying the stabilizing effect of the cavity method remains unclear. This study relates to the thermal management of micro-structured cavity-stabilized burners. The design factors affecting combustion stability and flame characteristics of hydrocarbon-air mixtures in millimeter-scale cavity-stabilized burners are determined. The results indicate that the flow velocity, wall thermal conductivity, channel height, and heat loss are critical factors in assuring combustion stability within the cavity-stabilized burner. The axial heat transfer from the post-flame to the pre-flame via wall conduction widens stability limits and can enable the construction of smaller burners. Axial heat transfer also enhances the benefits of operating under high admixture inlet velocity and wall heat transfer coefficient conditions. Heat exchange through the structure can lead to a broadening of the reaction region, but heat loss to the environment decreases the broadening effect and eventually results in flame quenching. The maximum power density tends to increase with increasing the channel height. Fast flows can cause blowout and slow flows can cause extinction. The anisotropic wall thermal conductivity influences the maximum achievable power density. The channel height has an important effect on the stability and performance of the cavity-stabilized burner and should be included when estimating burner performance.

Section: Theoretical Methodsmentioning

confidence: 99%

WITHDRAWN: Combustion stability and flame characteristics of hydrocarbon-air mixtures in millimeter-scale cavity-stabilized burners with proper thermal management

2022

Preprint

“…The pre-reaction of a portion of the fuel stabilizes the main combustion process by providing preheat, reactive species from fuel partial oxidation or fragmentation, or both. This type of system is referred to herein as catalytically stabilized combustion, or simply catalytic combustion [11,12]. While the effectiveness, stability and low pollutant emissions, of catalytic combustion is well documented [13,14], commercial development of catalytic combustion requires resolution of unique new design issues introduced by the catalytic reactor [15,16].…”

Section: Introductionmentioning

confidence: 99%

WITHDRAWN: The importance of cavities to the stabilization of methane flames in micro-structured combustion systems

2022

Preprint

Modeling computations are performed to highlight the importance of cavities to the stabilization of methane flames in micro-structured combustion systems. The effects of different design factors on flame stability are investigated. A dimensionless number analysis is performed to better understand the heat transfer characteristics of the burner. The results indicate that the recirculating, hot combustion products continuously contact and ignite the incoming fuel-air mixture, thus anchoring the flame in the vicinity of the recirculation region. When a cavity is used, the flame is anchored because the cavity induces recirculation of hot combustion products within the combustor. Combustion is stabilized and flame stability is improved by recirculation of hot combustion products induced by the cavity structure. An additional effect of the cavity is to direct the incoming fuel-air mixture outwards toward the walls of the combustor, thus rapidly diffusing the mixture into the combustor, making effective use of the combustor's volume. At low flame temperatures, long combustor residence times are needed for combustion completion in the gas phase. Flame stabilization is dependent upon speed of the fuel-air mixture entering the combustion region where propagation of the flame is desired. A sufficiently low velocity must be retained in the region where the flame is desired in order to sustain the flame. A region of low velocity in which a flame can be sustained can be achieved by causing recirculation of a portion of the fuel-air mixture already burned thereby providing a source of ignition to the fuel-air mixture entering the combustion region. However, the fuel-air mixture flow pattern, including any recirculation, is critical to achieving flame stability. The inlet velocity and wall thermal conductivity are vital in determining the flame stability of the burner. Faster ignition and more efficient heat transfer can be achieved, but the design is challenging due to the loss of flame stability.

“…The major problem of the micro-structured burner is the high ratio of surface to volume [65], [66]. As the ratio of surface to volume increases, heat loss to the surroundings increases, which could eventually lead to flame quenching [67], [68]. Therefore, the effect of the dimensions of the burner, for example, channel height, is investigated to better understand the stability of flame in the cavity-stabilized burner.…”

Section: Effect Of Channel Heightmentioning

confidence: 99%

Combustion Characteristics and Flame Stability of a Methane-Air Mixture in Micro-Structured Systems

Engineering Science & Technology

2022

Cavities are effective in improving the ability of a flame in micro-structured systems, but the mechanism for increased flame stability remains unclear. Numerical simulations are performed to understand the overall small-scale combustion characteristics of a cavity-stabilized burner. The effects of inlet velocity, equivalence ratio, wall thermal conductivity, channel height, and heat transfer coefficient on flame stability are investigated. A dimensionless number analysis is performed to better understand the heat transfer characteristics of the burner. The results indicate that the cavity structure can induce recirculation of hot products, thereby improving flame stability. The wall thermal conductivity and inlet velocity are vital in determining the flame stability of the burner. Further improvement in flame stability can be achieved using anisotropic walls. Fast flows can cause a blowout and slow flows can cause extinction. There exists an optimum inlet velocity for the greatest flame stability. A critical issue of fuel-rich cases is the loss of combustion efficiency. Combustion at the microscale can offer many advantages. Faster ignition and more efficient heat transfer can be achieved, but the design is challenging due to the loss of flame stability.