Effect of wall material thermal conductivity on the butane flame
stability of microscale gas fired burners
The effect of wall material thermal conductivity on butane flame stability is investigated for microscale gas fired burners. The present study aims to provide a fundamental understanding of the butane flame stability of microscale gas fired burners at different wall material thermal conductivities. Particular emphasis is placed upon the stability limits over a range of equivalence ratios and the effect of wall material thermal conductivity on the butane flame stability. The results indicate that the wall material thermal conductivity is vital in determining the butane flame stability of the gas fired burners, as the walls are responsible for the majority of the upstream heat transfer as well as the external heat losses. The most effective way of increasing the lean stability limits of the burner is an increase in primary stream inlet temperature. Completing the combustion process near homogeneous stoichiometric conditions, by intensifying the mixing process, may increase nitrogen oxides emissions. To ensure that the combustion process of the furnace is not adversely affected by the presence of the device, the device should not adversely interfere with the flow of products of combustion away from the combustion zone for each burner. Low wall thermal conductivities result in large axial wall-temperature gradients and high maximum temperatures. High wall thermal conductivity leads to uniform temperature profiles without hotspots. Low wall thermal conductivities cause the flame to shift downstream. Increasing wall thermal conductivity has little effect on flame location unless there are significant external heat losses. Typical ceramics allow maximum external heat loss coefficients. Materials with lower wall thermal conductivities limit the upstream heat transfer. Materials with higher wall thermal conductivities result in enhanced heat transfer to the surroundings. The inlet flow velocity plays a competing role in flame stability. There is only a relatively narrow envelope of flow rates within which combustion can be stabilized. The maximum fluid temperature exceeds the adiabatic flame temperature of butane-air mixture computed for room temperature.
Keywords: Burners; Combustion; Emissions; Extinction; Flames; Stability