A study on the effects of air preheat on the combustion and heat transfer characteristics of Bunsen flames

Existing designs of small-scale burners and domestic cookers have relied on open flame, where a large amount of heat is lost with flue gas. Flue Gas Recirculation (FGR) has been an effective technique for energy saving for large-scale industrial furnaces. Against this background, an experimental study was conducted to investigate the thermal, emission and heating performance of CH₄-H₂/air flames in terms of their dependence on both dilution effect (CO₂ and N₂) and preheat effect (initial reactant temperature from 20 °C to 100 °C). Testing of flame stability showed detrimental and favorable effects of dilution and preheat, respectively. Examination of flame height showed that under constant Re and Ф, the reaction cone is shorter at higher initial reactant temperature, indicative of enhanced burning velocity caused by preheat. Emission of CO and NO_x was monotonically increased as the result of higher flame temperature due to preheat. The key question whether flame heating performance can be promoted or not is answered by detailed analysis of the radial heat flux configurations. The data reveals that the combined effects of dilution and preheat indeed induce higher heat transfer rate, thus theoretical evidence is provided for feasibility of adoption of FGR in domestic cookers.