Experimental study and analyzing the near-wall quenching behavior for lean-burned methane/air premixed laminar flames

Abstract

The flame quenching at the near-wall region plays the most significant role on the unburned hydrocarbon (UHC) emission in spark ignition engines. Especially for lean-burn nature gas engines, near-wall quenching results in a large amount of methane slip which leads to a unneglectable greenhouse effect. Research on flame quenching under ultra-lean combustion and at elevated pressures are lacking, which contributes to UHC prediction in lean-burn nature gas engines. In this study, quantitative measurements of the quenching distance for lean CH₄/air laminar premixed flames were carried out at elevated pressures. Deep insight of near-wall quenching behaviors and flame propagation characteristics for laminar premixed CH₄/air flames were proposed through simultaneous high-speed schlieren and CH* chemiluminescence micrography. Laminar flame thicknesses of different equivalence ratios were released to reveal the relation between the flame front structure and wall-quenching characteristics, and the effects of flame front structure on the near-wall quenching were evaluated. Normal thermal gradient of the boundary layer at the near-wall region was calculated through the measured heat flux by employing high-speed heat flux sensors. Furthermore, dimensionless parameters as Nusselt number (Nu) and Peclet number (Pe) were derived to clarify the relation between the wall heat transfer and near-wall flame quenching. The result shows that quenching distances decrease exponentially with larger equivalence ratios, while reaching a significantly large distance near the lean limit (ϕ = 0.5) in this study. Although thermal gradients increase rapidly with larger equivalence ratios, the higher reactivity resulting from larger equivalence ratios overweighs the thermal gradient effect, leading to smaller quenching distances. The quenching distance is inversely proportional to the initial ambient pressure at the range of 0.5∼3 MPa in this study. The wall thermal gradient, however, enhances with the increase of the initial ambient pressure. Due to the dimensionless analysis, the wall heat transfer shows a strong effect on the near-wall flame quenching significantly under elevated pressures and lean combustion conditions.