An experimental investigation of the thermal flame structure during side-wall quenching of a laminar premixed flame

The thermal flame structure (i.e., the temperature distribution in the preheat and reaction zones) is a core element that describes fundamental flame behavior and chemical reactions. This work utilizes a suite of laser diagnostics to capture the consequential changes of the laminar flame structure resulting from wall heat transfer during premixed flame-wall interactions. Hybrid (fs/ps) rotational coherent anti-Stokes Raman spectroscopy (HRCARS) is used to measure 1D spatially-resolved gas temperature (Tgas) and its gradients throughout the flame. OH laser induced fluorescence (LIF) is used to measure the flame distribution, and phosphor thermometry (PT) is used to measure wall temperature. HRCARS coupled with OH-LIF provides a representative flame temperature (Tflame,OH), while HRCARS coupled with PT measures the wall heat flux (Qwall). Measurements are performed in a side-wall quenching burner operating with a lean premixed mixture CH4-air of Phi = 0.83 under laminar conditions. When the flame is farther than 1 mm from the wall, the flame structure is qualitatively similar to an adiabatic flame with the maximum temperature gradient (VTmax) in the preheat zone occurring at 1000 K. As the preheat zone reaches the wall, heat loss reduces VTmax in the preheat zone to a minimum, at which point the location of VTmax decouples from its established 1000 K temperature and approaches the wall where lower Tgas resides. VTmax undergoes dynamic changes, but remains attached to the wall until quenching. As VTmax reaches a minimum, Tflame,OH undergoes a non-linear decrease from 1750 K to 1400 K before the flame quenches. Normalized quantities are presented alongside non-dimensional heat flux, where a 30 % decrease in VTmax occurs at Qwall/Q Sigma = 0.2, followed by a 15 % decrease in Tflame,OH as Qwall/Q Sigma reaches 0.25. Findings report the modified thermal flame features that describe the stages of flame weakening that lead to flame quenching.