Abstract
The focus of this work is to visualise the regions of CH2O and heat release (HR) of an unconfined turbulent premixed bluff body stabilised ethylene-air flame at conditions approaching lean blow-off using simultaneous imaging of OH- and CH2O-PLIF. The HR regions are estimated from the product of the OH and CH2O profiles. At conditions near blow-off, wide regions of CH2O are observed inside the recirculation zone (RZ). The presence of CH2O and HR inside the RZ is observed to follow fragmentation of the downstream flame parts near the top of the RZ. The presence of wide regions void of both OH and CH2O inside the RZ at conditions very close to blow-off indicates the possible entrainment of un-reacted gases into the RZ. The behaviour of the lean ethylene-air flame with Lewis number (Le) greater than 1 is compared to that of a lean methane-air flame with Le of approximately 1. For both fuels, qualitatively similar observations of flame fragmentation downstream followed by build-up of CH2O and HR inside the RZ are observed at conditions near lean blow-off. Also, a similar trend of flame front curvature conditioned on HR was observed for both the ethylene-air and methane-air flames, where the magnitude of HR was observed to increase with the absolute value of curvature.
Highlights
In the combustion literature, simultaneous imaging of PLIF of OH and CH2O has been demonstrated to be a useful technique to visualise regions of heat release (HR) in premixedFlow Turbulence Combust flames [1]
Vortex-like structures appear along this region, causing the width of the CH2O and HR profiles to increase locally in the region between the flame surfaces created as preheated reactants are transported towards the flame front by these structures
If extinction of the premixed flame begins at the burnt side, the consumption of CH2O, which occurs in the high temperature region of the flame, may be frozen, causing the CH2O radicals to survive for some duration. This may explain the overlap of wide regions filled with both OH and CH2O inside the recirculation zone (RZ), where the surviving overlapping OH and CH2O may indicate the presence of quenched gases
Summary
Flow Turbulence Combust flames [1] It allows for a qualitative study of the reaction zone, while the individual profiles of CH2O- and OH-PLIF provide useful information concerning regions of partially reacted and burned gases respectively. Such measurements are useful when investigating combustion processes characterised by finite rate chemistry effects, such as lean extinction, where the presence of CH2O allows for identification of regions undergoing low temperature combustion processes. Paul et al [2] demonstrated that the profile of the pixel-by-pixel product of simultaneously obtained OH- and CH2O-PLIF images correlates well with HR rates in premixed flames. Taking the product of their PLIF profiles allows for a qualitative visualisation of the HR regions in premixed flames [2]
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