Flame Characteristics And Blow-out Instability Of Lean-premixed Swirl Combustion

Lean-Premixed Combustion is a new clean combustion technology that performs at lower stoichiometric ratios and is effective in reducing emissions of nitrogen oxides,hydrocarbons and carbon monoxide.In recent years,the instability of lean-burn premixed combustion?such as thermoacoustic oscillation,blow-out,tempering,etc.?has become a research hotspot with the development and utilization of clean energy such as natural gas,hydrogen,and syngas.The macroscopic transformation of flame in lean-burn premixed combustion is considered to be a precursor to combustion instability.Based on the full literature research,the data collection and control system for lean-burn premixed swirl combustion experiments has been designed and completed in this paper.2DPIV and OH*-Chemiluminescence combined with laser diagnostic experiments is carried out.Digital image analysis and processing procedures are compiled to extract combustion characteristics parameters.The mechanism of flame macrostructure transition and lean blow-out instability is studied.The main work and results of this paper include:?1?The data acquisition and control system for lean-premixed swirl combustion has been designed and completed which can display and record the gas pressure and temperature upstream and downstream of the nozzle synchronously.It's also can be used to monitor and control the valve opening of the electric three-way valve to obtain a suitable concentration of tracer particle.?2?The 2DPIV and OH*-Chemiluminescence combined with laser diagnostic experiments were carried out.It was found that the macrostructure of the flame changed significantly with the decrease of the equivalence ratio,followed by the"tightening"shape,the"M"shape,the"V"shape and the"column"shape.Based on the digital image analysis method,the reaction process variable and the flame surface density were calculated.The flame surface density was linearly related to the reaction process variable,that is,the flame surface density decreased with the decrease of the process variable.In particular,in the interval where the equivalence ratio is0.50⁰.60,when the flame exhibits"M"and"V"type transitions,the flame surface density decreases by more than 80%and then the flame blows out.Therefore macrostructure of the flame can be used as a predictor of blowout instability.?3?Aiming at the swirl flame,the coupling effect of flow field and reaction heat release on flame strain has been studied.It was found that as the equivalence ratio decreases,the swirl flame front is initially away from the high vorticity region located in the inner and outer shear layers,and gradually overlaps with the high vorticity region so that the flame front is subjected to a high strain.The maximum possible strain of the flame?_max decreases as the equivalence ratio decreases and the flame extinguish strain?_ext is also greatly reduced.The ratio of them?_{ma x}?_{ex t} is used as a character of the flame blowout.The swirl flame is likely to undergo local fracture when the ratio is high,and eventually the flame surface is completely detached from the bluff body and blown out.The research in this paper helps to reveal the interaction discipline between the flame and the flow field and optimize the safe operation interval of the combustion chamber.