Research On Partially Premixed Flame-Wall Interaction By Means Of Optical Diagnostics

In spatially confined flame environments,such as the combustion chamber of diesel engines,transient flame impinging on the wall is very common.Flame-wall interaction has a significant impact on the propagation of flame and combustion characteristics,which will influence the thermal efficiency and unburned hydrocarbon emissions of engines subsequently.In order to further understand the transport phenomena and reactions near the wall,flame quenching effects were investigated here.Optical diagnostic methods were used to study the flame development and the formation and oxidation process of soot emissions in this paper.Firstly,the near-wall flame temperature and KL factor were calculated by the two-color method.When the wall is raised up,the maximum flame temperature increases slowly with the wall height.The high temperature region broadens gradually and shifts from the outside to the inside of the flame.Besides,the flame front is also narrowed by the width.As the wall temperature rises,the flame temperature increases and the high temperature zone grows at the same time.The KL factor results show that the soot near the wall is mainly distributed at the outer side of the flame.After that,the qualitative measurement of formaldehyde,hydroxyl(OH)and polycyclic aromatic hydrocarbon(PAH)was carried out by laser induced fluorescence method.Wall parameters have different effects on these species distribution:(1)The formaldehyde signal on the LIF images is basically consistent with the low temperature region.When the height of the wall increases,the peak value of the formaldehyde signal decreases,and the formaldehyde rich zone shifts from the upper of near-wall flame to the vicinity of the flame center axis.Additionally,the formaldehyde distribution range of the flame becomes wider.When the wall temperature rises,the formaldehyde signal decreases and the overall distribution range becomes smaller.(2)The OH fluorescence signal appears on the outside of the nearwall flame.As the height of wall increases,flame-wall interaction weakens and OH gradually gathers toward the inner part of the flame and the fluorescence signal gets enhanced.The OH concentration in the flame is the lowest when the wall is located 10 mm above the burner.Distribution of OH near the wall with wall temperature of 30 °C and 60 °C is not much different,but the apparent increase of OH concentration can be seen when the wall temperature rises to 125 °C.(3)The fluorescence signal of PAH enhanced first and then weakened when the wall was raised up from 10 mm to 35 mm,and the signal intensity was the highest at 20 mm and 25 mm.At different heights,the wall temperature has the opposite effect on the distribution of PAH: when the wall is low,PAH increases as the wall temperature increases,and when the wall is higher,PAH decreases as the wall temperature increases.Finally,the volume fraction of soot in the flame was quantitatively measured by laser induced incandescence method.The distribution of soot is similar to that of PAH,indicating that PAH can effectively predict the process of soot formation during flame-wall interaction.As the wall surface raises,the soot volume fraction near the wall increases first and then decreases,and the total volume fraction is the highest at 25 mm.When the wall is low,raising the wall temperature makes the soot more.When the wall is high,the soot volume fraction can be reduced by 27.5% by raising the wall temperature.