Numerical Analysis Of The Three-Dimensional Effect On Flame Stability In Micro-and Meso-scale Channels

Numerical simulation has been widely applied to the investigation of factors affecting flame stability in micro-combustion.Two-dimensional models are commonly deployed to reduce the complexity of theory and hence computational cost.However,additional effect exists in three-dimensional simulations,which will affect flame stability.Thus,in this thesis 3D simulation was used to analysis following three problems:The combustible ranges of both equivalent ratio and inlet velocity of the square tube are wider than that of the circular tube under same hydraulic diameter.Numerical analysis reveals that there are three reasons for these differences in flame stabilization ability.First,the heat loss ratio of square tube is slightly lower than that of the circular tube.Second,the square tube has a longer circumference and a very low velocity zone in four corners which can stabilize the flame.Third,the strain rate at the flame tip in the square tube is smaller and thus the flame suffers a weaker stretch effect.With fixed height,the function of blow-off limit of methane/air flame to aspect ratio is non-monotonic,and the maximum value is 0.66 m/s with aspect ratio being 3.Two reasons were proposed for the non-monotonic behavior in this thesis.On the one hand,increasing aspect ratio causes the hydraulic diameter to increase,which leads to smaller surface-tovolume ratio and heat loss.These are beneficial to the flame stability.On the other hand,increasing aspect ratio will also cause the ratio of width to flame thickness to increase,which is prone to result in hydrodynamic instability.With increasing aspect ratio and fixed height,the stability of hydrogen/air flame does not change significantly,and the flame shape always changes in the direction of height at the beginning.The blow-off limit of hydrogen/methane/air flame in the rectangular channel with aspect ratio of 4 increases monotonically with the rise of hydrogen addition ratio.When hydrogen addition ratio is higher than 20%,the W-shaped flame will disappear.With further increase of the inlet velocity,the flat flame will change into U-shaped flame.There are mainly two reasons as follows: on the one hand,more OH group is generated with higher hydrogen ratio which will speed the combustion reaction rate,and thus inhibit the generation of Wshaped flame.On the other hand,excess hydrogen will reduce the Lewis number of the mixture,increasing the curvature of the flat flame until it finally changes into U-shaped flame.In this thesis,the 3D effect on flame stability in micro-and meso-scale channels was studied.The results are helpful for the researchers to select better model in simulations and may guide the design of micro-and meso-scale burners.