Laminar Burning Research Of Natural Gas-Air-CO₂ Mixture Based On Constant Volume Combustion Bomb

Today,under the background of energy crisis and environment pollution,natural gas has been considered as the most promising alternative fuel of engines because of its many advantages such as rich reserves,clean combustion and high thermal efficiency.Among many technical methods of natural gas engine,equivalence combustion combined with EGR technique is regarded as the primary means to satisfy the emission regulation of next stage,owe to advantages of little modification to original engine,etc.The burning velocity of natural gas is slower when mixed with EGR gas,and engine’s thermal efficiency is also lower.But,the flame propagation speed will be faster if the initial temperature of combustible mixture is elevated.Laminar burning is an important part of combustion fundamental research,the fundamental of complicated turbulent burning research.Based on constant volume combustion bomb test platform,with CO₂ instead of EGR gas in engine,this article experimentally studied the laminar burning characteristics of natural gas-air-CO₂ mixture,emphasizing on the analysis of the equivalent effect of initial temperature and dilution rate on unstretched flame propagation speed.Firstly,the influences of initial temperature,initial pressure,fuel air equivalence ratio and CO₂ dilution rate on flame propagation characteristics parameters and combustion characteristic parameters of combustible mixture was analyzed.Conclusions were drew as follows: stretched flame propagation speed,unstretched flame propagation speed and laminar burning velocity all increased with the increasing of initial temperature,decreased with the increasing of initial pressure and CO₂ dilution rate,previously increased then decreased with the increasing of equivalence ratio,the peak equivalence ratio of stretched flame propagation speed was 1.1 around,the peak equivalence ratio of unstretched flame propagation speed was 1.2 or 1.3,the peak equivalence ratio of laminar burning velocity was 1.1 or 1.2 around.Markstein length increased with the increasing of equivalence ratio,decreased with the increasing of initial temperature and initial pressure.The effects of dilution gas on Markstein length depend on equivalence ratio,Markstein length increased with the increasing of CO₂ dilution rate whenφ≥1.2,decreased with the increasing of CO₂ dilution rate whenφ<1.2.Both flame development period and combustion duration period increased with the increasing of initial pressure and CO₂ dilution rate,decreased with the increasing of initial temperature,previously decreased then increased with the increasing of equivalence ratio,corresponding equivalence ratio of the shortest flame development period and combustion duration period was 1.1 or 1.2 around.Then,the comprehensive influences of initial temperature and CO₂ dilution rate on unstretched flame propagation speed,flame development period and combustion duration period was analyzed with contour tool.The results showed that,for three temperature interval（323K 348 K,348K 398 K and 398 K 423K）,the effects of temperature on three parameters above gradually weakened.Detailed combustion parameters of two pairs of operation condition which had very similar unstretched flame propagation speed was compared.The results showed that,two unstretched flame propagation speeds whose deviation was less than 2% led to CA10,CA50,CA90 and combustion duration period with deviation less than 7%.However,as to the highest pressure,temperature and NOx emission of combustion,operation conditions with low temperature and CO₂ dilution rate prevailed over those with high temperature and CO₂ dilution rate.With regard to 348 K series and 398 K series operation conditions,the deviation of all of unstretched flame propagation speed were less than ± 6% when CO₂ dilution rate was malposed matched（operation conditions with high temperature had 2% more CO₂ dilution rate）.With regard to 323 K series and 373 K series operation conditions,the deviation of all of unstretched flame propagation speed were less than ± 7% when the same malposed match as above was made,except for 2 pairs of operation conditions.