Study on the Ignition and Combustion Mechanism of High Cetane Number Fuels Impacted by Methanol Addition
|Course||Power Machinery and Engineering|
|Keywords||Methanol Diesel N-heptane Chemical Kinetics LowPressure Premixed Flame Simulation|
The ignition of methanol by diesel is an economical, efficient and cleancombustion method; it is helpful to solve the energy and environment problems. Tooptimize the combustion of diesel/methanol fuels is the key stage for the widely usingof this engine work mode, so it is necessary to study the ignition and combustion ofdiesel/methanol based on the chemical kinetic. In this paper, n-heptane as a kind ofreference fuel of diesel is used to simulate diesel, simulation and experiment methodare utilize to study the low temperature oxidation（ignition） and high temperatureoxidation（combustion） of methanol/n-heptaneFor the low temperature oxidation, an experiment was conducted in acommon-rail diesel engine to study the ignition of premixed methanol by diesel and ina constant vessel which was used to simulate the ignition of diesel injected intomethanol-air gas mixture. The results both show that the ignition delay become longercompared with normal conditions, meanwhile the suppression effect of ignitioncaused by methanol become weak when the temperature increased. In order to seekthe chemical effect of that the ignition of diesel suppressed by methanol, the0-Dmethanol/n-heptane combustion chemical kinetic simulation was performed with adetailed mechanism. Simulation result shows that by comparing with n-heptane singlefuel, the mixing of methanol changes the production and consumption paths of OHand HO2in the radical pool and show a negative effect to the production of OH in thelow temperature oxidation region, so the two stages heat release are delayed and thelow temperature heat release is suppressed. The direct action between methanol,n-heptane and their oxidation intermediates can be neglected. The reaction paths ofn-heptane oxidation are also not disturbed by the methanol additive. Sensitive analysisshows that the dehydrogenation by OH from n-heptane, methanol and formaldehydeis the most important reaction. In the low temperature reaction region, thechain-branch reactions of n-heptane are more important; in the negative temperaturecoefficient （NTC） region the chain-carry reactions and the production andconsumption of HO2and H2O2are more important; before the high temperature heatrelease, the production and decomposition of H2O2are more important. The ignitiondelay of single fuel is longer than it of dual fuel when the initial temperature is lowerthan1000K, but it is reverse when the initial temperature exceeds1000K. Finally, aglobal mechanism involved17reaction was created preliminary.For the high temperature, an experimental study of the premixed methanol/n-heptane/oxygen/argon flame at25Torr with the methanol blend ratio of0%,11%,28%,50%was performed with the tunable synchrotron VUV （vacuumultra-violet） photoionization and molecular-beam sampling mass spectrometry.62kinds of combustion intermediates and final products were detected and their molefractions were also calculated. The results show that the consumption rate ofn-heptane and the dissociation of large molecular are not impacted by the methanoladdition, the effect of methanol behavior at the small molecular. The oxidation rate ofmethanol is larger than that of n-heptane, so the overall reaction rate will be enlarged.The equivalent concentrations of methyl, acetylene, ethyl, ethane and ketene decreasewhen methanol addition, but the equivalent concentrations of formaldehyde andacetaldehyde increase. Besides, the maximum mole fraction positions of methyl, ethyl,ethane and propane move away from the burner with methanol addition, but forformaldehyde it is in the opposite. The maximum mole fraction positions of otherhydrocarbon do not changed by methanol addition. Finally simulation will be on thebasis of experiment data in the future.