提高柴油机燃烧过程仿真计算精度的标定方法

Calibration method on improving simulating accuracy of combustion process in diesel engine

为提高柴油机燃烧过程仿真计算精度,该文提出了新的仿真模型标定方法:首先对仿真计算的喷雾液相贯穿距、总的喷雾贯穿距(含气相)和喷雾形态进行试验验证;然后基于喷雾过程标定结果,选择合适的燃烧模型,对仿真计算的缸压和放热率曲线进行试验验证。按照上述方法应用Fire软件研究了WAVE破碎模型、Dukowicz蒸发模型和ECFM-3Z燃烧模型相关参数对计算结果的影响,分别利用喷雾自由射流和单缸柴油机试验结果标定了喷雾和燃烧模型,并与仅利用缸压和放热率曲线标定仿真模型的传统方法进行了比较。结果表明:仅利用缸压和放热率曲线标定仿真模型的方法不能保证喷雾过程模拟的准确性,后续燃烧、排放模拟的准确性亦不能保证;采用新提出的仿真模型标定方法,能准确模拟单次和分段喷射条件下柴油机的缸内喷雾和燃烧过程,拓宽了应用范围,提高了仿真计算精度。

To improve the computation accuracy of combustion process simulation in diesel engine, a new calibration method of numerical model was proposed in this paper, and the calibration of spray process and combustion process were combined together. For the calibration of spray process, the key parameters of WAVE spray breakup model and Dukowicz evaporation model should be calibrated with experimental results, including spray liquid penetration, spray penetration and spray profile, which were measured in constant volume combustion bomb. Based on the calibration results of spray process, the simulation of combustion process required appropriate combustion model, the key parameters of which were calibrated by the experimental data in a single cylinder diesel engine, including the in-cylinder pressure and heat release rate. To validate the proposed method, a spray mesh and a moving mesh were established in AVL Fire software to calibrate the spray models and combustion model respectively. As with the meshes for the spray process, spray models parameters （including WAVE model parameter C2, Dukowicz parameters E1 and E2 ） were calibrated. To obtain proper value of C2, E~ and E2, the effects of spray model parameters on simulating results were studied and the eventual results were validated by experimental spray liquid penetration, spray penetration and spray profile, which were measured in experiment of free jet spray. Based on the calibration results of spray process, the optimal C2, E1 and E2 were applied in the simulation of combustion process. Firstly, eddy break-up （EBU） model was selected and calibration of relevant parameter A was completed in different injection conditions. The results showed that the EBU model was precisely appropriate for single injection but not for split injections, for the chemical reaction kinetics was not considered in combustion process simulation as couldn＇t predict combustion of fuel in pilot injection accurately. Then ECFM-3Z model was adopted and the model parameters include auto-ignition model parameter （Amp）, mixing model parameter （Mmp）, chemical reaction time （Crt） and extinction temperature （Et）. The effect of those parameters on numerical results was also studied. Then based on the results, the value of all parameters was verified and the accuracy of combustion model was validated by experimental results of a single cylinder diesel engine with single injection and split injections respectively. The calibration results showed that the numerical model could predict combustion process precisely both in single injection and in split injection conditions. It could be concluded that the ECFM-3Z model was appropriate for single injection and split injections. As an important reference, the common calibration method was also adopted to calibrate the numerical model. The results showed that the common calibration method couldn＇t simulate the spray process accurately. The common calibration method only considered the in-cylinder pressure and heat release rate, as may lead much uncertainty in spray development, evaporation and the fuel-air mixture formation, which influence combustion process a lot. Obviously, the application of the new calibration method could accurately simulate the whole working processes including spray, evaporation and combustion both in single injection and split injections, which improved the simulation accuracy and extends the application range compared with the common calibration method.