内燃机附壁油膜蒸发的一维解析

An analytical solution for wall-film evaporation in internal combustion engines

内燃机中附壁油膜是产生污染物排放的重要原因,针对该问题,本文提出了一个用于预测附壁油膜蒸发率的方程,并建立了一维非稳态数学模型来分析附壁油膜的加热和蒸发特性。该模型考虑了油膜与壁面的热传导,与空气的对流换热以及自身蒸发所释放的热量。应用所建立的模型预测了正十二烷在高温高压环境中的瞬态加热蒸发过程,考虑了油膜物性随温度的变化,得到了不同时刻不同位置附壁油膜温度分布的解析解。结果表明:附壁油膜的加热蒸发过程大致可以分为3个阶段,即初始表面快速加热阶段,稳定加热蒸发阶段和末尾阶段;在较薄的初始油膜,较高的环境温度和对流换热系数以及较低的环境压力下,油膜蒸发较快。

The uncombusted film of fuel on the piston chamber walls of an internal combustion engine is a primary source of pollutant discharge. In order to solve this problem, an equation for predicting the evaporation rate of the wall film was proposed. In addition, a one-dimensional unstable-state mathematical model was established to evaluate the heating and evaporation characteristics of the wall film. The conduction between the film and the wall, the convection between the film and the ambient air as well as the quantity of heat transferred by heat convection and e- vaporation of the wall film were considered in this new method. The transient behavior of an n-dodecane fuel at high ambient temperature and pressure was predicted by the new model, which provides an analytical solution for the temperature distribution within the fuel film. The results show that the process of wall-film heating and evaporation can be divided into three distinct stages, including an initial rapid heating stage, a stable heating and evaporation stage, and a final stage. The investigations show that the evaporation rate is higher for conditions of a thinner initial wall-film thickness, higher ambient temperature and convection rate, and lower ambient pressure.