船用低速柴油机缸内工作过程数值模拟的广义自适应策略与技术研究

As the reduction of pollutant from diesel exhaust progress, the requirement of more detailed understanding of the combustion process in the cylinder becomes more and more important since the resultant composition of pollutant is open related to the details of in-cylinder physico-chemical process starting from fuel injection to post combustion cooling. Although directly simulating the complete in-cylinder process of diesel engine is still not possible, the transition from model based computer simulation towards direct simulation has already started in the field of CFD. The objective of the current project is to develop the computational strategy and relevant technique so that the numerical simulation of in-cylinder process can closely follow the development of computer technology and fruitfully transfer itself from the current modeling based simulation to direct simulation in the future. The project is aimed at direction simulation by applying the strategy of self adaptation to the physical processes of fuel injection and subsequent combustion. By dividing the linear space resulting from discretisation of governing equations into subspaces with nearly equal norm, the final form of the set of linear equations can be solved in massively parallelizing fashion with balanced load. The self adaptation in the current project has a much more general implication that is not limited to geometrical mesh and time step but also include the physical models adopted. An algorithm is to be developed for the selection of physical, chemical as well as geometrical parameters to satisfy the criteria of space division defined by the norm of the sub-matrix as the representation of a sub-space.

柴油机减排研究表明，排放物组成的数值预测应包含缸内油气混合及燃烧细节。这就提出了对缸内工作过程直接模拟的要求。尽管目前的计算机软硬件技术尚不能满足柴油机直接模拟的需要，但CFD由经验模型向直接模拟的过渡已经开始。本项目从当前柴油机模拟现实出发，以缸内工作过程的全面直接模拟为目标，以喷雾及燃烧过程为具体对象，研究此类模拟的过渡策略及相关技术，以便该领域的数值模拟在有效地跟随计算机技术发展的同时也能不断地为柴油机减排研究提供更好地服务。在向直接模拟的过渡中，本项目拟采用广义自适应策略，将已有网格自适应概念推广至喷雾及燃烧反应空间。在计算实现上，将离散后的线性空间划分为等范数子空间。这项划分不仅仅定义在几何空间，也包括物理及化学反应空间。划分的目的在于取得各子空间的同步收敛，为大规模并行计算做理论准备。本项目将开发新的计算方法，用以遴选满足广义子空间划分判据（子矩阵范数）的物理、化学及几何参数。

随着船用柴油机进一步向高速、大功率方向发展，柴油机的喷雾特性及其混合过程对燃烧和排放问题更加显著。为降低柴油机主要排放污染物，发展新型柴油机燃烧技术，对喷雾燃烧过程进行深入分析是必不可少的环节。柴油机减排研究表明，排放物组成的数值预测应包含缸内油气混合及燃烧细节。本项目从当前柴油机模拟现实出发，以缸内工作过程的全面直接模拟为目标，以喷雾及燃烧过程为具体对象，研究了并行计算算法，确定其求解策略。建立了几何体数据管理系统，将柴油机燃烧过程各个部分的材料、作用、所处热力学状态环境的不同分别管理。在数据结构和数据流设计中，采用面向对象程序设计技术，分别设计几何体类和流体类，使得程序结构更贴近人对发动机工作过程的认识。完成了几何体网格自适应程序设计，柴油机参数化设计系统设计，几何模型表面网格优化研究，几何模型网格自适应的VB程序实现。通过对流场网格的自适应研究，完成了带有微小边界的气相化学反应的变温变压液相蒸发的数值模拟计算软件。本项目采用阴影法对单孔喷嘴自由喷雾特性进行实验，得到不同参数对柴油喷雾性能的影响，主要有不同喷射压力下贯穿距离、喷雾锥角随时间的变化，以及环境气体温度对自由喷雾的影响。综合上述结果，本研究推进了缸内工作过程CFD由经验模型向直接模拟的过渡，为该领域的数值模拟在有效地跟随计算机技术发展的同时不断地为柴油机减排研究提供更好地服务。