纳米流体在活塞冷却油腔内部周期性振荡冲击传热特性的基础研究

The nanofluid is used to replace traditional engine oil to enhance the heat transfer effect of the piston cooling gallery during reciprocating motion. However, the introduction of periodic inertia force totally destroys the flow and heat transfer mechanisms in tubes under turbulent state. There’ll be some new mechanisms for oscillating flow and heat transfer of nanofluid inside the piston cooling gallery. Therefore, the current project plans to employ the experimental, theoretical and numerical methods to explore the flow and heat transfer characteristics of nanofliud inside an annular piston cooling gallery. To begin with, the flow visualization experiment will be used to capture the dynamic flow patterns and further investigate its evolution mechanism. The nondimensional correlation for transient heat transfer will be established based on the Bayesian posterior probability. On this basis, the flux equation of nanoparticles will be solved together with traditional conservation equations to further reveal the migratory mechanisms of nanoparticles under the action of viscosity gradient, high-speed shear and reciprocating inertia effects. The enhancement mechanism of heat transfer of nanofluid, in the mainstream area and near-wall region inside the piston cooling gallery, will be discussed in details. As a supplementary method, the numerical simulation will be adapted to obtain more detailed information that can’t be gained from the experiments and theory. With all the investigations mentioned above, the cooling process of pistons is able to be precisely controlled and designed based on its particular heat dissipation requirement, which provides a basis for the application of nanofluid as a new cooling medium of pistons.

采用纳米流体代替传统机油，用来强化活塞冷却油腔的往复振荡冲击传热效果。然而周期性惯性力的引入完全破坏了纳米流体在常规管内湍流条件下的强化流动与换热机理，其在冷却油腔内部的振荡混合流动与换热过程将会存在新的物理机制。为此，本课题拟采用实验、理论与模拟相结合的手段，探索纳米流体在环形冷却油腔内部的流动与换热特性。首先通过可视化实验考察纳米流体流型的动态分布特征及其演化机制，基于贝叶斯后验概率建立振荡冲击传热过程的非稳态准则关联式。在此基础上，引入纳米颗粒通量方程并与传统守恒方程进行耦合求解，探索粘度梯度、高速剪切和往复惯性作用下产生的纳米颗粒迁移机制，揭示纳米流体在冷却油腔内部主流区和近壁区的强化传热机理。最后建立往复振荡条件下多相混合物的数值模拟方法，作为补充手段获取实验和理论无法得到的细节信息。从而针对活塞的具体散热需求，精确地控制和设计冷却过程，为纳米流体在活塞冷却油腔中的应用奠定基础。

随着内燃机动力性能的不断提升，整体热负荷急剧增加，如何迅速冷却高温部件是保证内燃机稳定运行的关键。在目前不改变活塞材料、冷却油腔基本结构及位置的前提下，采用散热性能更好的纳米流体来代替传统冷却机油是一种更好的选择。本课题采用实验、理论与数值模拟相结合的手段，对纳米流体强化活塞冷却油腔流动与传热的基础问题进行了研究：（1）改进和完善了课题组现有的实验平台，考察了冷却介质在往复振荡条件下的瞬态流型分布特征，从宏观可视化角度探索往复振荡条件下多相流体强化流动与换热过程的有效机理，依次考察曲轴转角、振荡雷诺数、无量纲径高比、充液率、纳米流体浓度等对流型分布特征的影响；（2）研究了周期性往复惯性力作用下，纳米颗粒在冷却油腔主流区和近壁区的迁移新机制、纳米颗粒进入粘性底层后与高温壁面的相互作用机理、以及纳米流体动态壁面附着层内部强化传热的物理机制。准确建立了活塞往复振荡条件下，纳米流体流动与换热过程的数学物理模型，并通过数值模拟进一步的验证、修改和完善；（3）采用分别求解每一相守恒方程的Eulerian方法，配合纳米颗粒的通量方程，描述了多相混合物的周期性流动与换热过程。以纳米流体在主流区和近壁区的数学物理模型为理论基础，构建了纳米流体振荡冲击传热过程准确可靠的数值模拟方法，定量计算了流型及换热系数随曲轴转角的时间和空间分布特征；（4）提出了基于贝叶斯推理和深度学习的原位虚拟校准方法，在多相流体的流动和换热过程中有效地降低了传感器测量值的不确定性，实现了对测量偏差的有效补偿。