纳米流体在内燃机冷却水腔中应用的基础问题研究

The rapid increasing of engine specific power will inevitably generate higher combustion temperature, which results in more heat transferring to the combustion chamber components. And the resulting problem of cooling in internal combustion engines is more and more prominent which has become the bottle-neck of developing highly strengthened internal combustion engines. In view of the excellent heat transfer performance of nanofluids, the present work attempts to apply nanofluids in the cooling water jacket of engine in order to enhance heat transferring. Furthermore, as the heat transfer performance of nanofluids is influenced by several influencing factors including material, size, concentration, and shape of nanoparticles, the heat transfer quantity of the cooling water jacket could be controlled by changing these influencing factors. By means of molecular dynamics simulation on nanofluids combining with visualized and quantitative experiments on flow of nanofluids, the present subject intents to reveal the physical essence of heat transfer enhancement in nanofluids from the viewpoint of microcosmic and macrocosmic, therefore build the mathematical and physical model of convective heat transfer in nanofluids, and propose the effective method to carry out numerical simulation on convective heat transfer of two phase nanofluids flow. Furthermore, the basic research results on convective heat transfer of nanofluids is applied to the study on enhanced heat transfer in engine cooling gallery by using nanofluids. And suitable mathematical and physical model and numerical simulation method for convective heat transfer of nanofluids which is appropriate for the need of designing engine heat transfer are proposed, which lays a theoretical foundation for application of nanofluids in internal combustion engine to enhance heat transfer.

随着内燃机升功率的大幅提高，必然会产生更高的缸内温度，从而造成更多的热量传递给燃烧室部件，由此产生的内燃机冷却问题变得越来越突出，已成为制约内燃机高强化发展的一个瓶颈。鉴于纳米流体良好的传热性能，试图将其用于内燃机冷却水腔，以达到强化传热的目的。且由于纳米流体的传热性能受纳米粒子的种类、大小、浓度、形状等因素影响，因此，可以通过改变这些因素，控制内燃机冷却水腔的传热量。课题拟通过纳米流体的分子动力学模拟，以及纳米流体流动的可视化与定量化实验研究，从微观和宏观两个层面上，揭示纳米流体强化传热的物理本质，进而建立纳米流体对流换热的数学物理模型，给出纳米流体两相流对流换热数值模拟的有效方法。并将有关纳米流体对流换热的基础研究结果，用于内燃机冷却水腔纳米流体强化传热研究，试图给出适于内燃机传热设计需要的纳米流体对流换热数学物理模型和数值模拟方法，为纳米流体在内燃机强化传热中的应用奠定理论基础。

课题的主要研究内容有：1）纳米流体分子动力学模拟研究；2）纳米流体实验研究；3）纳米流体对流换热两相流数学物理模型和数值模拟研究；4）纳米流体在内燃机冷却水腔强化传热中的应用研究。取得的创新成果有：1）用分子动力学模拟证实了纳米颗粒表面液体吸附层、纳米颗粒微运动是纳米流体导热强化的微观机制；并提出纳米流体总体呈现出类似固体的微观结构特征，也是纳米流体导热强化的微观机理；建立了纳米流体导热系数预测模型，模型预测结果得到了实验验证。2）提出用纳米颗粒中所含高能原子比例的高低作为判据，来判断纳米颗粒强化纳米流体导热系数能力的强弱。3）提出以单个纳米颗粒为核心，由液体吸附层、旋转液体层、有限存在空间构成的旋转流体微元结构，且由无数旋转流体微元构成了纳米流体动态微观结构；揭示了导热强化、流体内部掺混加强、以及传热方式发生根本改变是纳米流体流动换热强化的机理。4）纳米流体流动与传热MD研究发现，纳米流体的速度脉动相对于基础流体增强，纳米颗粒与连续相间存在速度滑移；纳米颗粒微运动和导热系数增加对纳米流体对流换热性能提高的贡献比例相当。5）从固液两相的动量守恒方程出发，推导出了适用于纳米颗粒的虚拟颗粒粘度模型，该模型模拟结果明显好于传统模型。6）可视化实验发现：纳米流体由层流向湍流转变的转捩点提前，从而证实了纳米流体通过改变流场，进而增强流体扰动，最终达到强化传热的效果。7）由纳米流体的热量与动量传递实验结果发现：纳米流体导热系数的增大对强化传热的贡献有限；纳米颗粒的微运动导致基础流体的动量传递强化，是纳米流体强化传热的主要原因。8）纳米流体内燃机冷却水腔应用研究表明：Euler-Euler两相流模型更适于内燃机的实际应用，但纳米流体粒子流的粘度模型至关重要，是影响数值模拟精度的关键因素。上述主要研究结果为纳米流体在内燃机冷却水腔中的实际应用奠定了理论基础，未来可根据纳米流体的具体特性来强化或控制内燃机冷却水腔的传热。