纳米颗粒在多孔介质内的迁移机理及其宏观控制方程

Nanofluid is an innovative cooling medium that can effectively increase the performance of heat exchange equipment. In order to develop advanced heat transfer technology and heat exchange equipment with high efficiency and low energy consumption, in-depth study of fluid flow and heat transfer processes of nanofluids in heat exchange equipment has become a hot spot of current research. Due to the complexity and irregularity of internal structure of heat exchange equipment, porous media model and theory have been widely used to investigate the fluid flow and heat transfer processes. However, there are still many imperfections of theory related to macroscopic governing equations of nanofluid-saturated porous media, since fluid flow and heat transfer processes of nanofluid in porous media involve complex transport phenomena. Moreover, the study associated with effective coefficients of macroscopic governing equations of nanofluid-saturated porous media has not been touched so far. In this project, the macroscopic governing equations of nanofluid-saturated porous media will be derived with nanofluid-saturated porous media as the research object, so as to elucidate the migration mechanism of nanoparticles and investigate the fluid flow and heat transfer processes in nanofluid-saturated porous media. Based on various unit cell structures, the effective coefficient correlations of macroscopic governing equations of nanofluid-saturated porous media will be obtained to discuss the influence of physical geometry and solid phase contact on the effective coefficients. In addition, the proposed macroscopic governing equations of nanofluid-saturated porous media will be applied to study the fluid flow and heat transfer processes of nanofluids in microchannel heat sink, and verify their availability, which will provide a mathematical tool and theoretical basis for the development of nanofluid-cooled microchannel heat sink with high efficiency and low energy consumption.

纳米流体是一种能有效提高换热设备性能的传热冷却工质。为了开发高效、低能耗的换热技术与设备，深入开展纳米流体在换热设备内的流动与传热研究，已成为当前研究的一个热点。由于实际的换热设备内部结构复杂且不规则，多孔介质模型和理论被广泛应用于研究换热设备的流动与传热过程。纳米流体在多孔介质内的流动与传热过程涉及复杂的传输现象，基于纳米流体的多孔介质宏观控制方程的理论尚不完善，尤其是其封闭方法目前还较少涉及。本研究以多孔介质内的纳米流体为研究对象，建立基于纳米流体的多孔介质宏观控制方程，阐明纳米颗粒在多孔介质内的迁移机理；针对几种不同的单元体，拟合得到宏观控制方程的有效系数表达式，探讨物理结构和微观骨架之间的接触对有效系数的影响。通过拟建立的多孔介质宏观控制方程研究纳米流体在微通道热沉中的流动与传热过程，验证该宏观控制方程的有效性，为设计高效、低能耗的微通道热沉提供数学工具和理论支持。

纳米流体是一种能有效提高换热设备性能的传热冷却工质。为了研究纳米流体在微通道内的流动和传热特性，考虑微通道内的尺寸效应和纳米颗粒在微尺度下的四种迁移机理，建立纳米流体在微通道内的微观数学模型。分析结果表明在微通道内布朗扩散和热泳力仍然是纳米颗粒迁移的主导因素。基于体积平均理论和纳米流体的微观控制方程，推导获得基于纳米流体的多孔介质宏观控制方程，并以泡沫金属骨架为特征单元获得有效系数，实现宏观方程的封闭。最终，我们利用该宏观方程研究了纳米流体在泡沫金属内的流动和强化传热特性。分析结果表明，混合强化的换热系数是纯水换热系数的80倍。由于单一纳米流体往往会存在传热越强，阻力增加越大的瓶颈，我们提出使用混合纳米流体。基于单一纳米流体的微观控制方程，建立了基于混合纳米流体的微观控制方程，深入研究混合纳米流体的种类、体积分数比值、颗粒直径比值、混合纳米流体的总体积分数等参数对混合纳米流体的传热和水力学特性的影响。结果表明，氧化铝-氧化钛/水混合纳米流体不仅比氧化铝-氧化锆/水混合纳米流体具有更好的传热性能，而且阻力更小。与此同时，采用PEC指数进行性能评价氧化铝-氧化钛/水混合纳米流体，结果表明在一定泵功率下存在着最优的体积分数比值和颗粒直径比值。