Thermal convection of nanofluids

纳米流体是一种在胶体溶液中含有少量悬浮纳米颗粒的液体，其尺寸小于100纳米，可以有效地增强对流换热。自从十多年前报道了纳米流体有效导热系数的异常大幅增强以来，纳米流体引起了人们的广泛关注。在乙二醇中加入体积分数小于1%的铜纳米颗粒，可使其导热系数提高40%。纳米流体还具有其他理想的特性，如增强润湿和扩散，以及在沸腾条件下增加临界热通量。本文旨在阐明纳米流体的特性及其增强对流换热的机理。为了涵盖广泛的物理和实际条件，将从理论上和实验上检查三种不同的配置：(1)两个平板之间的自然（浮力驱动）对流或瑞利-贝纳德（RB）对流，以及(2)两个同心圆柱体之间的对流，以及(3)通道中的调制（强制）对流。虽然是最简单的，但RB配置允许检查基本方面，例如流体特性对对流开始的临界条件的影响（传导和对流之间的稳定性交换），三维和瞬态模式的形成，向湍流的过渡以及对流的速率。环形对流在基态的性质上表现出很大的不同，与RB平衡传导态不同，基态涉及流动剪切。强制对流可能是最接近于人们在实践中所能期望的，它增强了流动和传热之间的非线性耦合。对于控制纳米流体传热的本构定律，目前还没有普遍的共识。经典现象学能量方程的两相系统方法和两相滞后型方法，将进行探讨。实验也将在勒阿弗尔的法国国家科学研究中心实验室进行。

Convective heat transfer can be enhanced effectively by using nanofuids, which are liquids that contain a small volume fraction of suspended nanoparticles, with sizes smaller then 100 nm, in a colloidal solution. Nanofluids have attracted much attention since anomalously large enhancements in effective thermal conductivities were reported over a decade ago. An amount less then 1% volume fraction of copper nanoparticles distributed in ethylene glycol can increase its thermal conductivity by 40%. Nanofluids also have other desirable properties such as enhanced wetting and spreading, as well as increased critical heat fluxes under boiling condition. The aim of this proposal is to clarify the characteristics of nanofluids and the mechanism of the enhancement of the convective heat transfer. Three different configurations will be examined theoretically and experimentally in order to cover a wide range of physical and practical conditions: (1) natural (buoyancy driven) convection between two flat plates or Rayleigh-Benard (RB) convection, as well (2) between two concentric cylinders, and (3) modulated (forced) convection in channels. Although the simplest, the RB configuration allows the examination of fundamental aspects such as the influence of fluid characteristics on the critical conditions for onset of convection (exchange of stability between conduction and convection), formation of three-dimensional and transient patterns, transition to turbulence, and rate of convection. The annular convection presents a major difference in the nature of the base state, which, unlike the RB equilibrium conduction state, involves flow shearing. Forced convection is perhaps closest to what one can expect in practice, with enhanced nonlinear coupling between flow and heat transfer. There is no general consensus as to the constitutive laws that govern the heat transfer in nanofluids. Both the classical phenomenological energy equation with a two-phase system approach, and a dual-phase-lagging type approach, will be explored. Experiments will also be conducted at the French CNRS laboratory in Le Havre.