Thermal convection of nanofluids

通过使用纳米流体可以有效地增强对流传热，纳米流体是在胶体溶液中含有小体积分数的悬浮纳米颗粒的液体，其尺寸小于100 nm。自从十多年前报道了纳米流体在有效热导率方面的巨大增强以来，纳米流体引起了人们的广泛关注。小于1%体积分数的铜纳米颗粒分布在乙二醇中可以增加其热导率40%。纳米流体还具有其他期望的性质，例如增强的润湿和铺展，以及在沸腾条件下增加的临界热通量。本研究的目的是阐明纳米流体的特性及其强化对流换热的机理。为了涵盖广泛的物理和实际条件，将从理论和实验上研究三种不同的配置：（1）两个平板之间的自然（浮力驱动）对流或Rayleigh-Benard（RB）对流，以及（2）两个同心圆柱体之间的自然（浮力驱动）对流，以及（3）通道中的调制（强制）对流。虽然最简单，RB配置允许检查的基本方面，如流体特性的对流（传导和对流之间的稳定性交换），形成的三维和瞬态模式，过渡到湍流的临界条件的影响，和对流率。环形对流提出了一个主要的区别，在性质的基础状态，这与RB平衡传导状态，涉及流剪切。强制对流可能是最接近人们在实践中所能期望的，具有增强的流动和传热之间的非线性耦合。关于纳米流体中的热传递的本构关系，目前还没有普遍的共识。经典的唯象能量方程与两相系统的方法，和双相滞后型的方法，将被探讨。实验也将在法国勒阿弗尔的CNRS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.