Thermal convection of nanofluids

使用纳米流体可以有效地增强对流换热，纳米流体是在胶体溶液中含有体积分数较小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有效导热系数异常大幅度的增强以来，纳米流体引起了人们的极大关注。体积分数小于1%的纳米铜颗粒分散在乙二醇中，可使其导热系数提高40%。纳米流体还具有其他理想的性质，如增强润湿和铺展，以及在沸腾条件下增加临界热流密度。该方案的目的是阐明纳米流体的特性和强化对流换热的机理。为了涵盖广泛的物理和实际条件，将从理论和实验上检验三种不同的构型：(1)两平板之间的自然(浮力驱动)对流或Rayleigh-Benard(RB)对流，以及(2)两个同心圆筒之间的对流，以及(3)通道中的调制(强迫)对流。虽然RB配置最简单，但它允许检查基本方面，如流体特性对对流开始的临界条件(传导和对流之间的稳定性交换)的影响、三维和瞬变模式的形成、向湍流的过渡以及对流速度。环状对流在基态的性质上有很大的不同，与Rb平衡导电态不同，基态涉及流动剪切。强迫对流可能最接近于人们在实践中可以预期的，因为流动和换热之间的非线性耦合得到了增强。对于控制纳米流体中热传递的本构定律，还没有达成普遍的共识。我们将探讨经典唯象能量方程的两相系统方法和双相滞后型方法。实验也将在位于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.