Thermal convection of nanofluids

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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.