Thermodiffusion in nanofluids under different gravity conditions

Abstract

A convective transport model is developed to study the role of thermal diffusion, or the Ludwig–Soret effect, in nanofluid systems with temperature gradients. The study deals with a fluid suspension of nanoparticles enclosed between two differentially heated horizontal, relatively closely spaced plates (Bénard configuration). An order-of-magnitude analysis is performed to identify the relevant parameters of the problem. Three-dimensional simulations are performed taking into account different conditions, including normal or microgravity conditions, gravity orientation, and positive or negative Soret effect. Different modes of convective instabilities are shown to be present in the system, which are associated with the gravity force and the density differences induced by concentration gradients. The characteristic flow patterns and instability developments are in agreement with the experimental findings obtained by independent investigators on colloidal suspensions. The onset of instabilities, their characteristic time scales, and flow patterns corresponding with different geometrical configurations, gravity levels, and gravity orientation are shown.

Boundary Layer Analysis and Heat Transfer of a Nanofluid

“…Previous studies have evaluated D B , D T using some reference temperature and volume fraction, whilst allowing T, φ to vary everywhere else in the equations, see for example (Buongiorno 2006;Savino and Paterna 2008). To clarify the dependence on temperature and volume fraction in all subsequent equations we will use constants that do not involve T, φ, i.e.…”

“…Evans et al (2006) concluded that Brownian motion has a negligible effect on the thermal conductivity. Savino and Paterna (2008) study buoyancy driven flow in a 1mm wide channel, they conclude that Brownian motion and thermophoresis do affect the flow, but only over a time-scale of 27 hours, with the results most noticeable when gravity is 10 −6 of its normal value. The real goal in the development of nanofluids for cooling purposes is to enhance the heat transfer and energy removal from a given surface.…”

“…The HTC is a surprisingly poorly defined quantity so, in the following section, we will begin by examining the HTC and defining it in a way that reflects the correct heat transfer from a surface. We will then develop a similar model to that of Buongiorno (2006), Savino and Paterna (2008) via the energy and momentum equations defined in chapter 3 of Bird et al (2007). Standard boundary layer scaling will be applied to reduce the equations and, in particular, demonstrate that Brownian motion and thermophoresis are negligible within the boundary layer.…”

Characterization and modeling of thermal diffusion and aggregation in nanofluids.

“…Savino and Paterna [76] modeled convective transport of nanofluids subjected to thermal diffusion in a temperature gradient between two differently heated parallel plates. They showed that the combination of concentration and thermal gradients could create circulation based on the orientation of the cell.…”

Green–Naghdi fluid with non-thermal equilibrium effects

“…Specific details of this idea in thermal convection in a porous medium may be found in Nield & Bejan (2006, p. 31), Nield (2002), Nield & Kuznetsov (1999), Banu & Rees (2002) and Malashetty et al (2005), and in the nonlinear case in Straughan (2006). Current theories for nanofluid behaviour (see Savino & Paterna 2008;Tzou 2008;Kuznetsov & Nield 2010, in press) all assume the fluid to be Newtonian. However, there is clear evidence in the literature (e.g.…”