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In this paper developing turbulent forced convection flow of a water–Al2 O3 nanofluid in a square channel submitted to a constant and uniform heat flux at the wall is numerically investigated. A single model and two-phase models (discrete particles model and mixture model) are employed with constant temperature properties. The investigation is accomplished for a size particles equal to 38 nm. The CFD commercial code Fluent is employed to solve the problem by means of finite volume method. Convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement is increasing with the particle volume concentration but it is accompanied by increasing wall shear stress values. The effect of Reynolds number is great for higher concentrations. The results obtained by tree different models are presented in terms of temperatures and velocity distributions, relative increasing of heat transfer coefficient, hr , and Nusselt number profile.Copyright © 2008 by ASME

The enhancement of heat transfer represents a very important issue regarding several industrial heating or cooling equipments. This goal can be achieved by means of different techniques; for example, by adopting passive methods like changing flow geometries or by enhancing the thermal conductivity of the working fluids. Thus, an innovative way of improving the thermal conductivity of base fluids consists into introduce suspended small solid nanoparticles. In this paper a numerical investigation on laminar mixed convection with Al2O3/water based nanofluids in a triangular channel is presented. Duct surfaces were heated by means of a constant and uniform heat flux. The analysis was performed in laminar steady state regime for particle size in nanofluids equal to 38 nm, by adopting a single-phase model approach. The base fluid was water and alumina (Al2O3) nanoparticles were dispersed at different concentrations. Different Richardson number values were considered in order to solve a 3-D model of triangular duct. Results are presented for the fully developed regime flow in terms of surface shear stress and heat transfer convective coefficient, Nusselt number and required pumping power profiles. Comparisons with results related to the fluid dynamic and thermal behaviors are carried out in order to evaluate the enhancement due to the presence of nanoparticles in terms of volumetric concentration. The increase of average convective heat transfer coefficients and Nusselt number values for increasing values of Richardson number and particle concentration is observed by analyzing the obtained results. However, also wall shear stress and required pumping power profiles increase as expected.

In this paper a numerical investigation on confined impinging slot jets working with a mixture of water and Al2O3 nanoparticles is described. The flow is laminar and a constant temperature is applied on the target surface. A single-phase model approach has been adopted because the particle concentrations are low. Different geometric ratios and nanoparticle volume concentrations have been considered at different Richardson numbers in order to take into account also the buoyancy effects. The aim consists into study the behaviour of the system by means of average and local Nusselt number, convective heat transfer coefficient and required pumping power profiles, temperature fields and stream function contours. The dimensionless stream function contours showed that the intensity and size of the vortex structures depend on the confining effects, given by H/W ratio, Richardson number and particle concentrations. Furthermore, for increasing concentrations, nanofluids realize increasing fluid bulk temperature, as a result of the elevated thermal conductivity of mixtures. The local Nusselt number profiles show the highest values at the stagnation point and they depend on Richardson number and particle concentrations.