Convegni ATI - Accesso riservato soci CTI

Autore: O. Manca, S. Nardini, D. Ricci

Collana: CA - 66 - Rende 2011

Note:
The enhancement technologies in convection heat transfer allow the effectiveness improvement of heat exchangers. It is possible to achieve the enhancement targets passively by adopting different solutions. A possible solution is to employ rough surfaces. When a fluid flows in a channel, ribs break the laminar sub-layer and create local wall turbulence, due to flow separation and reattachment between consecutive ribs. This behavior reduces the thermal resistance and greatly augments the heat transfer. However, higher friction losses are expected. Another heat transfer enhancement technique is represented by the introduction of additives in the base fluids, for example nanoparticles, characterized by dimension lower than 100 nm. The thermal conductivity of the working fluids results to be increased as well as pressure drops.
In this paper a numerical investigation is carried out on forced convection with nanofluids (water-Al2O3) in a two-dimensional channel with constant heat flux applied on the external walls. Fluid properties are considered constant and the mixture model is employed. The particle size is set equal to 38 nm and different nanoparticle volume fractions are considered. The flow regime is turbulent with Reynolds numbers ranging from 20000 to 60000. Furthermore, square and rectangular and arrangement of ribs are analyzed in terms of different pitches of elements. The investigation is accomplished by means of the commercial code Fluent.
Results are presented in terms of temperature and velocity fields, average Nusselt number, heat transfer coefficients, PEC index and required pumping power profiles. The aim is to find arrangement of ribs such to give high heat transfer coefficients and low pressure drops in presence of water-Al2O3 nanofluids. Heat transfer enhancement increases with the particle concentration, but it is accompanied by increasing pumping power. Heat transfer improves, as Reynolds number increases, but also an increase of pumping power is observed.