## THERMAL SCIENCE

International Scientific Journal

### NUMERICAL SIMULATION OF FLOW REVERSAL AT HIGH RAYLEIGH NUMBER IN A VERTICAL SQUARE DUCT - AN APPLICATION OF THE SPECTRAL METHOD

**ABSTRACT**

In this paper we numerically study the mixed convection in hydrodynamically as well as thermally developed incompressible laminar flow of nanofluids in a vertical square duct subject to the thermal boundary condition of constant heat flux per unit axial length with constant peripheral temperature at any cross-section, using the spectral method and the finite difference method. We have considered three different water based nanofluids containing alumina, titanium oxide or silver nanoparticles. We observe that the Rayleigh number remarkably reduces the fluid velocity and even the flow reversal may occur while developing an equi-temperature region for a small region in the center of the duct. It has also been noted that, for silver-water nanofluid, the nanoparticle volume fraction increases the Nusselt number more rapidly while slowly decreasing the product of friction and Reynolds number.

**KEYWORDS**

PAPER SUBMITTED: 2016-01-02

PAPER REVISED: 2017-05-31

PAPER ACCEPTED: 2017-06-12

PUBLISHED ONLINE: 2017-07-08

**THERMAL SCIENCE** YEAR

**2018**, VOLUME

**22**, ISSUE

**6**, PAGES [2897 - 2907]

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