Fluid Flow and Heat Transfer of Opposing Mixed Convection around Rectangular Cylinder Placed in Uniform Downward Flow of Forced Convection

Abstract

Experimental investigations have been carried out on opposing mixed convective flows of water induced around a horizontal rectangular cylinder in the range of the Reynolds and modified Rayleigh numbers, Re_D =500-3000 and Ra_D^*=7x10⁷-10¹⁰. The flow and temperature fields around the cylinder were first visualized with dye and liquid crystal thermometry. The results showed that longitudinal vortices of which axes parallel to the flow direction appear over the upper surface of the cylinder when the buoyancy force beyond critical. The flow along the upper surface, then, separates at the edges of the cylinder. Thus, the side and bottom surfaces of the cylinder are covered by separation bubbles and wake. The local and overall heat transfer coefficients from the cylinder were subsequently measured. The coefficients from the upper and side surfaces showed marked increase with the buoyancy force. This yields higher overall Nusselt numbers than those of the forced convections. It was also found that the non-dimensional parameter (Gr_D^*/Re_D^2.5) well correlates the present overall Nusselt numbers.