General Correlations for Laminar and Transition Natural Convection Heat Transfer from Vertical Triangular Cylinders in Air

Abstract

Experimental investigations have been reported on steady-state natural convection heat transfer from the outer surface of vertical triangular cylinder in air. Five cylinders have been used with equilateral side lengths of 0.044, 0.06, 0.08, 0.10, and 0.13 m. The cylinders are heated using internal constant heat flux heating elements. The temperatures along the vertical surface and the peripheral directions of the cylinder wall are measured. Axial (perimeter averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition to turbulent regimes. Axial (perimeter-averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for the laminar and transition regimes using the vertical axial distance as a characteristic length. Critical values of the modified Rayleigh numbers are obtained and correlated for transition to turbulent. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers for all ducts. The local axial (perimeter-averaged) heat transfer coefficients are observed to decrease in the laminar region and increase in the transition region. Laminar regimes are obtained at the lower half of the ducts and the chance of their appearance decreases as the heat flux increases. It is also observed that the laminar regime is area ratio dependent.