A common basis for the correlation of forced and natural convection to horizontal cylinders

Abstract

Abstract Natual and forced convection have always been correlated on different bases although the two phenomena have much in common. Accordingly, a method for correlating both types of convection on the same basis, at least for heat transfer outside horizontal cylinders, would be of interest. A tentative method of accomplishing this for transverse convection is presented, involving certain simplifying assumptions relating to drag and buoyancy, from which an “effective velocity” for natural convection is calculated (by means of the well‐known drag correlation) and incorporated in a Reynolds number. The Nusselt number for natural convection is then correlated in terms of the Prandtl number and this Reynolds number in much the same manner as that for McAdams's well‐known correlation for forced convection. For 150 different combinations of independent variables covering seven different fluids and wide ranges of diameter, surface temperature, and bulk fluid temperature, the transformed natural‐convection data agree with Douglas and Churchill's recent refinement of McAdams's relationship for forced convection with an average deviation of about ±10%.