Laminar Free convection in Vertical Concentrc Annular Ducts

Abstract

A theoretical investigation is presented for free convection in vertical concentric annular ducts with uniform temperatures specified on inner and outer walls, taking account of the temperature dependence of the thermophysical properties of fluid. The perimeter average Nusselt numbers and dimensionless average axial velocities in the ducts of various cross sections are well correlated by usingξ as a characteristics length ; and the Nusselt numbers of the annular ducts show very good accordance with those of parallel plates rather than those of a circular duct. A general representation of the average Nusselt numbers is presented for wide ranges of the radius ratio and Prandtl number. When the fluid physical properties are evaluated at the average temperature of the fully developed flow, the variable property solutions correlate with the constant property solutions.