Experiments on natural-convection heat transfer from the outer surface of a vertical cylinder to liquids

Abstract This paper describes the results of experimental investigations on natural-convection heat transfer from the outer surface of a vertical cylinder one metre high to water, spindle oil and Mobiltherm oil. The characteristics of the experiments are as follows. The experimental apparatus is contrived to enable us to measure local heat transfer coefficients directly; sufficiently long turbulent boundary layers are made to appear under various conditions of temperature and heat flux of the heated cylinder; correspondences of local heat transfer coefficients with flow patterns are clarified from the observations of the boundary layer by means of “mirage method” or other methods, and from the measurements of temperature profiles in it; the influence of Prandtl number upon heat transfer is found in its range from 2 to 2600. The boundary layer develops through laminar, vortex-street, transition-turbulent and turbulent flow pattern, and each flow pattern has respective characteristics of heat transfer. Vortex-street flow provokes abrupt increase of heat transfer coefficients in the transitional region from laminar to transition-turbulent. Especially with respect to water, no distinction was found between transition-turbulent and turbulent flow, and its cause is explained by an indeterminate character of the flow pattern in the transitional region. Local heat transfer coefficients are correlated nondimensionally about each flow region in the cases of uniform wall temperature and uniform heat flux. In each case, two kinds of experimental equations are proposed, respectively, by using the physical properties at a reference temperature and by using the supplementary terms referred to the variation of kinematic viscosity. Non-dimensional equations of average heat transfer coefficients are also proposed for the case of uniform wall temperature. Furthermore, some remarks on the ranges applicable to each equation are presented. The shapes of vortex-pairs, laminar sub-layers and turbulent lumps in the boundary layer as well as their development, are clearly taken in “mirage” photos. The transitions of flow patterns are also concretely described. Temperature fluctuations in the boundary layer are revealed and the time-mean temperature profiles are represented by a non-dimensional parameter. “Quasi-steady state” is defined experimentally as a state equivalent to a steady state with respect to heat transfer coefficients.