Natural Convection around a Smooth Cylinder and a Spiro Tube: Experiments andCalculations

摘要

A study on laminar natural convection around a smooth cylinder and a Spiro tubeis described in terms of experiments as well as calculations. As a practical application of this study, the optimum design of the Spiro tube was also performed. An experimental set up was established, and the temperature distributions and the heat transfer around the smooth cylinder and the Spiro tube were measured. The measured data were used to verify the calculations. The calculations were carried out by means of the Galerkin finite element method. The governing equations, which originate from the related conservation principles, were solved in the fluid primitive variables. Two cases were distinguished in the calculations: the smooth cylinder and the Spiro tube in infinite space and within confining plates. As proved by the calculations, the confining plates have a strong effect on the heat transfer. The wires at the outside of the Spiro tube are complex in geometry. A special model was developed in the calculations to simulate their contribution to the flow and heat transfer. According to this model, the flow resistance and the heat transfer rate provided by the wires are approximated by drag forces and heat sources. The distributions of drag forces and heat source indicate where the wires have the maximum flow resistance and where the wires are effective in heat transfer. According to these distributions, improvement can be made in the design of the Spiro tube. An empirical formula was proposed, in order to conveniently calculate the heat transfer around the Spiro tube. Based on the experiments and the calculations, this formula relates the mean Nusselt number to the Rayleigh number as well as to various geometrical parameters of the Spiro tube. Then the optimum design of the Spiro tube was performed. The optimization predicts the best geometry for the heat transfer. This considerably reduces the manufacturing cost of the Spiro tube.