Natural convection heat transfer from a bundle of in-line heated circular cylinders immersed in molten solar salt

Abstract

A two-dimensional steady-state natural convection heat transfer from a bundle of closely spaced isothermally heated cylinders in the in-line arrangement was investigated in this study for a laminar flow regime using numerical analysis. It was determined that flow patterns and heat transfer are strongly affected by the transverse and longitudinal cylinder-to-cylinder spacing ST and SL, Rayleigh number, and thermo-physical properties of the heat storage fluid. Performance maps in the form of contour diagrams for the average Nusselt number Nu and dimensionless heat transfer volumetric density q ̃, and corresponding correlations are presented for an in-line bundle of 9 × 5 horizontal cylinders immersed in a molten solar salt.The variation of the heat transfer enhancement for the in-line bundle of horizontal cylinders shows that there is an optimal cylinder spacing resulting in the highest heat transfer enhancement. The heat exchanger geometry was optimized by determining the optimal set of the tube (cylinder) spacings SL/D and ST/D in the bundle resulting in the maximum volumetric heat transfer rate q ̃, and smallest size and lowest cost of a heat exchanger. A set of correlations were developed for the maximum and optimum dimensionless heat transfer volumetric densities as functions of the Rayleigh number RaD. The analyzed geometries are important in the solar heating and storage technology, nuclear reactor safety, and waste heat recovery applications.