Laminar and turbulent natural convection from a stack of thin hollow horizontal cylinders: A numerical study

Three dimensional natural convection from a stack of thin hollow horizontal cylinders has been investigated numerically in both laminar and turbulent regimes of Rayleigh number (Ra) spanning in the range 10(4) to 10(8) and 10(10) to 10(13), respectively. In the present study, the length to diameter ratio (L/D) of the cylinders is considered in the range 0.5-20. Thin horizontal hollow cylinders of three, six, and ten in numbers are arranged in a triangular manner to form three different types of stacks of same length scale. The full Navier-Stokes equation along with the energy equation are solved to predict the flow pattern and heat loss from the cylinders. The present computational study is able to capture very interesting buoyancy plume structures around the stack of short and long hollow cylinders. The average Nusselt number (Nu) shows a positive dependence on Ra for all L/D. In addition, Nu for a stack of three cylinders is found to be marginally higher than the stack of six cylinders followed by ten cylinders. Empirical correlations are developed for Nu as a function of Ra and L/D, which would be beneficial to academic researchers as well as practicing engineers.