Experimental Study of Heat Transfer by Natural Convection through Vertical Cylinders

Abstract

Miniature Neutron Source Reactor (MNSR) is a passively cooled system operating in Natural convection heat transfer mode. A typical MNSR located at the premises of PIEAS (Pakistan Institute of Engineering and Applied Sciences) called as Pakistan Atomic Research Reactor-II, PARR-II. From safety analysis point of view the literature lacks any experimental study or information that can be used to predict the outer surface temperatures along the axial length of a fuel rod in the reactor core. Similarly the literature also lacks any information regarding the prediction of the fluid exit bulk temperature as a function of the reactor thermal power. The thermal power range for the operation of this particular MNSR studied ranges from 5.4 to 27 KW. The current experimental study is a pioneering effort to find a solution to the above mentioned problem. Hence, steady state heat transfer by natural convection was investigated experimentally from an enclosed assembly of thin vertical cylinders at high Grashof numbers. The published literature lacks experimental data regarding such a study in the turbulent boundary layer regime. An enclosed assembly consisted of a 3 x 3 array of vertical cylinders immersed in a large volume tank of water was used. All the cylinders were electrically heated. Various uniform heat fluxes were applied to each cylinder and the surface temperature at different positions along the cylinders were measured. The experimental results show that the surface temperature increases axially up to a certain length, then decreases due to some extra mixing which increases the heat transfer. However, such a behavior is expected to have little effect if the enclosed assembly consists of a large number of thin vertical cylinders. A criterion has been proposed for the determination of the onset of turbulent boundary layer in an assembly. The local heat transfer coefficient, local Nusselt number and local modified Rayleigh number have been presented for the experimental data. It has been found that a much better representation of experimental data results, if the Nusselt number is presented as a function of modified Grashof number and Prandtl number separately instead of modified Rayleigh number. This representation includes the effect of different fluids and L/d ratios. Similarly empirical correlations between overall Nusselt number and average modified Rayleigh number have also been developed based on the data of the assembly of cylinders used in the current study. This empirical correlation for an assembly * developed in current study is valid for 1.28 x 10 12  RaL  1.18 x 1013. A much better generalized correlation has also been proposed for natural convection heat transfer from a single vertical cylinder in an infinite medium which shows a better fit to all the currently available experimental data. The literature also lacks such a generalized correlation. The results obtained in this study have been utilized to predict the axial surface temperatures as well as the fluid bulk outlet temperature of PARR-II. Hence the correlations developed from current study are applicable to all thermal power ranges in PARR-II type MNSR reactors or any other assembly of vertical heated thin cylinders.