Turbulent fluid flow, heat transfer and onset of nucleate boiling in annular finned passages
Shim, Sang Yong
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Fins are often used in the energy industry for nuclear fuel or compact heat exchanger tubes to enhance the heat transfer rate. Information on turbulent fluid flow and heat transfer in finned passages is rather limited in the literature. This research was motivated to produce a theoretical means of predicting the pressure drop, heat transfer rate and onset of nucleate boiling (ONB) in finned flow passages. A finite element model was fo mulated to solve the governing conservation equations of momentum and energy. The finite element method was chosen for ease of representing accurately the irregular geometry under consideration. The turbulence model used is based on a classical mixing length theory which was extended to be applicable for finned geometry. The numerical model simulated experiments and analyses for annuli and finned annuli available in the literature. This was to show the accuracy of the numerical model and the validity of the turbulence model to the finned annulus geometry. The validated model was then applied to predict the ONB in finned annuli and to study the geometric effects of fin height and number of fins. Agreement of the present analysis with available experiments and analyses is quite reasonable for fully developed turbulent flow and heat transfer conditions in both annuli and internally finned annuli. The predicted ONB results in conjunction with the Davis and Anderson criterion show good agreement qualitatively and quantitatively with the Atomic Energy of Canada Limited (AECL) data. Both the measured and predicted ONB occurred at the sheath midway between fins. The predicted ONB followed the trends of the measured data such that the ONB power increases with increasing flow velocity, subcooling or pressure. The parametric study shows that heat transfer in finned annuli is generally more effective than that in the unfinned annuli for nealy all cases. However, an exception was seen with a tall 8-fin geometry such that heat transfer is slightly less effective than the unfinned annulus, particularly for high flows. The pressure drop increased with the increase of fin height or number of fins for a given mass flow rate (or for a given flow velocity). The ONB for the finned annuli was found to occur at higher powers than that for the unfinned counterparts for the same flow conditions. The ONB heat flux increased with increasing fin height or number of fins. The increase of the ONB heat flux was found more pronounced with low flows.