Graduate Thesis Or Dissertation

Natural Convection Heat Transfer and Boundary Layer Transition for Vertical Heated Cylinders

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  • In recent years, the global nuclear industry has placed a greater emphasis on passively safe reactor designs. In particular, much attention and design work has been applied to engineered systems for passive cooling of nuclear fuel, whether it be loaded in a core or stored in spent fuel pools. The most common nuclear fuel geometry takes the shape of vertically oriented cylindrical rods. Given the emphasis on passive cooling and the prevalence of cylindrical nuclear fuel, one would anticipate a rather full and developed body of knowledge regarding natural convection heat transfer phenomena from vertical cylinders. However, the current body of knowledge centered on this topic is lacking a comprehensive understanding of several key phenomena that would facilitate more accurate modeling and design of passive heat exchange systems of this geometry. These phenomena include the relationship between heat transfer rate and cylinder diameter in both the laminar and turbulent regimes, as well as the relationship between boundary layer regime transition and diameter. Given that turbulent heat transfer rates are nearly an order of magnitude greater than the laminar regime, characterizing and being able to predict regime transition is essential to understanding heat transfer from a heated cylinder as a whole. The importance of this point is further underscored by considering how, due to the disparity in heat transfer rate between regimes, the cylinder temperature at the location just prior to transition may be the highest of the whole cylinder – directly feeding into the thermal design limits of the reactor fuel. One of the many difficulties in empirically deriving a heat transfer correlation is determining the bounds of the correlation. With respect to natural convection heat transfer from vertical cylinders, there are several interdependent phenomena that need to be considered simultaneously. The current experimental study addresses these phenomena using five heated cylinders with a range of diameters. Data from these heated cylinders is obtained through a combination of thermocouples, distributed temperature sensors, and particle image velocimetry. The results of this study are dimensionless, diameter-dependent, natural convection heat transfer correlations for the laminar, transition, and turbulent regimes. Additionally, the dimensionless bounds of applicability for these correlations (the regime transition points) as a function of diameter have also been experimentally determined using boundary layer theory and empirical data.
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