A study is presented for laminar natural convection from horizontal cylinders with annular fins, which are referred to as annular-fin heat sinks in the following context. To simplify the analysis, the surfaces of the heat sinks are assumed to be isothermal. The heat sinks used for experimental studies in this work were made from aluminum, with a large ratio of fin thickness to fin diameter, thereby providing surfaces which were very close to isothermal. Previous studies on these types of heat sinks have not included the effects of fin thickness and end surfaces in the derived correlations.;A comprehensive model is presented for predicting natural convection heat transfer from these annular-fin heat sinks. It overcomes the shortcomings of the previous correlations. The model makes the distinction between the external and internal surfaces of the heat sink, allowing the heat transfer mechanisms that dominate on each of these surfaces to be more easily applied.;An approximate heat transfer solution for fully developed flow between the annular fins is obtained. The fully developed flow solution and an existing boundary layer solution for laminar natural convection are employed to construct the model. The model accounts for heat transfer from all surfaces of the heat sink, including the end surfaces and the rim of the fins. The Nusselt number of the heat sink is given as a function of the modified Rayleigh number or Elenbaas number. To account for the radiative heat loss, a radiation model for annular-fin heat sinks is also given.;Measurements for natural convection heat transfer from five heat sinks were performed and are used to validate the model. Radiative heat losses for these heat sinks were obtained from radiation measurements performed in a vacuum chamber.;The model is compared with the present experimental data, previous correlations and experimental data, including the two limiting cases, i.e. disk channels and circular cylinders, plus a set of apple core data. Good agreement is observed in all comparisons. It confirms that the isothermal simplification is justified.;The model provides engineers with a useful tool for designing heat sinks.
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