Analytical, numerical, and experimental characterization of air cooled cylindrical heat pipes under forced convection for various extended surfaces

Abstract

The objective of this dissertation is the characterization of heat transfer from cylindrical heat pipes (HPs) in cross flow with attached extended surfaces. The extended surfaces investigated include uniform thickness fins, open-cell highly porous metal foam, and an array consisting of periodic layers of metal fins and open-cell metal foam. First, benchmark direct numerical simulations are used to compare existing analytical models for predicting convection heat transfer coefficients associated with a finned HP. The benchmark simulations are also used to identify the shear stress transport (SST) turbulence model as the preferred numerical model for the prediction the convection heat transfer from a finned HP. The SST external model is then employed in conjunction with a multiphase 2D internal HP model by coupling these internal and external models. Predictions from the resulting coupled numerical model are compared with experimental measurements for validation. A previously unreported phenomenon, localized depression of temperatures in the heat pipe wall is identified. A novel analytical model of annular foam-only arrays is subsequently developed, along with a generalized expression for the fin efficiency of square arrays based upon corresponding annular efficiencies. Numerically predicted thermal efficiencies compare favorably with the predictions from the novel expressions for both the annular and square arrays. Comparison of experimentally measured heat rates and predicted heat rates found with the new expressions for a square metal foam array provides further validation. Finally, novel expressions for the thermal resistance associated with annular composite fin-foam arrays are developed. Experimentally measured heat rates for such an array are compared to corresponding predictions generated with the new expressions to verify the model. A preliminary comparison of predicted heat transfer for the annular fin array, the foam-only annular array, and the new combined fin-foam array is generated, demonstrating the improved thermal performance associated with the metal foam-based configurations.