A Boiling Heat Transfer Experiment for a Senior-Level Engineering Laboratory

This paper presents a simple experiment that can be executed in an undergraduate laboratory to observe the boiling curve, including the transition and film boiling regimes. The experiment involves submerging an instrumented copper sphere into liquid nitrogen and measuring the evolution of the temperature of the sphere. Besides providing excellent visualization of the film, transition and nucleate boiling regimes, from the measurements the boiling curve can be obtained. Quantitative results and uncertainty analysis are presented for the critical heat flux and minimum heat flux. A paradox is presented when insulation is added to the sphere, since it can prevent the creation of a vapor film, maintaining the more efficient transition and nucleate boiling heat transfer regime and thus cooling the copper sphere faster than with no insulation. As expected, when the copper sphere is insulated further, heat transfer diminishes, offsetting the gains made by maintaining nucleate heat transfer. The experiment also evaluates temperature gradients inside the sphere and the validity of the assumption of uniform sphere temperature. This experiment is suitable for a senior-level experimental class or as a part of a heat transfer course.