Instability of vapor films around a sphere during free convection



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An apparatus capable of disturbing a vapor film has been developed. The apparatus was used to investigate the destabilization of the vapor film boiling around 3/4 inch in diameter silver-plated copper sphere. The results show an increase in heat flux in the film boiling regime caused by bubble-induced oscillation. Data were obtained in distilled water for bubble rates of 3 and 5 bubbles/sec, and initial sphere temperature range of 800 to 1100 F. Data also were taken for different sizes of nozzles which were 1/2 and 1/4 mm inside diameter, and water temperature ranges from 60 to 140 F were investigated. The nature of bubble disturbances and transient vapor film behavior was investigated with a high-speed movie camera during quenching. All experiments were done at atmospheric pressure. It was found that a transition which was named a pseudotransition will appear before the regular transition boiling. This pseudotransition is characterized by nucleate-like bubbles covering the front portion of the sphere. The effect of subcooled water upon the onset of transition temperature was investigated. It was found that decreasing subcooling results in lower heat flux. It was found that higher initial sphere temperature tends to increase the transition temperature. Higher bubble rates tend to increase the heat flux in the film boiling regime.