An investigation of transient heat transfer in the region of flow boiling dryout with Freon-12 in a heated tube

Power transient experiments using vertical round tube test sections have provided information on the heat transfer characteristics associated with a change from pre-dryout to post-dryout flow boiling conditions. The test sections were heated by passing electric current along the tube wall, and cooled internally by Freon-12 flowing upwards through the tube. Seven steel tubes of various sizes were used (internal diameters in the range 7.1-26.6 mm, wall thicknesses 0.9-2.0 mm, and lengths of 0.9-3.9 m). Data were obtained for coolant mass fluxes in the range 150-3270 kg m-2 s-1, at a nominal pressure of 1.0 MPa, with exit qualities in the range 0.3-1.0. The transients were intiated by small increases in power input to the test section. Heat transfer characteristics were determined by calculating wall temperature reponses as functions of time and comparing these with the corresponding temperature traces recorded in the experiments. In relation to the temperature responses of the tube wall under these transient conditions, the results show that transition boiling has only a slight effect and that film boiling has very significant effects. (orig.)