An experimental and analytical study of transient heat transfer in the region of dryout for a heated tube using Freon-12 as coolant.

Power transient experiments have been performed using vertical round tube test sections to determine 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. Five steel tubes of various sizes were used (internal diameters in the range 7.1-21.3 mm, wall thicknesses 1.1-2.0 mm, and lengths 2.86-3.94 m). Heat transfer results are reported for coolant mass fluxes in the range 270-3270 kg m"2 s~', at a pressure of 1.04 MPa, with exit qualities in the range 0,3-1.0. The transients were initiated by small step changes, corresponding to increases of a few per cent, in power input to the test section, after initial steady-state conditions had been established with heat fluxes just below the corresponding critical heat flux value. The heat transfer characteristics associated with the transients were determined from measurements of the wall temperature transients. A computer code (THETRAN) was developed for analysing the experimental heat transfer results by combining established heat transfer codes. The new code takes account of both transient thermal diffusion within the tube wall and transient thermal energy transfer to the coolant. The analysis involved predicting wall temperature variations as a function of time and comparing these predictions with the corresponding temperature traces recorded in the experiments. The sensitivity of this technique was investigated in relation to the various relevant input parameters, including, for example, changes in the magnitude of predicted critical heat flux. Analysis of the experimental results has shown that post-dryout heat transfer characteristics can be considered in terms of four regions. In order of increasing surface temperature from the onset of boiling crisis conditions, these are: (i) a transition boiling region in which surface heat flux decreases (from the critical heat flux value) with increase in surface temperature, (ii) a minimum heat flux film boiling region, in which the heat flux may be constant over a substantial range of surface temperature values, (iii) an intermediate film boiling region, and (iv) a high temperature film boiling region. In relation to the temperature response of the tube wall under these transient conditions the transition boiling region had only a slight effect; the minimum heat flux film boiling region had a very significant effect and sometimes remained while substantial increases in surface temperature occurred. (Examination of the data obtained for this region indicated that this minimum heat flux is flow-dependent in a relationship of the form qmm (Re) where n ~ 0.5.); and the intermediate and high temperature film boiling regions also had significant effects on the temperature response. (Published film boiling correlations have been slightly modified and then utilised for correlating the data from these regions.) Critical heat flux (CHF) values deduced from experimental data are shown to be in reasonable agreement with a modified Groeneveld CHF correlation