Boiling Heat Transfer Around a Horizontal Cylinder and in Tube Bundles

Abstract

<p>From the analysis of a postulated loss-of-coolant and loss-of-emergency cooling accident in a typical horizontal pressure tube nuclear reactor, the need was established for a systematic investigation on the boiling heat transfer coefficient to be expected on a 12.7 cm diameter cylinder and a bundle of these cylinders under saturated and subcooled conditions. To accomplish this, a three-part experimental program was instigated.</p> <p>In the first part, a successful technique based on a quenching (transient) procedure and the use of a heat flux meter was developed to measure directly the local boiling heat flux density around a large horizontal cylinder (12.7 cm diameter) as a function of wall temperature under saturated and subcooled conditions. These experiments were complemented by pertinent theoretical analyses. Although the surface averaged critical heat flux densities are shown to be predicted very well by known correlations, these experiments and analyses do reveal that there is considerable variation of the local critical heat flux density (CHF) around the cylinder. It was shown by high speed motion pictures (1000 frames/s) and from quenching experiments on partially insulated cylinders that this variation results from the interaction of the upflowing liquid-vapour boundary layer with the boiling phenomena. These results for the partially insulated surface and for steam injection underneath a single copper cylinder are also presented.</p> <p>In the second portion of the experimental program, a five row by three column array of instrumented copper cylinders was quenched to determine the interaction effect on the boiling curve of the vapour-liquid mixture rising from tubes located below. This bundle was investigated under saturated and subcooled conditions. Results show that there is a significant critical heat flux increase over the lower part of the cylinder and no appreciable effect over the other parts. The critical heat flux increase is relatively independent of the amount of upflowing vapour.</p> <p>In the third part of the program, a steam-heated tube, 2.67 cm outer diameter, was used to study the steady-state boiling characteristics of heptane on single horizontal tubes. The average boiling heat flux for each of these steam-heated tubes in a four row by three column array was determined and compared with that observed when each was used alone. The results show a substantial increase in the critical heat flux value and essentially no increase in the nucleate boiling regime on any tube when the lower tubes were boiling. When all tubes were at their critical heat flux, the average CHF in the bundle was about 20% higher than the one obtained in a single tube experiment.</p> <p>It is recommended to initiate a study to calculate the pressure drop in a horizontal boiler. It is suggested that the apparent decrease in the overall average critical heat flux for a bundle of tubes, which has been reported in the literature for horizontal reboiler systems, may not be caused by vapour blanketing but may result from insufficient liquid being drawn into the bundle, by natural convection or the thermosyphon effect, to satisfy that required to generate the vapour if all tubes are operating at the critical heat flux. Some recommendations for future work relating to this problem are presented.</p>