Experimental investigation of steam bubble condensation in vertical large diameter geometry under atmospheric pressure and different flow conditions

摘要

The condensation of steam bubbles injected into a large diameter vertical pipe (100 mm) with flowing low-subcooled water at low pressure conditions (～1.1-1.5 bar) was experimentally investigated. The analysis of the experimental investigations will contribute to a better understanding of bubble condensation at low pressures, which is of interest to the safety analysis of nuclear power plants, and any industrial activity in which bubble condensation is of importance. The test section is a transparent 1 m long plastic pipe surrounded by a 18 × 18 cm rectangular "aquarium" filled with water for refraction correction. High-speed camera (HSC) recording was used to gather data about condensing bubbles including: bubble diameter, shape and rising velocity. The isolation of the steam condensation from evaporation in these experiments enabled us to concentrate on the condensation process alone with well-defined flow conditions. Steam was injected via three different injection nozzles directly near the center of the test section. The present experiments were carried out at three different steam superficial velocities, water superficial velocities and water temperatures for each injection nozzle. Raw images were recorded at 1500 frame/s and 0.32 mm/pixel spatial resolution for a total time of 2 s in each measurement. The raw images were processed by self-developed programs in order to identify and track the condensing bubbles after detachment, and to measure their diameter, velocity, position, and aspect ratio. The measurements made possible the calculation of both the bubble Reynolds number and the Nusselt number and, thus, a Nu-Re correlation was developed. The measurements extend the already available database for the validation of interfacial heat and mass transfer correlations. These are used for instance in CFD simulations where steam bubble condensation plays an important role. The analysis of the results presented shows also new and interesting observations for large bubbles with high Reynolds number such as: large deformation, deviation from spherical form, rough and vibrating bubble surfaces, and enhanced heat transfer coefficient. The experiments show a large effect of bubble surface structure, bubble rising velocity, and initial injection velocity upon the condensation rate. Based on the analysis of the results, a new correlation of the Nusselt condensation number has been proposed for the investigated range of bubble diameter and Reynolds number.