Experimental study on the effects of a flow obstacle on annular flow in relation with CHF

Abstract

There is ample evidence in literature that the insertion of flow obstacles in the coolant flow of a boiler postpones the f i lm dryout emergence on the heated surfaces. Specifically, the deposition of droplets forced by the obstacle is beheved to increase and is studied in this research. The two objectives of the present research are (i) to confirm the increase in the film flow rate, this by (ii) building an appropriate fiow-loop equipped with suited techniques for film thickness and film velocity measurements. A cylinder is inserted in the center of a vertical water-air annular tube flow. Variations in liquid film characteristics induced by the obstacle, especially the film thickness and interfacial parameters as disturbance waves velocity and frequency, are measured through (i) a conductance probes technique and (ii) a flow visualization technique. The LDA technique is also tried to be adapted to velocity measurements in the thin film. The presence of an obstacle in an annular fiow leads to an increase in the film thickness, and does not affect the interfacial behavior of the film. An increase in the droplet deposition constant is observed and agrees reasonably well with predictions from the Windecker et al. (1999) correlation. The fiowloop and the measurements techniques are suited for extensive droplet deposition measurements, provided the recommendations are adapted.