Experimental Measurement and Force Balance Analysis of Sliding Vapor Bubble Behavior on a Horizontal Tube

Abstract

Boiling on the outer surface of a horizontal tube occurs in various systems including PAFS, which is the passive residual heat removal system of APR +. In order to predict the boiling heat transfer based on a kinematic model such as the heat partitioning model, it is important to understand the behavior of the vapor bubbles. Particularly, in the case of the lower inclined surface such as the lower half of a horizontal tube, the accuracy of the heat transfer prediction depends on the evaluation of the sliding bubble motion. However, previous studies on bubble behavior mainly focused on the horizontal plate or the vertical channels, and the experimental and analytic studies on the sliding bubble behavior on a horizontal tube were insufficient. Therefore, the purpose of this study is as follows. First, measuring the sliding bubble behavior parameters experimentally, required for the improvement of the heat transfer model. Second, analyzing the sliding bubble behavior using the improved force balance model to establish the foundation of the predictable model. In order to observe the sliding bubble behavior on the outer surface of a horizontal tube heater, the experimental facility with a horizontal tube heater was constructed. In addition, the stereoscopic measurement technique using the two synchronized high-speed cameras was established. The experiment was performed using nearly saturated water under atmospheric pressure for various conditions

liquid velocity (11.2 – 27.6 mm/s), wall heat flux (26 – 66 kW/m2), and the location of nucleation site (0 – 180° from the bottom of the heater). A thin film heater with a narrow heating strip proposed in this study generates boiling bubbles in a restricted region on a horizontal heater. Therefore, the quality of the visualization of the sliding bubbles improved significantly. Furthermore, an artificial cavity created on the heating strip could control the location of the nucleation cavity so that it improved the surface condition difference between the polyimide-based heater and the metal heater. The two synchronized high-speed cameras captured the behavior of the bubbles from two perpendicular measurement angles. The configurations of the bubbles were identified by an image processing method based on shadowgraphy from the two images, and the results of the image processing includes various boiling bubble parameters, such as the departure and lift-off of bubble, bubble volume transient, bubble velocity, bubble frequency, etc. In particular, the volume of the non-spherical bubble was calculated by a three-dimensional reconstruction method, which defines a specific cross-section configuration at each elevation using stereoscopic images. The two verification steps confirmed that the reconstruction method has allowable errors and the monoscopic visualization method has a limitation for deformed bubble measurement. Based on the experimental observations, the force balance analysis of the sliding bubble on the horizontal tube was performed. The previous force balance model for the sliding bubble on a horizontal plate was improved for sliding bubble on the horizontal tube by introducing additional forces and representing them in cylindrical coordinates. The circumferential force balance analysis estimated the local liquid velocity, not obtained in experiment. The radial force balance analysis estimated the local liquid velocity gradient and predicted the lift-off point of the sliding bubble. As a result, the dominant forces determining bubble behavior confirmed in this study are the surface tension, contact pressure force, buoyancy and added mass force. Meanwhile, the lift force, buoyancy and added mass force are dominant near the lift-off. Finally, the study based on the experimental observation and analysis results figured out the transients of the bubble behavior parameters. It could be the foundation of developing a prediction model of bubble behavior by simplifying and improving on several parameters.