Measurement of sliding bubble behavior on a horizontal heated tube using a stereoscopic image processing technique

The significance of the sliding bubble effect on boiling heat transfer has been reported by a number of precedent studies through experiments and numerical analyses. However, the experimental data and mechanistic boiling heat transfer models considering sliding bubbles on horizontal tube outer surfaces are insufficient. In the present study, therefore, the behavior of sliding bubbles on a horizontal tube heater is measured at nearly saturation temperature and atmospheric pressure for various flow conditions; liquid velocity (14.7-27.6 mm/s), wall heat flux (67-129 kW/m2), and the location of nucleation site (45° and 90° from the bottom of the heater). A thin film heater with a narrow heating strip was proposed to generate boiling bubbles in a restricted region on a horizontal heater. This improved the quality of the visualization of the sliding bubbles. Two synchronized high-speed cameras were used to capture 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 elicited various boiling bubble parameters, such as the bubble diameter transient, bubble velocity, bubble frequency, etc. In particular, the volume of the nonspherical bubble was measured by a three-dimensional reconstruction method which defines a specific cross-section configuration at each elevation using stereoscopic images. Two verification steps confirmed that the reconstruction method has allowable errors and also indicated the limitation of the monoscopic visualization method for deformed bubble measurement. According to the result of this measurement, the location of the nucleation site is a principal factor affecting the life cycle of a sliding bubble on a horizontal tube. Under the present experimental conditions, a bubble generated on the lower half of the tube showed the lift-off from the heating surface twice during its life cycle; the first is due to the contact pressure force and the inertia of the surrounding liquid caused by bubble shape change shortly after the nucleation, and the second is mainly due to the lift force after being accelerated by the sliding motion. In contrast, bubbles occurring at the side of the tube did not show sliding motion, but rather rose without confinement by the wall after the lift-off. The effects of the bulk liquid velocity and wall heat flux on bubble behavior were investigated and the results are discussed in this paper. This experimental observation can be used to improve understanding of the boiling heat transfer mechanism on a horizontal tube and the bubble dynamics of a sliding bubble on a downward heating surface.