Predicting the sliding bubble velocity on the lower part of a horizontal tube heater under natural convection based on force balance analysis

The main objectives of this work are to examine the sliding bubble behaviour outside a horizontal tube through a visualisation experiment and to establish a force balance model for predicting the sliding bubble velocity. Of interest here, the sliding bubble velocity is included in a mechanistic boiling heat transfer model and it varies with the bubble volume, location of the nucleation site, generation frequency of the bubble, etc. In order to predict the sliding velocity of the bubbles generated under the various conditions, a model developed based on the physical mechanism is required. This paper presents experimental techniques and measurement results for observing vapour bubble sliding phenomena. A specially devised flexible heater was fabricated and attached on a rod in order to control the bubble generation location. With the heater and a digital image processing technique, accurate measurement of the bubble volume and velocity became facilitated. Through the experiment, the life cycles of a single bubble was clearly visualised including the birth, departure, sliding, and lift-off. The force balance model which includes the curvature effect in cylindrical coordinate was derived to predict the sliding bubble velocity in the experiment. The equation of the model was based on the balance of circumferential directional forces acting on a sliding bubble with a correction of the local liquid velocity to consider the wake generated by a preceding bubble. The force balance analysis showed that the dominant forces are the buoyancy, quasi-steady drag, and added mass forces. The proposed model was validated by comparing the predicted bubble velocity with the experimental data and it predicted the bubble velocity with satisfactory accuracy. In addition, the force balance analysis found that the sliding bubble velocity correlates well with the bubble generation location when it was non-dimensionalised in a form of Froude number.