Characteristics of sliding bubbles in subcooled flow boiling in a narrow rectangular channel under natural circulation condition

Abstract

Narrow rectangular channels are widely used in many fields due to its characteristics of boiling heat transfer enhancement, and the bubble behaviors have been in particular interest for decades because of its significant contribution to understanding the mechanism of heat transfer enhancement. In the present work, visualization experiments in upward subcooled flow boiling in a narrow rectangular channel under natural circulation condition were carried out. A sequence of image processing algorithms was used to deal with the original bubble images to get relevant bubble parameters, including bubble diameter and bubble velocity, etc. The experiments were performed at pressures ranging from 0.1 MPa to 0.3 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. Effects of the liquid subcooling and heat flux on the bubble stochastics and mean characteristics were investigated. In natural circulation, the bubble diameter distribution approximately conforms to lognormal distribution while the bubble velocity distribution agrees well with normal distribution. These are in contrast to both of the bubble diameter and velocity follow normal distribution in forced circulation. The present experiment data shows a good linear relationship between the mean bubble velocity and mean bubble diameter. A correlation for the mean bubble diameter as a function of local subcooling, heat flux, mass flux, and gap width size were obtained. Moreover, we found that all the bubbles will slide along the heating wall after generation in narrow rectangular channel, and two types of sliding bubbles can be observed in our experiments. The growth curve of the Lift-type bubbles and the Sliding-type bubbles conform to a dual model and a power model, respectively.