Visual observation of nucleate boiling and sliding phenomena of boiling bubbles on a horizontal tube heater

Abstract

In the estimation of the mechanistic boiling heat transfer named wall heat flux partitioning model or RPI model, detail bubble behavior significantly influences the results. This mechanistic model is believed to have advantage from reflecting phenomena more correctly and be applicable to wider range of geometric and conditions by its constituting principle. Consequently, visualizing the phenomena over wide range of heating conditions around the bubble is important. But in conventional experimental approaches, there are some limitations. At high heat flux that exceeding the ONB(Onset-of-Nucleate Boiling) heat flux, visual observation becomes restricted by a number of bubbles are generated on a heater surface. In this study, we newly designed a heater based on the Flexible Printed Circuit Board (FPCB) for a better visualization of the nucleate boiling and sliding phenomena of boiling bubbles. It has narrow heating widths and it make possible to local heating of the surrounding fluids and locally generates bubble without any visual interference along the axial direction. Furthermore, a measurement technique was established in the interest of reconstruct the boiling bubbles and quantify their behavior based on an image processing. Two synchronized high-speed cameras were arranged at an angle of 90° with each other to observe the bubble nature precisely. The bubble motion is simultaneously recorded in axial and side directions through these cameras from their birth to lift-off. Using the measurement technique, important parameters of the nucleate boiling are measured including bubble departure frequency, and bubble volume.