Visualization of Micro Vapor Bubble Dynamic Behaviors in Open Capillary Microgrooves

In this paper, a visualization investigation on micro bubble dynamic behaviors in vertical rectangular capillary microgrooves is conducted. With the help of a high-speed digital camera with the maximum speed of 30000 frames per second, a CCD camera and a stereomicroscope, the processes of formation, growth, coalescence and collapse of micro vapor bubbles in the microgrooves are observed. Bubble profile and size can be determined by image processing technique with MATLAB. The results show that micro vapor bubbles have four evolution modes. They include individual micro bubble growth and collapse along a microgroove, individual micro bubble growth and collapse across neighboring microgrooves, micro bubble coalescence and collapse along the same microgroove, and micro bubble coalescence and collapse across neighboring microgrooves. Whichever mode occurs, micro bubble doesn’t depart from the bottom of microgroove before it collapses. Experimental results about time-varying micro bubble sizes are distinctly different from those of theoretical calculation results by dynamic microlayer model for nucleate boiling heat transfer. Micro bubble breakup always occurs during the initial growing stage of bubble and collapse time is far shorter than the end time of the initial growth of individual bubble obtained by the dynamic microlayer model. Theoretical analysis indicates that interface effect and geometric structures of the microgrooves are responsible for the differences.