Bubble Dynamics on Artificial Nucleation Sites in Boiling Heat Transfer

Abstract: Pool boiling heat transfer is a complicated scientific problem due to the complex phase change process. In recent years, with the development of nuclear technology, microelectronics and space exploration, more attention has been paid to boiling which present great potentials for the thermal management in these applications because nucleate boiling has much better heat transfer performance than single-phase fluid, while bubbles, including their departure frequency and diameter and the interaction between bubbles plays an important role in determination of the heat transfer performance. Therefore, it is very important to study the bubble growth characteristics to explore the boiling heat transfer mechanism and improving the heat transfer performance in practice. Bubble dynamics in pool boiling heat transfer was experimentally studied in this thesis. The frequent coalescence of vapor bubbles in the vertical direction was observed, which has not been reported in detail in the open literature. The change in bubble frequency and size, as well as the collision and coalescence behaviors between bubbles were studied, and the images were processed by MATLAB and ImageJ.  The results show that with the increase in wall heat flux, the bubble growth rate increases, and the period of bubble growth cycle decreases. In a whole bubble cycle, the initially formed bubble is larger than the succeeding sub-bubbles, but in the succeeding sub-bubbles, the latter formed bubble is usually larger than the previous one. Furthermore, the departure frequency and diameter of bubbles are highly related. The wall superheat and the depth of cavity on the smooth copper surface can affect bubble frequencies as well as the bubble detaching size, while the diameter of cavity has a less influence on the bubble behavior according to the experimental results. During bubble growth, the bubble vibrates in the horizontal direction before its departure from the heating surface.