Visualization of Boiling Heat Transfer on Copper Surface with Different Wettability

In this work, a pool boiling visualization experiment platform was built to study the influence of wettability on the nucleate boiling heat transfer. The copper surface was prepared by chemical etching method, and three types of copper surfaces(hydrophilic, hydrophobic, and smooth) were obtained. The boiling heat transfer characteristics of these three surfaces were discussed based on the obtained heat transfer curves and the heat transfer coefficient(HTC) curves. It was found that the HTC still increases when the boiling heat transfer closes the critical heat flux(CHF). To better evaluate the boiling heat transfer efficiency, the concept of heat flux growth rate per unit superheat is introduced. The dynamic behaviors of single and multiple bubbles were also recorded to study the mechanism of the boiling heat transfer. The bubble behavior characteristics show good consistent with the proposed heat flux growth rate. By comparing the heat transfer curves and the bubble dynamic behavior, the following characters of boiling heat exchange can be found: At low heat flux, the heat exchange is dominated by the bubble coverage on the surface, and the hydrophobic surface has better boiling heat transfer efficiency than the hydrophilic surface. As the heat flux increases, the vapor channel connecting a big bubble and the wall surface plays an important role in enhancing the heat transfer. The polymerization of bubbles on the hydrophobic surface forms bubble films and breaks the vapor channel, leading to a significantly smaller HTC and a lower CHF than that of the hydrophilic surface. The HTC of hydrophilic surface in this work can reach up to 54.85 kW/(m^(2)·K), which is 2.3 times the hydrophobic surface.

Bubble Interaction in Thermal Boundary Layers

The boiling process is characterized by strong nonlinear interactions between bubbles in the thermal boundary layer. The present paper uses a heat balance approach to derive the interaction length between bubbles based on boiling heat transfer mechanisms and bubble dynamics. The effects of some important parameters on the interaction length are then analyzed. Interestingly, the whole range of fluxes from low heat flux nucleate boiling to high heat flux boiling, departure from nucleate boiling (DNB), critical heat flux (CHF), and transition boiling, can be well understood by the concept of interaction length. The present paper also uses the concept of interaction length to present a unified description of DNB and CHF phenomena through a better understanding of nucleate boiling.