Abstract
Nucleate boiling is an essential heat exchange method in industry. Due to the complicated mechanisms of the boiling process, prediction of its heat transfer performance has always been challenging. Studying the growth and heat transfer characteristics of a single bubble is of great significance for understanding the internal mechanisms of nucleate boiling. In this paper, single bubble growth in saturated pool boiling was numerically studied. The microlayer evaporation at the bubble base was calculated and added to the simulation via UDFs. The effects of contact angle, wall superheat, and microlayer initial thickness distribution were investigated. The results show that a thinner microlayer has a greater evaporation rate, resulting in an increased bubble growth rate and departure diameter. The bubble growth rate, departure time, and departure diameter increase with the increase in the contact angle and wall superheat. As the wall superheat increases, there is a competition between microlayer evaporation and phase interface evaporation, which ultimately leads to an extremum of the total contribution of microlayer evaporation to bubble growth. The heat transferred by microlayer evaporation accounts for at least 67.3% and up to 83.6% of the bubble's latent heat at departure, indicating that microlayer evaporation has a significant impact on bubble growth.
Highlights
As an efficient heat transfer approach, nucleate boiling has been widely used in refrigeration, electric power, aerospace, chemical, and other industrial fields
The following reasons may explain the results: the increase in wall superheat will, on the one hand, increase the maximum contact area between the bubble and the wall, so the total amount of evaporated microlayer increases, which leads to an increase in Qml; on the other hand, as can be seen from Fig. 16(a), at the late stage of bubble growth, the slope of the Qb–t curve increases with the increase in wall superheat, which indicates that evaporation at the vapor–liquid interface is more intense at higher wall superheats, and the bubble will absorb more heat from the surrounding superheated liquid
The results indicate that the contribution of microlayer evaporation to bubble growth cannot be ignored when the wall superheat is not too high
Summary
As an efficient heat transfer approach, nucleate boiling has been widely used in refrigeration, electric power, aerospace, chemical, and other industrial fields. In addition to the microlayer initial thickness, the contact angle and wall superheat are two important parameters that affect bubble behavior and heat transfer characteristics. To better understand the influence of different parameters on the boiling process, the growth and detachment of a single bubble on the heating surface during saturated pool boiling were numerically simulated using the computational fluid dynamics (CFD) method in this paper, and the effects of microlayer thickness, contact angle, and wall superheat were investigated. The growth and heat transfer characteristics of a single bubble under multi-parameter influence were obtained, which helps to further understand the mechanisms of nucleate boiling
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