As it stands, the boiling process has some of the highest potential for rapid and small-scale, heat and mass transfer applications that are relevant to numerous industrial and chemical applications. Unfortunately, this process is not widely used since some of the involved phenomena are not fully understood and have proved difficult to reliably predict. As such, a suitable universal model for boiling heat transfer has been pursued for a very long time. Following recent technological advances and numerous researches, some success in predicting the entire heat transfer from a surface under different boiling levels has been achieved. Nonetheless, the reported semi-empirical models are yet to be proven as truly universal, owing to some inherent drawbacks that relate to lumping of multiple individual stages and phenomena together.
In preceding similar studies, bubble growth and departure have been shown to be coupled and have been dealt with simultaneously. Intense research has also revealed that for the case of departure, the force balance is relatively straightforward, depending on a suitable drag coefficient, while as for growth, the thermal aspects and boiling dynamics are rather more complicated. Therefore, it is imperative that a universally valid approach be developed to help realized the full potential of boiling for various applications.
Recently, Dr. Herman D. Haustein at Tel Aviv University introduced a novel approach that not only draws from a 1D energy balance, but also explains the need for correction of the effective superheat, based on kinetic theory and consideration of the vapor Knudsen layer conditions. The Israeli based scientist built on the insight provided by previous studies so as to capture the most important underlying physical mechanisms, and thus develop a universally suitable approach for the growth and departure of a single bubble. Addressing this key aspect of boiling, lays the foundation for the prediction of the boiling heat transfer over the entire surface, under all relevant conditions. His work is currently published in the research journal, International Journal of Multiphase Flow.
In brief, Dr. Haustein employed an integral, simplified, force balance for departure, and a simplified (one-dimensional) energy balance for the Macro-scale bubble growth. Next, the growth model was extended to incorporate the micro-scale aspect of vapor reabsorption, following the gas kinetic theory. For the bubble growth the theoretical analysis was used to identify the dominant dimensionless parameters, while their relative influence (weighting) was found statistically from an exhaustive amount of literature data. This study embodies an important effort to merge macro- and micro-scale aspects, in order to faithfully represent the complexity of boiling.
The author observed that his approach offered a new explanation for the additional pressure dependence found in bubble departure and confirmed the transition in trend at high pressure. Additionally, his work was able to extend the range typically considered, to include 19 liquids/liquefied gases, up to near critical pressures, from sub-millimeter to several centimeter bubbles and gravity levels down to 1/16th that of earth, thereby providing good prediction over the entire range. Moreover, the study demonstrated the importance of the kinetic parameter, in addition to the nominal superheat, in mapping-out the boiling regimes. Therein, he noted that the range was limited at one end by the dominance of microlayer evaporation and substrate conduction at lower superheats, and at the other by bubble interaction – thermal robbing and bubble coalescence.
In summary, the study by Dr. Herman D. Haustein demonstrated a novel analytical/empirical model for single bubble growth and departure in nucleate pool boiling. Despite being based on several simplifing assumptions, the developed approach was able to yield accurate predictions in a consistent manner over a very wide range of conditions.
Herman D. Haustein. A kinetics-based universal model for single bubble growth and departure in nucleate pool boiling. International Journal of Multiphase Flow, volume 105 (2018) page 15–31.Go To International Journal of Multiphase Flow