The not-so-subtle flaws of the force balance approach to predict the departure of bubbles in boiling heat transfer

Abstract

We present a critical evaluation of the force balance approach in predicting the departure of rapidly growing bubbles from a boiling surface. To this end, we conduct separate effect bubble growth experiments in a carefully controlled environment. We use high-speed video to quantify experimentally all the external forces acting on a growing bubble through the profile of the liquid-vapor interface. Our experimental data show that the momentum conservation equation is always rigorously satisfied, as it should, if the various forces are precisely quantified. However, based on our analysis and our observations, we come to the conclusion that force balance models cannot be either robust or accurate for the purpose to predict bubble departure. They are not robust because the rate of change of the bubble momentum, i.e., the key quantity that force balance models aim at evaluating as the sum of the external forces, is orders of magnitude smaller than each of the force terms in the momentum conservation equation throughout the entire bubble life cycle. Thus, the slightest error on one of the external forces leads to very different predictions for bubble departure. The approach is also not accurate because the analytical expressions used to estimate the external forces are riddled with questionable assumptions (e.g., on the bubble growth rate, added mass coefficient, contact line length, and contact angle) and uncertainties that are, once again, orders of magnitude larger than the rate of change of the bubble momentum itself.

Comprehensive simulations of boiling with a resolved microlayer: validation and sensitivity study

“…We have introduced the axisymmetric cylindrical geometric volume-of-fluid (GVOF) method for interface tracking (Bureš, Sato & Pautz 2021), and coupled it with the existing sharp-interface heat-and mass-transfer model within PSI-BOIL, and with the phasic velocity extrapolation algorithm of Malan et al (2021), but with several modifications (Bureš & Sato 2021b). As reference data for validation, we have acquired comprehensive measurements of first-bubble growth in a microlayer evaporation regime taken at the Massachusetts Institute of Technology by Bucci (2020), partially presented in Bucci, Buongiorno & Bucci (2021). In the present study, we have used the data for which the experimentalists had the highest confidence in the fidelity of the reported results, corresponding to a situation with an applied heat flux of 425 kW m −2 .…”

“…(The reader should note that, in this paper, the work of Professor M. Bucci (Bucci et al 2016) and the work of Mr M. Bucci (Bucci 2020), as well as their joint publication (Bucci et al 2021), are referenced. We bring this to the reader's attention to avoid possible confusion of these two researchers.…”

“…(2021), but with several modifications (Bureš & Sato 2021 b ). As reference data for validation, we have acquired comprehensive measurements of first-bubble growth in a microlayer evaporation regime taken at the Massachusetts Institute of Technology by Bucci (2020), partially presented in Bucci, Buongiorno & Bucci (2021). In the present study, we have used the data for which the experimentalists had the highest confidence in the fidelity of the reported results, corresponding to a situation with an applied heat flux of .…”

Microlayer dynamics of hydrodynamically interacting vapour bubbles in flow boiling

“…Unfortunately, the formulations that calculate the growth force (which is usually considered to account for liquid inertia and added mass effect) suffer serious shortcomings (Bucci et al. 2021) and cannot be used to reasonably estimate the bubble dynamics.…”

“…They all reported a wedge-shaped microlayer, which increased monotonously from the nucleation site (real contact line) to the outer periphery. In the last decade, quantification of microlayers has been done using thin-film interferometry (TFI) (Gao et al 2012;Suryanarayan & Srivastava 2021) and infrared (IR) thermal imaging (Jung & Kim 2015;Bucci, Buongiorno & Bucci 2021) in pool boiling. Sinha, Narayan & Srivastava (2022) conducted experiments on a vertical flow boiling channel and analysed microlayer thickness evolution beneath a single vapour bubble using TFI.…”