Session R03: Bubbles: Growth, Heat Transfer and Boiling (5:00pm - 5:45pm CST)

Nucleate boiling in the presence of an oscilalting torus

Direct Numerical Simulations have been conducted to study the effect of a solid oscillating torus on nucleate boiling heat transfer at low superheats in the isolated bubble regime. We use a sharp interface dual grid level set method (SI-DGLSM) to carry out the numerical simulations for nucleate boiling and level set based immersed boundary method (LSIBM) for the fluid-structure interaction. A semi-implicit projection method is used to solve the mass, momentum and energy conservation equations. For nucleate boiling under the influence of oscillating torus, a lock-on regime has been observed, during which the frequency of bubble departure synchronizes with the frequency of torus oscillation over a range of oscillation amplitude and frequencies. The upward motion of the torus pushes the surrounding fluid down which in turn pushes the liquid-vapor interface leading to bubble departure and thus lock-on. The downflow cased by the bubble rise and evaporation of liquid increases in speed due to shorter area close to the wall due to presence of torus thereby leading to thinning of thermal boundary layer and increase in Nusselt number.   

Understanding the coupled mechanisms of microlayer evaporation and single vapor bubble dynamics

Results of an experimental study conducted to understand the role of microlayer evaporation process towards single vapor bubble growth from a heated substrate subjected to constant heat flux (50 to 80 kW/m$^{\mathrm{2}})$ conditions have been presented. Quantitative visualization of the bubble growth process has been conducted through simultaneous use of two optical techniques, a) rainbow schlieren deflectometry (thermal field and dynamics of the vapor bubble) and b) thin film interferometer (microlayer evaporation). Three distinct stages of bubble growth process have been observed; a) initial growth (hemispherical vapor bubble and expanding microlayer), b) transition growth (microlayer with flattened periphery), and c) diffusion growth (receding contact line and microlayer with a dry spot). Analysis of thin film interferograms showed that the contribution of microlayer evaporation towards the bubble growth process is no more than $\sim $ 15{\%}. While, the localized heat flux dissipation rates in the microlayer are found to be $\sim $ 1 MW/m$^{\mathrm{2}}$. Experimental results also showed the presence strong coupling between the bubble dynamics and the microlayer evaporation process.   

Effects of surface nanostructure and wettability on pool boiling: A molecular dynamics study

Effects of surface topology, surface chemistry, and wall superheat temperature on the onset of boiling, bubble nucleation and growth, and the possible formation of an insulating vapour film are investigated by means of large-scale MD simulations under controlled pressure. The simulations reveal that the presence of a nanostructure triggers the bubble formation, determines the nucleation site and facilitates the energy transfer from the hot substrate to the water. On the other hand, the surface chemistry governs the shape of the formed bubble. A hydrophilic surface chemistry expedites the bubble nucleation, however, decelerates the bubble expansion, thus postpones the formation of the film of vapour. Therefore, a hydrophilic surface provides better energy transfer from the hot wall to the water. By analysing the system energy, we show that irrespective of wall topology and chemistry, there is a wall temperature for which the amount of transferred energy is maximum.   

Sub-cooled flow boiling in impacting drops on hot surfaces

We experimentally observed several different types of sub-cooled boiling in impacting drops on heated solid surfaces. With the increase in the surface temperature, the boiling regime in an impacting water drop changes as follows: moderate nucleate boiling, micro-bubble emission boiling, circular-bubble-wave travelling boiling, oscillatory boiling, and Leidenfrost boiling. For the variation of the boiling regime, we proved that dynamic hypothesis is unlikely as the phenomena drastically changes only by altering the initial drop temperature. We have constructed a spherical bubble model describing the violent sub-cooled boiling in an impacting drop by taking into account the evaporation of micro-liquid layer beneath an expanding vapor bubble and the temperature distribution in an impacting drop.   

Thin film interferometric mapping of microlayer dynamics during the growth process of single vapour bubble under nucleate flow boiling regime.

The enhanced heat transfer associated with nucleate boiling is explained by two widely accepted heat transfer mechanisms; micro-convection and microlayer evaporation. Various researchers postulated that when a vapor bubble grows quickly on a solid heated substrate, it traps a thin layer of superheated liquid between bubble base and heated wall. This thin liquid layer evaporates and feeds vapor bubble to sustain its further growth. This thin liquid layer is of the order of micron and hence termed as microlayer. Heat transfer through microlayer evaporation contributes significantly to the overall heat transfer rate from the heater surface. The present work reports one of the first attempts to map the real time dynamics of microlayer during formation of single vapor bubble under subcooled flow conditions using the principles of thin film interferometry. Flow boiling experiments have been performed in a vertically oriented channel with water under atmospheric pressure conditions. Thin film interferometer and the corresponding data reduction algorithm have been developed in-house to elucidate the real time dynamics of microlayer in terms of interferometric fringes and quantify the microlayer thickness with time and Reynolds number.   

Gravity Effects on Pool Boiling

Nucleate pool boiling simulations are carried out to study the effect of variable gravity on the dynamics of bubble formation, bubble departure and bubble interaction. The focus is on identifying dominant trends in flow and thermal behavior at different gravity levels at fixed values of wall superheat and liquid subcooling. From the characterization of turbulence intensity and time averaged fluctuations of velocity and heat flux it is found that the wall heat flux conditioned by the wetted surface area - the liquid heat flux - provides a useful indication of high heat flux hot spots. The significance of near-wall, small scale bubble dynamics is reflected in the transition of vorticity structures from hairpin type vortices in the near-wall region to vortex ring patterns away from the wall. Our observations reveal the significance of small scale dynamics at low gravity levels that were previously thought to be dominated mostly by large bubbles.   

Deep Learning Strategies for Critical Heat Flux Detection in Boiling

In this talk, we present image-based deep learning models that enable detection of critical heat flux (CHF) using pool boiling experimental images. Classifiers that identify pool boiling regimes are trained using conventional Convolutional Neural Network (CNN) and Transfer Learning (TL). Results show both models accurately identify the boiling regimes using the same dataset for training and validation. To assess the generalization of each model, a cross-dataset experiment is conducted. It is found that the TL model can detect the CHF with 93.6{\%} accuracy compared to 58.8{\%} for conventional CNN on a second double-blind test set. In addition, in comparison to the CNN model, only 10{\%} image frames fed for CNN training are used for training the TL model, demonstrating the potential of TL for handling data scarcity commonly encountered in engineering applications.   

Effect of subcooled temperature on the nucleate pool boiling

Boiling is a common but complex method to dissipate a large amount of heat. To understand the physics involving the heat transfer during pool boiling, the vapor bubble dynamics and flow field of surrounding liquid are experimentally investigated in this study, focusing on the effect of subcooled temperature ($\Delta $T$_{\mathrm{sub}})$. High-speed two-phase particle image velocimetry measurement is performed under the subcooled condition ($\Delta $T$_{\mathrm{sub}}=$ 2-15${^\circ})$ when a polished flat stainless steel plate is heated to maintain a constant temperature located on the bottom of the water tank. A growth of bubble in subcooled condition is also analytically modeled based on the heat transfer by micro evaporation, heat diffusion, and condensation. In a highly subcooled condition ($\Delta $T$_{\mathrm{sub}}$ \textgreater 8${^\circ})$, small-sized bubble departs from the wall and rapidly shrinks by condensation. Both bubble velocity and aspect ratio are peaked shortly before the complete dissipation. However, in less subcooled condition ($\Delta $T$_{\mathrm{sub\thinspace }}$\textless 8${^\circ})$, the bubble departs in larger size (2 - 3mm), and is significantly deformed by liquid inertial force. Induced by the exchange of surface elastic energy and kinetic energy, the bubble velocity and aspect ratio fluctuate while the departed bubble is rising.   

A critical comparison between computational models of evaporation and boiling

The phase-change of liquid to vapor may be as volatile as film boiling, or as calm as surface evaporation. Several physical models have been proposed to represent these phenomena, e.g., energy jump, energy-concentration jump, and Schrage. Here, we present a comparative study between these models and their variants and discuss their range of application and their accuracy specifically for cryogenic flow conditions. The temperature of the liquid-vapor interface is another point of contention for phase-change models. We measure the contribution of the pressure jump, surface curvature, and specific volume difference and cross-compare their effects on the mass and energy transfer at the interface. The implementation of the models in the multi-material multi-phase computational code and its validation with theoretical and experimental results are shown. Finally, the suitability and accuracy of these models are assessed in surface evaporation, nucleate pool boiling, and film boiling in 2D and 3D configurations.   

Gibbs effect on evaporation of a vapor bubble.

In numerical simulations of boiling, the interface temperature is generally assumed to be the same as the saturation temperature at the system pressure. While this is a reasonable assumption for stable boiling, it may not be correct for rapid evaporation, where the highly unsteady nature of the physics can lead to the deviation of the interface temperature from the saturation temperature. In this study we explore the effect of the interface temperature on the stable and unstable growth of a vapor bubble in a superheated liquid. To this end, we use a comprehensive Gibbs-Thomson equation to determine the interface temperature and explore the effect of local variation of this parameter on the bubble dynamics.