Prediction of Bubble Departure in Forced Convection Boiling with a Mechanistic Model That Considers Dynamic Contact Angle and Base Expansion

Setoodeh, Hamed; Ding, Wei; Lucas, Dirk; Hampel, Uwe

doi:10.3390/en12101950

Cited by 15 publications

(4 citation statements)

References 32 publications

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“…This assumption, while not always clearly stated, seems to be at the basis of other force balance models appearing in the literature (e.g., see Refs. [5][6][7][8][9][10][11][12][13][14][15][16]). However, experimental investigations of quasi-static gas bubbles ejection from orifices (e.g.,…”

Section: Force Definition Modelmentioning

confidence: 99%

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

2021

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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.

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Section: Force Definition Modelmentioning

confidence: 99%

“…In summary, there is no shortage of such models in the literature (e.g., see also Refs. [9][10][11][12][13][14][15][16]).…”

Section: Introductionmentioning

confidence: 99%

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

2021

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“…In recent years, many studies have been conducted on bubble formation, , bubble motion, − and bubble removal and departure. − Bubble dynamics are affected by many factors, such as the buoyancy force and surface tension . The electric field is another remarkable influential factor in bubble dynamics. − The influence of electrohydrodynamics under different voltage profiles, such as direct current (DC)/alternating current (AC) or nonuniform DC, has been investigated.…”

Section: Introductionmentioning

confidence: 99%

Activating Bubble’s Escape, Coalescence, and Departure under an Electric Field Effect

2020

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In this paper, we present the results of applying an electric field to activate bubbles' escape, coalescence, and departure. A simple electrowetting-ondielectric device was utilized in this bubble dynamics study. When a copper electrode wire inserted into deionized water was positioned on one side of single or multiple bubbles, the bubble tended to continuously escape from its initial position as the voltage was turned on. Contact angle imbalance at different sides of the bubble was observed, which further promoted the bubble's escape. An analysis model with an electromechanical framework was developed to study the charging time difference on two sides of the bubble, which generated a wettability gradient and capillary force to propel it away from the electrode. Sine, ramp, and square alternating current waveforms with 60 V amplitude and 2 Hz frequency were tested for comparison. It was shown that all waveforms promoted the bubble's escape; the square wave shape manifested the farthest escape capability, followed by sine and ramp waves. An upper view of several bubbles aligning in triangle, square, pentagon, and hexagon shapes demonstrated that the bubbles tended to move outward when the electrode is placed at the geometric centers. Experiments with an electrode on one side and several bubbles positioned in a line were conducted. In these cases, the bubbles closer to the electrode reacted faster than those farther from the electrode, resulting in coalescence. Once the bubble size became larger, it departed either by overcoming the disjoining pressure in a thin film of water or via the buoyancy force in a thick film of water. Controlling bubble dynamics by the electric field, including escape, coalescence, and departure provides an active and reversible approach to move bubbles or increase departure frequency in many fluid mechanics and heat transfer studies.

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“…In the phase change process of boiling, bubble dynamics play a vital role in determining the overall heat transfer rate. Many studies have been conducted on bubble formation [109,110], bubble motion [111][112][113], and bubble removal and departure [114][115][116]. Bubble dynamics are affected by many factors, such as the buoyancy force and surface tension.…”

Section: A1 Introductionmentioning

confidence: 99%

Water harvesting and condensation heat transfer enhancement induced with electrowetting-on-dielectric

Yan¹

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As demand for the world's natural resources continues to rise, energy saving has become an urgent topic. Water harvesting and condensation heat transfer enhancement represent two vital energy-saving objectives. Many researchers have focused on alternating surface wettability by employing advanced materials or complex surface structures to achieve such goals; however, most of these approaches operate in a passive manner. In terms of active methods, electrowetting-on-dielectric (EWOD) has become a popular option owing to its excellent contact angle reversibility, switching speed, and long-term reliability in altering surface wettability. This dissertation presents a study of the EWOD effect on water harvesting and condensation heat transfer. It describes experimental and analytical studies concerning various characteristics such as EWOD-induced droplet dynamics, water capture capability, and heat transfer performance. It also quantifies water harvesting and condensation heat transfer enhancement. This dissertation is divided into four main studies, each of which considers different aspects of the effects of EWOD on water harvesting and condensation heat transfer. The first part of this dissertation (Chapter 2) describes microfabrication technologies to obtain EWOD devices, including low-pressure chemical vapor deposition, photolithography, sputtering deposition, and lift-off and spin coating. Mask designs with different electrode configurations and a device microfabrication protocol are also described. The second part of this dissertation (Chapter 3) presents an experimental investigation of EWOD-induced water harvesting enhancement. EWOD devices were tested in a high-humidity environment under mist flow. Compared with an uncharged EWOD device, the water capture capability of charged devices improved significantly. These results are of great importance, as they indicate strong potential for improvement in water-harvesting applications. The third part of this dissertation (Chapter 4) describes a visualization study of EWOD-regulated condensation droplet distribution. Side-by-side experiments were performed to compare charged and uncharged devices. Charged devices exhibited a regulated droplet distribution, faster droplet growth, more dispersed droplet distribution, and more large droplets. These experimental results introduced a novel approach to actively influence droplet distribution on microfabricated condensing surfaces and showed promise for improving the condensation heat transfer rate via EWOD. The fourth part of this dissertation (Chapter 5) discusses the EWOD effect on the condensation heat transfer coefficient and heat flux. The heat transfer coefficient and heat flux were compared on uncharged and charged (40V DC) EWOD devices. Experimental results demonstrated a positive effect of EWOD on condensation heat transfer. This approach could be incorporated into many industrial applications (e.g., heat exchanger fin surfaces, condensing surfaces of waste heat recovery systems, and components of electronic cooling packages) requiring high-efficiency heat dissipation. In summary, this work makes valuable contributions to the field of water harvesting and condensation heat transfer, proposing a new approach to research in these areas. Findings also detail a new tool to achieve water harvesting and condensation heat transfer enhancement via an active EWOD method.

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Prediction of Bubble Departure in Forced Convection Boiling with a Mechanistic Model That Considers Dynamic Contact Angle and Base Expansion

Cited by 15 publications

References 32 publications

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

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

Activating Bubble’s Escape, Coalescence, and Departure under an Electric Field Effect

Water harvesting and condensation heat transfer enhancement induced with electrowetting-on-dielectric

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