EHD effects on periodic bubble formation and coalescence in ethanol under non-uniform electric field
Introduction
Bubble formation process plays an important role in a wide range of industrial applications, such as cooling nuclear reactors, beer productions, emulsion preparation, water treatments and separation equipment (Cui et al., 2001, Hepworth et al., 2003, Pan et al., 2016, Poullikkas, 2003, Xu et al., 2015). The enthusiasm for bubble formation has always abundant during the exploration of multiphase flows and fluid dynamics. The gas-liquid interfacial area is a key parameter that determines the heat and mass transfer. Fundamentally, the interaction between the bubble and the surrounding liquid is dominated by a balance between facilitating and restraining forces (Di Bari et al., 2013, Georgoulas et al., 2015, Tsuge et al., 1992). In this context, the gas initial momentum, the buoyancy, and the pressure difference forces are facilitating the bubble formation and detachment process. By contrast, the surface tension, viscous and inertial forces tend to maintain the bubble attached to the orifice.
Both electric and magnetic fields can induce motion and conversion in fluid systems, many researchers have made outstanding contributions in the related areas (Di Marco and Grassi, 2009, Di Marco and Grassi, 2002, Maatki et al., 2013, Selimefendigil and Öztop, 2017). Particularly, imposing an external electric field on bubble generation systems is vital to enhancing the heat and mass transfer coefficient of gas-liquid phases. With regards to this situation, the EHD would inevitably influence the bubble geometry as well as the bubble dynamic characteristics. If integrated a thermal field on dielectric liquid layer further, the physical problems in fluid flow will become more complex (Hassen et al., 2017). It is a new balance by the combined hydrodynamic and electrostatic forces. In particular, the response of the bubble near the electrode to the strength gradients of the electric field results in distinct features in terms of the bubble shape as well as formation and detachment relative to the bubble located elsewhere (Iacona et al., 2007, Di Marco, 2012). Taking the coalescence and breakup of the bubbles into account, the treatment of the bubble under the electric field would be rather difficult (Liu et al., 2006). Essentially, even understanding of the formation and detachment of an isolated bubble under electric field is far from sufficient, since the variation on electric field factors such as types of supplied voltage, electrode structure and gradients of field strength, making it difficult to reveal the mechanisms. As a general agreement in the field-free situation, the bubble formation process involves three consecutive stages, including the nucleation stage, stable growth stage, and necking stage (Zhang et al., 2017). In this view, however, there is a lack of consensus on bubble evolution partitions under the effect of an electric field. Furthermore, the bubble growth parameters, such as the growth time and departure volume observed in field-free can be modified as the EHD effect. In a set of experiments, Kweon et al. (1998) found that the bubble departure volume decreased continuously in the non-uniform field but remained nearly constant in a uniform electric field. Based on the results, the importance of the electric field uniformity was established by a series of researchers (Cattide et al., 2007, Dong et al., 2006, Herman et al., 2002). Siedel et al. (2011) experimentally investigated the growth, departure and vertical rise of isolated bubbles with and without an electric field. They proposed that increasing the electric field strength led to a gradual decrease in the bubble curvature radius with little variations in the bubble growth time and departure frequency. However, Diao et al. (2014) investigation demonstrated that the electric field decreased the bubble growth time significantly.
Regarding the electrical forces on the bubble surface, it impresses the bubble growth and shapes with the bubbles being squeezed and stretched in the horizontal and vertical direction, respectively (Chen et al., 2008). Meanwhile, the forces influence the internal pressure of bubbles as well as the distribution of the electric field, which causes a strong coupling between the electrostatics and mechanics of this kind of problem. Considering the parameters of electrical permittivity, electric field strength and gravity, Wang et al. (2016) numerically investigated the behavior of bubbles detached from a capillary under the effects of a non-uniform electric field. The results indicated that an additional driving force, supplied by the strongest intensity regions near the orifice, replaced the buoyancy force to accelerate the bubble detachment process. They pointed out that this force would drive the bubble from the field with stronger strength to a field with weaker strength. Actually, this force is induced by the different dielectric constants of liquid and gas under a non-uniform field which strongly depends on several parameters including the frequency of the electric field, the bubble size, shape as well as the liquid and gases electrical properties (Liao et al., 2016, Mauro, 1978, Ming et al., 2013).
Here, an experimental apparatus is established to produce bubbles through a submerged capillary. Ethanol is selected as the liquid medium due to its conductivity. An external non-uniform electric field is applied between the metal capillary electrode and ring electrode. The aims of the present work are in the following. Essentially, previous investigations do lack the description of the fundamental knowledge of bubble microscopic behaviors, especially for the non-uniform electric field with varying strengths. In addition, the electric field strength changes the coalescence of bubbles at the relatively high gas flow rate but its effects on the bubble dynamics and characteristics still remain unknown. Thus, the first objective of this study is investigating the effect of field strength on the periodic bubble formation phenomenon by employing enhanced high-speed photography with high temporal and spatial resolutions. The bubble forming volume evolution, and the curvature radius can be therefore quantified with accuracy. The second one is the coalescence bubbling regime at relatively high gas flow rate under the electric field can be revealed and elaborated.
Section snippets
Apparatus and measurement system
The major components of the electrostatic dispersion experimental system are schematically displayed in Fig. 1. A rectangular container made of transparent plexiglass is used to accommodate the conductivity experimental media of ethanol. The container is in the shape of square cross-section with sides’ length of 60 mm and a height of 100 mm. A stainless-steel capillary installed at the symmetric center of the container bottom wall is simultaneously considered as a ventilation device and an
Effect of an electrical forces on bubble formation
Theoretically, the most generally accepted expression for the EHD force (Fe) generated by an electric field exerted on a conductive fluid is described by Landau and Lifshitz (1984):
Here, ρe represent the free charge density of the fluid, ρl is the fluid density and ε is the dielectric permittivity. The subscript, T, expresses a constant temperature case. The first part of the right side in Eq. (1) describes the Coulomb force. This force produced by the interaction of
Bubble formation mechanisms
In the present study, the experimental results show that the bubbles emersion, growth, and detachment from the capillary orifice are kept in a single and reciprocating cycle when the Weber number is low. As shown in Fig. 3, the bubble behavior during formation and detachment at different electrical Bond numbers in ethanol is captured in a series of images at a constant Weber number of 0.0006 for all cases.
In the absence of the electric field, it can be seen that the bubble initially emerges
Conclusion
In the present study, we have systematically investigated the characteristics of the bubble generated on a submerged capillary in conductive media of ethanol under non-uniform electric field, emphasizing on the effect of EHD. The parameters are taken in the range of electrical Bond number 0 ≤ BoE ≤ 5.52 and Weber number 0.0006 ≤ We ≤ 0.1365. High-speed images show that the bubble formation characteristics in the electric field are notably different compared with that of the bubble under
CRediT authorship contribution statement
Wei Zhang: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original Draft. Junfeng Wang: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration. Bin Li: Validation, Investigation, Formal analysis. Hailong Liu: Resources, Investigation. Christian Mulbah: Investigation, Data Curation. Dongbao Wang: Investigation, Data Curation. Piyaphong ongphet: Resources, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are grateful for the financial support from the Natural Science Foundation of China under Grant No. 51761145011 and No. 51876086, and Postgraduate Research & Practice Innovation Program of Jiangsu Province under Grant No. KYCX19_1599.
References (45)
- et al.
Study of the behavior of a bubble attached to a wall in a uniform electric field
Int. J. Multiphase Flow.
(1996) - et al.
Gas and solids between dynamic bubble and emulsion in gas-fluidized beds
Powder Technol.
(2001) - et al.
Electric field effect on the bubble behavior and enhanced heat-transfer characteristic of a surface with rectangular microgrooves
Int. J. Heat Mass Transf.
(2014) - et al.
A numerical study of quasi-static gas injected bubble growth: Some aspects of gravity
Int. J. Heat Mass Transf.
(2013) - et al.
Influence of electric field on single gas-bubble growth and detachment in microgravity
Int. J. Multiphase Flow.
(2003) - et al.
Motivation and results of a long-term research on pool boiling heat transfer in low gravity
Int. J. Therm. Sci.
(2002) - et al.
Adiabatic bubble growth in uniform DC electric fields
Exp. Therm Fluid Sci.
(2013) - et al.
Bubble shape under the action of electric forces
Exp. Therm Fluid Sci.
(2013) - et al.
An investigation of behaviours of a single bubble in a uniform electric field
Exp. Therm. Fluid Sci.
(2006) - et al.
Coalescence and conjunction of two in-line bubbles at low Reynolds numbers
Chem. Eng. Sci.
(2016)
An experimental investigation on effects of an electric field on bubble growth on a small heater in pool boiling
Int. J. Heat Mass Transf.
Numerical investigation of quasi-static bubble growth and detachment from submerged orifices in isothermal liquid pools: The effect of varying fluid properties and gravity levels
Int. J. Multiphase Flow.
Modelling the effect of liquid motion on bubble nucleation during beer dispense
Chem. Eng. Sci.
Experimental study on nucleate boiling enhancement and bubble dynamic behavior in saturated pool boiling using a nonuniform dc electric field
Int. J. Multiphase Flow.
Study on the deformation and departure of a bubble attached to a wall in d.c./a.c. electric fields
Int. J. Multiphase Flow.
Electrophoretic motion of a liquid droplet and a bubble normal to an air-water interface
Colloid Surf. A.
Visualization of bubble detachment and coalescence under the influence of a nonuniform electric field
Exp. Therm Fluid Sci.
Effects of magnetic field on 3D double diffusive convection in a cubic cavity filled with a binary mixture
Int. Commun. Heat Mass Transf.
A theory of coalescence
Chem. Eng. Sci.
Effects of two-phase liquid-gas flow on the performance of nuclear reactor cooling pumps
Prog. Nucl. Energy.
Effects of electric field on microbubble growth in a microchannel under pulse heating
Int. J. Heat Mass Transf.
Studies in bubble formation—I bubble formation under constant flow conditions
Chem. Eng. Sci.
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