EHD effects on periodic bubble formation and coalescence in ethanol under non-uniform electric field

https://doi.org/10.1016/j.ces.2019.115451Get rights and content

Highlights

  • The strengthening effect of EHD on the formation of periodic bubbles was confirmed.

  • Varied electrical Bond numbers (BoE) and Weber numbers (We) was considered to separate the EHD effects.

  • The bubble dynamic behaviors were compared in the presence and absence of the electric field.

  • We found that the coalescence bubbling regime exists in high We was restricted by the electric force.

  • Mechanism of the EHD effects was explained by a qualitative force analysis.

Abstract

This paper focus on the effects of electrohydrodynamic (EHD) on bubble dynamic characteristics generated on a submerged capillary in ethanol under non-uniform electric field, using high-speed photography and with a particular interest in the effect of electrical Bond number (BoE) on the bubble behaviors. The obtained results ranges of BoE from zero to 5.52 and Weber number (We) from 0.0006 to 0.1365 show that the bubble formation process associated with three stages at the low electric field strengths, including the nucleation, stable growth and necking stages, while the stable growth stage is absent for the high field strengths. With the increase of both We and BoE, the bubble period and waiting times are shortened rapidly, while the frequency is accelerated remarkably. The bubble dynamic behavior, which is characterized by the bubble volume and curvature radius, depends primarily on the BoE, and the increase in the We slightly raises the bubble volume and curvature radius at the departure time. Furthermore, it is found that with the increasing of BoE, bubbles coalescence evidenced at high We is weakened and eventually disappeared. Nevertheless, this phenomenon has the tendency to subsequently reappear at the higher We and BoE.

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):Fe=ρeE-12E2ε+12E2ερltρl

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)

  • M. Gao et al.

    An experimental investigation on effects of an electric field on bubble growth on a small heater in pool boiling

    Int. J. Heat Mass Transf.

    (2013)
  • A. Georgoulas et al.

    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.

    (2015)
  • N.J. Hepworth et al.

    Modelling the effect of liquid motion on bubble nucleation during beer dispense

    Chem. Eng. Sci.

    (2003)
  • Y.C. Kweon et al.

    Experimental study on nucleate boiling enhancement and bubble dynamic behavior in saturated pool boiling using a nonuniform dc electric field

    Int. J. Multiphase Flow.

    (2000)
  • Y.C. Kweon et al.

    Study on the deformation and departure of a bubble attached to a wall in d.c./a.c. electric fields

    Int. J. Multiphase Flow.

    (1998)
  • C. Liao et al.

    Electrophoretic motion of a liquid droplet and a bubble normal to an air-water interface

    Colloid Surf. A.

    (2016)
  • Z. Liu et al.

    Visualization of bubble detachment and coalescence under the influence of a nonuniform electric field

    Exp. Therm Fluid Sci.

    (2006)
  • C. Maatki et al.

    Effects of magnetic field on 3D double diffusive convection in a cubic cavity filled with a binary mixture

    Int. Commun. Heat Mass Transf.

    (2013)
  • G. Marrucci

    A theory of coalescence

    Chem. Eng. Sci.

    (1969)
  • A. Poullikkas

    Effects of two-phase liquid-gas flow on the performance of nuclear reactor cooling pumps

    Prog. Nucl. Energy.

    (2003)
  • X. Quan et al.

    Effects of electric field on microbubble growth in a microchannel under pulse heating

    Int. J. Heat Mass Transf.

    (2011)
  • S. Ramakrishnan et al.

    Studies in bubble formation—I bubble formation under constant flow conditions

    Chem. Eng. Sci.

    (1969)
  • Cited by (29)

    • Polarization motion of bubbles in a non-uniform electric field

      2023, Chemical Engineering Journal
      Citation Excerpt :

      Cold LED light source (ILP-2, Olympus, Japan) with sufficient frequency was set across the camera to brighten the motion region of the bubbles. Such an experimental system was also used to investigate the growth and dispersion characteristics of bubbles in a leaky-dielectric liquid medium [13,14]. The latter refers to the electrostriction stress, which is negligible in this case due to the minimal influence of fluid compressibility on the bubble formation and motion.

    • Experiment and prediction model study on pool boiling heat transfer of water in the electric field with periodically changing direction

      2022, International Journal of Multiphase Flow
      Citation Excerpt :

      This was used to detailly analyze the mechanism of the influence of electric field on the bubble dynamics behavior. They indicated that bubble departure diameter and bubble release period would be decreased with the increment of electric field intensity Zhang et al. (2020). suggested that the decreased bubble departure diameter resulted from the reduction of bubble surface tension, which was caused by the electric field.

    View all citing articles on Scopus
    View full text