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  •  L. Sun, leechengsun@sohu.com; H. Liu, echoliu215@sina.com

    A review on bubble generation and transportation in Venturi-type bubble generators

    Jiang Huang, Licheng Sun (), Hongtao Liu (), Zhengyu Mo, Jiguo Tang, Guo Xie, Min Du

    State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource & Hydropower, Sichuan University, Chengdu, China Abstract Venturi-type bubble generators own advantages of simplicity in structure, high efficiency, low power consumption, and high reliability, exhibiting a broad application potential in various fields. This work presents a literature review of recent progress in the research concerning Venturi-type bubble generators, with a focus on the performance evaluation, bubble transportation, and breakup mechanisms. Experimental studies employing flow visualization techniques have played an important role in exploring the bubble transportation and breakup phenomena, which is vitally necessary for clarifying the bubble breakup mechanisms and understanding the working principle and performance of a Venturi channel as a bubble generator. A summarization was carried out on both experimental and theoretical work concerning parameters influencing the bubble breakup and the performance of Venturi-type bubble generators. Based on the geometric parameter optimization combined with appropriate flow conditions, it is expected that Venturi-type bubble generators can produce bubbles with controllable size and concentration to satisfy the application requirements, while a further work is required to illustrate the interaction between the liquid and gas bubbles.

    Keywords Venturi-type bubble generator

    performance

    bubble transportation

    bubble breakup mechanism

    Article History Received: 6 August 2019

    Revised: 7 September 2019

    Accepted: 8 September 2019

    Review Article © Tsinghua University Press 2019

    1 Introduction

    Bubbly flow is a typical and fundamental flow pattern of gas–liquid flow, characterized by the gas or vapor phase dispersed in a liquid continuum. Due to the large gas–liquid interfacial area beneficial to heat and mass transfer and efficient mixing processes, bubbly flow is immensely important in many applications, such as bubble columns, flotation cells, spargers, fluidized beds, and electrochemical reactors, etc. With the increasing applications of bubbly flow, a well control of bubble size and concentration has become a key issue and attracts much attention (Fujikawa et al., 2003; Mills and Schlegel, 2019a). For flotation technology, microbubbles with diameter of 100 μm or less tend to remove numerous pollutants (e.g., colloids, metal ions, microorganisms, proteins, oil emulsions, fine and ultrafine particles) in waste water treatment (Rodrigues and Rubio, 2007); however, com- paratively large bubbles with diameter of 500–2000 μm are employed for lifting many large particles in mineral flotation (Reay and Ratcliff, 1973; Rodrigues and Rubio, 2003). To satisfy various application requirements, many methods for

    the generation of fine bubbles have been developed so far. Venturi structure is employed by many devices for generating bubbles in a gas–liquid flow system, it can provide a wide size range of bubbles from tens of microns to millimeters (Fujiwara et al., 2003; Yoshida et al., 2008; Sun et al., 2017; Zhao et al., 2019).

    A classic Venturi structure consists three main parts: a converging section, a throat, and a diverging section. It operates on the Bernoulli principle of the conservation of energy. When a fluid flows through a Venturi channel, its increase in velocity (kinetic energy) in the throat is accompanied by a fall in pressure (pressure energy), while in the diverging section, it regains most of its pressure energy by the reduction in kinetic energy. In practical applications, Venturi structure can apply to short contact time and high intensity gas–liquid reactions, and usually takes the two forms. One is the above- mentioned conventional Venturi (Gabbard, 1972; Fujiwara et al., 2007; Sun et al., 2017; Zhao et al., 2018), with which the introduced gas bubbles or cavitation bubbles are well-broken into finely dispersed bubbles in the divergent section. The other is jet-Venturi or Venturi ejector (Jackson, 1964;

    Vol. 2, No. 3, 2020, 123–134 Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-019-0049-3

  • J. H, L. Sun, H. Liu, et al.

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    Kandakure et al., 2005; Balamurugan et al., 2007; Sharma et al., 2018), mainly consisting of a throat and a diverging section. A typical jet-Venturi or Venturi ejector consists of a liquid nozzle, a suction chamber, a mixing tube, and a divergent section (Gourich et al., 2007). Liquid is pumped into the system at high velocity through the nozzle, and the gas phase is sucked into the chamber. The two phases get mixed in the chamber and the mixing tube and bubbly flow is subsequently created in the divergent section. Versatility in applications has been well proved for Venturis. Venturis have been suggested to be as gas absorption devices (Bauer et al., 1963; Zhou and Smith, 2000; Baawain et al., 2007), bubble generators or gas distributors (Kress, 1972; Huynh et al., 1991; Briens et al., 1992; Havelka et al., 2000; Kawamura et al., 2004), and chemical reactors for the fast reaction inside (Cramers and Beenackers, 2001; Gourich et al., 2005). Liquid-driven type Venturi is often used as gas-inducing devices, such as bubble generators and gas distributors, while gas-driven type Venturi is used as scrubbers (Ali et al., 2013; Gulhane et al., 2015; Zhou et al., 2016; Bal et al., 2019).

    Recently, Venturi bubble generators receive increasingly attention in the fields of application and research, and the performance is the most important concern. The size and distribution of generated bubbles are the primary indexes for its performance. In a Venturi channel, it takes advantage of the intensified interaction between the gas and liquid two phases to produce a large number of fine bubbles. In view of increasingly concern to Venturi bubble generators, this paper reviews the progress of fundamental research involved, for a better understanding of complex interaction between gas and liquid in Venturi channels.

    2 Bubble generation methods

    Bubble technology is expected to be a cost-effective and environmentally friendly technology with great potential in many applications. In order to meet the application requirements mentioned above, an appropriate method for generating bubbles becomes the key, on which extensive research has been motivated. Generally, hydrodynamic method (Agarwal et al., 2011), acoustic or sonication (Xu et al., 2008), electrochemical method (Wu et al., 2008), and mechanical agitation (Xu et al., 2008) are the four basic ways for generating fine bubbles. For all these methods, much smaller and higher number density bubbles are desired, and moreover, they are controllable in size according to purposes of practical applications. Duo to low-energy consumption and widely applicable to various application environments, the hydro- dynamic method is the most frequently encountered in industrial applications and several typical bubble generators have been developed, such as spiral liquid flow type, pressurized dissolution type, ejector type, and Venturi type (Terasaka

    et al., 2011), as shown in Fig. 1. For a spiral liquid flow type bubble generator, micro or fine bubbles are generated due to centrifugation effect and strong shear force caused by a high-speed rotating liquid flow (Fig. 1(a)). When the gas in saturated high-pressure water is released into ordinary pressure water, a large number of bubbles will be formed, so that a pressurized dissolution type bubble generator is named consequently. The ejector type and Venturi type bubble generators take advantage of intensified pressure change of high-speed flow arsing from the variation in the cross-section area to generate fine bubbles. With these principles, a variety of bubble generators have been inves- tigated. Sadatomi et al. (2005, 2012) placed a spherical body and an orifice plate in the flow paths, respectively, taking advantage of the throttling effect to accelerate the fluid and cause a highly-turbulent and shear flow, thus breaking bubbles into pieces.

    A Venturi type bubble generator, as shown in Fig. 1(d), owns advantages of simple structure, installation convenience, having no internal moving parts, less maintenance, and good reliability. Most importantly, low power consumption makes it an economic alternative for enhancing gas–liquid mass transfer (Terasaka et al., 2011; Basso et al., 2018). A Venturi bubble generator is capable of generating a high number density of micro or fine bubbles normally with a mean diameter below 100 μm (Fujiwara et al., 2003; Kawamura et al., 2004; Yoshida et al., 2008; Kaneko et al., 2012), and the concentration and size of produced bubbles in a wide range from tens of microns to millimeters are controllable by operating the liquid and gas flow rates. For a Venturi bubble generator, gas is normally introduced through the throat under conditions of free gas suction and forced gas supply. The latter can provide larger gas feed rate, but limit the gas dispersion efficiency at high gas flow rates

    Fig. 1 Several typical bubble generators.

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    (Zahradnik et al., 1997). While for the former, the gas supply circuit is open to the environments, mak