Visual Observation of Flow Regime Transition in Downward Vertical Gas-Liquid Flow Using Simple Mixer

1. Introduction

Correct calculation of important parameters in two-phase flow like pressure drop, mass transfer and heat depends on correct prediction of phase distribution or flow pattern. In many engineering applications in this case the oil and gas industries, biomechanics and environment pollution analysis of a two-phase contribute highly to good process design (Siddiqui et al., 2016). For this reason, the two-phase flow in industrial applications has been and is the focus in many studies. Most of the two-phase flow studies focus on the analysis of horizontal and upward vertical flow, while the focus on downward vertical flow is rare.

There exist various methods for determining the two-phase flow patterns like direct observation and high speed photography, ultrasonic, neural networks etc. In high-speed photography and similar emerging techniques, the flow patterns can be observed directly. The present method is less expensive than complicated methods with easy implementation. Arosio and Guilizzoni introduced some of the practical methods for a two-phase flow structure visualization (Arosio & Guilizzoni, 2006).

Kendoush and Al-Khatab detected flow regimes in downward two-phase flow through the isothermal air-water system and identified the three distinct flow regimes, of bubbly, slug, and annular. They found that flow pattern transitions are not sharp, but fairly gradual (Kendoush & Al-Khatab, 1994). Ishii et al. identified flow regimes for vertical co-current downward air-water two-phase flow through impedance void meter together with a self-organized neural network. The experimental test sections consist of round pipes with internal diameters of 25.4 and 50.8 mm (Ishii, Paranjape, Kim, & Sun, 2004).

Bhagwat assessed the flow patterns and void fractions in vertical downward two-phase flow in an experimental manner and applied flow visualization to study the flow pattern. The flow visualization confirmed the existence of four major flow patterns in downward two-phase flow: the bubbly, slug, and falling film or froth and annular) (Bhagwat & Ghajar, 2012). Jha et al. presented an ultrasonic method for flow pattern classification. They stated that their new method can be adopted in a variety of industrial applications (Jha, Ray, Mukherjee, & Chakraborty, 2013). Almabrok studied the upward and downward gas-liquid flow in vertical pipes using high speed photography in an experimental manner. He assessed the effect of 180° bends exert on the flow in the straight part of the pipe and liquid film behavior close to the bends on the characteristics and development of downward and upward gas-liquid two-phase flow in large diameter pipes (Almabrok, 2013). The different flow patterns of downward two-phase in pipe are shown in Fig. 1 (Bratland, 2010).

Figure 1. Different Downward Two-Phase Flow Patterns (Bratland, 2010)

In addition to the importance of flow patterns determination method, another significant issue here is how to produce different flow patterns. In the previous studies air and water are blended in a device named mixer. There exist different mixers with complex studies through which the two-phase flow patterns are generated, Fig. 2.

Figure 2. Mixer Structure

The structure of mixer and how it mixes two fluids, has a direct effect on the two-phase flow parameters (Almabrok, 2013). The objective of flow patterns study is to develop flow map plot(s), Fig. 3 (Shoham, 2006).

A simple mixer is applied in features of: generating downward two-phase flow regimes in an easy manner and no significant effect on pressure drop is studied here. Direct observation and high speed photography method are applied here to obtain gas liquid downward two-phase flow patterns in (374 points) different points, which are then compared with previous experimental data.

Figure 3. Downward Two-Phase Flow Map in Pipe of 51mm Diameter (Shoham, 2006)

2. Experimental Setup

Different tests are run on two-phase flow are in the experimental loop, Fig. 4. Air and water are used as the gas and liquid phases in all experiments. The main vertical pipe of 50 mm internal diameter at 4 m height is made of transparent plexiglas to allow visual observation.

Figure 4. Schematic of the Experimental Setup

The test section is located around 3.5 m downstream of the mixer and 3 m downstream of the final bend, allowing the flow to have enough distance to reach the fully developed regime, Fig. 4. Based on the turbulent flow correlation, the entrance length is about 0.5 m which is much less than the available length in the experiment. The visual observation, confirms that the entrance effect is important only in the initial 0.5 meter of the pipe after which the flow is fully developed and stable.

To prevent vibrations, six shock absorbers are installed at the inlet, outlet and under the pump. The water flow rates are regulated by four valves and measured using two calibrated rotameters with accuracie of 2%. The air and water rotameter are within 0-350 lit/min and 0-400 lit/min range, respectively. The water is pumped from the main tank through a centrifugal pump with 4 kW power.

Up to 8 bars of compressed air pressure is fed to the device by a compressor. The air flow rate is measured by four calibrated rotameters with accuracies of 2%. Air and water are mixed in a simple mixer located at the top of the main pipe. To build a simple mixer, the most convenient manner for air injection, air stone which is used mostly in home aquariums, is considered, Fig. 5.

The air stone of a hard material is appropriate for various operations in CNC machines. Inside of the air stone inlet should be drilled to allow air hose. This stone is placed in a metal frame welded inside the pipe wall.

At the outlet of the pipe, the air and water mixture is drained to the main tank where they have enough room to separate. The air is discharged into the atmosphere and the water remains in the main tank.

Figure 5. Schematic of the Mixer

3. Results and Discussion

3.1. Study of Mixer Performance

To begin with, first, it should be checked that if the mixer has the ability to produce downward two-phase flow patterns. To record the mixer performance a high speed digital camcorder (CASIO EX-F1) with 1200 fps (frame per second) shutter speed equipped with visualization, flow regime identification and flow recording apparatus is installed. An image processing computer code is considered in order to improve quality of the images of two-phase flow (Hanafizadeh, Ghanbarzadeh, & Saidi, 2011). For bubbly flow, results of these processes are shown in Fig. 6.

Figure 6. Image Processing Steps for Improving Image Quality

(for Example: Bubbly Flow)

Since most researchers apply the superficial phase velocities of water (V_SL) and air (V_SG), these parameters are involved as the coordinates of flow map, defined as follows:

In these equations A_P is the pipe diameter, Q_L and Q_G are the volumetric flow rates of liquid and gas, respectively. The flow patterns in downward vertical gas-liquid two-phase flow, after image processing run on four flow regimes are shown in Figs. (7-9).

As observed in Figs. (7-9) all expected flow patterns are obtained; therefore, it can be deduced that the new simple mixer has the ability to produce two-phase flow patterns in a proper manner. Falling film images are presented in Figs. (7a-9a).

Figure 7. Final Processed Images of the Four Main Flow Patterns in Downward Flow (series 1)

Figure 8. Final Processed Picture of Four Main Flow Patterns in Downward Flow (series 2)

As observed in Fig. 10, no change occurs at different superficial air velocities, since the water superficial velocity is lower than a constant value, where liquid phase moves in the form of layer on the pipe wall and fall down. It should be worth noted that this constant value (Vs_L) varies on the researchers’ choice: Kendoush and Al-khatab 0.5 m/s, Al-mabrook, Barnea et al. 0.6m/s (Barnea, Shoham, & Taitel, 1982), Raeiszadeh et al. 0.4 m/s, Bhagwat 0.2 m/s and in this study, it is 0.42 m/s.

When superficial liquid velocity is greater than this constant value, the liquid phase occupies the whole pipe volume and gas phase appears in bubbles, Figs. (7b -9b). If air velocity is increased more from a specific value, air bubbles accumulate and larger bubbles in the scale of pipe diameter appear. This flow pattern is named ‘slug’, Figs. (7c-9c). It is notable that this determined value varies for different superficial water velocities, for V_SL=0.48 m/s, for V_SG=0.048 m/s, for V_SL=1 m/s, for V_SG=0.12 m/s, for V_SL=1.7 m/s and for V_SG=0.6 m/s.

Eventually, by an increase in air velocity the Taylor bubbles collapses, thus formation of a very turbulent mixture of two-phase. This pattern is named the ‘froth flow’, Figs. (7d-9d). Here, it can be deduced that mixer performance is acceptable due to the fact that different patterns of two-phase flow are generated completely. In the next step the mixer accuracy is examined in generating two-phase map. The map preparation is the objective of such studies. Thus comparison of the obtained flow map with the existing ones constitutes the bases of this assessment.

In this flow pattern map the superficial air velocity is 0.008-2.5 m/s and water velocity was 0.01-2.5 m/s. This map is drawn through the results obtained by direct observation of 374 test cases of variation in V_SL and V_SG, Fig. 10.

Figure 9. Final Processed Picture of Four Main flow Patterns in Downward Flow (series 3).

Fig. 11 compares the present flow map with other references. Flow map is compound of several parts. Each part shows transition between two flow patterns as shown in Fig. 12.

Figure 10. Flow Pattern Map for Vertical Downward Two Phase Flow in 50mm Pipe for Present Study

Figure 11. Comparison Present Flow Map with other Researchers

Figure 12. Several Parts of Present Flow Map