Experimental study on a swirl-vane separator for gas–liquid separation

Chemical Engineering Research and Design 1 5 1 ( 2 0 1 9 ) 108–119

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Experimental study on a swirl-vane separator for gas–liquid separation Gang Wang, Changqi Yan, Guangming Fan ∗ , Jianjun Wang, Junxiu Xu, Xiaobo Zeng, Antai Liu Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin, Heilongjiang 150001, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Experimental research on a swirl-vane gas–liquid separator has been conducted with the

Received 24 December 2018

aim to explore its application in downhole gas–water separation systems. With the help

Received in revised form 13 June

of a high-speed camera, the effect of gas content, Reynolds number and flow conditioning

2019

elements on the separation performance were investigated. The change of flow pattern will

Accepted 1 September 2019

lead to the oscillation of air core inside the separator, which has significant influence on

Available online 13 September 2019

the separation performance. Experimental results show that both the increase of Reynolds number and the arranging of flow conditioning elements can only restrain the oscillation of

Keywords:

air core to a certain degree. In addition, according to the evolution of air core, the flow pattern

Gas–liquid separation

can be divided into: rod air core flow, tadpole-shaped air core flow, oscillating air core flow

Swirl-vane separator

and swirling annular flow. Compared with the gas–liquid flow pattern map in vertical tube

Flow pattern

with the same inner diameter, it can be found that the flow pattern of gas–liquid mixture

Air core evolution

before entering the separator has significant influence on the state of the air core. In the end,

Operational envelope

the operational envelope limits of the separator were determined experimentally. Due to the effect of flow pattern, the separator is more suitable to be operated at low gas content. This research leads to a better understanding of hydraulic characteristics in this kind of separator. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

In the process of natural gas extraction, the problem of produced water, which is produced to the surface from hydrocarbon-bearing formations during gas extraction, has attracted more and more attention (Ogunsina and Wiggins, 2005). The traditional treatment is to collect

producing formation (Kolle et al., 2008; Liu et al., 2018). Compared with traditional methods, the idea of DGWS is appealing, but the further research is needed, especially the separator which becomes an important factor restricting the development of DGWS system. At present, hydrocyclones and spiral separators are generally used in DGWS systems for downhole gas–liquid separation (Zhao et al., 2017). There are

the downhole fluid to the surface, after some treatment, and then reinject it to the stratum. There are many problems with this treatment, such as high costs, environmental pollution (Ghaffarkhah et al., 2017).

many researches on hydrocyclones, but they mainly focus on oil–water

In the 1990s, Canadian C-FER (the Centre for Frontier Engineering Research) first proposed the idea of “downhole gas–water separation

of gas–liquid separation has not yet been conducted (Wang et al., 2015). In addition, limited by space, most of the hydrocyclones are asymmet-

(DGWS)”. The basic principle of DGWS system is: in a high-water well, a separation device is used at the bottom of the well to separate the fluid

rical “single-entry” structures, which leads to an asymmetrical internal

(gas and water in gas wells) produced in the producing zone, and then the gas-rich stream is lifted to the surface, while the water-rich stream

The experimental results show that the spiral separator can maintain high separation efficiency only in a specific range of gas–liquid mixture flow rate. Besides, due to the geometric characteristics of spiral

is directly injected into a selected formation usually deeper than the

∗

separation. The geometrical parameters of hydrocyclones are mainly designed for oil–water separation, and the in-depth qualitative analysis

flow field, and then affects the separation efficiency (Schütz et al., 2009).

Corresponding author at: 3A Laboratory Building, 145 Nantong Street, Harbin, Heilongjiang 150001, China. E-mail addresses: [email protected] (G. Wang), [email protected] (G. Fan). https://doi.org/10.1016/j.cherd.2019.09.003 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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flow is much less disturbed in axial cyclones, which contributes to the increase of separation efficiency (Nieuwstadt and Dirkzwager, 1995). In

Nomenclature

axial cyclones, the fluid is changed from linear motion to swirl motion

General symbols F1 The centripetal force on the element (N) The centrifugal force on the element (N) F2 Fr Resultant force on the element (N) The radial distance of the element from the r center (m) p The pressure at r(Pa) The width of the element (m) l v Tangential velocity of the element (m/s) Superficial velocity (m/s) V Flow rate (m3 /h) Q Diameter (m) D R Radius (m) 2 a Gravitational acceleration  2 V Ro(m/s  ) l l Reynolds number =  Rel



l

=

1 1 2 1−

S

Swirl number

lE

Entrance length (m)

  R 4  R 2 tan ˛ i

1− R i o

1− R o

Greek symbols  Density (kg/m3 ) Blockage coefficient Outlet angle of swirl element (◦ ) ˛ Inlet angle of swirl element (◦ ) ˇ Angle of the element (◦ ) ˚ ı Gas volume fraction Dynamic viscosity (Pa s)  Porosity of wire mesh ε  Surface tension (N/s) Subscripts g Gas Liquid l i Inner o Outer Hub h

with the action of swirl vanes. The axial cyclone also has many forms. Some researchers in Japan conducted deeply experimental research on swirl vane separators that are applied to the separation of steam and water for steam generators in boiling water reactors (BWRs) and presented some improvements. Their experimental results showed that this type swirl-vane separator has the characteristics of low resistance and high separation efficiency, which is suitable for gas–water separation under high gas contents (Matsubayashi et al., 2012; Kataoka et al., 2009, 2008). In the development of thorium molten salt reactor (TMSR), to solve the problem of fission gas, Oak Ridge National Laboratory (ORNL) proposed a new type of separator which can achieve continuous inline gas–liquid separation by the cooperation of multistage swirl elements (Rosenthal et al., 1970). Further research indicates that the increase in operating pressure is beneficial to improve the separation efficiency of this kind of separator (Yin et al., 2016, 2015). Vane-type pipe separators (VTPS), have been studied by many scholars, which has the advantages of small size and light weight. In addition, the separation performance of VTPS has been verified experimentally in both gas–liquid separation and liquid–liquid separation (Shi and Xu, 2015; Shi et al., 2012; Hannisdal et al., 2012). During natural gas extraction, as the gas–water mixture rises, the pressure gradually decreases, which causes the gas to expand. This will result in an increase of gas volume fraction in the gas–liquid mixture, which in turn will affect the flow pattern of gas–liquid mixture (Kelessidis and Dukler, 1989). The flow pattern of gas–liquid mixture before entering separator has significant influence on the hydrodynamics inside the separator. Therefore, it is necessary to consider the impact of flow pattern changes during the design of separators. When the gas–liquid mixture flow upward in a vertical tube, it can be divided into four basic flow patterns: bubble flow, slug flow, churn flow, annular flow (Taitel et al., 1980). Different flow patterns have different effects on the state of the two-phase fluid inside the separator. In view of these problems, a new kind of kind of gas–liquid separator is proposed in this work. Considering the change of gas content, the influence of flow pattern must be taken into consideration. The formation of a stable air core is the key to efficient operation in this kind of separator (Nieuwstadt and Dirkzwager, 1995; Krishna et al., 2010). The state of the air core inside the separator is largely affected by the gas–liquid flow pattern before entering separator, but not much attention has been paid to this in previous research. To have a deeper understanding about hydraulic characteristics of this new type sepa-

Abbreviations USC Upstream swirling chamber DSC Downstream swirling chamber DGWS Downhole gas–water separation

rator, the evolution of the state of air core at different gas content is the first part of our study. After that, the influence of Reynolds number, flow conditioning elements on the state of air core were investigated respectively. According to the change of the air core under different conditions, the air core was classified, and the corresponding flow patterns were summarized. At last, the operational limits of the separator were obtained experimentally.

separators, there are certain difficulties in the manufacture of spiral separators (Dixit et al., 2015). Given the small space in the downhole, centrifugal separator, which have been widely used in lots of fields (Wang, 2011; Yang, 2001), is a good choice for DGWS systems. In addition to the hydrocyclone and spiral separator, there are many other types of centrifugal separators. According to the way that fluid enters in separator, the centrifugal separator can be divided into two types: the first is called tangential flow cyclone and the other is axial cyclone (Nieuwstadt and Dirkzwager, 1995). In addition to hydrocyclone, gas–liquid cylindrical cyclone (GLCC) is an another kind of tangential injection separator. A significant advantage of the tangential injection separator is its high swirl intensity and stable separation performance. As a disadvantage of such separator, it should be noted that the flow in this kind of separator is highly turbulent and nonstationary, which limits the further application in two-phase flow separation (Hreiz et al., 2014a,b). In most axial cyclones, the fluid is put into swirl motion by swirl generator, which results in a relatively low swirl intensity of the fluid. Compared with tangential flow cyclones, a clear advantage is that the

2.

The separation principle of separator

With the effect swirl elements in the separator, the gas–liquid mixture is changed from linear motion to swirl motion. Fig. 1 shows an element of the swirling fluid in the separator (Cai et al., 2014). The pressure at radius r is denoted as p and the  pressure at radius (r + dr) is expressed as

p+

dp dr dr

.

Neglecting the second order terms, the centripetal force F1 of the fluid element between r and (r + dr) can be calculated by the following method:

F1 =



p+



dp dp dr (rd˚ + drd˚) l − prld˚ ≈ rl drd˚ dr dr

(1)

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When the gas–liquid mixture flows through the swirl elements of the separator, the two-phase flow is changed into the swirling flow regime. Since the density of the liquid l is greater than that of the gas g , the gas moves toward the center of the separator and form an air core with the effect of radial resultant force. In addition, according to the results of the numerical simulation, some scholars concluded that the separation length of small bubbles, which is the distance one single bubble takes form the periphery to the air core, is longer than that of large bubbles (Nieuwstadt and Dirkzwager, 1995; Yin et al., 2016; Pisarev et al., 2012; Qian et al., 2018). This conclusion has been confirmed by experiment (Yin et al., 2017).

Fig. 1 – An element in the swirling flow field.

Meanwhile, the centrifugal force F2 of the fluid element between r and (r + dr) can be calculated by the following method: F2 =  (r + 0.5r) ld˚dr

v2 v2 rld˚dr = r + 0.5r r

(2)

In Eq. (2),  is the fluid density. According to the calculations of centripetal force F1 and centrifugal force F2 , the resultant force Fr of the fluid element in the radial direction can be obtained (Zhang et al., 2017).

Fr = F1 − F2 =

dp v2 − dr r

 rld˚dr

(3)

3.

Experimental apparatus

3.1.

The configuration of the separator

The separator configuration, shown in Fig. 2, consist of three stage swirl elements and two swirling chambers. According to flow direction of the gas–liquid mixture, we named the three swirl elements as the first stage swirl element, the second stage swirl element and third stage swirl element respectively. Similarly, the two swirling chambers are named upstream swirling chamber and downstream swirling chamber, which inner diameters are 50 mm. The outer cylinder of the separator is made of transparent organic glass, which allows us to observe and record the two-phase flow phenomenon inside the separator. These visual observations of two-phase flow phenomenon taking place inside swirling chambers allow us to identify key mechanisms that govern the operational limit of the separator. In addition, this leads to a better understanding of the links between the hydrodynamics and the separation performance.

Fig. 2 – The configuration of the separator.

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number was used according to previously research (Yin et al., 2015; Pisarev et al., 2012; Krishna et al., 2010; Qian et al., 2018; Yin et al., 2017). The swirl number is defined as the ratio of the tangential to axial momentum flow from the vanes normalized by R0 and can be calculated as follows: 1 1 S= 2 1−

Fig. 3 – The geometry of swirl elements.

The swirl elements displayed in Fig. 3 are important parts of the separator. In order to ensure the accuracy of geometric parameters, each swirl element was processed by 3D printing. Fig. 3(a) and (b) show the detailed geometry of the first and the second stage swirl elements that were installed at the inlet and middle of the separator respectively to form a swirling flow in swirling chambers. The third stage swirl element, displayed in Fig. 3(c), was installed at the outlet of the separator and its function is to reconvert the fluid into linear motion, so that restore the pressure at the end of the separator. The hub of each swirl element has a center hole to separate the gas inside swirling chambers. There is only one inlet in the center hole of the first and third stage swirl elements. While the center hole of the second stage swirl element has two inlets and the center hole is divided into two parts. Outer diameter of each vane is the same as inner diameter of swirling chambers. The diameter of each hub(Di ) is 18 mm. The center hole diameter of first and second stage swirl elements(Dh ) is 10 mm, and that of the third stage swirl element is 4 mm. Each swirl element has five identical vanes fixed to the hub, each of which is twisted and has a fixed inlet and outlet angle. ␣ and ␤ represent inlet angle and outlet angle of each swirl element respectively, which are all angles to the central axis of the separator. Outlet angle of the first and second stage swirl elements is 35◦ , and inlet angles are 0◦ and 15◦ respectively. In third stage swirl element, the inlet angle and outlet angle are 25◦ and 0◦ respectively. When the gas–liquid mixture flows through the swirl element, the mixture rotates with the guidance of this vanes. Under these conditions, the mixture has different swirl intensity when it passes through swirl elements with different parameters. To reflect the influence of different swirl element parameters on the swirl intensity, the swirl

1− 1−

 R 4 i

Ro  R 2 tan ˛

(4)

i

Ro

where Ri and Ro is radius of the hub and swirling chamber respectively. ˛ is outlet angle of the first and the second stage swirl elements. denotes the blockage coefficient, which is defined as the ratio of the passing area at the trailing edge of the vane to that of the upstream inlet pipe, indicating the influence of vane thickness on axial velocity. As shown in Fig. 2, the new separator has three-stage swirl elements and two swirling chambers, which allow this separator to achieve a secondary separation. As a result, it can maintain high separation efficiency at higher gas contents. When the gas–liquid mixture goes through the first stage swirl element, the fluid is changed into the swirling flow regime in upstream swirling chamber. Under the action of radial pressure gradient, the gas is concentrated to the center and an air core is formed in upstream swirling chamber. And the gas will be separated out of the separator through the center hole of hubs in the first and second stage swirl elements. During this process, the gas enters center the hole through the inlet 1 at the second stage swirl element. Under the condition of low gas content, the gas in the gas–liquid mixture can be separated completely in the upstream swirling chamber. With the increase of gas content, due to the change of flow pattern, a part of the gas is not successfully separated in upstream swirling chamber and enters downstream swirling chamber. After passing the second-stage swirl element, the remaining gas and water recover sufficient swirl intensity and an air core is formed in downstream swirling chamber. The remaining gas will be separated from the hubs of the second and third stage swirl elements. In this separation process, the gas enters the center hole through inlet 2 of the second stage swirl element. After flowing through the third stage swirl element, the fluid reconverts into the linear motion. At least, pure phase water will leave the separator and eventually the entire separation process is completed. As shown in Fig. 2(a), a valve is installed at each outlet to control the flow rate of the fluid discharged from each gas outlet. During the operation of the separator, the opening of valves V1, V2, V3 and V4 on separator are only in two states: fully closed or fully open. According to the geometry of each swirl element and operation principle of the separator, it can be known that V1 and V2 are mainly used to control the discharge of fluid in the upstream swirling chamber, while V3 and V4 are mainly used to control the discharge of fluid in the downstream swirling chamber. It should be noted that V1 and V2 are always kept on state of fully open. V3 and V4 are opened only when a part of the gas is not separated in the upstream swirling chamber and enters the downstream swirling chamber.

3.2.

Experimental system

In our experiment, the air and water at room temperature (18 ◦ C) were used for a series of experiments on the separator. The experimental system, shown in Fig. 4, mainly consists of a

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Fig. 4 – Experimental system. venturi type air–water mixer which can produces bubbles with different diameters, the water supply system, the air supply system and a separator. A transparent organic glass tube with a length of 0.3 m is placed upstream of the separator, which is used to observe the flow pattern of gas–liquid mixture before entering the separator. In the air supply system, the air was compressed by an air compressor and stored in a gas tank, which can maintain a stable air supply pressure. The air was supplied to the separator through the gas tank and the air flow rate was adjusted by a control valve. The water in the water tank enters the venturi type air–water mixer through a multi-stage centrifugal pump and was mixed with air before entering the separator. The phenomenon inside the separator was recorded by a high-speed camera. After the separation is completed, most of water was returned to the water tank for recycling. A stream of air–water mixture with high concentration of gas was discharged into the air–water separation tank and separated by gravity. The flow rates of gas and water can be measured by flow meters and denoted as Qg and Ql respectively. The relative deviation of gas and water flow meters are 0.075%.and 0.5% respectively.

4.

Experimental results

4.1. The influence of different factors on the state of air core The state of air core is affected by several factors, which directly determine the separation performance. The formation of a rod-like air core is conducive to the operation of the separator, which can maintain the separation efficiency at a high level. In order to have a better understanding about

hydraulic characteristics of the separator, the influence of Reynolds number, gas content, and the layout of flow conditioning elements on the state of air core were investigated in the experiment. In this research, the Reynolds number is defined as follows: Rel =

2l Vl Ro l

(5)

where Vl is liquid superficial velocity(m/s), l is the density of liquid (kg/m3 ), l denotes the dynamic viscosity of liquid (Pa s). The gas volume fraction ı is calculated as follows: ı=

Qg × 100% Qg + Ql

(6)

where Qg is the flow rate of gas and Ql is the flow rate of water (m3 /h). The influence of gas content. Previous researches on this kind of separator mainly focused on low gas content conditions with gas volume fraction ␦ not more than 0.5% (Yin et al., 2015; Nana et al., 2013). At this gas content, the flow pattern of gas–water mixture before entering the separator is bubble flow. The influence of flow pattern on the separator performance has not attached enough attention in previous research. Thus, the evolution of the state of air core at different gas contents is the first part of our study. The change in gas content of the gas–water mixture causes a change in flow pattern before entering separator. Experimental results show that flow pattern has significant influence on the state of air core inside swirling chambers. Fig. 5(a) shows the state of air core inside the upstream and downstream swirling chamber at different times when

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Fig. 5 – The air core state at different gas volume fraction when Rel = 26,832, S = 0.39. the gas volume fraction ı was 10%. In each set of figures, the upstream swirling chamber(USC) is on the left and the downstream swirling chamber(DSC) is on the right. Before entering the separator, the air gathered into bubbles with different diameters. Under this condition, the flow pattern of gas–water

mixture is bubble flow. When these bubbles enter the separator in sequence that leads to the formation of gas pulses, the gas slug appears in the upstream swirling chamber and form a tadpole-shaped air core, accompanied by a spiral tail. As a result, the air core in the upstream swirling chamber was not

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continuous and unstable. However, due to the low gas content, the gas was completely separated in the upstream swirling chamber, and there was no air core in the downstream swirling chamber. With the further increase of gas content, compared with the gas volume fraction ␦ of 10%, the characteristics of air core inside swirling chambers changed significantly due to the change in the flow pattern before entering the separator. By comparing the states of the air core in Fig. 5(a)–(c), it can be found that the instability of air core is further enhanced with the increase of gas volume fraction. In the case where the gas volume fraction ␦ is 20%, the average diameter of air core in upstream swirling chamber is increased due to the increase of gas content. A continuous air core is established inside the upstream swirling chamber. As shown in Fig. 5(b), a larger gas slug enters the separator and gradually moves downstream in upstream swirling chamber. Because of the further development of flow pattern, the gas gathers into larger bubbles before entering the separator, and the volume of gas slugs in the separator is increased. The diameter of gas slug exceeds the inner diameter of the center hole in hub, thus part of the gas is not separated in upstream swirling chamber and enters the downstream swirling chamber. As a result, a very fine air core appears in the downstream swirling chamber. With the continuous increase of gas content, the flow pattern of the gas–water mixture gradually changes from the bubble flow to the slug flow before entering the separator. As shown in Fig. 5(c), the air core in the swirling chambers shows significant oscillation correspondingly. The specific performance is as follows: the unevenness of the air core distribution is further increased, and the difference in air core diameter at different positions is increased. It is worth noting that part of the gas is not separated due to the oscillation of air core in the downstream swirling chamber, and the separation efficiency begins to decrease. As the gas content increases, the flow pattern of gas–water mixture before entering separator changes accordingly. The effect of the change in flow pattern is that the air core instability is increased, which further lead to the decrease in separation efficiency. Therefore, during the design of separator, the flow pattern of gas–liquid mixture before entering the separator must be taken into consideration. The influence of Reynolds number. The instability of air core inside the separator is a key factor limiting the separation efficiency. Reducing this instability is the key to improving the separation performance. The effect of the Reynolds number on this instability was studied. It should be note that in the experiment, the gas volume fraction in the gas–liquid mixture before entering separator did not change. Therefore, while increasing the Reynolds number, the mass flow rate of gas is also increased. Compared the air core states at different times in Figs. 5(a) and 6 , a remarkable feature is that with the increase of Reynolds number, the volume of the gas slug in upstream swirling chamber decreases and the air core fracture phenomenon disappears. When the Reynolds number reaches 70435, the gas slug and air core fracture phenomenon disappear completely. An air core with a stable and clear interface is formed inside the separator, and the stability of the air core is increased. The cause of this phenomenon can be explained by the fact that as the increase of Reynolds number, the effect of turbulent force gradually increases, which act to break and disperse the gas into small bubbles. As a result, the gas enters the separator in a more uniform form. In addition, the swirl intensity of the air–water mixture inside the separator is also

increased with the increase of Reynolds number, which lead to an increase in centrifugal force. Therefore, in terms of practical application, working at higher Reynolds number is beneficial to the separator. In addition, it can be found that with the increase of Reynolds number, there are more and more tiny bubbles at exit of the first stage swirl element. This is the result of fluid shear. A high shear rate is normally encountered where there is an abrupt change in flow velocity (Bowers and Brownlee, 1998). Fluid shear is increased with the increase of swirl intensity. Exposing the gas–liquid mixture consisting of bubbles with different diameter dispersed in water to high shear rates will lead to a significant reduction in bubble size. Thus, more and more tiny bubbles appear at the first stage swirl element with the increase of Reynolds number. The influence of flow conditioning elements. From the above experimental results, it can be seen that the flow instability of the gas–liquid mixture before entering the separator will cause the oscillation of air core. Therefore, suppressing the oscillation of the air core is a direct way to improve separation efficiency. The addition of flow conditioning elements at the inlet of channel can suppress the flow instability of the twophase fluid (Tong and Weisman, 1996). When the oscillating gas–liquid fluid flows through the flow conditioning element, a certain pressure drop is generated and a part of the oscillating energy will be dissipated. It is expected to dissipate the energy of the gas–liquid mixture during the oscillation in this way. After many attempts, the combination of an orifice plate and a wire mesh pad was finally selected as the flow conditioning elements, which were placed at both ends of the visual observation section respectively. The orifice plate is expected to dissipate the energy of two-phase fluid oscillation and the main function of wire mesh pad is to make the distribution of gas in liquid as uniform as possible. The orifice diameter (D) of the orifice plate is 25 mm, and the porosity (ε) of wire mesh is 88.65%. Fig. 7 shows the state of air cores before and after the arrangement of flow conditioning elements when the Reynolds number Rel = 53,665 and the gas volume fraction ı = 44.79%. By comparison, it can be found that under the same gas content, the oscillation of the air core is significantly weakened. After using flow conditioning elements, although there are still gas slugs in the upstream swirling chamber, the volume of the gas slug is significantly reduced, and the diameter distribution of the air core becomes more uniform. The separation of the gas is completed in the upstream swirling chamber and there is no visible air core in the downstream swirling chamber. These phenomena prove that the oscillation degree of gas–liquid fluids is decreased and the distribution of two-phase fluids becomes more uniform before entering the separator. This indicates that more gas is separated from the first stage swirl element and the separator can be operated with higher gas content after using flow conditioning elements.

4.2.

The flow pattern inside the separator

Through the previous discussion, it can be found that under different gas and water flow rates couples, the air core inside the separator will present different states, which have significant influence on the separation efficiency of the separator. Therefore, the identification of different air–water flow regimes that taken place in the swirling chambers has impor-

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Fig. 6 – The change of the air core state when Rel = 70435, S = 0.39 and ı = 10%. tant significance for the reasonable selection of the separator operating conditions. According to the state of air core inside the separator, four different flow patterns were identified by visual observation: (a) rod air core flow (b) tadpole-shaped air core flow (c) oscillating air core flow (d) swirling annular flow. Each flow pattern is discussed below and the corresponding flow pattern is summarized in Fig. 9. Rod air core flow (Fig. 8(a)): In this case, the air core presents a rod shape with a stable and clear interface. This kind of air core usually appears in low gas content conditions. The air–water mixture is usually in bubble flow before entering the separator, where the gas is approximately uniformly distributed in the continuous liquid in the form of discrete bubbles (Kelessidis and Dukler, 1989). Since a stable air core can be formed, the gas is completely separated in the upstream swirling chamber and there is no air core in the downstream swirling chamber. Tadpole-shaped air core flow (Fig. 8(b)): With the increase of gas content, small bubbles accumulate into larger bubbles, and the flow pattern before entering the separator gradually changes from bubble flow to slug flow. As a result, the tadpoleshaped air core appears in the upstream swirling chamber, accompanied by a spiral tail and there is a thin air core in the downstream swirling chamber. In this state of air core, most of the gas can be successfully separated, and the separation efficiency is maintained at a high level. Oscillating air core flow (Fig. 8(c)): As the gas content is further increased, the flow pattern of the gas–liquid mixture before entering the separator changes accordingly, and the air core inside the separator begins to oscillate. According to the experimental observation, the flow pattern of gas–liquid mixture before entering the separator is slug flow or churn flow, which both can lead to the oscillation of the air core. In our experiment, whether the air core in the upstream swirling chamber oscillates or not is selected as the criterion to distinguish tadpole-shaped air core flow and the oscillating air core flow. Due to the violent oscillation of the air core, the separation efficiency drops sharply. Therefore, the performance of the separator is severely reduced under this flow pattern. Swirling annular flow (Fig. 8(d)): This flow pattern is similar to the annular flow. The gas forms a continuous air core

throughout the separator and the liquid exists as a liquid film. This flow pattern is characterized by the phenomenon that both the liquid and the gas are swirling in the separator. The separator in this research cannot work under this flow pattern, but the corresponding separators have been developed and widely used in the field of nuclear energy (Matsubayashi et al., 2012; Kataoka et al., 2008; Funahashi et al., 2016; Liu and Bai, 2016; Xiong et al., 2014, 2013). In the study of Taitel et al. a flow pattern transition model of gas–liquid two-phase flow for vertical tubes was established and matched very well with the experimental results in the 50 mm inner diameter vertical tube (Taitel et al., 1980). According to his work, the algebraic equations of different flow pattern transition boundaries can be expressed as follows and the corresponding flow pattern map of air–water two-phase flow in the 50 mm inner diameter vertical tube is shown in Fig.10. Bubble flow to slug flow:

Vl = 3.0Vg − 1.15

a (l − g ) 

1/4 (7)

l2

Bubble flow to dispersed bubble flow:

 Vl + Vg = 4.0

0.089 D0.429 (/l ) o

l0.072



a (l − g ) l

0.446  (8)

Slug flow to churn flow: lE = 40.6 Do

V + V g l



+ 0.22 √ aDo

(9)

Annular flow: 1/2



Vg g

a (l − g )

1/4 = 3.1

(10)

A comparison of the flow pattern map divided by the state of the air core inside the separator with the general flow pattern transition boundaries under the same pipe diameter is

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Fig. 7 – The influence of flow conditioning elements on the state of air core when Rel = 53,665, S = 0.39 and ı = 44.79%. shown Fig. 11. In the Fig. 11, curves 1–3 represent the flow patterns transition boundaries of the bubble to slug, the slug to churn, and the churn to annular respectively. It can be found that the upper boundary of the rod air core flow area is in good agreement with the transition boundary of the bubble flow to the slug flow. This proves that the separator can form a stable rod-shaped air core under the condition of bubble flow before entering the separator. With the increase of superficial gas velocity, the small bubbles accumulate into larger bubbles, and the state of air core in the separator gradually changes from the rod-shaped air core to the tadpole-shaped air core. Tadpole-shaped air core is mainly distributed in the low superficial velocity region of the slug flow. With the increase of the gas and liquid superficial velocity, tadpole-shaped air core flow is changed to other air core states gradually, and its existence region is small. In the area of the slug flow, with the increase of superficial gas velocity, the volume of the gas slug is continuously increased. The gas slug and the liquid slug

enter the separator in turn, which cause the air core inside the separator to oscillate. With the increase of superficial water velocity, the tadpole-shaped air core gradually disappears, which is the result of multiple factors, such as the flow pattern of the two-phase flow before entering the separator, the air and water superficial velocity, the geometry of the swirl element, etc. The fluid shear and turbulent force of the liquid is increased with the increase of superficial water velocity. With the combined action of the turbulent force and the fluid shear in the swirling flow field, the integrity of the gas slug is destroyed and the air core in the separator begins to oscillate. Under the condition of the tadpole-shaped air core, the air core state is relatively stable. It is worth noting that part of the oscillating air core flow area is distributed in the slug flow area and the other part is distributed in the churn flow area. It is obvious that the air core inside the separator is inevitably oscillating when the flow pattern of the gas–liquid mixture before entering the separator is

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Fig. 9 – Flow pattern map for the separator when S = 0.39, solid lines represent the transition boundary determined experimentally and dash lines represent maximum available superficial velocity in this experimental apparatus.

Fig. 10 – Flow pattern map of air–water two-phase flow in a 50 mm inner diameter vertical tube, where the shaded portion indicates the range of air and water superficial velocities in this research. Fig. 8 – Four different flow patterns in the separator.

churn flow. However, under the conditions of slug flow, the air core may also oscillate. This phenomenon can be explained as follows: with the increase of superficial gas velocity, the volume of the gas slug increases gradually, and the gas slug and liquid slug enter the separator alternately. This will lead to the uneven distribution of gas–liquid mixture inside the swirling chamber, so the stable air core cannot be formed in the separator. Before the flow pattern of gas–liquid mixture is changed into annular flow, the air core inside the separator has been transformed into swirling annular flow, which is the result of centrifugal force. After flowing through the swirl element, the gas–liquid mixture is changed from linear motion to swirl motion, and the centrifugal force is generated. The presence of centrifugal force helps the gas gather into the center of swirling chambers, and the liquid accumulates on the inner wall.

Fig. 11 – The comparison of flow pattern map inside separator with general flow pattern, dash lines represent maximum available superficial velocity of our experimental apparatus.

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ments is decreased gradually. This proves that the inhibitory effect of flow conditioning elements to the oscillation of the air core is only apparent at lower gas content.

5.

Fig. 12 – Operational envelope limits of the separator when S = 0.39.

4.3.

The Operation envelope limitation of the separator

Different types of separators have their own scope of application, and this scope can be defined from different perspectives. Separation efficiency is an important performance indicator of the separator. Therefore, the operational envelope of the separator is defined from the perspective of separation efficiency. When the gas content in the gas–liquid mixture before entering the separator is lower than the limit of operational envelope, all of the gas can be separated by the separator. The area below this operational envelope curve corresponds to normal operating conditions. There is also some gas that are not separated near the operational envelope limit, but their amount is small and can be ignored. In the front part of this paper, the influence of gas content, Reynolds number and flow conditioning elements on the separator flow regimes were discussed. As a summary, we obtained the operational envelope limits through a series of experiments. Fig. 12 shows the operational envelope of the separator with and without flow conditioning elements. In Fig. 12, curves 1–3 represent the transition boundaries for several flow patterns in the separator, respectively. Curve 1 represents the transition boundary of rod air core flow to tadpole-shaped air core flow and oscillating air core flow. Curve 2 denotes the transition boundary of tadpole-shaped air core flow to oscillating air core flow. And curve 3 represents the transition boundary of oscillating air core to swirling annular flow. As shown in Fig. 12, the separator can maintain a high efficiency in entire rod air core flow area. When the flow pattern inside the separator is changed to oscillating air core flow or tadpole-shaped air core flow, the separation efficiency of the separator begins to decrease. And the separator is unable to be operated in the swirling annular flow. This result is consistent with the characteristics of each flow pattern. In addition, the operational envelope limit curve increases with the rise of superficial water velocity. With the increase of superficial water velocity, the swirl intensity is increased accordingly. As the swirl intensity rises, the stability of the air core is increased. Thus, the separator can be operated at higher gas content. Comparing the two curves in Fig. 12, the application of flow conditioning elements makes operational envelope limit of the separator increase to a certain extent, especially at the low superficial water velocity. However, with the increase of superficial water velocity, the effect of flow conditioning ele-

Conclusion

In this research, a new type of separator was invented and an experimental study was conducted on the performance of the separator. During the study, with the help of a high-speed camera, the effect of gas content, Reynolds number and flow conditioning elements on the separation performance were investigated. A stable air core can be formed in the separator at low gas content where all visible gas can be completely separated. With the increase of gas content, the flow pattern of the gas–water mixture is changed before entering the separator, and the air core starts to oscillate, which leads to a sharp drop in the separation efficiency. The key to further improve the separation efficiency of the separator is suppressing the oscillation of air core in the separator. Experimental results show that both the increase of the Reynolds number and the application of flow conditioning elements can only restrain the oscillation of the air core to a certain degree. The flow pattern of gas–liquid mixture before entering separator must be taken into consideration in the design of the separator, which has significant influence on the separation efficiency. With the increase of gas content, according to the shape of air core in the separator, the flow pattern can be classified as: rod air core flow, tadpole-shaped air core flow, oscillating air core flow and swirling annular flow. And compared with the flow pattern in a vertical tube at the same inner diameter, it can be found that: when the flow pattern before entering the separator is bubble flow, a stable rod-shaped air core can be formed in the separator. Under the conditions of slug flow and churn flow, the instability of the air core is increased gradually. And the swirling annular flow can be formed in the separator under conditions of churn flow or annular flow, not necessarily annular flow. At last, operational envelope limits of the separator with and without flow conditioning elements were determined experimentally. The operational envelope limit curves increase with the rise of superficial water velocity. The increase in separation performance by the application of flow conditioning elements is more pronounced at low flow rates.

Acknowledgments The authors greatly appreciate support from the National Natural Science Foundation of China (Grant No. 11875117) and support from the Fundamental Research Funds for Central University of Ministry of Education of China (HEUCF181505, 3072019CF1506).

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