1. Introduction
Within the aquaculture industry, aeration and water circulation are among the most essential needs to maintain the proper dynamics of a lake [
1]. Both of these processes assist in sustaining and prolonging the life of a body of water, while simultaneously improving the water quality, as well as the health and the production capacity of the farmed aquatic animal [
2]. Water circulation is typically used to keep the water temperature consistent, reduce stratification, increase nutrient solubility, and reduce the buildup of organic substances at the bottom of the tank [
3]. In contrast, aeration, which is the addition of oxygen into the water, is used to support the aquatic life within the system by providing adequate aerobic conditions [
1]. Airlift pumps are proven to be effective systems within this industry due to their ability to aerate and circulate water simultaneously.
An aerator’s main function is to supply a pond with the proper concentration of dissolved oxygen in order to improve the energy efficiency of the oxygen transfer process [
4]. A study testing the oxygen transfer within an airlift system concluded that if designed properly, an airlift pump can reach greater efficiencies for oxygen transfer than a diffused aeration system [
5], eliminating the need for an added aeration device. Some important oxygen transfer parameters to evaluate the oxygenation occurring in the system are the standard oxygen transfer rate (SOTR) and the standard aeration efficiency (SAE). The SOTR can be defined as the mass of oxygen that can be added to the body of water per unit time at standard conditions (20 °C water, 0 mg/L initial DO concentration and 1 atm pressure in clean water) [
4]. The SAE on the other hand is the STOR per unit of power [
4]. There are various aspects of an airlift pump that could affect the oxygen diffusion within the system. Conditions, such as bubble size, gas–liquid interfacial area and flow patterns, each influence the oxygen transfer rate and aeration efficiency in different ways. As bubble size is reduced, an increase in mass transfer is typically observed [
6]. This is due to the available gas–liquid interfacial area [
7]. The gas–liquid interfacial area can be described as the surface area available for the two phases to either coexist or interact with each other [
8]. Therefore, compared to a few larger bubbles, a multitude of smaller bubbles increases the total surface area for oxygen to diffuse between the two phases. This was observed in a study conducted by Calderbank and Moo-Young [
9], in which expressions for the mass transfer coefficient were determined for large gas bubbles, and the oxygen supply rate by aeration was consequently found to increase by expanding the gas–liquid contact area. A larger number of small bubbles can be achieved by using a sparger with a large number of holes [
10].
Given that bubble sizing and their distribution directly affect the formation of flow patterns [
11], it is therefore important to observe these factors in comparison to the mass transfer rate. The four common flow regimes categorized for two-phase flow in a vertical pipe are bubbly flow, slug flow, churn flow and annular flow. Bubbly flow consists of a multitude of small-sized bubbles that are less efficient in pumping water but increase mass transfer capabilities. Slug flow is the most efficient flow pattern in terms of pump performance due to its piston-like bubbles that allow more water to be lifted. Due to the larger bubble sizes in a slug flow pattern, the gas–liquid interfacial area is diminished compared to bubbly flow and therefore reduces its oxygen transfer rate. However, the increased turbulence at the tail end of each slug also helps increase the oxygen transfer rate. In the experiments conducted by Reinemann [
12], this effect in the slug regime allowed the bubbly and slug flow to have similar gas transfer rates, and this allowed him to draw the conclusion that flow pattern did not have a significant effect on oxygen transfer. The chaotic nature of the churn flow pattern is an advantage as it decreases the mass transfer resistance between the liquid film on the bubbles’ surface to the bulk of the liquid, which in turn increases mass transfer [
13]. The last of the flow patterns is annular flow, which occurs when the velocity of gas increases to a point where it pushes the liquid into a film against the interior tube wall [
14].
Another factor that has an influence on mass transfer within an airlift pump is the superficial gas velocity. Superficial gas velocity refers to the ratio of airflow rate to the cross-sectional area of the riser pipe [
15]. As concluded in various studies, an increase in superficial gas velocity results in an increased gas–liquid mass transfer coefficient [
13,
16,
17]. This is because with a higher airflow rate, gas hold up, which is the amount of gas within the column at a given time, increases and decreases the average bubble diameter which subsequently expands the gas–liquid interfacial area available for mass transfer to occur [
13]. Furthermore, by increasing the airflow rate, the bubbles start to rise at a higher velocity which creates more turbulence in the system, also helping with mass transfer [
13]. Similarly, a study conducted by Kumar and Vinod [
18] examined the effect of the airflow rate on the mass transfer coefficient and found that the mass transfer coefficient increased as the airflow rates of the system were increased [
18].
The airlift technology is so versatile that the system design is often manipulated to be used in a variety of different industry systems. There are a number of papers in the literature that distinctly focus on variations of the airlift design and how it could affect the mass transfer occurring inside the system. This could include the geometry of the pump itself, a change in the riser or bubble column or even a different setup of the system. In Siegel’s study [
19], the interrelationship of the three main components in an airlift reactor being the riser, the downcomer as well as the gas–liquid separator, were compared with the mass transfer in the reactor [
19]. Twenty different reactor geometries were tested, and it was found that the gas–liquid separator plays an important role in the reactors’ behavior and therefore should be highly considered in the design. Furthermore, it was determined that a correlation exists between the pneumatic power gas input per total dispersion volume, the riser superficial gas velocity for each condition tested and the overall mass transfer coefficient [
19]. Another set of experiments conducted by Drandev et al. [
20] explored the ratio of the cross-sectional area of the downcomer to the riser (Ad/Ar) in airlift reactors and its effect on the oxygen mass transfer of the system. The airlift reactor used in this experimental setup changed the conventional reactor shape to a rectangular one. The results concluded that a rectangular-shaped reactor had better oxygen mass transfer characteristics at a cross sectional area ratio of 2.0 [
20].
Another method of changing the airlift system design that could affect the oxygen transfer coefficients is by adjusting the riser pipe diameter or shape. In a study conducted by Pi et al. [
21], the idea of a trumpet-shaped riser was investigated. This trumpet-shaped riser was placed inside an airlift reactor where it could act as a modified internal-loop reactor [
21]. From the experiments performed, the volumetric oxygen transfer coefficient was greatly influenced by the ratio of riser height to the static fluid head above the spargers, the ratio of the area of the cross-section of the riser to the downcomer (Ar/Ad) and the superficial gas velocity within the riser [
21]. This uniquely formed riser provided optimal conditions for fluid circulation which enhanced the efficiency of the oxygen transfer and an oxygen transfer rate of (2.17 ± 0.11) × 10
−5 kg m
−3 s
−1 was recorded with an oxygen mass transfer coefficient of (27.88 ± 1.12) × 10
−3 s
−1 [
21].
In the present study, a dual injector airlift pump designed by Ahmed and Badr [
22] is examined to determine its aeration capabilities. The two different injection geometries (axial and radial) of the pump are experimentally evaluated at various airflow rates to obtain the standard oxygen transfer rate and the aeration efficiency. Observations of bubble size, flow pattern and flow behavior around a bubble are used to further validate findings through flow visualization imaging, and particle image velocimetry (PIV) measurements.
2. Materials and Methods
The experimental setup for an airlift pump that uses water and compressed air for the two-phase flow is depicted in
Figure 1. This looped system pumps water into a supply tank from a 125 L reservoir tank using a sump pump. An adjacent tank allows the water to overflow, keeping the water level at a specified head. This setup is adjustable to allow for changes in the submergence ratio, also known as the water head, over the total length of the riser pipe (H
S/L). For the purpose of representing the most common operating conditions used at the aquaculture facilities, a submergence ratio of 0.7 was selected for all present tests. From the supply tank, the water enters an airlift pump that is connected to a 1.6 m riser pipe with an ID of 31.75 mm, creating a lift height of 0.48 m. Using compressed air, the airlift pumps the water in this pipe up to the delivery tank. At this point, any excess pressurized air is released into the atmosphere while the water is transported to a collecting tank. From there, flow rate measurements can be taken before the water is returned to the reservoir tank where the cycle can continue. The water used in the system was deionized water to ensure minimal contaminants and mineral deposits.
For the pneumatic portion of this setup, the supplied air is fed through a pressure regulator using a 6 mm hose. The airline is then split to account for the axial and the radial injection points, each of which is connected to a needle valve to control the flow rates. The air then passes through digital mass flow meters to regulate the airflow rate and collect temperature and pressure readings before injecting the air into the pump. For the dissolved oxygen testing, the water was purged of oxygen by injecting nitrogen into the reservoir tank. This was achieved using a nitrogen tank connected to a 6 mm hose that had a small 25 mm air stone attached at the end to help with diffusion.
In order to perform a flow visualization analysis of the two-phase flow patterns occurring in the system, a high-speed camera setup was utilized. This setup required a light and a diffuser sheet to be placed behind the riser pipe in order to adequately capture the flow image. The recorded videos were then transferred onto the MiDAS computer software where the high-speed footage could be further analyzed.
Throughout the experiments, two mass flow meters ranging from 0–500 SLPM were used to monitor the airflow rates entering the system. Each of these mass flow meters has a reading error of ±0.8% as well as an error of ± 0.2% of full scale. An uncertainty of ±1.2% was therefore calculated for the air mass flow rate. The water mass flow rate was determined using a stopwatch with increments of 1 millisecond to record the time it took to fill the collection tank in increments of 1 L. From this, the water mass flow rate uncertainty was calculated to be ±3%. The efficiency of the airlift pump was determined using Equation (1) and had a calculated uncertainty of ±0.3%. Additionally, the DO measurements were recorded using a DAQ system and a LabVIEW program that was designed to receive, record and analyze the collected data. Instantaneous DO measurements were collected at a sampling frequency of 1 kHz over a period of 100 seconds in order to ensure that the received measurements are statistically adequate to present the variations in these measurements.
2.1. Pump Design
The 31.75 mm airlift pump being tested in this study is an optimized dual injector model designed by Ahmed and Badr [
22] as shown in
Figure 2. This design consists of two main geometries: axial and radial. The radial inlet consists of a perforated tube of the same inner diameter as the riser pipe. There are 180 holes evenly distributed radially along the circumference of this component, each hole measuring 1.7 mm in diameter. The purpose of the radial inlet is to create bubbles in the form of slugs, which allows for a larger surface area to raise the water. The axial injection geometry consists of a smaller diameter pipe within the pump that extends approximately 12 mm above the injection site. This design forces the injected air to hit the inner pipe, creating a shear force upwards along the wall. For the purpose of these experiments, the axial and the radial geometries were tested separately to compare their oxygen mass transfer rates.
In this study, the efficiency of the pump can be found using a modified definition of Nicklin’s equation as follows [
23]:
In this equation,
is the water discharge and
is the volumetric flow rate of air, both measured in m
3/s. The pressures were recorded in Pascals where
is the injection pressure of air and
is the atmospheric pressure. The variables
and
are the water density (kg/m
3) and the gravitational acceleration (m/s
2), and
and
measured in meters are the length of the riser pipe and the static head respectively. These variables are also defined visually in
Figure 1.
2.2. Capacitance Sensor
The void fraction is defined as the percentage of the riser pipe that is occupied by air. A capacitance sensor uniquely designed for this experimental setup that was developed by Elsaftawy et al. [
24] was used to measure the void fraction. This design is comprised of three main components: two capacitance sensors, a meter circuit, and a LabVIEW interface [
24]. The LabVIEW program was designed to receive, record and graph the capacitance signal as well as find the average void fraction. When performing the experiments, the void fraction data were collected at a sampling frequency of 2.5 kHz over a period of 100 seconds. The uncertainty of the void fraction calculated using static calibration was found to be ±6%.
2.3. Oxygen Probe and Circuit
To measure the dissolved oxygen in these experiments, a galvanized dissolved oxygen probe was used. The dissolved oxygen probe was fixed in the reservoir tank as this allows time for the pumped water to fully mix back into the system before readings are taken. Therefore, this provides a better representation of the time it would take the entire system to reach saturation. The probe works by allowing oxygen molecules to diffuse through a membrane where they can be reduced when they reach the cathode, producing a small voltage that increases as the oxygen increases. A circuit was designed using an embedded dissolved oxygen circuit in order to process the data recorded by the probe and record the findings through the LABVIEW interface. As stated on their data specification sheets, the uncertainty of both the probe and the embedded dissolved oxygen circuit was ±0.05 mg/L [
25,
26].
2.4. PIV System
A planar PIV system was used to measure the water velocity field in a vertical plane downstream of the air injector.
Figure 3 illustrates the setup configuration of the PIV system. The velocity field was measured in a vertical plane containing the pipe centerline. The plane was illuminated by a light sheet obtained from a set of optics and a laser source. Polymer Seeding Particles (PSP) were seeded into the supply tank and present within the liquid in the pipe. Each round seeding particle has a diameter of 20 μm and is white in color. The airlift loop is left to run for a sufficient period of time to ensure homogeneous distribution of the seeding particles within the liquid. A high-speed camera along with several lenses and filters are required for optical recording to capture successive images of the laser sheet that illuminates the flow section under investigation. The purpose of the camera is to detect the locations of seed particles in the flow field illuminated by the laser light sheet. The utilized camera is the SpeedSense Lab 320 provided by DentecDynamics. This high-speed camera can reach a frame rate of 1380 frames per second with a sensor resolution of up to 1920 × 1200 pixels
2. For each run, the camera focus and aperture are adjusted such that the particles are observed clearly within the flow with no distortions in order to ensure high image quality. A synchronizer connected to the laser, the camera and the host computer is used to control the timing of the laser pulses and the captured images.
To operate the PIV setup, compressed air is injected with an extremely low flow rate through the radial air injector only, in order to observe and measure the behavior of the flow field. This is achieved by allowing a single slug bubble to be injected by opening the airflow control valve for a short burst of time. Following the air injection, air bubbles will start to rise, and the seeded particles will pass by the illuminated laser sheet. The camera is positioned perpendicularly to the laser sheet into the page while the laser sheet captures the particles passing through a vertical plane in the middle of the pipe. This allows the camera to capture images separated by a constant time interval of Δt. The captured images can then be stored and processed, and the liquid velocity vectors can be determined by using the cross-correlation method. During experimentation, several factors can be controlled and adjusted in order to best track the interrogation areas and observe clear particles. These factors include the exposure time, triggering rate, image resolution, interrogation area size, laser sheet intensity, camera position, seeding particles concentration, and laser sheet thickness. It is important to mention that at low airflow rates, the shape of the bubbles and the rate at which they propagate through the liquid change.