Experimental Conditions

All experiments were conducted using river sand particles, air, and water as the working fluids, under atmospheric pressure and at a room temperature of 24ºC. Further detailed information regarding these experimental conditions is summarized in Table 1.

Experimental Setup

The experimental setup depicted in Fig. 1 illustrates the apparatus for the airlift system. It comprises an airlift pump, a lifting pipe, an air supply pipe, a water supply pipe, and a slurry delivery pipe. The airlift pump features a suction port with a 70 mm diameter at its base and a flange structure at the upper- end for seamless connection with the lifting pipe. The lifting pipe, made of standard seamless steel pipe SCH20 with dimensions of 110 mm outer diameter, 100 mm inner diameter, and 2000 mm length, ensures stable flow conditions during experimentation. Its lower end connects to the airlift pump via a flange, while the upper end links to the outlet sampling pipe. To facilitate observation of the airlift pump’s bottom flow field and post-operation sand pit morphology, the experimental setup includes a 3800 mm×2520 mm×3000 mm (length×width×height) test pool filled with approximately 400 mm thick river sand, as specified in Table 1. Before commencing experiments, the submergence ratio is set by adjusting the pool’s water level via a water injection pipe. The submergence ratio is defined as γ = L2/L3, as illustrated in Fig. 1. Positioned above the pool is a three-axis (X, Y, Z) moving platform allowing controlled movement of the lifting pipe and airlift pump in three dimensions using screw drive mechanism v_xyz = 4.5 mm/s. The airlift pump can move linearly at a constant speed along both horizontal and vertical directions. Compressed air, provided by two M40A screw air compressors and stored in an air tank, is regulated for precise adjustment of the airflow rate using a thermal gas mass flow meter and a pressure regulating valve installed on the air pipe at the airlift pump inlet. Subsequently, air is injected into the airlift and mixed with water in the lifting pipe. The resulting gas-liquid-solid three-phase mixture flows upward in the lifting pipe and is discharged into the sampling trough via the sampling pipe. Within the sampling trough, air is separated from the liquid-solid mixture and released into the atmosphere to complete the sampling process. Throughout the experiment, the water level is meticulously adjusted using the water injection pipe and the three-axis (X, Y, Z) moving platform to maintain an almost constant submergence ratio. This comprehensive setup constitutes the experimental platform for airlift pumping with a lifting pipe diameter of 100 mm.

In the experiment, the submergence ratio can be controlled within the range of 0.5 to 0.8, while the air intake can vary from 0 to 240 m³·h⁻¹. To obtain the inlet air flow rate accurately, a thermal gas mass flow meter (model: JL-LDR-80) with an accuracy of ±1.5% is installed near the airlift pump. The JL-LDR thermal gas mass flow meter operates based on the principle of thermal conduction for flow rate measurement. It employs a bridge loop, with one sensor measuring the flow temperature and another sensor maintaining a constant temperature difference higher than the fluid temperature, allowing for precise flow rate measurement even under high-temperature and high-pressure conditions.

Fig. 1.

Schematic Diagram of Airlift Pump Experimental Platform.

Experimental Methodology

The experimental process is as follows:

1. Before commencing the experiment, thorough consideration is given to the confining effect of deep water on sediment particles. To closely simulate real-world engineering conditions, a sand supply method akin to practical scenarios is employed. Initially, experimental river sand is meticulously spread across the bottom of the experimental water tank, forming a sand layer approximately 400 mm thick, as depicted in Fig. 1. To ensure that the pump bottom remains parallel to the sand surface throughout the experiment, an experimental auxiliary apparatus is employed. The X-axis of the three-axis (X, Y, Z) moving platform is maneuvered back and forth to address any irregularities in the surface of the sand layer. Additionally, a level is used to verify the evenness, guaranteeing that subsequent sand pit extractions result in a smooth sand pit profile and minimizing experimental inaccuracies.

2. On the fixed frame of the three-axis (X, Y, Z) moving platform, mark the scales for the X, Y, and Z axes to facilitate adjustment of the simulated construction position. Label the positions of the x and y axes for the locations of vertical construction points. Mark the scale position of the z-axis as the position where the bottom of the pump aligns with the sand surface. Then, adjust the Z-axis position to the maximum stroke value. Finally, open the water inlet valve of the water injection pipe to inject experimental water into the reservoir to the appropriate depth. Collaboratively adjust the submergence rate to 0.8 by adjusting the Z-axis position and the water inlet valve.

3. Turn on the air compressor and simultaneously observe whether the air pressure in the air reservoir tank is fully charged. Once filled, adjust pressure regulating valve 3 to stabilize the working pressure required for the experiment.

4. Use the scale markings on the three-axis moving platform and move the airlift pump to the corresponding position to locate the simulated construction point of the airlift pump. Move to the sampling point, and after the pressure gauge on the air reservoir tank stabilizes, take samples at that point.

5. Sampling process: Adjust the Z-axis position downward to the position where the bottom suction port of the airlift pump aligns parallel to the bottom sand surface before the experiment starts, even when H = 0, as shown in Fig. 2a). Begin sampling into the sampling trough using the sampling tube. The sampling time is 20 seconds, with sampling for 10 seconds at H = 0, followed by moving the Z-axis downward for another 10 seconds. Immediately after 20 seconds of sampling, move the Z-axis to the top end of the stroke.

6. Continue adjusting the air intake volume to other values of air flow rates and repeat steps (2-5).

7. To ensure that there is no mutual interference between the sampling points, ten samples can be taken at the bottom of the airlift pump for each construction point. After sampling ten points, open the drain valve of the water reservoir tank to drain the water from the reservoir tank. After draining the water from the reservoir tank, ten sand pits are left on the bottom sand layer, as shown in Fig. 2b).

8. Then, measure the morphology of the sand pits. As shown in Fig. 2a), measure the height of the sand pits under the diameter of the sand pits, namely (D/10, h1), (D/5, h2), (D3/10, h3), (D2/5, h4), and (D1/2, h). Where D is the diameter of the sand pit.

9. Repeat the above steps to complete the sampling and measurement of the bottom sand pit morphology for different air intake volumes at submergence rates of 0.5, 0.6, and 0.7.

In addition, the sampling method used in the experiment follows the sampling method employed in practical engineering applications by our research group. Throughout the experiment, to maintain a constant submergence rate, the water level is observed for changes in submergence rate after each sampling. Once the water level is adjusted to be consistent, the next set of experiments begins. To minimize randomness and reduce experimental errors, three sets of samples are taken under identical conditions. After completing each experiment, the position of the sand pit at the bottom is numbered accordingly.

Fig. 2.

a) Schematic diagram of sand pit measurement in this experiment; b) Positions of bottom sand pit sampling points.

Uncertainty Analysis

This experiment utilizes various techniques to calibrate all measuring devices. Additionally, uncertainty analysis is conducted based on the multivariate Taylor series method. Table 2 summarizes the values obtained through uncertainty analysis.

Analysis of Solid Conveying Performance of Airlift Pumps

At varying submergence ratios, Fig. 3 illustrates the solid particle lifting performance of the airlift pump. The results of this experiment are consistent with the trends observed in the study by Kassab et al. Results indicate that the efficiency of solid particle lifting by the airlift pump is significantly influenced by the submergence ratio. The mass flow rate of solid particles shows a consistent pattern with changes in air flow rate across all submergence ratios. Under constant submergence ratio conditions, the solid conveying capability of the airlift pump increases with the rising air flow rate until reaching a peak at a specific air volume, after which it stabilizes. When the air mass flow rate remains constant, the mass flow rate of solid particles rises with increasing submergence ratio. Notably, the increase in solid mass flow rate with rising submergence ratio is more pronounced at lower air volumes, gradually becoming less prominent at higher air volumes. Furthermore, as the submergence ratio increases, more airflow is needed at the origin of the coordinate axis to lift solid particles. This phenomenon may be attributed to the heightened static head at the pump inlet under high submergence ratio conditions, necessitating less air volume to initiate the lifting of solid particles and thus facilitating their earlier flow.

Analysis of the Suction Range at the Bottom of the Airlift Pump

To validate the theoretical analyses presented in this paper, we compared the theoretical results with our experimental findings. To investigate the suction range of the airlift pump, we conducted a characterization analysis of the sandpit morphology observed during the experiment. Using Matlab, we analyzed the data obtained from measuring the main height and diameter of the sandpit, as illustrated in Fig. 4. It’s evident from Fig. 4a) that, at γ = 0.8 and Q_g = 30 m³·h⁻¹, the sandpit exhibits a regular circular shape with uniform contours around its perimeter. Fig. 4b) illustrates the morphology of the sandpit derived from the measured height, assuming a regular circular profile. Our experimental observations consistently revealed circular profiles for the sandpits under varying submergence ratios and air intake rates, with uniformly distributed contours along the perimeter.

Fig. 3.

Comparison of the experimental results of this study with those of Kassab et al. [4].

Fig. 4.

a) A Study of Sandpit Integrity at γ = 0.8, Q_g = 30 m³·h-1; b) Characterization of Sandpit Morphology.

Based on the aforementioned findings, to conduct a more precise analysis of the suction range at the bottom of the airlift pump, a characterization analysis of the three-dimensional sandpit profiles was performed using profile data points. Initially, an analysis of the general trends was conducted for various submergence ratios and air intake volumes.

As can be seen from Fig. 5, it’s evident that the relationship between sandpit size and air intake volume was investigated across different submergence ratios. Variations in sandpit size exhibit slight differences between high submergence ratios (γ = 0.8, 0.7) and low submergence ratios (γ = 0.6, 0.5). As depicted in Fig. 5, at an submergence ratio of 0.8, both sandpit diameter and depth increase with rising air intake volume. The diameter ranges from 300 mm to 370 mm, while the depth escalates from 55 mm to a maximum of 79 mm. The smallest sandpit exerts its influence over an area 18 times that of the airlift pump intake, whereas the largest sandpit’s range extends to 28 times the intake area. Fig. 5 illustrates a sudden upsurge in sandpit size from low to high air intake volumes at high submergence ratios. This phenomenon could be linked to the solid conveying curve of the airlift pump. At higher submergence ratios, the elevated liquid level necessitates less energy to reach the top of the lifting pipe, as per the law of conservation of energy. Consequently, under similar air intake volumes, the solid conveying rate is poised to increase. Moreover, escalating air intake volumes might augment gas flow velocity, intensifying aerodynamic effects and lifting more particles, thus enlarging the sandpit diameter. Larger air intake volumes may also alter the liquid-solid interaction, rendering particles more susceptible to gas-induced lift, thereby influencing sandpit morphology. Likewise, from Fig. 5, it’s apparent that variations in sandpit size at submergence ratios of 0.7 and 0.8 exhibit similar trends. While the sandpit diameter displays an upward trajectory, the depth shows relatively minimal fluctuation without any discernible pattern. Nonetheless, at an submergence ratio of 0.7, sandpit size changes appear more uniform, lacking the abrupt differences observed elsewhere.

At low submergence ratios, as depicted in Fig. 5, no sandpit formation is observed at an submergence ratio of Q_g = 30 m³·h^-1, attributable to insufficient lifting energy resulting in a limited bottom influence range. However, as shown in Fig. 6, sandpit contours are discernible without actual sandpit formation, with surface sand accumulation evident. This indicates that at low submergence ratios and low air volume values, the critical threshold for fully lifting particles is not reached. Although particles are set in motion at the bottom, they might oscillate up and down, traversing into the lifting tube. Consequently, only a minor portion of particles is conveyed outward, while the bulk settles at the bottom, forming small sand mounds. For low submergence ratios, specifically when Q_g=30 m³·h^-1 or higher, increasing the air volume leads to a slightly reduced bottom influence range, with less variation observed in sandpit diameter compared to higher submergence ratios. This phenomenon could be attributed to the stronger binding of particles by surface tension at lower submergence ratios, imposing greater constraints on the movement of bottom particles and resulting in smaller sandpit formations.

Regarding the morphology of the sand pits, as observed in Fig. 5, it is evident that nearly every sand pit’s center at the bottom formed a small mound, ranging from a point to a triangular shape. This feature is particularly pronounced at a submergence ratio of 0.8, with the size of the mounds gradually leveling off with increasing air intake. This phenomenon may be attributed to the fact that after the experiment ends and the airlift head ascends, a small portion of particles are still lifted at the bottom. As a result, the distance between the bottom of the airlift and the sand surface increases, leading to significant losses in both friction and local resistance along the path. Consequently, a large number of particles accumulate at the suction port and the lower end of the lift pipe, resulting in collisions and friction, which in turn leads to significant energy losses and the formation of small mounds. With an increase in air intake, the size and volume of the sand pit gradually increase, leading to a larger area at the base of the formed mounds and, consequently, a decrease in their height. However, a unique phenomenon was observed in the experimental results: at submergence ratios of 0.7, 0.6, and 0.5, with Q_g = 150 m³·h^-1, small mounds were absent at the bottom of the sand pits, which instead exhibited a semi-elliptical shape. This may be attributed to the fact that, under the experimental system parameters, Q_g = 150 m³·h^-1 is more conducive to the lifting of bottom particles, leading to an optimal lifting state. Combined with Fig. 5, it can be inferred that the airlift pump’s solid lifting performance is optimal at Q_g = 150 m³·h^-1.

In investigating the influence of the submergence ratio on the bottom’s affected range, sandpits were examined at four submergence ratios (0.8, 0.7, 0.6, 0.5) with an air intake of Q_g = 150 m³·h^-1. As indicated by the aforementioned analysis of the airlift pump’s performance, the optimal lifting performance of solid particles was observed at Q_g = 150 m³·h^-1, hence, this air intake was chosen for the analysis. From Fig. 7, it is evident that under the same air intake conditions, the bottom’s affected range gradually expands with an increasing submergence ratio. The diameter of the bottom sandpit increases with the rising submergence ratio, and similarly, the depth of the sandpit exhibits an increasing trend. This may be attributed to lower submergence ratios potentially leading to easier particle sedimentation at the bottom, resulting in the formation of smaller sandpits. Conversely, under higher submergence ratios, particles may be more prone to being flushed by airflow, leading to the formation of larger sandpits. Additionally, with the increase in submergence ratio, the distance between the end of the lift pipe and the liquid surface, ΔL(ΔL = L₃ – L₂), decreases. Consequently, less energy is required for solid particles to overcome gravity as they ascend to the outlet. Since the air intake remains constant, according to the principle of energy conservation, the analysis of the airlift pump’s performance indicates an increase in the mass flow rate of solid particles at the outlet, thereby enlarging the bottom sandpit’s affected range.

Fig. 5.

The relationship between sand pit size and air intake under different submergence rates.

Fig. 6.

Bottom morphology at γ = 0.5 and γ = 0.6, Q_g = 30 m³·h^-1.

Fig. 7.

Relationship between the size of sandpits and submergence ratio at Q_g = 150 m³·h^-1.