Excerpt from the Feb. 2021 Vol. 13 of Measurement: Sensors research paper by LinZhao, Zhuang Chang, Zhenduo Zhang, Rui Huang, and Denghui He
The centrifugal pump is extensively used in the fields of petroleum, chemical industry, nuclear power, agriculture, etc. The bubble flow is an important flow pattern that exists in the working process of a centrifugal pump. When bubbles are evenly distributed in the impeller channel, the pump performance is less affected by them. As the inlet gas volume fraction (IGVF) increases, bubbles may accumulate in the impeller, and thus, affect the pump performance. When bubbles coalesce further, it may form an air mass around the impeller inlet. With the increased gas concentration, the pump is prone to surge, resulting in a sudden drop in head, which is also accompanied by strong vibration and noise. In serious cases, the impeller channel will be blocked, resulting in “gas lock” effect, which makes the pump idle. This not only greatly reduces its service life, but also affects the normal production. Therefore, it is significantly important to study the flow pattern in the pump under bubble inflow condition for the safe and stable operation of pump.
Murakami and Minemura classified four typical flow patterns in the pump impeller, i.e., the Bubble Flow (BF), the Agglomerated Bubble Flow (ABF), the Gas Pocket Flow (GPF) and the Segregated Flow (SF). For the pump operating under the bubbles inflow, the bubble behavior will affect the gas-liquid flow pattern in the pump, and thus, affect the pump performance. However, limited literature on the bubble behaviors in the pump are publicly available. Minemura and Murakami investigated the effect of the pressure gradient force, the drag force, the buoyancy force and the inertia force on bubbles motion in a centrifugal impeller. They reported that the governing factors for the bubble motion are the pressure gradient force and the drag force, and the effect of the inertia force increases as the diameter of bubble increases. Sterrett developed an analytical model for the motion of a single bubble through a pump impeller. The results showed that the Coriolis and buoyancy forces are important in describing the kinematics of gas phase.
The bubble motion is also influenced by the pump suction pressure. Barrios and Prado measured the bubble size inside the impeller channel of an Electric Submersible Pump (ESP) by using a high-speed instrumentation. They found that the majority of the bubbles inside the pump are not spherical. With the increase of the IGVF, the tendency of bubble coalescence as well as the bubble size increases. Similar results on the variation of bubble size were also reported by Cubas et al. Finally, the stagnant bubble at the impeller inlet causes the pump surging. Shao et al observed that for the BF and ABF patterns, the bubbles in the impeller rotate with the impeller, and the bubbles in the volute move along the volute channel. Once the IGVF reaches a critical value, some bubbles will flow back to the impeller near the volute tongue. When the flow pattern transits from the GPF pattern to the SF pattern, some bubbles in the discharge pipe return to the impeller near the volute tongue. When the height of the gas in the inlet pipe reaches the critical value, sometimes the bubble will flow into the volute, sometimes it will flow back to the impeller at a small velocity.
Stel et al experimentally and numerically investigated the bubbles motion in a centrifugal pump impeller. The effects of the bubble diameter and the liquid flow rate on the bubble trajectories were evaluated. The results indicated that the bubble movement is hindered by the bubble diameter and impeller rotational speed but facilitated by the liquid flow rate increasing. They also demonstrated that the behavior of the bubble inside the impeller is mostly dominated by a balance between the pressure gradient and the drag forces. This balance determines whether the bubble leaves or stays in the impeller. The bubble behaviors and their effects of the gas-liquid distribution in the impeller are still need further exploration.
2. Experimental setup and method
2.1. Experimental setup
Fig. 1 shows the gas-liquid two-phase flow loop employed in this study. The tap water and compressed air were used as the fluids. The experiment method and the parameters of the measurement devices employed in the present experiment are available in Ref. . The measurement locations of the inlet pressure (Pin) and the pressure increment (ΔP) of the pump was shown in Fig. 1.
2.2. Centrifugal pump
The primary parameters of the centrifugal pump are shown in Table 1. As shown in Fig. 2, the pump inlet and the motor were designed in the same side to record the gas-liquid distributions in the whole impeller. The water flows into the pump through an annular inlet. The flow in the pump can be filmed from the hub of the impeller. The inlet part of the pump, the impeller and volute were made of polymethyl methacrylate (PMMA) (Fig. 3). The volute was designed with rectangular external shape and circular internal section to reduce the reflection and refraction of the light during the experiment, and thus, improving the shooting quality of high-speed photography. The pictures of the impeller and part of the volute are displayed in Fig. 3.
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