1. Introduction
The blade optimization of a microbubble pump has been performed to enhance the pump performance in the present study. The microbubble pump can produce microbubbles of high precision without any additional mechanical device while the manufacturing cost is relatively high due to precision parts processing. Microbubble pumps have been applied in a variety of fields including water treatment, biogas refining, green algae processes, fisheries, and life health. Microbubbles pumps generate microscale gas bubbles due to the strong mixing with the gas and liquid induced by the rotation of the impeller inside the pump. The pump impeller is rotated by a motor and needs to produce a high pressure in order to generate microbubbles, thus requiring a lot of motor power. To reduce the need for such motor power, it is necessary to develop high-efficiency pump impeller. Conventional impeller blades of microbubble pumps have an open channel type impeller and require a more rigid blade shape due to the high pressure inside the impeller blades. Open channel type impellers, one of those being the regenerative pump impeller, are becoming more widely selected in the industrial field due to the relatively lower manufacturing and maintenance costs.
A variety of research on regenerative pumps has been carried out in order to understand the theoretical phenomena concerning pump performance by analytical, experimental and numerical methods [
1]. For example, Wilson et al. [
2] proposed a momentum exchange theory, which describes a spiral flow pattern in a regenerative pump. Senoo [
3] compared the turbulent shear force theory with the momentum exchange theory using mathematical models and showed that the two theories were interchangeable. Badami and Mura [
4] also proposed a theoretical model of a regenerative blower utilizing two components: a tangential component, which determines the effective flow rate, and a meridional component that determines the circulatory flow rate.
Parametric designs of regenerative pumps have been performed to enhance pump performance using design parameters obtained by experimental measurements and numerical simulations. Shimosaka and Yamazaki [
5] carried out experiments on a regenerative pump by introducing design parameters: the dimension of the flow channel, and the impeller clearance. They showed that allowable clearance is decided by the dimension of the flow channel and pump performance depends on a combination of blade length, thickness, and channel dimension. Kanase et al. [
6] performed an experimental and numerical analysis to identify the effects of the geometric parameters in the performance of a regenerative pump. They showed that higher pump efficiency can be obtained by the optimal selection of the number of blades and the chevron angle of those blades. Nejadrajabali et al. [
7] found that a regenerative pump with symmetric angles and forward-facing blades have better performance than radial blades under design flow conditions. Pei et al. [
8] analyzed the effects of the wrapping angles of a regenerative pump on the rise in pump pressure and efficiency. They showed that the size of the wrapping angle has a significant effect on the pump pressure rise while the pump efficiency increases slightly. Choi et al. [
9] performed an experimental study on the effect of the blade angle of a regenerative pump. Using a 10-blade configuration, with 7 inclined blade angles and 3 chevron angles, it was found that the radial blades and the chevron blades with an angle of approximately 30 degrees impart a comparatively high pump efficiency.
With the rapid development of computing processes over recent decades, the optimal design method using computational fluid dynamics has been introduced to improve the performance of regenerative blowers and pumps. The optimal design method coupled with computational fluid dynamics is promising and can effectively improve the performance of turbomachinery. Wang and Choi [
10] studied the effects of the impeller blade angles and the hub configurations on the efficiency of regenerative blowers by introducing the optimal design method. The results showed that the shape of a hub has a greater effect on the pump efficiency than the outflow angles of blades. Jang and Lee [
11] introduced an optimal design for a regenerative blower using the RSM (responses surface method) and numerical simulation. They selected two design variables, the extension angle and the number of impeller blades, to enhance pressure and showed that the extension angle is more effective to increase blower efficiency. Heo et al. [
12] performed the optimization of a regenerative blower using a hybrid MOEA (multi-objective evolutionary algorithm) and RBNN (radial basis neural network) surrogate modeling to improve the blower efficiency under design flow condition. They reported that the blower efficiency successively increases by reducing locally high-velocity regions generated inside the blades using three design variables: the blade height, the blade width and the angles between the inlet and outlet ports. Jeon et al. [
13] studied the performance of regenerative pumps to increase pump efficiency using the optimal design method and the RSM with two design variables, which were the impeller upper blade height, and the hub height. They showed that the hub height is more effective than the upper blade height to increase pump efficiency. As described above, all of the research on the regenerative blowers and the pump are conducted with the blades having a cantilever shape. Therefore, the microbubble pump which operates at high pressure by mixing the air and water is required to be more structurally robust and to have the optimum design with high efficiency.
In the present study, the optimization of the impeller blades used for a microbubble pump was performed to obtain a higher efficiency pump using numerical simulations. A three-dimensional numerical simulation is introduced to analyze the internal flow and to predict the performance of the pump. The SST turbulence model is employed to estimate the eddy viscosity. For the optimal design of the pump impeller blades, the response surface method is adopted with three design variables: the number of blades, the blade thickness and the radius of the hub rib. An experimental apparatus was designed and fabricated to verify the computational results. The pump efficiency is selected as an objective function in the present study. Comparisons of internal flow and the performance of the reference and optimal pumps are also investigated and discussed in detail.
2. Microbubble Pump with Novel ‘S-shape’ Impeller
A reference microbubble pump is a fluid machine that produces microbubbles by mixing air and water inside the pump, which has a regenerative impeller, as shown in
Figure 1. The pump has the capacity of 1.1 kW and consists of a regenerative impeller, two inlets, one each for water and air, a casing, a cover, and an outlet. The specifications of the pump under operating conditions are summarized in
Table 1.
The flow coefficient (
) and the pressure coefficient (
) of the pump are defined as
where
is the volumetric flow rate of water,
is the pressure rise,
is the impeller tip speed,
is the density of water, and
is the cross-sectional area at the outlet duct of the pump.
In regenerative pumps, such as the reference microbubble pump, Choi [
14] described the flow inside the pump by dividing it into two aspects based on the momentum exchange theory proposed by Wilson [
2]. First is through (or tangential) flow surrounded by the impeller, and the other is the recirculating flow formed inside an impeller, as shown in
Figure 2. The recirculating flow passes through the impeller and the channel in centrifugal and centripetal directions, respectively. It is noted that the recirculating flow including the axial, radial and tangential components and is responsible for energy transport, which causes violent mixing between the internal flow inside impeller and the channel flow. Considering the momentum exchange, the Euler head for the average streamline radius of the recirculating flow due to the rotational speed of the fluid and the average tangential velocity due to the rotation of the impeller are expressed as Equation (3).
where
is the gravitational acceleration,
is the Euler head,
U is the average rotational velocity, and
is the average tangential velocity of the recirculating flow due to the rotation of the impeller.
In a typical turbomachine, the hydrodynamic power transmitted by the rotating impeller to the working fluid is derived by calculating the product of the recirculating flow and the Euler head. The major difference between a regenerative machine and an axial or centrifugal one is that the hydraulic power of the centrifugal or axial machine is determined by the total flow rate while the regenerative type power is determined by the recirculating flow rate.
As shown in
Figure 2, the recirculating flow involves flow mixing inside the impeller for the open typed regenerative pump impeller, which causes internal and tangential pressure losses. A large pressure loss due to mixing on the interference plane between two recirculation flows between impeller blades is generated as well as the rotational flow on the channel side. To enhance pump efficiency, it is desirable to design the impeller having a separate internal space between the blades to increase the pressure head due to the large rotational momentum while reducing pressure losses generated between two circulating flows. In the present study, the authors proposed a novel ‘S-shape’ pump impeller having a separate space between the impellers to improve pump performance as shown in
Figure 3. Unlike conventional open type regenerative impellers, the new pump impellers are connected to each other on three sides, and thus can improve not only pump efficiency, but also the structural stability.