Optimal design of inlet device of multistage centrifugal pump

In the article the authors present a new type of inlet device for multistage centrifugal pumps. An overview of the existing types of inlet devices is given, their advantages and disadvantages are observed. The form of the proposed type of inlet device is described, which makes it possible to combine good cavitational properties of the pump with low losses in the inlet device. The results of hydrodynamic modeling of the proposed type of inlet device and its comparison with the existing ones are given.


Introduction
An important factor within the process of creation of modern multistage pumps is the designing of inlet chambers. The geometry and nature of fluid flow in these chambers directly affect pump characteristics such as NPSHR [1][2][3], the efficiency rate, the steepness of the pressure characteristics in the area of low flow rates [4][5][6][7][8][9]. Moreover, steeply decreasing characteristic is especially important for the pumps used in thermal power engineering and nuclear power engineering [10]. Now the following types of inlet devices are used for these pumps: -annular chambers; -annular chambers with a separating edge; -semi-spiral chambers; -channel inlets (deflectors) with annular chamber. All these devices have their advantages and disadvantages. Thus, the pumps with an annular chamber have a higher efficiency rate due to its relatively small losses in the inlet devices. But when operating in the low feed rate area, the pump head decreases due to the reverse currents in the inlet devices, which leads to the characteristic's falling-off.
The integration of a separating edge of sufficient size to the inlet device allows to minimize this phenomenon, but also reduces the efficiency rate of the pump, as well as its cavitation properties [11][12][13].
The appliance of semi-spiral inlet devices, in turn, allows to improve the cavitation property of the pump, which is especially important for the multistage pumps designed for large feeds, but at the same time reduces the pump head (due to appearance of the moment of speed at the inlet to the first stage) and its efficiency rate. Channel inlet devices are used, on the contrary, for the pumps with low feed rates, but the losses in them are large compared to the other types of inlet devices. Figure 1 shows the lateral semi-spiral and annular inlet devices. The purpose of this study was to create such a geometry of the inlet device order to ensure the smallest losses at the inlet to the first pump stage.

Methods
To determine the losses in the flow part of the inlet device, the 3D models of the original intake system and the optimized one were built. For th carried out [14]. As the initial model an annular inlet of centrifugal pump type CNS 300 taken.
Considered inlet systems are shown in figure 2. The distinguishing feature of the suggested inlet device lies in the fact that in addition to the reduced volume massive streamlined separator with a small curvature is used instead. It is assumed that this solution will reduce the volume of stagnant zones and impr The purpose of this study was to create such a geometry of the inlet device of a multistage pump in order to ensure the smallest losses at the inlet to the first pump stage.
To determine the losses in the flow part of the inlet device, the 3D models of the original intake system and the optimized one were built. For these models, hydrodynamic modeling of fluid flow was carried out [14]. As the initial model an annular inlet of centrifugal pump type CNS 300 Considered inlet systems are shown in figure 2. The distinguishing feature of the suggested inlet vice lies in the fact that in addition to the reduced volume, it does not have thin massive streamlined separator with a small curvature is used instead. It is assumed that this solution will reduce the volume of stagnant zones and improve the shaft streamlining as a whole.
3D models of the original inlet system and the optimized one. To determine the losses in the flow part of the inlet device, the 3D models of the original intake ese models, hydrodynamic modeling of fluid flow was carried out [14]. As the initial model an annular inlet of centrifugal pump type CNS 300-120 was Considered inlet systems are shown in figure 2. The distinguishing feature of the suggested inlet , it does not have thin-walled edges, and a massive streamlined separator with a small curvature is used instead. It is assumed that this solution ove the shaft streamlining as a whole.
3D models of the original inlet system and the optimized one.
The method of numerical simulation is based on solving discrete analogs of the basic = const) this is: where  j u -the average value of the fluid velocity in the projection on the j-th axis (j = 1, 2, 3).
The equation of conservation of momentum (Reynolds averaging): where U, P -the average velocity and pressure; The introduction of the Navier-Stokes equation, averaged according to Reynolds, makes the system of equations not closed, since the additional unknown Reynolds stresses appear. To solve this system, in this problem a semiempirical model of k-ω SST turbulence was used, which introduced the additional equations needed: the equations of transfer of the kinetic energy of turbulence and the relative dissipation rate of this energy: The flow part is divided into a set of finite cells, for each of which the discrete analogues of continuous equations are composed. The complex of all discrete analogs forms a closed system of algebraic equations.
As the boundary conditions, the velocity values at the inlet device and the pressure at the outlet from it were taken. The speed was set in such a way as to ensure the flow through the intake system equal to 300 m 3 /h.
The computational grid for each model consisted of about 200 thousand cells. Figure 3 shows the computational grids for the original model and the optimized one.

Results
Based on the above modeling methods, a comparative numerical study of the flow in the original and proposed inlet devices was performed. The comparison criteria were the magnitude of the losses at the pump inlet, the even distribution of the absolute velocity at the inlet to the pump impeller. Figure 4 shows the vector fields of the absolute velocity distribution in the meridional section of the original inlet device and the optimized one. Then, after processing the results of the numerical experiment, the hydraulic losses of both devices were determined. With fluid flowing through the original intake system, the losses, expressed in terms of the differential pressure, were about 5000 Pa. The optimized model showed a result of about 3000 Pa.
The coefficient of evenness of the velocity distribution (Coriolis coefficient) allows to quantify the real distribution of velocities over the flow cross section: , for the optimized one 1.1137   . Figure 5 shows the fields of the velocities at the cross-sections, where the Coriolis coefficient is determined.
From the data analysis it can be seen that the proposed configuration of the inlet system allows a 30-40% reduction in losses at the pump inlet compared to an annular chamber with a separating edge. And also it allows to reduce the Coriolis coefficient at the inlet to the first stage of sectional pump with 1.2814   до 1.1137   , which improves the conditions of flow impingement on the impeller blades and improves the cavitation property of the pump. 1. As the numerical study has shown, the proposed design of the inlet system can significantly improve the flow structure at the in rate; 2. Currently, work is underway to manufacture an experimental pump with the geometry of the inlet device given, for carrying out its comprehensive tests and determining the proposed design use.

Published under licence in Materials
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of th and the title of the work, journal citation and DOI. The distribution of the velocity field at the cross-section at the pump inlet.

References
As the numerical study has shown, the proposed design of the inlet system can significantly improve the flow structure at the inlet to the first stage of a multistage pump and increase its efficiency Currently, work is underway to manufacture an experimental pump with the geometry of the inlet device given, for carrying out its comprehensive tests and determining the As the numerical study has shown, the proposed design of the inlet system can significantly let to the first stage of a multistage pump and increase its efficiency Currently, work is underway to manufacture an experimental pump with the geometry of the inlet device given, for carrying out its comprehensive tests and determining the prospects for the