CFD for Centrifugal Pump

What is a Centrifugal Pump?

In this pump model, rotational kinetic energy is converted to hydrodynamic energy. Figure below shows a schematic cutaway of a centrifugal pump. This pump has 4 main parts: suction pipe, discharge pipe, impeller and casing. The flow enters the pump from the suction pipe (duct 1 in the figure) and the flow leaves the pump with a higher pressure from the discharge pipe (duct 2 in the figure). The rotating member of the pump, Impeller, increases fluid pressure. The flow from the suction pipe is transferred vertically to the center of the impeller and by going to the outer radii of the impeller, its velocity decreases and its pressure increases until it finally exits the Discharge Pipe. Usually, the impeller is driven by an electric motor. Centrifugal pumps are a type of turbomachine.

The advantages of this pump model include its simple design and operation, high efficiency for a wide range of flow rates and pressures, and the need for relatively low maintenance. This pump model is used in water and sewage systems, HVAC systems, oil and petrochemical industries, and food industries

Cutaway schematic of a typical centrifugal pump. Adopted from “Fluid Mechanics” by Frank M. White

 

Applications of CFD in Centrifugal Pumps

There are various analytical and experimental methods to study pumps. In this method, the domain of fluid movement is discretized and for each cell, discretized unit, the equations governing the fluid flow are solved by the computer. Each method has its strengths and weaknesses. Analytical methods are simple and fast, but they explain the details of the pump’s performance with little accuracy, and many idealizations and assumptions are used in their calculations, which cause errors in the results. Experimental tests give accurate results but are expensive and it is very difficult to record the results inside the pump in this way.

The shape of the impeller vanes can be different depending on the pump application and the fluid for which it is designed.

 

Another method is CFD simulation. This method requires more knowledge than analytical methods. There is also a need to validate CFD simulation results with experimental results. The strength of this method is the detailed description of pump performance at every point inside the pump. In the following, we will review the CFD analysis of centrifugal pump.

• Pump Design and Optimization: Fluid velocity and pressure are well calculated by CFD simulation at any point of the pump domain. In this way, it is determined which parts of the pump are not working properly. Is the impeller suitable? Is the length of the impeller suitable? Is the speed of the impeller suitable for producing the desired pressure? Does part of the fluid pressure inside the pump decrease for various reasons? Such questions are answered by CFD simulation.
• Cavitation Assessment: If the fluid pressure in some points of the pump is less than the fluid vapor pressure, then the fluid vaporizes and bubbles are formed in the flow. When these bubbles collide with the impeller, they scratch its surface and damage its blades. This phenomenon can be seen in experimental tests from the damaged shape of the impeller, but the origin and point of bubble formation can be found by CFD simulation.
• Thermal Analysis: Heat is generated inside the pump for various reasons, and it is very important to transfer this heat to the environment. Investigating and finding heat generation points and finding the way of heat transfer is possible with CFD simulations.

The parts inside the red rectangles are the vanes that are used to increase the heat transfer between the pump body and the ambient air.

 

Centrifugal Pump CFD Simulations by ANSYS Fluent

 ANSYS Fluent is the right software for CFD centrifugal pump simulations. We will examine some of the features and capabilities of this software which is suitable for simulation in this field.

• Turbulent flows: Inside centrifugal pumps, there is usually a turbulent flow, and it is very important to accurately simulate this flow regime. ANSYS Fluent offers wide possibilities in this field, including all kinds of turbulence simulation models and the possibility of setting their details. In some cases, eddies may form in the fluid flow that disrupt the fluid flow, so that the new flow does not enter and exit the vortex, and the vortex occupies a part of the domain inside the pump, which makes the passage of the fluid flow smaller. This problem can be studied with the help of CFD simulations of turbulent flow with ANSYS Fluent.
• Fluid-Structure Interaction: In centrifugal pumps, sometimes it is necessary to check and analyze the stress. The load that enters the impeller shaft is asymmetric, which causes its deflection to change. Also, in various cases, there may be vibrations in the pump that cause unpleasant noise and even break the pump bases. Such issues can be simulated in ANSYS. With ANSYS Fluent, the fluid flow is simulated and the pressure distribution at different points of the pump is obtained, and with this data, the stress analysis in the pump structure is performed with the help of ANSYS Mechanical. See “What is Fluid-Structure Interaction?” for more information.
• Multiphase flows: In many applications of centrifugal pumps, the flow is multiphase, such as sewage flow. Flow contents may settle in the pump and block the passage. The design of the pump must be in a special way for each multiphase flow to create the appropriate pressure in the fluid. Simulation of multiphase flow with ANSYS Fluent is possible with various methods and features. See “Using ANSYS Fluent for Multiphase Flow Simulations: Techniques and Tips” for more information.

Click to access the Multiphas CFD

 

Calculation of Pump Performance Curves by ANSYS Fluent

Pump Performance Curves will be given to you at the time of purchase for each pump. As in the figure below, in this curve it is shown that the pump with the size of the impeller you want produces what head at each flow rate. Head is the unit of pressure based on the weight of the fluid column. In addition, the required power of the motor pump and its efficiency are drawn on such shapes. You may also have net positive-suction head (NPSH) as shown in the figure below. This parameter represents the minimum fluid pressure before entering the pump to prevent fluid vaporization and cavitation.

Measured-performance curves for a model of a centrifugal water pump with a basic casing with three impeller sizes. Adopted from “Fluid Mechanics” by Frank M. White

The following equation is used to calculate the head produced by the pump in ANSYS Fluent:

   

ΔP is the pressure difference before and after the pump [Pa], ρ is the fluid density [kg/m³], g is the acceleration due to gravity [m/s²], and H represents the pump head [m]. This equation is usually defined in the form of an expression in Fluent. In the simulation, H is recorded in each iteration until its value converges.

Centrifugal blood pump CFD simulation. This project is just one of the simulations of centrifugal pumps by ANSYS Fluent, carried out by CFDLAND experts.

 

Conclusion

One of the methods of designing, studying and optimizing centrifugal pumps is CFD simulation. In this article, we have stated several advantages and strengths of CFD simulations in the field of centrifugal pumps and we have seen why ANSYS Fluent is the right software for these simulations.

CFD simulation of centrifugal pumps is a specialized work and requires a lot of knowledge and experience. CFDLAND specialists are experts in this field and have done many projects, some of which you can see in turbomachinery cfd SHOP. Ease your work and order CFD analysis and simulation projects of centrifugal pumps with ANSYS Fluent to CFDLAND with confidence in our abilities.