Centrifugal pump design and analysis

Aim : Centrifugal pump design and analysis using solidworks. Obtain the relationship between Pressure ratio and mass flow rate and explain the results.

Introduction :

CFD :

Computational fluid dynamics is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows.

Centrifugal pump :

A centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers. Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips. The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet. The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge.

Basic theory on the simulation :

How does a centrifugal pump work?

The impeller is the key component of a centrifugal pump. It consists of a series of curved vanes. These are normally sandwiched between two discs (an enclosed impeller). For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred (Figure 1).

Fluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes. The impeller, on the opposite side to the eye, is connected through a drive shaft to a motor and rotated at high speed (typically 500-5000rpm). The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing.

There are two basic designs of pump casing: volute and diffuser. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure.

In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet. This design causes the fluid pressure to increase towards the outlet (Figure 2).

The same basic principle applies to diffuser designs. In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller (Figure 3). Diffuser designs can be tailored for specific applications and can therefore be more efficient. Volute cases are better suited to applications involving entrained solids or high viscosity fluids when it is advantageous to avoid the added constrictions of diffuser vanes. The asymmetry of the volute design can result in greater wear on the impeller and drive shaft.

What are the main features of a centrifugal pump?

There are two main families of pumps: centrifugal and positive displacement pumps. In comparison to the latter, centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids, down to 0.1 cP. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, there are a number of applications for which positive displacement pumps are preferred.

What are the main applications for centrifugal pumps?

Centrifugal pumps are commonly used for pumping water, solvents, organics, oils, acids, bases and any ‘thin’ liquids in both industrial, agricultural and domestic applications. In fact, there is a design of centrifugal pump suitable for virtually any application involving low viscosity fluids.

Type of centrifugal pump	Application	Features
Canned motor pump	Hydrocarbons, chemicals where any leakage is not permitted	Sealless; impeller directly attached to the motor rotor; wetted parts contained in can
Magnetic drive pump		Sealless; impeller driven by close coupled magnets
Chopper/grinder pump	Waste water in industrial, chemical and food processing/ sewage	Impeller fitted with grinding teeth to chop solids
Circulator pump	Heating, ventilation and air conditioning	Inline compact design
Multistage pump	High pressure applications	Multiple impellers for increased discharge pressures
Cryogenic pump	Liquid natural gas, coolants	Special construction materials to tolerate low temperatures
Trash pump	Draining mines, pits, construction sites	Designed to pump water containing solid debris
Slurry pump	Mining, mineral processing, industrial slurries	Designed to handle and withstand highly abrasive slurries

Summary

A centrifugal pump operates through the transfer of rotational energy from one or more driven rotors, called impellers. The action of the impeller increases the fluid’s velocity and pressure and directs it towards the pump outlet. With its simple design, the centrifugal pump is well understood and easy to operate and maintain.

Centrifugal pump designs offer simple and low cost solutions to most low pressure, high capacity pumping applications involving low viscosity fluids such as water, solvents, chemicals and light oils. Typical applications involve water supply and circulation, irrigation, and the transfer of chemicals in petrochemical plants. Positive displacement pumps are preferred for applications involving highly viscous fluids such as thick oils and slurries, especially at high pressures, for complex feeds such as emulsions, foodstuffs or biological fluids, and when accurate dosing is required.

Entire case setup :

Create a 3D Model in Solidworks.

Go to flow simulation and create lid at inlet and outlet.

Select the Wizard and choose Rotation and Fluid as water.

The Computational Domain is created.

Modify the Components in the Components Control. Uncheck the rotation zone.

The Rotating Region is selected and the speed is set to 1000 rpm.

The Inlet Boundary Condition is set as Environmental Pressure .

The Outlet Boundary Condition is the Exit Velocity of 10 m/s.

The Surface Goal is set in the Inlet and the Outlet faces.

Then the Mesh is created for the model and the Automatic mesh level is set into 3 and Advanced Chanel refinement is selected.

Run the Simulation and the cut plots are created for the Pressure and Velocity and get the values for the goals.

The above explanation is for the Baseline simulation.

Then After, Run A parametric Study as Below:

Results :

Design Study 1 :

Velocity cut plot :

Velocity Flow Trajectory:

Pressure cut plot :

Pressure Flow Trajectory:

Design Study 2 :

Velocity cut plot :

Velocity Flow Trajectory:

Pressure cut plot :

Pressure Flow Trajectory:

Design Study 3 :

Velocity cut plot :

Velocity Flow Trajectory:

Pressure cut plot :

Pressure Flow Trajectory:

Design Study 4 :

Velocity cut plot :

Velocity Flow Trajectory:

Pressure cut plot :

Pressure Flow Trajectory:

Design Study 5 :

Velocity cut plot :

Velocity Flow Trajectory:

Pressure cut plot :

Pressure Flow Trajectory:

Observations:

Pressure ratio = Outlet total pressure/ Inlet total pressure

Velocity (m/s)	Mass flow rate (kg/s)	Pressure ratio
10	-0.494185098	9.9919
15	-0.74152159	10.0405
20	-0.972229268	9.7827
25	-1.237114123	10.4834
30	-1.482209913	8.9239

Pressure ratio vs. Mass flow rate plot:

Conclusion :

When the velocity increases the mass flow rate increases.

Negative mass flow rate indicates the flow is aginst the casing. (No importance for the negative sign it is only indicates flow is going out from the model).

Due to Model Geometry are not so accurate the Desired Graph and results are not archieved . Otherwise , The pressure ratio is high for low velocity and pressure ratio starts to decrease when the velocity increases.

Due to Model Geometry are not so accurate the Desired Graph and results are not archieved . Otherwise , The Pressure ratio Decreases with increase in the mass flow rate.

References : https://www.michael-smith-engineers.co.uk/resources/useful-info/centrifugal-pumps