Centrifugal Pump Design Optimization with CFD
The Engineering Problem
Centrifugal pumps are mainly used for pumping water in industrial and residential properties. These machines move liquids with the help of kinetic energy that is stored in the motor. Although, their main function is to move water and cause the liquid to flow, centrifugal pumps are also used in sewage, food processing plants, water treatment plants, manufacturing plants, chemical and petroleum industries and have become extremely popular down the years. Centrifugal pumps find their applications in pumping all types of low viscosity fluids at moderate pressure. Then can also easily handle liquids having high proportions of suspended solids present in them.
The industrial pump is the slave to the pumping system. The system governs the pump. The system is composed of the suction and discharge vessels plus all pipes, elbows, valves, filters, fittings, and instrumentation. The pump reacts when the system changes. If the pump is forced to do what it cannot do, then it fails frequently and prematurely. We call it mysterious pump failure, reactive maintenance, or unscheduled downtime. So, how do you know what your pump can do within the system? The answer is simple, but not always realistic. The pump curve should be available and understood by everyone involved with the pump, although it rarely is.
This turbo-machine essentially consists of an impeller rotating in a casing called volute. Fluid enters the eye of the impeller (the center of the impellers) and exits though the space between the impeller blades to the space between the impeller and casing walls. The velocity of fluid elements is in both tangential and radial directions, as the impeller rotates. The velocity as well as the pressure, both increase, as the fluid flows through the impeller. Since the rotational mechanical energy is transferred to the fluid, at the discharge side of the impeller, both the pressure and kinetic energy of the water will rise. At the suction side, water is getting displaced, so a negative pressure will be induced at the eye. Such a low pressure helps to suck fresh water stream into the system again, and this process continues.
Impeller is the most vital part of a centrifugal pump. Successful impellers have been developed with many years of analysis and developmental work. The vanes (blades) of the impeller are backward curved. Backward curved vanes have the blade angle less than 90 degree. Backward curved vanes are the most preferred vane type in the industry due to its self stabilizing power consumption characteristics. This means with increase in flow rate power consumption of the pump stabilizes after a limit.
CFD for design optimization study
The complexity of the flow in a turbomachine is primarily due to the 3-D developed structures involving turbulence, secondary flows, unsteadiness etc. Initially, the design of a centrifugal pump was based mainly on empirical correlation, combination of model testing and engineering experience. Nowadays, the design demands a detailed understanding of the internal flow during design and off-design operating conditions. CFD has successfully contributed to the prediction of the flow through pumps and the enhancement of their design.
CFD simulation makes it possible to visualize the flow conditions inside a centrifugal pump, and provides valuable information about the centrifugal pumps hydraulic design. Simulation results are used to calculate and predict the performance of a centrifugal pump to replace the experiments in the process of pump design. A great deal of labour and facility is saved, as well as it helps in shortening the design cycle. Therefore, great improvements on centrifugal pump design can be achieved by CFD analysis of inner flow inside a centrifugal pump and following application of its results in pump design processes.
This project simulates a typical centrifugal water pump using the **Steady State, Multi-Reference-Frame(MRF) method and K-Omega SST** turbulence model using SimScale. The pressure-velocity coupling is performed through the SIMPLE algorithm. The MRF zone is assigned a rotational velocity of 157.08 rad/s (1500 rpm). The project is concerned with the **influence of the outlet blade angle (β2)** and the **number of blades** in the performance of a centrifugal pump. The design and off-design performance characteristic curves, the local and global flow variables are numerically predicted using SimScale for impellers with 3 different **outlet blade angle i.e. 13, 23 and 33 degrees** and 3 different **number of blades i.e. 6, 8 and 10.**
The Centrifugal Pump model considered has inlet and outlet diameters of 150mm and 151.5mm respectively and it’s impeller diameter is 340mm. The domain is the geometry itself and was meshed using the ‘Snappy-Hex-Mesh’ on the SimScale platform. The resulting mesh consists of approximately 4.5 million cells and is shown in the figure below.
Effect of Outlet Blade Angle Variation (β2)
**β2 = 13, 23 and 33 Degrees**
**Flow Rate: 540 m^3/h**
**Number of Blades: 8**
The pressure contour results show that maximum pressure difference (208.4 KPa) between the pump inlet and outlet occurs in the pump with blade outlet angle of 33 degrees and least for 13 degrees (116.6 KPa). (Pump outlet set as Pressure Outlet with fixed value - 0 Gauge Pressure boundary condition)
Effect of Variation of Number of Blades
Number of Blades: 6, 8 and 10
β2 = 33 Degrees
Flow Rate: 540 m^3/h
From the pressure contour results it can be seen that maximum pressure difference (230.5 KPa) between the pump inlet and outlet occurs in the pump with 10 blades and least for 6 blades (161.04 KPa).
Performance Comparison between Pumps with 8 and 10 blades (β2 = 30 degrees)
Best Efficiency Point
Best Efficiency Point is the flow at which the pump operates at the highest or optimum efficiency for a given impeller diameter. BEP for this pump is obtained at flow rate of 432 cubic meters per hour. And, the maximum efficiencies obtained for pumps with 8 blades and 10 blades are 60.5% and 62.04% respectively.
Head at Best Efficiency Point
Since the BEP occurs at a flow rate of 432 units, that flow rate also intersects the pump curve at a point equal to head of 26.65 meters and 28.33 meters for pumps with 8 blades and 10 blades respectively.
Flow Rate vs. Pressure Difference Curve
Flow Rate vs. Torque Curve
Flow Rate vs. Head Curve
Velocity Vectors and Streamlines