Centrifugal pump design and analysis
AIM
Design and analysis of a centrifugal pump.
THEORY
Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.[1] The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from which it exits.
Like most pumps, a centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid. A portion of the energy goes into kinetic energy of the fluid. Fluid enters axially through eye of the casing, is caught up in the impeller blades, and is whirled tangentially and radially outward until it leaves through all circumferential parts of the impeller into the diffuser part of the casing. The fluid gains both velocity and pressure while passing through the impeller. The doughnut-shaped diffuser, or scroll, section of the casing decelerates the flow and further increases the pressure.
Centrifugal pumps are unique because they can provide high or very high flowrates (much higher than most positive displacement pumps) and because their flowrate varies considerably with changes in the Total Dynamic Head (TDH) of the particular piping system. This allows the flowrate to be "throttled" considerably with a simple valve placed into the discharge piping, without causing excessive pressure buildup in the piping or requiring a pressure relief valve. Therefore, centrifugal pumps can cover a very wide range of liquid pumping applications.
OBJECTIVE
The objective of this project is to design a centrifugal pump in SolidWorks and then perform flow simulation and analysis. By varying the mass flow rate, the performance diagram is obtained. It is the plot between the pressure ratio and the mass flow rate.
Pressure ratio = outlet total pressure / inlet total pressure.
First, the impeller and the casing is designed using suitable dimensions. It is made sure that merged results is unchecked. The inlet is extruded for the fluid to be sucked.
The outlet to the flow is created using swept boss/base by extruding a cylinder along the specified line. Again, it is made sure that the merged results is unchecked.
Using direct editing feature, the outlet and the casing are combined.
Shell command is used to create the shell of thickness 1mm between the inlet and the outlet.
A rotating zone is designed around the impeller to allow SolidWorks to do the calculation. In order to so, a circle is created in the top plane such that it just encloses the impellers, without touching them and the volute casing. This circle is then extruded using the mid-plane option such that it does not come in contact with the casing. Again, it is made sure that the merge results check box is unchecked.
The lids are placed at the inlet and the outlet and the design is saved.
Flow simulation
A flow simulation is created from the wizard. Selecting the internal flow, the rotation checkbox is checked and local region(s) averaging is selected. For the fluid, water is selected and the setup for the simulation is completed without changing the preset initial conditions.
Right clicking on input, the rotating region is unselected from the component control option. A rotating region is inserted and this region is selected. An angular velocity of 1000 rad/s is provided.
For the boundary conditions, environmental pressure is provided to the inlet and outlet velocity is provided to the outlet.
Right clicking on global mesh, advanced channel refinement is selected to automatically create the mesh for smaller areas.
The surface goal plots are added for the outlet and average value of total pressure and mass flow rate is selected.
Once this is done, from the input data, calculation control options is selected and the iterations is set to 500 to get the results. Rest everything is unselected.
The simulation is run. The flow trajectory and cut plots are obtained.
Parametric study
A new parametric study is selected and outlet velocity is selected as the variable input. The velocity is varied from 10 m/s to 30 m/s and there are 5 scenarios.
For the output, the surface plot for total average pressure at the outlet is selected. The study is run.
The performance curve is generated from the obtained results and the results are interpreted.
RESULTS
Outlet velocity = 10 m/s
Velocity cut plot
Pressure flow trajectory
Velocity flow trajectory
Average value of total pressure at inlet and outlet and the pressure ratio
Mass flow rate for different outlet velocities
OBSERVATIONS AND CONCLUSION
From the data, we can see that the mass flow rate increases with increase in outlet velocity. It is because more fluid flow takes place for a given amount of time. The value of mass flow rate is negative because the flow is leaving the system.
The pressure ratio decreases with increase in the mass flow rate except at one point. It may be due to the use of inaccurate geometry and clearances.
REFERENCES
https://www.dultmeier.com/technical-library/how-does-a-centrifugal-pump-work.php