Centrifugal pump design and analysis
AIM: To do flow simulation for the offset centrifugal pump at different boundary conditions.
Objective:
To create a centrifugal pump with assuming appropriate dimensions.
To calculate the pressure and mass flow rate at the outlet at different outlet velocities.
To do parametric study for the centrifugal pump.
To obtain a relationship between the pressure ratio and the mass flow rate.
Introduction:
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.
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.
Applications:
Common uses include water pumping, sewage water pumping, agriculture based pumping works, petroleum pumping and petrochemical pumping.
Centrifugal pumps are often chosen for their high flow rate capabilities, abrasive solution compatibility, mixing potential, as well as their relatively simple engineering.
A centrifugal fan is commonly used to implement a vacuum cleaner. The reverse function of the centrifugal pump is a water turbine converting potential energy of water pressure into mechanical rotational energy.
Modelling:
To create a model of the centrifugal pump, first of all we have to start with modelling a impeller.
To create impeller go to the top plane, start sketch and draw two concentric circles of appropriate diameters.
Then, select the three point arc, create an arc in between the circles, as we are not doing a real case study so we will not consider the inlet and outlet angles.
Similarly, then we would create an another arc at small offset distance, but without using offset tool.
Now, as we create two arc joining the circle, then we would trim the outer circle by using the power trim tool.
Then we would select the circular pattern and enter the number of the vanes as we want.
Then, after that we want that all the pattern vanes and the inner circle to be a single sketch therefore, we will trim of the inner circle that creates a boundary to the vanes.
Then we will extrude the impeller as per our required dimension.
Here, one thing we should have to note that while extruding the impeller we must use extrude via mid plane as it will be easier for us to create an extrusion of the further sketches like pump casing.
Now, as the impeller is created successfully, now we would draw a pump casing.
To create an pump casing we use the same part file.
We start a new sketch on the top plane and create a circle of diameter larger than the impeller outlet diameter. Note that the diameter for the cassing should be large enough.
Then we will extrude the circle again using mid plane option.Note that the extrusion for the casing should be more from the impeller's extrusion so that there would be enough spacing in between them.
Now, as the extrusion is completed we will create a inlet and outlet pipe on the casing.
To create an inlet pipe, we would create an circle on the casing surface plane, then create a circle on the surface of appropriate diameter and then extrude it upto a certain point.
Afterwards we would create a outlet pipe on the casing, to create an outlet pipe we would start sketch on the top plane. then create a centre-line tangent to the casing of an appropriate length.
Then create a line offset to the line of the same length, the offset distance taken is equal to the radius of the outlet pipe. Note here the line is not a centreline.
Now, we would create an refrence plane at the end of this line. The refrence plane can be made by selecting refrence geometry option from the features menu.
Now, we would create a circle at the centre of the new plane upto the end-point of the centreline.
Now, we will use the loft command to extrude the circle.
Here, point to be noted that while extruding the casing, pipes we should unselect the merge results option as they are separated parts.
Now, we will use the combine command and combine the casing, inlet and outlet pipe.
Now, we would create a shell in the casing to create passage for fluid to flow inside the pump and get compressed and get outs from the pump.
We would appropriate the dimensions for the shell.
As, the pump modelling is completed we would check that the pump impeller is not get merged into the casing so that we use the section command to check the cross section of the model.
Here as you can see that, I have done some changes in the centrifugal pump, I am trying to create an offset between the pump impeller and the casing.
So to do that I select the pump casing and select edit sketch, and provide some appropriate vertical and horizontal distances from the centre of the plane.
Actually to create an offset centrifugal pump two involutes circle are created and the impeller and the casing draw on the profile accordingly with appropriate dimensions.
Then exit sketch the model automatically creates an offset in between the impeller and the casing.
The Model finally gets compelted and it is given below:
Simulation:
Now, to do flow simulation through the pump, we initally go the flow simulation add-ins and select flow simulation.And then we get the tools menu for the flow simulation.
Then we create the lids at the outlet and the inlet pipe sections to create a water-tight model.
Now, we run the wizard, name the project as per our wish, select the default SI unit system, then select the internal flow, and the rotation option for the rotation effect for the impeller and after that a new project will displays on the left side.
Now, here have provided the rotation option but we did not rotate the impeller, we create a rotation zone around the impelller and rotate that zone only.
To create a rotation zone, we would again to the sketch option, and create a circle of appropriate diameter on the top plane that would cover the oulet radius of the impeller and then extrude it via mid-plane option so that it would cover the entire impeller in it.
Again before clicking tick mark to extrusion, we have to de-select the merge option.
This will create a same effect as of the impeller but we have to do some simple steps, that we have to right click on Input data and on the component control we have to de-select the extruded sketch of the zone.
Now, we will provide some boundary conditions and the rotation inputs and some goals.
We would provide a environmental pressure at the inlet and the outlet velocity at outlet.
Also, we select the goals for the total averge pressure at the outlet.
We would select the rotation zone and provide an angular velocity of 2000 rad/s.
And then we would run the simulation.
This is our baseline simulation done at outlet velocity equals to 10 m/s.
After the simulation gets over, we would load the results.
In the results section, we would select the velocity and the pressure plots in cut plots section.
The Velocity plot obtain from the baseline simulation is shown below:
Similarly, the Pressure plot obtain from the baseline simulation is shown below:
Also, we would run the flow trajectory for the baseline simulation so that we can view that how the fluid actually would flow in the pump.
Parametric Study:
Now, as the Baseline simulation is done, we would now create a new parametric study for the centrifugal pump by doing so we can change anyone of the parameters and then we would get the different results accordingly and compare them at the same time.
Here, I am changing the outlet velocity provided at the outlet.
Select the output parameters such as total average pressure at outlet in goal section.
Also we select the velocity and the pressure cut plots in results section.
The Pressure plot for the parametric study is shown below:
Similarly, the Velocity plot for the parametric study is shown below:
Results:
We can easily evaluated the results for the Pressure at outlet and the mass flow rate through the pump while looking at the goal summary:
The Pressure Ratio vs Mass flow rate is plotted below:
As you can see that, in the goal summary the pressure ratio does not have general trend for the parametric study and similarly the mass flow rate does not have a genearal trend.
So, we can't say much about the relationship between the mass flow rate and the pressure ratio but calculating more parametric study can help us to reach us at a conclusion but my computer is not able to do much calculation continuously.
Now, looking at the flow trajectory of the baseline simulation, there is an dark blue section at the middle portion and looking towards the colour, it changes from inside to outside till the casing.
This shows the variation in the pressure through the pump.
The dark blue colour represents the suction of the fluid.
The Red colour and the orange shade represents the compressed fluid flowing out of the outlet pipe.
The Pressure values at outlet is already been evaluated above in the goal summary.
The Flow Simulation for the Centrifugal Pump is given below:
The Flow Simulation for the Centrifugal Pump is given below: