Centrifugal pump design and analysis

AIM:

To create a centrifugal pump geometry to perform the flow simulation using Solidworks and obtaining the analysis results to view the relationship between the mass flow rate and pressure ratio.

THEORY:

1.1 Centrifugal Pump-

Centrifugal pumps are used to induce How or raise the pressure of a liquid. Its working is simple. At the heart of the system lies impeller. It has a series of curved vanes fitted inside the shroud plates. The impeller is always immersed in the water. When the impeller is made to rotate, it makes the fluid surrounding it also rotates. This imparts a centrifugal force to the water particles, and water moves radially out. In Fig this process is illustrated. 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 freshwater stream into the system again, and this process continues.

From foregoing discussions, it is clear that the negative pressure at the eye of the impeller helps to maintain the flow in the system. If no water is present initially, the negative pressure developed by the rotating air, at the eye will be negligibly small to suck a fresh stream of water. As a result, the impeller will rotate without sucking and discharging any water content. So the pump should be initially filled with water before starting it. This process is known as priming. The impeller is fitted inside a casing. As a result, the water moves out will be collected inside it, and will move in the same direction of rotation of the impeller, to the discharge nozzle.

What are the limitations of a centrifugal pump?

• The efficient operation of a centrifugal pump relies on the constant, high-speed rotation of its impeller. With high viscosity feeds, centrifugal pumps become increasingly inefficient: there is greater resistance and higher pressure is needed to maintain a specific flow rate. In general, centrifugal pumps are therefore suited to low pressure, high capacity, pumping applications of liquids with viscosities between 0.1 and 200 cP.
• Slurries such as mud, or high viscosity oils can cause excessive wear and overheat leading to damage and premature failures. Positive displacement pumps often operate at considerably lower speeds and are less prone to these problems.
• Any pumped medium that is sensitive to shearing (the separation of emulsions, slurries, or biological liquids) can also be damaged by the high speed of a centrifugal pump’s impeller. In such cases, the lower speed of a positive displacement pump is preferred.
• A further limitation is that, unlike a positive displacement pump, a centrifugal pump cannot provide suction when dry: it must initially be primed with the pumped fluid. Centrifugal pumps are therefore not suited to any application where the supply is intermittent.
• the pump produces a variable flow; a positive displacement pump is insensitive to change pressures and will provide a constant output. So, in applications where accurate dosing is required, a positive displacement pump is 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. There is a design of a centrifugal pump suitable for virtually any application involving low viscosity fluids.

Mass Flow Rate-

The mass flow rate of a system is a measure of the mass of fluid passing a point in the (m) system per unit time. The present study aims to analyze the fluid flow in a flow bench and perform the grid dependency and valve lift test and observe the results.

Pressure Ratio-

The pressure ratio is the ratio between the outlet pressure and the inlet pressure.

Pressure ratio = Outlet total pressure/ Inlet total pressure

PROCEDURE:

3D MODEL:

1. To create the centrifugal pump geometry, starting the sketch from the top plane in Solidworks.
2. Two circles that are concentric with the origin are sketched with diameter 0.04m and 0.08m respectively.
3. Constructing two arcs between the two circles for the vanes of the impeller that are concentric.
4. Trimming the extra outer circle and circular patterning the vanes as 5 concerning the origin. Now extruding the sketch from midplane to get the profile.
5. From the top plane, drawing another circle from its origin and extruding from the midplane to get the geometry. Now from the top plane, a new circle is sketched and extruded.
6. With the centreline, a tangent over the circle from the top plane and a line is drawn parallel and equal to the centreline inside the circle at a distance of 0.005m. A reference plane is drawn on the endpoint of the line drawn and a circle is drawn on that plane and sweeper into the circle. From the top face of the profile, the circle is drawn and extruded for the inlet cavity.
7. Using the direct editing tab, combining the profile into a single geometry.
8. Using the shell, the thickness is provided to the geometry.
9. Now from the top plane, the circle is drawn outside the edges of vanes end extruded in the midplane such that it does not interfere with vanes and other parts.

BASELINE SIMULATION:

• Units: SI units
• Simulation Type: Internal Flow
• Fluid: Water
• Environment Pressure = 101325 pa
• Outlet Velocity = 10 m/s

1. For the watertight body, the lid is created on both the circular profile of the centrifugal pump. In the wizard menu, the above parameter is provided as inputs for the baseline simulation for the centrifugal pump.
2. Rotating Region = Outside the vanes (Boss Extrude 4) with 1000 rad/s
3. In the component control, boss extrude is excluded from selection, and the number of iteration for the solver is set as 1000 in the calculation control option.
4. Insert Surface Goals of Avg Total Pressure = Outlet Profile (LID2) and a mass ow rate is provided for both the lids.
5. Global Mesh is refined at level 3 and advanced channel refinement. Then the simulation is performed and the cut plots are created for the Pressure and Velocity and get the values for the goals.
6. After the completion of the baseline simulation, the parametric study is calculated.

PARAMETRIC STUDY:

1. To perform the parametric study on the compressor pump, the followings are provided:
2. Input parameters = Velocity normal to face (outlet velocity)
3. The input values are ranged from 10m/s to 30m/s with number of values 6 (i.e.) 10m/s, 14m/s, 18m/s, 22m/s, 26m/s & 30m/s.
4. Output Parameters = All goals and all results are inserted.
5. Running the simulation, we get the following results.

Case 1.

Outlet velocity 10 m/s

Velocity plot –

Pressure plot –

Flow trajectory –

Case 2 –

Outlet velocity 15 m/s

Velocity plot-

Pressure plot –

Flow trajectory –

Case 3.

Outlet velocity 20 m/s

Velocity plot –

Pressure plot –

Flow trajectory –

Case 4.

Outlet velocity 25 m/s

Velocity plot-

Pressure plot –

Flow trajectory –

Results

BASELINE SIMULATION:

Goal Name	Unit	Value	Averaged Value	Minimum Value	Maximum Value	Use In Convergence	Delta
SG Av Total Pressure Outlet	[Pa]	2937645	3002310	2816078	3151638.32	Yes	187959.7
SG Av Total Pressure Inlet	[Pa]	298255	311344.7	298255	324434.93	Yes	23872.64
SG Mass Flow Rate Outlet	[kg/s]	-0.4892	-0.4892	-0.4892	-0.4892	Yes	1.79E-05
SG Mass Flow Rate Inlet	[kg/s]	-4.1887	-4.4052	-4.6564	-4.1648	Yes	0.3607

PARAMETRIC STUDY: Table of various design points with various goals-
Goal	Design Point 1	Design Point 2	Design Point 3	Design Point 4	Design Point 5	Design Point 6
Velocity normal to face (Outlet Velocity 1) [m/s]	10	14	18	22	26	30
Environment pressure (Environment Pressure 1) [Pa]	101325	101325	101325	101325	101325	101325
SG Av Total Pressure Outlet [Pa]	2032861	5982077	1321818	1027223	1086635.19	995890.55
SG Av Total Pressure Inlet [Pa]	194570.5	665362.6	115928.8	114364.1	110036.04	125608.32
SG Mass Flow Rate Outlet [kg/s]	-0.4936	-0.692	-0.8904	-1.0888	-1.2867	-1.4803
SG Mass Flow Rate Inlet [kg/s]	-2.3353	-8.902	-0.3852	-0.2469	-0.0429	-0.3111
Pressure Ratio	10.44794	8.9907	11.40198	8.98204	9.875266	7.92855

CONCLUSION:

1. Thus the Pressure ratio vs Mass Wow rate was obtained by sketching and performing the flow simulation for the centrifugal pump. The following are observed during the analysis of the centrifugal pump:
2. From the plots we can see that initially, the velocity is less so, the mass Wow rate is less when the velocity increases the mass Low rate increases.
3. We also can see that the pressure ratio is high for low velocity and pressure ratio starts to decrease when the velocity increases.
4. Inlet pressure remains the same for all the cases but the outlet pressure varies. When we compare mass flow rate with the pressure ratio we can see from the graph that it is inversely proportion, that is we can see that with the increase in mass Row rate the curve first rises, and then it falls continuously. We conclude that as an increase in mass Row rate, the pressure ratio value decreases up to certain limits after that pressure will become constant.
5. Here the variation is not linear as because the geometry of the centrifugal pump is not sketched with the standard centrifugal pump i.e. with dimensions and values. If the standard values are used, the resulted plot will be linear and accurate i.e. the pressure ratio will decrease in an increase in velocity at the outlet. The negative sign indicates the Mow of the fluid at the outlet port.