Centrifugal pump design and analysis

AIM: Analysis and simulation of the centrifugal pump and obtaining pressure ratio, 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.
Common uses include water, sewage, agriculture, petroleum, 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.

THEORY: 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.

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 of 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.

The same basic principle applies to diffuser designs.  In this case, the fluid pressure increases as fluid are expelled between a set of stationary vanes surrounding the impeller.  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 driveshaft.

MAIN FEATURES OF 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.

LIMITS OF 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.  Additionally, if the feed pressure is variable, a centrifugal 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.

SIMULATION SETUP: 

The CFD simulation is done in Solidworks flow simulation.

Geometry:
The impeller of thickness 20mm is made with a diameter of 90mm and six blades. Then this impeller is closed in a cylinder with height 30mm, diameter larger than the impeller, and merge result option is deselected. A suction pipe of 25mm diameter is extruded to a height of 70mm. Outlet pipe of diameter 10mm is swept through a line of 80mm such a way that it is tangent to the cylinder and the merge result option is deselected. A boolean addition operation is performed on a suction pipe, cylinder, outlet pipe so that the thickness from inlet to outlet is 1mm using shell command.
For the rotation of the impeller, a cylindrical chamber is created enclosing the impeller.

Pre-Processing: 
Using the Wizard tool following initial conditions are selected.
1. Fluid type: water
2. Flow type: internal with rotation.

Under the component, control deselect the cylindrical chamber. Select the cylindrical chamber in Rotating regions and give an angular velocity of 1000rad/s. 
BOUNDARY CONDITIONS: 
1. Inlet: Environment Pressure.
2. Outlet: Output velocity of 10m/s.
GOALS: 
1. surface goal average Total Pressure: Inlet lid.
2. surface goal average Total Pressure: Outlet lid.
3. surface goal Mass flow rate: Outlet lid.

To avoid convergence too quickly perform the simulation for at least 500 iterations by selecting in calculation control options.

Run the above simulation.

Solving: After running the simulation create a new parametric study.
INPUT VARIABLES: Outlet velocity of 10m/s, 15m/s, 20m/s, 25m/s, 30m/s.
OUTPUT PARAMETERS: Cut plots, goals, and trajectories created after running the simulation.

Run the parametric study.

RESULTS AND INFERENCES:

Cutplot of Velocity = 10m/s


Trajectory


Cutplot of Velocity = 15m/s


Trajectory


Cutplot of Velocity = 20m/s


Trajectory

Cutplot of Velocity = 25m/s


Trajectory


Cutplot of Velocity = 30m/s


Trajectory

From the above cut plots and trajectories, the flow pattern can be observed.

GOAL RESULTS:

Inlet pressure:

Outlet Pressure:

Mass Flow Rate:

Pressure Ratio at Design Point 1: 5901414.19/757212.2479 =7.79.
Pressure Ratio at Design Point 2: 6332169.702/810530.4547 =7.81.
Pressure Ratio at Design Point 3: 4775120.412/460350.3163 = 10.37.
Pressure Ratio at Design Point 4: 6612477.285/997397.4969 = 6.62.
Pressure Ratio at Design Point 5: 5842902.36/702967.4291 = 8.31.

As Observed in the above plot there is variation in pressure ratio same as the variation in output pressure and input pressure. Generally, as the velocity increases the pressure decreases.

CONCLUSION:
There is a significant increase in the mass flow rate with an increase in velocity and pressure ratio.