Centrifugal pump design and analysis

Project:- Centrifugal Pump design and analysis

Aim & Objective:- To create a 3D CAD model of a centrifugal pump and perform a flow simulation. The objective is to obtain the relationship between Pressure ratio and mass flow rate to study the pump behavior for different conditions.

Introduction:-  A centrifugal pump is a mechanical device designed to move fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers.  Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips.  The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet.  The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge. A picture below shows different types of impeller

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 a high speed. The rotational motion of the impeller accelerates the fluid out through the impeller and 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 toward the outlet.

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.

As stated earlier, the objective is to establish a relationship between the mass flow rate and pressure ratio. Mass flow rate is defined as the mass of matter that passes through an area per unit of time. It is a measure of the rate of the movement of passing fluids (liquids and gases) through a defined area. Mass flow rate is a very important variable, which is commonly seen in engineering and fluid mechanics. Its standard SI unit is kilogram per second (kg/s). The mass flow rate equation can be easily derived by simply reviewing the definition of mass flow rate and recalling its standard unit. The units of mass flow rate are kg/s, mass per time, which indicates that mass flow rate is the ratio of the change in a fluid's mass to the change in time. The following is the mass flow rate formula:

Where  is the change in mass,  is the change in time.

The Pressure ratio is defined as the ratio between the outlet pressure and the inlet pressure of the pump where:- 

Pressure ratio= Outlet Pressure/ Inlet Pressure.

Setup:-  The centrifugal pump was designed using SolidWorks 3D CAD software. Note that the dimensions of the pump are purely made up and hence the results would be different from what the literature might show. A picture of the 3D model is shown below.

The next step was to set up the simulation using the flow simulation "Wizard" command with the following conditions:- 

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

The study also included selecting the "Rotation" with local regions for analysis type. The rotating region was for outside the vanes with 1000 rad/s. Thus that region was excluded from selection during the component control selection. 

The next stage was to set up boundary conditions, environmental pressure was selected for the inlet lid of the pump. A parametric study was conducted for the outlet velocity, these values included 10,15,20,25, and 30 m/s. 

Once the boundary conditions have been defined, surface goals have been defined as well, these included the total average pressure and mass flow rate at both inlet and outlet lids of the pump. An equation goal of the pressure ratio was also added for the parametric study.

For the mesh, a global mesh was chosen with a refinement of 5 and a local mesh for the volute casing with a refinement of 5.

For this project, 500 iterations have been chosen to ensure that the flow simulation will converge and meets all of the goals required.

Results:-  

The table and charts below shows the results of the parametric study:- 

Below is a picture of the velocity trajectory of the centrifugal pump 

Conclusion:- 

The table clearly shows when velocity increases, the mass flow rate increases in the outlet as well. The increase in outlet mass flow rate is almost linear when the velocity increases. Also, we can deduce that when the velocity increases, the pressure at the outlet decreases as well, and thus the pressure ratio of the centrifugal pump. There is an inversely proportional relationship between mass flow rate and the pressure ratio. This is because the kinetic energy of a liquid coming out of an impeller is obstructed by creating a resistance in the flow. The resistance is created by the pump casing which catches the liquid and slows it down. When the liquid slows down the kinetic energy is converted to pressure energy.