Centrifugal pump design and analysis

AIM:

To perform flow analysis on a centrifugal pump and find the variation of pressure ratio and mass flow rate and find relation between them.

INTRODUCTION:

In this simualtion, we first model a centrifugal pump with the most basic geometrry possible with made up dimensions and then, perform flow analysis on its geometry. Then, after defining motor motion, the simulation was run. After the simulation was run, results were obtained from the simulation with the help of goal plots isuch as mass flow rate at the outlet.

THEORY:

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy. The rotational energy typically comes from an engine or electric motor. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into volute, from which it exits. While working, the impeller rotates at high speed, creating vacuum about its center(inlet port) which results in suction of water from the inlet pipe. The water then rotates about the impeller, increases its rotational energy and then finally exits through the exhaust port.

The construction geometry of the entire setup greatly affects the parameters of the pump such as mass flow rate, velocity and pressure. Hence simulation is very helpful to check the geometry and modify it according to our needs.

CONSTRUCTION AND SIMULATION:

For the construction of the centrifugal pump, the impeller blades were designed first.

Then, the involute casing was designed around it with diameter slightly larger than the impeller blade domains. After that, the suction nozzle was built with a relatively smaller diameter on the face of the involute casing that acts as the source for the water feed into the centre of the impeller. Finally, we need to include the discharge nozzle. It is constructed with the help of a tangent to the outer surface of the involute and its offset. Then, a circle was swept with the offset line as the path after creating a plane on the other end of the tangent.

This brings an end to the modelling of the centrifugal pump. It should be taken care that while extruding the parts of the pump, we should make sure that the option "merge results" is always disabled. Else, we get a mixed geometry without any hollow geometry in the model.

Finnally, a cylindrical shape was extruded that would enclose the entire impeller region. It was done so, to create the rotational domain for the ratation of the impeller. It should be extruded in such a way that, it doesnt cross anything else. It should be only big enough such that it only encloses the impeller.

After the construction of the pump is done, we create a new flow wizard from the flow simulation window, with flow type as internal. Then, the boundary conditions were provided, with inlet port at a pressure of 101325 pa with an exit velocity of 10 m/s. After setting the boundary condtions, we now have to define the rotation of the impeller. To do so, we can define it using the rotating region feature, select the rotation region that we had extruded, and then set the speed to 1000 rad/sec.

It shoud be made sure that the rotation region that we had created, is unchecked in the component control menu or else, the calculation wiill treat that domain as a solid.

Then, we add the results that we need from the simulation such as avg total pressure at inlet and outlet, and the mass flow rate at the outlet, from the goals.

PLOTS AND RESULTS:

After defining all the parameters for the study, we now start a parametric study to get the results for different cases of outlet velocity. It was done, with varying the velocity from 10 m/s to 30 m/s with intervals at 5. After running the parametric study, we get the following results:

Where, the pressurre ratio is defined as the ratio of total outlet pressure to the total inlet pressure.

velocity cutplots:

case-1, outlet velocity = 10 m/s:

case-2, outlet velocity = 15 m/s:

case-3, outlet velocity = 20 m/s:

case-4, outlet velocity = 25 m/s:

case-5, outlet velocity = 30 m/s:

From the above results, we can plot the following graphs:

plot between the pressure ratio and the mass flow rate:

OBSERVATIONS AND CONCLUSIONS:

From the above plot, we observe that the pressure ratio and the mass flow rate at the outlet are inversely proportional. And the result can be justified as follows.

With increase in the velocity, the mass flow rate at the outlet increases. We know, from bernoullis equation that with increase in velocity, the pressure simultanously decreases on accordance to the conservation of energy. Hence, with increasing outlet velocity, the outlet mass flow rate increases and the outlet total decreases.

And since we know that, pressure ratio = total outlet pressure/total inlet pressure, the pressure ratio also decreases with increase in the velocity and we observe an inverse relation between the mass flow rate and the pressure ratio.