Centrifugal pump and compressor performance

INTRODUCTION

The study of Compressors, fans, blowers and turbines are in itself a subject in engineering and they are widely classified under TURBOMACHINERIES. A turbomachine is a device in which there is a continuous transfer of energy between rotor and fluid passing through rotor due to dynamic action which in turn results in pressure and momentum change of the fluid. They differ from the positive displacement machines and comes with certain advantages and disadvantages.

Pumps and compressors are both energy consuming turbomachine. The energy is supplied from external source such as a motor through a shaft which runs in the bearings and in turn powers rotor. As the name, suggest, rotor is the component which rotates, it has number of vanes or blades, the vanes draw the liquid from the suction flange and energy is imparted to fluid. This energy is used to increase the pressure or momentum or both to the fluid. When motor drives the shaft and the rotor, centrifugal force is imparted on water present between blades, this causes the water to moves radially out and hence the suction is created at the eye of the rotor, which draws fluid from the suction pipe. Kinetic energy and the momentum of fluid is used to raise the pressure of the fluid, as the fluid collects in the spiral casing, whose area increases, this is to further reduce velocity and increase the pressure.

Centrifugal pump has several advantages over the Reciprocating pumps and hence finds wide usage. Some of the important operating parameters while selecting the pump are Head which is the height to which the fluid is pumped, Discharge which is the flow rate, Speed of the rotation. Based on this, Specific speed, Manometric head, Euler efficiency specific work could be calculated.

Manometric head = Suction head + Delivery head + Friction Head + V^2/2g

Euler head = Euler work/g

Euler work = Rotor velocity * Horizontal component of fluid velocity.

Manometric efficicieny = Manometric head/Euler head

For Centrifugal compressors, thermodynamic relations are applied to find the efficiency, in terms of temperature change and is defined as the isentropic efficiency = Ideal work/Actual work

Pumps and compressors could be connected in series or parallel for increasing head or discharge respectively.

The performance curve has three important zones, Discharge or mass flow rate on x axis and Head or pressure ratio on y axis. As the mass flow rate increases, outlet pressure decreases for the same work, in this zone, if the operating point is designed, as the mass flow rate decreases, higher head is available, which in turn draws more fluid and hence the mass flow rate increases. This zone is hence a stable zone of operation. However, as the flow rate is decreased continuously, we attain the second zone where the slope is positive. This is termed as the surging zone, This is an unstable zone where a small disturbance can get amplified. This results in periodic stopping and revival of fluid flow and condition is referred as the surging. As we increase the fluid flow rate from the favourable zone, we reach zone 3, where the characteristic drops sharply, in this zone, any increase in work doesnt really cause appreciable increase in mass flow rate and the zone is termed as the choking zone. This is also characterised by sharp drop in the efficiency.

So the compressor and pump has to operate between two zones Surging and Choking. Surging is disastrous and Choking doesnt help achieve the goal.

CFD SIMULATION

GEOMETRY: Inlet suction pipe of ~28mm is chosen and delivery pipe of ~14mm is chosen. The shaft is coded to rotate at 1000rads/s which is 9550RPM. Blades are assumed to be radial both at the inlet and outlet.

MESHING: Grid depenency studies are carried out and Manual meshing with Nx = 40 Ny = 40 Nz = 100 is chosen

SOLUTION AND SOLVER SETUP

Study is carried on both air and water, The outlet velocity is varied as a parameter for the parametric study.

Inlet pressure is set to 101325Pa.

The solver is also set to complete 750 iterations minimum before the simulation is said to be converged, along with cycle to cycle variation reduing within tolerance

RESULTS AND PLOTS

The cut plots for the velocity and pressure could be seen.

But, plots dont help understand the performance characteristic of the device, hence the parametruc study is carried out by changing the outlet velocity. The curves are plotted

PERFORMANCE PLOT FOR AIR

PERFORMANCE PLOT FOR WATER

The outlet velocity is varied using the parameteric study, as the outlet velocity is closer to zero, all of the momentum of the fluid is used to raise the pressure and hence the pressure ratio is high. But since the work is the same, the mass against which the boost is obtained would be lower, hence at lower outlet velocity, pressure ratio should be high and mass flow rate should be low. On similar lines, at higher velocity, it means not entire momentum is converted to pressure energy, hence the Pressure ratio should progressively reduce, mass flow rate should progressively increase.

DISCUSSION ON RESULTS

1) The plots of water and air matches the trend expected in the middle portion but not entirely

2) When we compare the pressure rise of water and air, it shows, that more work is required to raise the pressure of air when compared to raising the pressure of water, This explains why pump is used in certain scenario and why compressor in the other scenario

3) Surging and choking is not evidently pronounced in the plots

4) This could be cause of the pump design shortcoming, hence some values are assumed.

5) The following plots show, recirculation in the suction flange which is the cause of poor design and could be the reason for not seeing surging in the simulation