Centrifugal pump design and analysis
Objective
1) To carry out analysis on made up centrifugal pump
2) To understand relationship between pressure ratio and mass flow rate
Theory
Centrifugal pump is device to create pressure difference across inlet and oulet. Surging is a one of criteria used for designing of pumps. It may be explained in the following way. If we suppose that the compressor is operating at a point \'D\' on the part of characteristics curve (Figure 1) having a positive slope, then a decrease in mass flow will be accompanied by a fall in delivery pressure. If the pressure of the air downstream of the compressor does not fall quickly enough, the air will tend to reverse its direction and will flow back in the direction of the resulting pressure gradient. When this occurs, the pressure ratio drops rapidly causing a further drop in mass flow until the point \'A\' is reached, where the mass flow is zero. When the pressure downstream of the compressor has reduced sufficiently due to reduced mass flow rate, the positive flow becomes established again and the compressor picks up to repeat the cycle of events which occurs at high frequency.
This surging of air may not happen immediately when the operating point moves to the left of \'B\' because the pressure downstream of the compressor may at first fall at a greater rate than the delivery pressure. As the mass flow is reduced further, the flow reversal may occur and the conditions are unstable between \'A\' and \'B\'. As long as the operating point is on the part of the characteristics having a negative slope, however, decrease in mass flow is accompanied by a rise in delivery pressure and the operation is stable.
Figure 1 The theoretical characteristic curve
Setup
A hypothetical CAD model of centrifugal pump is created having blaldes and rotation space in part . Fluid is given a rotaion of 1000rad/sec in Rotation space and boundary condition is applied having environment pressure of 101325 pa at pump inlet and output velocity at exit lid. A individual study is caried out for output velocity of 10 m/sec and flow trajectory is evaluated.Then a Parametric study is caried out for a range of output velocity to underastand effect of mass flow rate. Following table shows details of design point.
Table 1 Parametric study input Parameters
Results and Discussion
Table 2 Results of Parametric study
Table 3 Pressure ratio and mass flow rate variation with velocity
Figure 2 Pressure Ratio vs mass flow rate
From figure 1 and figure 2 it is evident that on increasing outlet velocity, pressure will decrease with mass flow rate upto certain limits after that pressure will become constant. It is clear from figure 2 that on left side of peak ,as flow rate decreases pressure ratio will drop. If this drop becomes significant then flow reversal can take place causing surging phenomenon.This can cause damage to pump blades.
Animation 1 Flow Trajectory inside centrifugal pump
Conclusion
1) Relationship between pressure ratio and mass flow rate is parabolic in nature.
2) Pressure ratio becomes almost constant on further increasing velocity.
3) Critical pressure ratio lies at mass flow rate of 0.24 kg/sec.
References
1) https://nptel.ac.in/courses/112104117/chapter_4/4_8.html