Repeat Prob. 14-60. but at a water temperature of 80°C. Repeat for 90°C. Discuss.
The maximum volume flow rate at which cavitation occur.
Answer to Problem 61P
The maximum volume flow rate at which cavitation occur is
Explanation of Solution
Given information:
The temperature of the water is
Write the expression for the required head using the energy balance equation.
Here, the initial pressure is
Write expression for the available net positive suction head at the pump inlet.
Here, the saturation pressure is
Write the expression for the minor losses.
Here, the minor loss coefficient due to sharp edge is
Write the expression for the velocity of the water.
Write the expression for the Reynolds number.
Here, the density of the water is
Write the expression for the minor losses.
Write the expression for the required net positive suction head.
Write the expression for the friction factor.
Here, the friction factor is
Calculation:
Refer to the Table-A-7E, "Properties of saturated liquid" to obtain the density of the water as
Substitute
Substitute
Use hit and trial method to obtain the volume flow rate.
Iteration 1 consider the volume flow rate as
Substitute
Substitute
Substitute
Substitute
Substitute
Since, the available net positive suction head is greater than the required net positive suction head hence, cavitation does not occur at this flow rate.
The different iteration are shown in the below Table.
S.No | |||
1 | |||
2 | |||
3 | |||
4 | |||
5 | |||
6 | |||
7 | |||
Draw the plot between the flow rate and available net positive suction head and the required net positive suction head.
Figure-(1)
Figure-(1) shows that the cavitation occur at volume flow rates above the
Refer to the Table-A-7E, "Properties of saturated liquid" to obtain the density of the water as
Substitute
Substitute
Use hit and trial method to obtain the volume flow rate.
Iteration 1 consider the volume flow rate as
Substitute
Substitute
Substitute
Substitute
Substitute
Since, the available net positive suction head is greater than the required net positive suction head hence, cavitation does not occur at this flow rate.
The different iteration are shown in the below Table.
S.No | |||
1 | |||
2 | |||
3 | |||
4 | |||
5 | |||
6 |
Draw the plot between the flow rate and available net positive suction head and the required net positive suction head.
Figure-(2)
From Figure-(2), available net positive suction head and the required net positive suction head curves are not crossing each other as the temperature of water is near to boiling temperature. The pump cavitates at any flow rate.
Conclusion:
The maximum volume flow rate at which cavitation occurs is
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Chapter 14 Solutions
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