Pressure after Pump with -0.4[bar], 0.8x104[N/m3], 3[m/s], D1/D2=1.4, HP=30[m]

  • Thread starter Thread starter Karol
  • Start date Start date
  • Tags Tags
    Pressure Pump
Click For Summary

Homework Help Overview

The discussion revolves around a fluid dynamics problem involving a pump, where the initial pressure is negative, and various parameters such as specific weight, velocities, and diameters are provided. The Bernoulli equation is applied to relate these quantities and determine the pressure after the pump.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of the Bernoulli equation with gauge pressures, questioning the validity of using negative pressure in calculations. There is an exploration of whether absolute or gauge pressures should be used consistently throughout the equation.

Discussion Status

Some participants have offered guidance on the setup of the problem, suggesting that the original poster's approach is correct but may contain calculation errors. Multiple interpretations regarding the treatment of pressures are being explored, indicating a productive dialogue without explicit consensus.

Contextual Notes

There is a mention of the negative pressure value, which raises questions about the assumptions made regarding gauge versus absolute pressure in the context of the Bernoulli equation.

Karol
Messages
1,380
Reaction score
22

Homework Statement


A pump is on a line. the data are:
P1=-0.4[bar]
\gamma=0.8\cdot104[N/m3]
V1=3[m/s]
\frac{D_1}{D_2}=1.4
HP=30[m] (head of pump)
See attached drawing.

Homework Equations


Bernoulli equation:
H_1+\frac{V_1^2}{2g}+\frac{P_1}{\gamma}+H_P=H_2+ \frac{V_2^2}{2g} +\frac{P_2}{\gamma}

The Attempt at a Solution


The velocity after the pump:
V_2=V_1\frac{D_1^2}{D_2^2}=3\cdot 1.4^2=5.9
Bernoulli equation:
\frac{-0.4\times10^5}{8000}+\frac{3^2}{20}+30=0.3+\frac{5.9^2}{20}+\frac{P_2}{\gamma}
\Rightarrow\frac{P_2}{\gamma}=27.9 \Rightarrow P_2=2.2\times10^5
The answer should be 1.87x105[pa]
 

Attachments

  • Pump.png
    Pump.png
    2 KB · Views: 480
Physics news on Phys.org
Hello.
The negative pressure for P1 is a clue that you're dealing with gauge pressures. [EDIT: However, it's ok to use gauge pressures in the equation. I think your setup is correct. Check the calculation again.]
 
Last edited:
so what? i can put, in bernoulli equation, either absolute or gauge pressures, as i understand, if i use the same type on both sides, which i have done here.
 
That's correct. I think you just made a mistake in carrying out the calculation.
 

Similar threads

Flow of water in a venturimeter
  • · Replies 2 ·
Replies
2
Views
2K