Flow of water in a venturimeter

1. Sep 6, 2012

Karol

1. The problem statement, all variables and given/known data
Water flows in a horizontal venturimeter. the diameter is 100 mm, and the throat is 50 mm.
The pressure at the entrance is 0.65[bar].
What is the maximum throughput that is allowed so that the absolute pressure in the throat will not drop under 0.3[bar]

2. Relevant equations
Bernoully equation:
$$H_1+\frac{V_1^2}{2g}+\frac{P_1}{\gamma}+H_P=H_2+ \frac{V_2^2}{2g} +\frac{P_2}{\gamma}$$

3. The attempt at a solution
The velocity V2 in the throat:
$$V_2=V_1 \frac{0.1^2}{0.005^2}=4V_1$$
Bernoully equation:
$$\frac{0.65E5}{1E5}+\frac{V_1^2}{20}=\frac{0.3E5}{1E5}+\frac{16V_1^2}{20}$$
Which produces negative velocity.
Even if i use absolute pressures, the same. i use 1.65 instead of 0.65.

2. Sep 6, 2012

TSny

Hi, Karol.

I don't see how you're getting a negative velocity here.

Also, are you sure that your value of $\gamma$ is correct?

3. Sep 7, 2012

Karol

you are right!
Thanks...

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