exit tank pressure 1

[tony1234]

Using Bernoulli's Eq, I have attempted to calculate the exit air pressure and velocity of a system which consists of a tank (~5 gallon), 3" long 0.25" dia pipe, and an exit nozzle that is basically a cone shape (0.25" to 6" diameter). The tank will be pressurized to 10 psi. Why is my pressure at the 0.25" pipe grossly negative? I have followed a similar example that assumes free jet at exit, but I want to determine the pressure at that point if I were to place any sort of permeable material in-line. Is this calculable?

 

[rbcoulter]

Schmilton is correct.

Bernoulli's equation can be applied in a "loose" fashion by assuming a pipe pressure. Most choose the pressure to be atmospheric. No fair to change it after that. Then the velocity can be calculated.

JStephen - Yes, you can have a situation where the absolute pressure would be a unrealistic negative number. In this case; however, the velocity is not pre-determined at the pipe.
If you assume that the velocity is some very high value then it could very well mean that a negative absolute pressure is needed in the pipe but this problem does not require that.
Maybe tony1234 should tell us how he got the high velocity.

 

[sailoday28]

correction to mine last post. For low atmos pressure Assume atmos at exit, then calculate exit velocity. Throat velocity is determined from area ratio of throat to exit. Then calculate throat pressure- which will be lower than exit pressure---either vacuum or negative.