3 replies to this topic

[rxnarang]

In adibatic flow of gas through a conduit of constant cross section, the sonic velocity is always reached at the end of the pipe. You are correct.

However, there is always energy loss involved due to friction, including entrance and exit losses, which are a form of energy loss. All these energy losses influence the energy profile of the fluid. Given that sonic velocity occurs at the end of the line, higher the losses, lesser the amount of fluid flow. Hence, taking the exit losses into account is important, as it will influence the amount of fluid which can flow in the pipe.

Crane Example 4-21 is correct in computing all the losses which occur.

Hope this helps.
Regards

[Taran]

Rajiv,

I can grasp the concept that an exit loss is required when discharging into a tank, etc when we are actually concerned with the pressure in the tank. However, when discharging to atmosphere, we don't care about the pressure of the atmosphere, only the pressure at the end of the pipe. By definition, sonic velocity is the velocity at which the downstream conditions have no effect on the upstream conditions because the information cannot be transferred upstream fast enough.

Therefore, if an exit loss occurs after the end of the pipe and sonic velocity occurs at the end of the pipe, why do we need to take into account an exit loss?

[rxnarang]

Taran,

Good question. This sent me into a tizzy and I read some more. Sad to say, I could not find any reference which justifies taking losses downstream of the point where Mach 1 occurs. In fact, all sources stated categorically that in sonic flow, discharge from end of pipe to atmosphere ( esssentially stagnant conditions) is through shock waves. Sonic flow occurs at the end of line.

Only in sub-sonic compressible flow and incompressible flow, sudden expansion loss needs to be taken into account. Essentially this loss is one velocity head.

I am still hunting for more information.

Anybody else with more ideas on this topic?