Flow through Convergent-Divergent Duct

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Discussion Overview

The discussion revolves around the behavior of flow through a convergent-divergent duct, specifically focusing on horizontal and inclined flows with incompressible fluids, as well as considerations for compressible fluids. Participants explore the implications of Bernoulli's equations, pressure head changes, and the effects of duct geometry on flow behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that in a horizontal convergent-divergent duct with incompressible flow, the actual total pressure head is less than the ideal pressure head due to frictional losses.
  • Another participant suggests that if the duct is inclined upward, the pressure head will increase upstream due to height differences, while the total head loss remains constant, leading to a condition where the velocity head becomes zero.
  • Several participants inquire about the behavior of compressible fluids in the duct, questioning the nature of pressure loss in such scenarios.
  • A participant shares a resource link for further reading on compressible flow through convergent-divergent nozzles.
  • There is a discussion about the angle between the convergent and divergent sections of nozzles, particularly in pressure relief valves, and whether this angle affects the nozzle's behavior.
  • Another participant mentions a coefficient of discharge for relief valves and references standards from ASME and API for further guidance.

Areas of Agreement / Disagreement

Participants express varying opinions on the effects of duct inclination and the behavior of compressible fluids, indicating that multiple competing views remain. The discussion does not reach a consensus on these points.

Contextual Notes

Some assumptions regarding fluid properties, flow conditions, and the specific geometry of the ducts are not fully addressed, leaving room for further exploration of these factors.

neostalker
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Guys, i would like to ask some opinion from u all. About the convergent-divergent duct, it is horizontal flow and the water is incompressible.

So, based on Bernoulli's equations, elevation and density is the same. Because of the frictional losses, the actual total pressure head, H is less than then the Ideal Pressure Head.
Any disagreement from you guys, or any other opinion.

If the convergent-divergent duct is inclined upward, what will happen?
 
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If the duct was inclined upwards the pressure head will increase at the upstream due to difference in height , but the total head loss will stay constant .
The flow will continue until the pressure head at upstream equals the elevation head at downstream thus velocity head will be zero and the fluid comes to rest.
 
Guys,

Following the quesrtion by neostalker, I would like to know what would happen if the flow is with compressible fluids ? What will be pressure loss in this case ?
 
johntyroger said:
Guys,

Following the quesrtion by neostalker, I would like to know what would happen if the flow is with compressible fluids ? What will be pressure loss in this case ?

Given the generality of your question, here is a good point to start reading:

http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html

CS
 
stewartcs said:
Given the generality of your question, here is a good point to start reading:

http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html

CS

Thanks for the response Stewart CS. The link was very helpful.

It was a very good start for me. In the case of the pressure relief valves, usually the convergent and divergent sections of the nozzle are perpendicular to each other. Is this angle between going to affect the behaviour of CD nozzle ?
 
Last edited:
johntyroger said:
Thanks for the response Stewart CS. The link was very helpful.

It was a very good start for me. In the case of the pressure relief valves, usually the convergent and divergent sections of the nozzle are perpendicular to each other. Is this angle between going to affect the behaviour of CD nozzle ?

With relief valves you'll have a coefficient of discharge from the manufacturer to account for the flow through the valve. The rule of thumb is 0.975 if I recall correctly (per ASME and API). However, the manufacturer should provide it to you. A good reference is API 520 and ASME BPV Code.

CS
 

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