Flow of gas into a vacuum system through an orifice

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SUMMARY

The discussion focuses on calculating the flow rate of gas entering a vacuum system through an orifice, emphasizing the concept of "choked flow" due to the pressure differential between ambient pressure and a near vacuum. Key resources include a detailed PDF on choked flow and relevant equations from eFunda and Wikipedia. A critical factor in these calculations is the discharge coefficient (Cd), which is influenced primarily by the orifice geometry rather than the gas velocity.

PREREQUISITES
  • Understanding of fluid mechanics principles
  • Familiarity with the concept of choked flow
  • Knowledge of orifice discharge coefficients (Cd)
  • Basic mathematical skills for applying flow rate equations
NEXT STEPS
  • Study the equations for calculating flow rates in choked flow scenarios
  • Research the factors affecting the discharge coefficient (Cd) for different orifice geometries
  • Explore advanced fluid dynamics concepts related to vacuum systems
  • Review practical applications of choked flow in engineering systems
USEFUL FOR

This discussion is beneficial for physics enthusiasts, engineering students, and professionals in fluid mechanics or vacuum technology who seek to understand gas flow dynamics in vacuum systems.

Bcreative
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Hello, I'm sorry if this is in the wrong section but I recently took real interest in physics, particularly fluid mechanics, so this is my first post. Consider the following:

A glass system has a pump pulling gas out of the system, which has an orifice to the atmosphere at the opposite end of the system. How do you calculate the flow rate of gas into the system through the orifice?
 
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Hi Bcreative. Welcome to the board. The pressure ratio across an orifice with ambient pressure on one side and something close to a hard vacuum on the other results in what's known as "choked flow".

Here's a good site that describes it in easy to understand terms:
http://www.okcc.com/PDF/Choked Flow of Gases pg.48.pdf

These two sites provide equations and a bit more detail:
http://www.efunda.com/forum/show_message.cfm?start=1&thread=16827&id=16827
http://en.wikipedia.org/wiki/Choked_flow

The hard part about calculating the flow rate is determining the discharge coefficient for your orifice (Cd). The discharge coefficient is a function of the geometry and less so a function of velocity.
 

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