# Incompressible and compressible flow.

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September 24, 2013, 08:53
#21
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,607
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Quote:
 Originally Posted by mercy.gh Hello, In addition to the interesting conversation; Measuring the Mach number also demands attention to the temperature of your environment, since the speed of sound would change correspondingly. I am trying to simulate a combustion chamber with a high velocity fuel -oxygen injection.Really close to the injectors(before combustion happens) flow is definitely compressible (velocity of injection is 500 and sound almost 380) but in a small distance, when combustion happens Temperature will rise highly and the velocity of sound will increase to 800 which makes the Mach number much less! Should I consider this compressible or in-compressible flow?
what about the velocity? if V/800 < 0.3 you can consider the flow incompressible in terms of acoustic wavew but you still have dependance of density due to high temperature gradients

September 24, 2013, 09:58
#22
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mersedeh
Join Date: Aug 2012
Location: stockholm
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Quote:
 Originally Posted by FMDenaro what about the velocity? if V/800 < 0.3 you can consider the flow incompressible in terms of acoustic wavew but you still have dependance of density due to high temperature gradients
True, I must consider this compressible. I just have never worked with that!
Thank you

 January 30, 2014, 07:32 Deflagration not Detonation #23 New Member   Richard Chinery Join Date: Jan 2014 Posts: 1 Rep Power: 0 with respect to the earlier posts on combustion effects on what would normally be incompressible flow, simple temperature correction accounts for change in density. you could use combustion intensity figures to plot the heat release and continually account for this? i don't see the need for use of compressible flow considerations unless you are considering detonation.

September 1, 2016, 10:52
#24
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larmes
Join Date: Aug 2016
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Quote:
 Originally Posted by Ananda Himansu ;31568 If the bulk compressibility of the fluid (the ratio of the change in specific volume to the change in pressure that causes it; this is a material property of the fluid) is small by comparison with the pressure variations encountered in the flow under consideration, then these pressure variations will cause only small changes in the density.
How high of a pressure difference do you need to have in order for the density to change due to the pressure difference?

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