When the volume flow rate decreases

[thebillsfan]

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TPR says that must mean the fluid is compressible. Why can't it be because the fluid is viscous?

 

[G1SG2]

TPR says that must mean the fluid is compressible. Why can't it be because the fluid is viscous?

Because viscosity wouldn't explain decreased flow rate better than compressibility. Flow rate Q=Area * Velocity. The viscosity would explain the decreased velocity, but you have to take the area into consideration as well, in which case I think compressibility would be a better explanation.

 

[thebillsfan]

alright, thanks

 

[wanderer]

I'm not 100% sure about this so if someone can confirm (or dispute) my reasoning that would be great.
The reason that volume flow rate works is that it is a proxy for mass flow rate. Since density is constant kg/s is proportional to m³/s, and obviously mass is conserved so mass flow rate and volume flow rate are constant. If the fluid is compressible kg/s is still constant, but the equation V=m/p does not yield the same number at different points due to compression so A*v is not constant.