Wednesday, 9 October 2019

fluid dynamics - What determines the dominant pressure-flow relationship for a gas across a flow restriction?


If one measures the pressure drop across any gas flow restriction you can generally fit the relationship to $$\Delta P = K_2Q^2+K_1Q$$ where $\Delta P$ is the pressure drop and $Q$ is the volumetric flow


and what I've observed is that if the restriction is orifice-like, $K_2 >> K_1$ and if the restriction is somewhat more of a complex, tortuous path, $K_1 >> K_2$ and $K_2$ tends towards zero.


I get that the Bernoulli equation will dominate when velocities are large and so the square relationship component. But what's determining the $K_1$ component behavior? Is this due to viscosity effrects becoming dominant? Does the Pouiselle relationship become dominant?





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