Q: Hi, I was wondering about your software — I was looking for something like it from years, but it looks wrong that changing the viscosity of the fluid from a light oil to a thicker one, like from 10 to 100 cSt (or mm^2/S) the flowing liters passing by the orifice increases instead to decrease. Can you explain if I”m wrong, or not?




A: Roberto, It may seem incorrect, but if you look at the equation for flowrate through an orifice, you can see that only in terms of discharge coefficient C, through Reynolds number, viscosity is treated. Also, in the equation for discharge coefficient, Reynolds number is in the denominator, which means if viscosity is higher, Reynolds number is lower, and the coefficient of discharge is also higher, which results in higher flowrate for the same pressure difference in front of and after the orifice.

That is a mathematical solution and confirmation for this problem, but what about the physical? I would say that that due to higher viscosity, the vena contracta effect is smaller, and the contraction of flow stream after the orifice is not as big as it is with less viscous fluid. As contraction of flow stream after the orifice is smaller, the flow cross section is bigger and flow rate is also higher.

To review the theory from ISO for orifice flow, see:

The preceding Q&A is based on an exchange on PipeFlowCalculations.com, a website featuring calculators and a forum that focuses on fluid flow issues.