In: Other
Is the value of Cd a function of the fluid which is flowing through the meter? What other effect does Cd accommodate in the case of viscous fluids (in addition to viscous or friction losses)?
No, the value of Cd (Coefficient of Discharge) is not a function of the fluid. But it depends upon the flow rate and the velocity of the fluid.
The coefficient of discharge is the ratio of the actual discharge flow rate to the theoretical discharge flow rate.
When we measure the pressure drops across pipes, we provide restrictions to flow in the form of venturi or an orifice which leads to sudden expansion and contraction in the area of the flow. In order to incorporate these losses in the flow, there is a range of coefficient of discharge so that the discharge flow rate remains the same as theoretical. The value of Cd lies between 0 and 1.
For the case of venturimeters, the value of Cd lies from 0.89 to 0.98 while for the case of orificemeter the value ranges from 0.68 to 0.72.
So, venturimeters have lesser losses as compared to orificemeter.
The Coefficient of Discharge is only used to accommodate these frictional losses that the equipments impose upon the flow.