How important are the dimensions of entry and exit cones of a venturi designed for low velocity water flow?
Follow-ups to a previous question: 1) Does the final diameter of a venturi exit cone need to match the entry diameter of the entry cone? 2) I read that to minimize drag the entry cone should be 30 degrees and the exit cone should be 5 degrees. If entry and exit cone diameters must match, the exit cone will obviously be much longer than the entry cone. 3) If total device footprint is a constraint, what is the best place to trade off: terminating the exit cone at a smaller diameter than the entry cone – but keeping the 5 degree angle…or increasing the exit cone angle to provide a cone length that fits the allowable footprint but matches the entry cone diameter?
I am designing this for low-velocity water flow, and maybe salt water. Device footprint is an issue. I am trying to determine what my critical design constraints are. How are flow rate losses (inefficiencies) affected if my design does not meet the dimensional requirements of the “ideal” venturi discussed above?
To answer you questions:
1. Yes, the exit diameter should be the same as the inlet diameter other wise the pressure drop across the venturi will not accurately measure the flow rate.
2. According to Eschbach[1] the ideal entry angle is 20 degrees and the ideal exit angle is 5-7 degrees.
3. Changing from these ideal and even within the ideal will change the Calibration coefficient which normally varies between .95 and .99 due to a variety of factors including the angles noted above, surface roughness, and the turbidity of your water. Since you measuring the flow of sea water this last factor may be the most serious since the turbidity of the sea water will not be a constant.
Niel
