Correction in Velocity
- Recalling
the fact that the measured value of the piezometric pressure
drop for a real fluid is always more due to friction than
that assumed in case of an inviscid flow, a coefficient
of velocity Cv (always less than 1) has to be
introduced to determine the actual velocity Vc
when the pressure drop
in Eq. (15.12) is substituted by its measured value in terms
of the manometer deflection
'∆h'
Hence,
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(15.13) |
where '∆h' is the difference in liquid levels in the manometer and ρm is the
density of the manometric liquid.
Volumetric flow rate
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(15.14) |
- If a coefficient
of contraction Cc is defined as,
Cc = Ac /A0, where A0
is the area of the orifice, then Eq.(15.14) can be written,
with the help of Eq. (15.13),
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The value of C depends upon the ratio of orifice to duct area, and the
Reynolds number of flow.
- The main job in measuring the flow rate with
the help of an orificemeter, is to find out accurately the value of C at the
operating condition.
- The downstream manometer connection should strictly
be made to the section where the vena contracta occurs, but this is not feasible
as the vena contracta is somewhat variable in position and is difficult
to realize.
- In practice, various positions are used for the manometer connections
and C is thereby affected. Determination of accurate values of C of an
orificemeter at different operating conditions is known as calibration of the
orifice meter.
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