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Chapter 6

Q. 6.P.9

Water flowing at 1.5 l/s in a 50 mm diameter pipe is metered by means of a simple orifice of diameter 25 mm. If the coefficient of discharge of the meter is 0.62, what will be the reading on a mercury-under-water manometer connected to the meter? What is the Reynolds number for the flow in the pipe? Density of water = 1000 kg/m³. Viscosity of water = 1 mN s/m².

Step-by-Step

Verified Solution

Mass flowrate, G=\left(1500 \times 10^{-6} \times 1000\right)=1.5 kg/s.
Area of orifice, A_0=(\pi / 4)(0.025)^2=0.00049 m².
Area of pipe, A_1=(\pi / 4)(0.050)^2=0.00196 m².
Reynolds number =\rho u d / \mu=d\left(G / A_1\right) / \mu

=0.05(1.5 / 0.00196) /\left(1 \times 10^{-3}\right)=\underline{\underline{3.83 \times 10^4}}

The orifice meter equations are 6.19 and 6.21; the latter being used when \sqrt{\left[1-\left(A_0 / A_1\right)^2\right]} approaches unity.

G=\frac{C_D A_0}{v} \sqrt{\frac{2 v\left(P_1-P_2\right)}{1-\left(A_0 / A_1\right)^2}}           (equation 6.19)

Thus:          \sqrt{\left[1-\left(A_0 / A_1\right)^2\right]}=\sqrt{\left[1-\left(25^2 / 50^2\right)^2\right]}=0.968

Using equation 6.21, G=C_D A_0 \rho \sqrt{(2 g h)} gives:

1.5=0.62 \times 0.00049 \times 1000 \sqrt{(2 \times 9.81 h)}, and h = 1.24 m of water

Using equation 6.19 in terms of h gives:

1.5=(0.62 \times 0.00049 \times 1000 / 0.968) \sqrt{(2 g h)}, and h = 1.16 m of water

This latter value of h should be used. The height of a mercury-under-water manometer would then be 1.16 /((13.55-1.00) / 1.00)=0.092 m or \underline{\underline{92  mm  Hg}}