Question

(b) Answer the following in relation to flow through orifices. (i) Explain why the coefficient of contraction is included in the discharge equation for flow through a small orifice. [3 marks] (ii) If a small orifice meter with a diameter of 0.016 m discharges flow at a rate of 6 x 10-4 m3 /s under a head of 1.6 m, what is the value of the discharge coefficient? [2 marks] (iii) Explain the hydraulic difference between a small orifice and a large orifice. [2 marks]

Answer 1

i) Coefficient of contraction is included in discharge because flow area at vena contracta is smaller the orifice area thats why we have to include coefficient of contraction in dicharge.

Ac = Cc Ao where Ac = flow area at vena contracta

Ao = orifice area

Whare Cc is always less than 1 .

ii)

Cd= coefficient of discharge

Ao = area of orifice =

= 0.7854× (0.016²)= 2.01×10^(-4) m²

h = head = 1.6 m and discharge Q = 6× 10^(-4) m³/s

Cd = 6× 10^(-4)/(2.01×10^-4)sqrt(2×9.81×1.6) =0.533 ans

iii) Classification based on shape:

Orifices are classified into small orifice and large orifice depending upon the size of orifice and head of fluid in that orifice. Small orifice is the one in which has the head of fluid from the centre of orifice is more than five times the depth of orifice. Also the large orifice is the one which has the head is less than five times the depth of orifice.