A mouth piece is a short pipe fitted externally or internally to an orifice. The length of the mouth piece is 2 to 3 times the diameter of the orifice to which it is fitted. Mouth pieces may be classified based on their position, their shape and the manner in which the mouth piece discharges.
Mouth pieces may be external or internal mouth pieces. An external mouthpiece projects outwards from the tank wall. An internal mouth piece is fixed so as to project internally. An internal mouth piece is also called reentrant or Borda’s mouth piece.
Mouth pieces may be cylindrical with a uniform diameter. If the diameter of the mouth piece varies along its length, they may be convergent or divergent mouth pieces. In a convergent mouth piece, the diameter decreases towards the outlet. In a divergent mouth piece, the diameter increases towards the outlet.
A mouth piece may be a freely discharging mouth piece or a fully discharging mouth piece. In a freely discharging mouthpiece, the jet after its contraction does not touch the inner surface of the mouthpiece. In a divergent mouthpiece, the jet after contraction within the mouthpiece, expands, and fills the mouth piece.
Types of Mouth Pieces:
An external mouthpiece consists of a short length of pipe fitted externally, to the orifice. The discharge through the orifice can be increased by fitting an external mouthpiece.
Fig. 8.47. shows an external mouthpiece of area a fitted to an orifice.
Let H be head of the liquid over the mouthpiece.
As the liquid enters the mouthpiece, the area of flow reaches a minimum value ac at a section CC. Beyond this section, a sudden enlargement occurs and the liquid fills the mouthpiece. The area of the jet at outlet is obviously equal to a.
If the head of water over the mouthpiece exceeds this limit, the mouthpiece will not discharge full i.e., the jet will flow without touching the walls of the mouthpiece and the device will only function as an orifice with a reduced value of the coefficient of discharge.
This is a mouthpiece whose area decreases towards its outlet. The inner profile of the mouthpiece almost matches with that of the jet. No sudden enlargement of the flow section occurs and hence, there will be no loss of energy head. The area of the jet at outlet is the same as that of the mouthpiece at outlet.
Hence, the coefficient of velocity and coefficient of contraction are all unity. Hence, the coefficient of discharge is also unity. However, in actual condition; a small loss of head occurs due to friction. The coefficient or discharge is 0.97 to 0.98.
We know, in the case of the cylindrical external mouthpiece, a loss of head occurs due to a sudden enlargement that takes place beyond a section C-C in the mouthpiece. If this sudden enlargement of the area of flow is prevented the loss of head is prevented. This condition is achieved in the convergent divergent mouthpiece.
This mouthpiece is converging up to a throat section C-C and beyond the section, the mouth piece is diverging so that the area of the mouthpiece gradually increases from the minimum area ac to the outlet area a.
Fig. 8.50. shows a convergent divergent mouthpiece fitted to an orifice of a tank.
Let H = head of liquid over the mouthpiece.
Applying Bernoulli’s theorem to the free water surface, section C-C and the outlet,
Fig. 8.52 shows an internal mouthpiece. In the mouthpiece shown in Fig. 8.52. the jet, after contraction, does not touch the sides of the mouthpiece. In this case, the mouthpiece is said to be running free. But, in the mouthpiece shown in Fig. 8.53. the jet, after contraction at a section, expands and fills the mouthpiece. In such a case, the mouthpiece is said to be running full.
Let a = area of the mouthpiece
ac = contracted area of jet in the mouthpiece
H = head of water surface above the centre of the mouthpiece
v = velocity of flow through the mouthpiece.