Contraction losses exceed expansion losses given the same tr

Contraction losses exceed expansion losses given the same transition geometry?

The pressure loss through the contraction is caused by

two consecutive processes:

(1) Contraction of the flow to the vena contract, and

(2) Expansion to the wall of the small pipe.

The latter is an uncontrolled expansion against an

adverse pressure gradient. Vena contracta is the point

in a fluid stream where the diameter of the stream is

the least, and fluid velocity is at its maximum, such as

in the case of a stream issuing out of a nozzle,

(orifice).The maximum contraction takes place at a

section slightly downstream of the orifice, where the

jet is more or less horizontal.The effect is also observed in flow from a

tank into a pipe, or a sudden contraction in pipe

diameter. Streamlines will converge just downstream

of the diameter change, and a region of separated

flow occurs from the sharp corner of the diameter

change and extends past the vena contracta.

The reason for this phenomenon is that fluid

streamlines cannot abruptly change direction. In the

case of both the free jet and the sudden pipe diameter

change, the streamlines are unable to closely follow

the sharp angle in the pipe/tank wall.

Sudden expansions are when the area of the pipe

increases suddenly along the length of the pipe (at a

90 degree angle). The downstream velocity will be

lower than the upstream velocity. In Energy lost is

because of turbulence. Amount of turbulence depends

on the differences in pipe diameters.

It is observed that

the flow is severely disrupted if there are contour

changes occurring in the downstream flow in the pipe.

Sudden enlargement creates more severe formation of

flow eddies than sudden contraction. Also, the losses

are more at the point where the enlargement in the

pipe begins.

In the sudden contraction, vena contracta’s

are formed at the point of contraction and this point is

the most susceptible point for pipe damage. So, to

increase the life of the pipe in cases of sudden

contraction the pipes must be designed in view of the

above observations making the corners more rounds

so as to minimize the losses in the pipes.

Therefore Contraction losses exceed expansion losses given the same transition geometry.