Water through a diffuser 1

[waseem19]

everybody knows that when water is moving from a small pipe to a big pipe the velocity will be reduced and area of flow is increased to the full area of the big pipe .

now consider an EMPTY straight pipe with a diffuser(pipe enlargment) in the middle, then we let water (incompressible fluid) into the pipe at a constant flow rate, when water reaches the section just before the diffuser we will have a certain velocity and a certain area of flow which is now the area of the pipe it self (pressurised system)

Water is now flowing through the diffuser the area of the pipe is increasing and as i'v read in the books so will the area of water and the velocity will decrease.

When we reach the full area of the pipe we will have a much bigger area of flow and much lower velocity.

My question is how can the water changes its area (increase it) while moving through the diffuser? if the original pipe dia is 100mm and the final is 10000mm how can water increases its area to fill the diffuser while its moving and to fill the 10000mm pipe eventually?

The reason why i said empty pipe in the beginning is if we have an already filled pipe with water, the water already in the 10000mm pipe will act as a break to the water ejecting from the 100mm pipe, and so what we all know will actually happen (area increase and velocity decrease) but not because we’ve increased the pipe area but because there is something braking the water and causing it to accumulate behind it and therefore slowing it down.

Please tell me what do you think about this.

 

[BobPE]

waseem19:

the big pipe fills because of a fundemantel rule of fluid dynamics called conservation of flow, it's learned early on in an engineering degree.

BobPE

 

[Dikkala]

Air in the empty pipe will first get displaced by incoming water. Gradually the water occupies that space before moving out, thereby losing some momentum (and velocity)! Once the larger is area is full same amount of water will continue to flow but with leass velocity so that the quantity outflowing is same as incoming.

 

[quark]

Waseem!

A good but very fundamental question. The thing which makes the water changes area is back pressure. Take a perfectly horizontal pipe, take first smaller dia. pipe and then much bigger dia. pipe. Now water gets partially filled in the pipe and the jet coming out of the pipe at exit will not have the dia. of the pipe. (and it all depends upon the pressure of water in the pipe and the respective areas)

Secondly take a bend and attach it up to the end of the bigger pipe. Now because of backpressure created by the gravity of water itself water will form full area.

Regards,

Repetition is the foundation of technology