water through a diffuser,BobPE 5

[waseem19]

hi bob and thank you for replying to my question previously about the same subject , it's very fundamental but it just won't add up in my mind . i was convinced by what quark said about it ( back pressure causes flow area to increase) it is the same what i have in mind that presence of water in the bigger pipe will force the water ejecting to slow down , but i wanted to comment on your reply .
i didn't question the conservation of mass principles ,however I’m questioning the way it is interpreted at diffusers and pipe enlargement ,I think that water attains the conservation by keeping the same velocity and area it had in the small pipe, which is exactly what happens to water coming out from a pipe to the air (garden hose),would the water ejecting in the air knows if you put a very big pipe around it ?will it increases it’s area after you put that pipe and reduce it again when you remove it ?
unless there is something forcing the water to slow down it won’t .
so actually water doesn’t slow down and increases it’s flow area because of the conservation of mass ,it does that because something is forcing it to do so ,but it in the same time it is still correct with respect to conservation of mass because flow rate is still the same.
I hope I explained what I’m thinking about.

 

[BobPE]

there is no such thing as back pressure really, it's just a term that circulates primarily in industry and makes its way out into the real world every now and then.

The energy existis in a closed pipe as the Energy gradient (EGL) and the Hydraulic gradient (HGL), PERIOD.

The EGL and HGL flow through a sudden enlargement and as such, the laws of conservation of energy state the energy can not be lost, it can only change form. So at the enlargement in the pipe, the energy changes form. Velocity is reduced and flow remains the same. So what changes, obviously the pipe area, so the EGL remains constant and the HGL is modified due to the changed velocity. There is no filling the pipe or stuff like that happening because the pipe is under pressure and not open to atmosphere which is the only way you can get air into the enlargement to allow it to work like you are thinking(the only time there is stuff like that happening is when you are really filling the pipe, but as soon as the filling allows that enlargement to go from gravity to pressure flow, the principals above apply.

The water at the end of a hose is not pressure flow as soon as it leaves the hose, so looking at that scenario is not the same. What happens at the end of the hose is that the EGL and HGL collapse to the very end of the hose producing the last energy loss in the pressure system which is the exit loss. Once out of the hose, you have a different matter since its now at atmoshperic pressure.

I hope I helped, believe me, the water flow will remain the same as the area increases, the velocity will go down, and the area will increase.....and the conservation of energy will make the whole thing work, every time!!!!! (if you designed it right lol)....

Your thoughts are not off the mark in any way, hydraulics is not an easy subject or concept.

take care

BobPE

 

[gumlog]

Waseem and BobPE,

Actualy the energy in the stream at the exit of the valve still exists and is not absorbed by the exit losses. Only the static pressure in the jet leaving the nozzle or valve is zero. The Total pressure is still there....it is the velocity pressure at the outlet. Remember this is how the Pelton wheel works. Good workout!

gumlog PE

 

[BobPE]

gumlog:

I think you are talking about jet flow as far as the stream when it leaves the confines of the system such as a nozzle. The EGL and the HGL of the system cease to be associated with the flow when it leaves the system (pipe) and thats what I was refering too. Gotta love those Pelton wheels, they are quite unique.

BobPE

 

[waseem19]

BobPE, gumlog

is it correct to say (scientifically) that velocity increased in the contraction because the resultant force on the fluid acting in the flow direction has increased ?since velocity has increased (accelerated) so the increased force must be the cause (F = m a)?

if that is correct what is the relation between this force that has increased and the pressure that has decreased in the contraction?

can i say that in pipe there is two pressures acting on the wall of the pipe and different one along the flow direction(which i think of as the cause of additional force in the contraction) ?

it is very fundamental but i can't get a solution that satisfies everything that happens in the contraction.

Thanks a lot .

 

[gumlog]

If you write the Euler equation: v1**2/2gc+p1/gamma+z1+work of the pump = v2**2/2gc+p2/gamma+z2+work of the turbine+head losses from point 1 to 2, and the points 1 and 2 are sufficiently far apart specifically that point 2 is far enough down stream that all local momentum effects have settled (about 6 pipe diameters) then the conservation of mass and energy will hold true. (**=exponent, gamma=bulk density like water 62.4 lb/ft**3) All terms are in lb-ft/lbm or feet of the fluid flowing.

If you have an abrupt enlargement on the order of 100 to 1000mm as you mentioned, and you take point 2 to be too close to the enlargement then you have a free field flow condition with eddys and vortices and must use free field theory to solve the problem. This theory is not very good for quantatative answers but for thinking processes only.

If you consider this situation before the pipe fills up with liquid, then you have two phase fluid flow which cannot be rigorously solved. The only two phase flow answers we have are strictly emperical, ie. air and water, steam and condensate.

Now to you and BobPE on the jet, the energy on the left hand side of the equation is in the jet on the right side as pure kinetic energy (velocity squared /2gc)when the jet is released to the atmosphere. Check it out by zeroing all the proper terms and you will see.

Bobpe do you mean by the egl that it is the total fluid energy and the hgl is the static pressure line?

gumlog PE

 

[BobPE]

waseem19:

the velocity you talk about is increased because of the decrease in area at the nozzle where Q=VA. Force has increased by the conversion of pressure head to velocity head which relates to F=ma. There is only one pressure inside the pipe acting in all directions, it has two components, pressure head and velocity head.

gumlog, its been a long time since I used the Euler equation!!! But I know what you are talking about. The key to the equation is that its for confined flow. When the flow leaves the pipe its not confined and the only energy in the stream is the velocity of the jet and the differential head that it is falling, Pressure is all gone since the jet is in the atmosphere. The key to the pelton wheel is to get the transfer off all the pressure head in the pipe to velocity head before it leaves the pipe.

BobPE

 

[waseem19]

bobPE
you said "Force has increased by the conversion of pressure head to velocity head which relates to F=ma" , what is the physical nature of this force and WHAT IS THE DIRECTION OF THIS FORCE ?

is it differnet than the force acting on the pipe wall in the contraction ?

 

[waseem19]

gumlog

when applying Euler equation between the surface of a tank and the discharge point of a very short length of pipe (lossess = 0 ) we get V=sq(2*g*H), H: height of water in tank

i get from the equation that (Assume that the H stays the same for any diameter ) the velocity is constant for any pipe diameter, is this right ?

can i say that the maximum flow i can get through a system with zero losses depends only on Head on outlet tank and area of the tank outlet ?

 

[BobPE]

waseem19:

yes you are right. The head in the tank is the maximum amount of energy in the system. The energy is then lost due to flow as fristion loss. Head available at a nozzle on that system is a summation of all the losses subtracted from that tank head. The area of the outlet and pipes in the system impact these losses and reduce the pressure available at the nozzle. This equation only works as long as the system is in confined flow (in a pipe). The velocity is constant for the tank discharg to atmosphere for any area orifice, independent of area. When it eneters a pipe system, concervation of flow takes place which may modify fluid velocities to maintain energy balances for the EGL of the system.

gumlog: The egl is the energy available in the system as pressure and it slopes through the system proportional to head loss from its maximum incurred through friction and entry exit loss. The HGL is the EGL-Velocity head. It tells you where the energy in the system is, in eithr pressure head or velocity head.

This is a great conversation!!!

BobPE

 

[gumlog]

I believe we are all on the same sheet of music and I do mean music. If any more discussion is desired I would be willing to talk about this as long as anybody wants to.
This has been a very good workout in concepts among three patient and courteous engineers and I have enjoyed it very much.
I have an interesting project on board now to design a raw water pumping station using hydro power. Will use water turbines to drive pumps. This will be a 6000+ HP installation.

Gumlog PE

 

[waseem19]

bobPE

Can you please comment on the other question i posted on the 19th ,,it was the one about the force direction .you commented on the second one only, your comment was very helpful and many thanks for that .

 

[KRSServices]

Wasseem, having read this thread, I was bound by curiosity as to the original question. In my opinion, and using the 10m diffuser example (from a 4&quot pipe, I am assuming the system is closed with a constant static pressure. once flow is introduced (a valve opens downstream of the diffuser) the water does not hit a wall (as a brake) or anything else like that because Q1(at the inlet to diffuser) = Q2(at the outlet of the diffuser). The pressure is different at the inlets and outlets because the pressure head is lost due to friction and turbulance. If you do not believe me, insert some baffles into the stream flow to introduce turbulance and note the pressure drop. If further proof is required, free flow a fire hydrant with and without a diffuser and place a pitot in the stream. the Q remains constant but the flow stream (distance) is dramatically different, why? The diffuser creates great turbulance and is quite effective at lowering the pressure head of the stream flow. The Q does not change in the diffuser, only the pressure head, velocity and area does. For information purposes, the behavior (scattering) of the flow stream at the point of discharge (nozzle) is called the hydraulic jump (supercritical to subcritical. I have seen this principal used to diffuse stormwater flow. KRS Services

http://www.krs-services.com

 

[BobPE]

KRSServices:

Sub and Super critical flow? That occures in gravity flow, not jet flow and not flow at the outlet of a nozzle, for a jump to occur there has to be a transition between these two gravity flow regeims. At the end of a nozzle, pressure head is converted to jet flow and unless it ends in a pipe and flows by gravity, sub and super cirtical flow and hydraulic jumps cannot occur.

waseem:

I am not sure what you are asking about the force. If you are asking about the force.

 

[waseem19]

bobPE
you said "Force has increased by the conversion of pressure head to velocity head which relates to F=ma" , what is the physical nature of this force and WHAT IS THE DIRECTION OF THIS FORCE ?

is it differnet than the force acting on the pipe wall in the contraction ?

i'v just copied my question again Bob,, it was on the 19th on reply to your posting on the 18th ,,if you scroll up you will see both of them ,,i totally agree with you about your comments on KRS.

 

[waseem19]

and guys ,, exactly how did i end up second top expert in this forum ??

i was only asking ?!!!!!

 

[rbcoulter]

The logic of how they rank the experts on these forums baffles me. Some move up in the forums even though they have not posted anything in several months.

 

[KRSServices]

Waseem,

You are quite correct regarding the terminology I used regarding flows and hydraulic jumps. They are indeed descriptions for gravity systems, particularly stormwater systems. I was trying to illustrate the behavior of the water stream flowing out the nozzle versus a diffuser. It is remarkably similar, not only in appearance but in behavior as well. My appologies KRS Services

http://www.krs-services.com