Francis Turbine And Kaplan Turbine Doubt

[ank_gl]

While doing experiments on a francis turbine, i noted that the discharge decreases when the load on shaft was increased, =>the speed was decreased. I was(in fact, I am) kind of confused about why it dropped, I did consult some text but didnt get any right reason. I thought that since the blade speed has decreased, the flow should have a lesser back pressure to tackle and should be more, but the reverse happens. Also, I thought since the speed is decreased and torque increased, flow must increase to account for the force, but again i was wrong(i eventually did some calculations and the increased torque problem was solved). Now then, here I stand with a doubtful explanation. I think, when the blade velocity decreases,the jet velocity is also decreased to maintain the inlet angle as close to blade angle as possible => ensuring shock less entry, this happens as the shock less entry will provide the minimum back pressure to the flow and thus flow drops a little bit. Is this reason good enough to explain the fall in discharge??

Again while doing experiment on a kaplan turbine, i noted that the blade's tip velocity is about 2 to 2.5 times the inlet jet velocity. How come that is possible? If it is, the machine should be a compressor!
How can an incoming jet transfer momentum to a blade if the blade is already moving faster? I know that the velocity of blade changes with the radial distance of concerned section, but then the blade angle takes care of this and in now way, any part of blade can travel faster than the incoming jet, because that makes it a compressor, right??

 

[ank_gl]

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Fine, just plain 2 questions
why does discharge decrease with decrease in speed for a Francis turbine?
How can a turbine blade travel faster than the water jet??

I hope to get some views atleast

 

[trambolin]

Well, I understood almost nothing about your comments, but let's see if this works :

I have an air pump, I block its inlet, should I expect read more current or less current than normal conditions on the pump motor terminals? Think about the load and the entropy!

 

[ank_gl]

since the pressure ratio increases when the inlet is blocked for the pump, motor will draw more current. Obviously the load increases in this case.
But how does it answer my questions??

 

[trambolin]

Nope, the load decreases. If you don't believe me (probably you won't), try it and you will see. Because an argument over takes too long...

 

[ank_gl]

got it wrong. discharge decreases with increase in speed. i saw the wrong graph, and the decrease is due to the increased centrifugal forces

 

[ank_gl]

Well, I understood almost nothing about your comments, but let's see if this works :

I have an air pump, I block its inlet, should I expect read more current or less current than normal conditions on the pump motor terminals? Think about the load and the entropy!

Hmm ok, i thought a bit and came out with this, pump is no more required to impart energy to incoming fluid, it just needs to sustain the vortex, so power input decreases.

And for doubts regarding the turbines, they both are reaction turbine and degree of reaction changes with velocity of wheel. I made the velocity diagrams for both and it was self explanatory.

Another doubt in centrifugal pump, I was studying the main characteristics where it said that graph is plotted bw power and speed(rpm) keeping the head and discharge constant. discharge can be kept constant simply by a valve,
how can head be kept constant, it solely depends on the discharge and speed, so if speed is varied(for this curve) and discharge is not, head ll also vary, right??
Or is there a misprint, i tried finding that curve on net, i didn't get anything

 

[eaaaazizo]

ok its very good topic but i have a project in relaited equipement i wish from u to help me about DERIAZ MACHINE

 

[trambolin]

I think same argument will apply for your last question, though this time I am not sure if I get the question.

Suppose you have a steady state flow with sufficient opening and discharge and head flows are constant. then without opening valve more you increase the speed. From the previous argument, you should see no significant power increase because it will (slight abuse of terminology) start skidding in the fluid, am I right or not?

 

[ank_gl]

Sorry eaaaazizo, i haven't studied much about DERIAZ MACHINE. But it would be nice if you throw out some doubts. It ll be good for both of us i guess.
Trambolin, thanks for response, unfortunately i am in a hurry, i ll think about it in an hour or two

 

[eaaaazizo]

pump storage plants ( reversable turbine pump )

so after i write my enquiry i still search so i find some thing about it
so
it is an educational equipment used to simiulate reverse turbine pump
eas.asu.edu/~holbert/eee463/pumpedstorage.html
this site show how reverse turbine pump working
and some attachments

 

[vkm]

Dear ank

Saw your questions as below

Q1 why does discharge decrease with decrease in speed for a Francis turbine?
Q2 How can a turbine blade travel faster than the water jet??

A1 Rule is that flow is always directly proportional to the speed ( Pumps & Turbines) everything else remaining unchanged. Speed increases - flow increses and vice versa.

A2 Blade velocity is the difference between the "tangential / whirl components" of absolute and relative velocities. Here what you refer to water jet is the absolute velocity. Blades are twisted so that
- Towards the lower diameter the tangential componenet of relative velocity is positive thereby reducing the blade velocity ( m/s) from the whirl componenet of the jet velocity.
- Towards the higher diameter the tangential componenet of relative velocity is negative thereby incresing the blade velocity ( m/s) from the whirl componenet of the jet velocity.

This gives rise to the apparent paradox.Since you are well aware of the shockless entry fundamental you will appreciate the above based on velocity triangles.

Regards

VKM