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• ## Re: [microhydro] Re: Relation of Qmax and runner length

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• ... This is correct - the formula for calculating discharge, needs an additional term, called the coefficient of discharge and denoted by C. Thus Qmax = C x
Message 1 of 6 , Jul 3, 2003
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> Your question is very interesting because I am myself studyING about
> that wheel
> Do you know the works of Clemson university about that wheel ?
> I have disvovered tAat there is a real question about Qmax and that
> question is also in relation with the true Qmax which is lower that
> the product of nozzle section (Width * throat) * Speed of water
>
> A MENDRET
> FRANCE

This is correct - the formula for calculating discharge, needs an additional
term, called the coefficient of discharge and denoted by C. Thus Qmax = C x
nozzle crossectional area x speed of water. This coefficient allows for losses
due to friction and losses due to contraction of the water stream as it passes
through the nozzle.

I do not know the value of this coefficient for crossflows, but have seen values
of 0.95 to 0.97 quoted for Pelton turbines.

Regards,

Max Enfield
Planetary Power
• ... mathematical ... length[L] in a ... can be as ... diameter. ... diameter. ... runner and has a ... sqrt [2 x g x ... head in ... Qmax is given ... Excuse
Message 2 of 6 , Oct 29, 2003
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--- In microhydro@yahoogroups.com, Max Enfield <max@p...> wrote:
> brijesh mainali wrote:
> >
> > Hi friends,
> > I will be grateful to you, if any of you could suggest me the
mathematical
> > expression [formula] for the maximum flow [Qmax] and runner
length[L] in a
> > cross flow turbine.
> >
> > Regards
> > Brijesh Mainali
>
> One of the nice things about a crossflow is that the runner length
can be as
> chosen to suit the application and is independent of the runner
diameter.
> However, usually the runner length does not exceed 3 x runner
diameter.
>
> At maximum flow the inlet stream occupies the full width of the
runner and has a
> thickness [T] designed to be 9% of the runner diameter.
>
> First calculate the velocity in m/sec using the usual formula V =
sqrt [2 x g x
> H] where g [gravitational constant] = 9.8 m/sec^2 and H is the net
> metres. Then Qmax = V x L x T. Using metric values throughout,
Qmax is given
> in m^3/sec.
>
> Regards,
>
> Max Enfield
> Planetary Power

Excuse me for the previuos mail incorrect
I am searching about crossflow and the works of Nadim m Aziz in
Clemson university
I cannot undestand the large lamda admission ( 90 degrees) and the
fact of the Q that couls pass in the nozzle is >> than the real Qmmax
So it seems that water fall in hard rain and not in compact jet in
the wheel
I have no good onformation on the eventual position of a vane

A MENDRET
FRANCE
• ... If what you say is correct, then the coefficient of discharge is about 0.5. I cannot accept that it is ever this low for a well designed turbine (or more
Message 3 of 6 , Nov 3, 2003
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>
> Excuse me for the previuos mail incorrect
> I am searching about crossflow and the works of Nadim m Aziz in
> Clemson university
> I cannot undestand the large lamda admission ( 90 degrees) and the
> fact of the Q that couls pass in the nozzle is >> than the real Qmmax
> So it seems that water fall in hard rain and not in compact jet in
> the wheel
> I have no good onformation on the eventual position of a vane
>
>
> A MENDRET
> FRANCE

If what you say is correct, then the coefficient of discharge is about 0.5. I
cannot accept that it is ever this low for a well designed turbine (or more
specifically a well designed nozzle).

In an earlier posting, I noted seeing values of 0.95 to 0.97 quoted for Pelton
turbines. It is probably a bit less than this for a crossflow, because the
rectangular nozzle has a larger wetted perimeter compared to a circular nozzle
with the same cross sectional area.

In our designs we have simplistically taken the discharge coefficient as 1.0.
If it were actually closer to 0.5 then our performance claims will have been
greatly overstated and we would by now have a lot of unhappy customers. This is
certainly not the case.

Regards,

Max Enfield
Planetary Power
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