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FORUM de MECAFLUX.com • Afficher le sujet - Problem with Heliciel analysis of Kaplan propeller turbines

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Nous sommes le Mar 26 Sep 2017 04:55

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MessagePosté: Sam 17 Juin 2017 04:49 
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Inscription: Sam 17 Juin 2017 04:21
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We are using Heliciel for design of Kaplan propeller hydro turbines and carefully following the tutorials for this application. To check the method we first use propeller data (number of blades, tip and hub diameters, flow, gross head and shaft power for known commercial turbines. For cases where the kinetic power is a large fraction of the hydraulic power--e.g., 60% or more--the Heliciel optimum power, speed and pressure drop compare favorably with the commercial specifications. However, we discovered that when kinetic power is a small fraction of hydraulic power--e.g., 15%--the Heliciel optimum shaft power, speed and pressure drop are much less than the commercial specification. We tried increasing the simulated guide vane induced tangential velocity beyond the optimum value to just counter the induced tangential velocity of the propeller but only a small improvement is made. In both large and small kinetic/hydraulic power cases we are simulating appropriate values of draft tube induced suction. Both cases employ similar programmed blade thickness profiles (profile method B) and blade shapes. Any assistance with overcoming this problem will be very much appreciated whether in English or French.


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MessagePosté: Sam 17 Juin 2017 12:05 
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Inscription: Mer 28 Jan 2009 12:31
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Bonjour

il me semble que certains principes de bases doivent être éclaircis:

l'helice de la turbine ne transforme que de l’énergie cinétique, uniquement de l’énergie cinétique et aucune énergie de pression!

une turbine de type bulbe montre comment travail un helice de captage dans le fluide, elle ne capte que l'energie cinétique du fluide.
Le système kaplan est une solution pour convertir de l'energie de pression en énergie de vitesse pour alimenter notre hélice de captage qui ne sait qu'utiliser l’énergie de vitesse (cinetique).

donc une turbine kaplan profite de l'energie de pression transformée en energie de vitesse par le distributeur qui met en rotation le fluide et immege l'hélice dans un flux tengentiel qui accélere la vitesse de rotation de l'hélice par rapport a un flux purement axial, ceci se traduit par un gain de puissance.

Mais ceci est expliqué très en détail sur la page:
en francais:
en englais:

donc si vous constatez des variations de précision entre diverses hauteurs de chutes (pression), c'est tres certainement parce que votre distributeur(guide vanes) ne transforme pas la hauteur de chute en flux tangentiel

le didacticiel en ligne explique en détail comment réaliser un distributeur mettant en rotation le flux (ce flux tangentiel amont sera entré en paramètre de l'hélice de captage pour obtenir la puissance a l'arbre) en générant la perte de charge égale a la hauteur de chute. :
francais:
anglais:

Pour résumer, si vous trouvez des variations importantes , il est probable que le rôle du distributeur soit mal ajusté et que l'apport d’énergie cinétique du flux tangentiel en amont de l'hélice de captage soit mal évalué.

cordialement
jf iglesias


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MessagePosté: Sam 17 Juin 2017 16:40 
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Inscription: Sam 17 Juin 2017 04:21
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Dear JF,

I appreciate your prompt reply to my post and understand the principles you describe---I have read the excellent tutorials on this subject many times!

For cases where the ratio of propeller input kinetic power (without guide vane assistance) / hydraulic power is large--e.g., > 60%--we obtain good agreement with commercial reference designs after setting simulated guide vane induced tangential velocity approximately equal to the tangential velocity induced by the propeller as described in the tutorial. An example would be 15 rad/s.

However, for cases where the input kinetic power / hydraulic power ratio is small ... e.g., < 15% ... to obtain expected power and speed of a known commercial design we would have to set the simulated guide vane induced tangential velocity (rad/s) to 5 or 10 times the value just to compensate the induced tangential velocity of the propeller ... e.g., 75 to 100 rad/s. We might accept the much greater guide vane head loss but are not sure such a high value of induced tangential velocity is possible -- we will make a Heliciel model to see if it is.

Does your experience suggest that such a high value of guide vane induced tangential velocity is attainable?

Kind regards,
AllanC


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MessagePosté: Dim 18 Juin 2017 10:20 
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Inscription: Mer 28 Jan 2009 12:31
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Sorry I do not have experience of the exact limitation of the tangential speed limit.

i just know that an important tengential speed is the transformation of an important pressure load (when kinetic power / hydraulic power ratio is small) by the guid vanne.
I think that the limit of pressure transformation in tengential speed , is reached when the guide vanne angle is to much important, so the efficiency of profiles (CZ/CX) of the guide vanne is not good anymore and give more head losses than tengential speed energy. in this case (angle of guide vanne more than 45°) the efficiency of the transformation of the head load in tengential speed is not a good system anymore(to much losses)
If think that the kinetic power / hydraulic power ratio is small and the guide vannes profiles are not working in hem good angles of efficiency, it is time to use a Pelton system...
wee should find this limit by comparing constructor efficiency data of kaplan and Pelton turbine: when the Pelton efficiency become as good as the Kaplan system.

I do not have this data, but if you get some you can share it here to go further...

i just have this Comparative efficiency (H constant) of types of turbines:




a: propellers
b: Turbine Francis
c: turbine kaplanTurbine Kaplan (propeller)
d: turbine peltonTurbine Pelton


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MessagePosté: Lun 19 Juin 2017 03:23 
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Inscription: Sam 17 Juin 2017 04:21
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Dear JF,

Thank you for your additional message. Based on limited experience synthesizing/analyzing guide vanes with Heliciel according to the tutorial procedure I suspect it would not be possible to generate much more tangential velocity than required to just compensate the induced tangential velocity of the propeller. For this troublesome case with low ratio of kinetic/hydraulic power we have an "existance proof" that it should be possible to obtain good efficiency with a fixed pitch Kaplan propeller in a bulb configuration. The existance proof is the data published by a reputable turbine manufacturer (Flygt). The reference data includes propeller pitch angle but not the NACA or other blade profile.

It came to mind that kinetic power should increase if the swept area is reduced while maintaining the same flow specification. For example, if the area is halved the inlet velocity will double. Since kinetic power is proportional to the cube of velocity and directly proportional to swept area the net effect should be a 4x increase of kinetic power. If this is true and proven by Heliciel then perhaps the problem is that the propeller dimensions I have been using are too large for the operating conditions. I will let you know the results of this experiment. Of course if it works there will remain the question of why the reference turbine dimensions produced an unfavorable ratio of kinetic/hydraulic power. But at least we will have made some progress!

Kind regards,
AllanC


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MessagePosté: Ven 4 Aoû 2017 10:49 
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;)


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