Hi Martin,
Unfortunately I was not able to conduct measurement for full flow operation
because its weir was not finished yet due to budgetary constraints (only 75%
released).
I also don't have material that pertains directly on pitch computation for
axial turbines. What I did is I assumed that wind turbine propellers operate in
the same principle as the axial turbines (since the formula is applicable for
all kinds of fluids, except for differences in Reynolds number).
Assuming that you have info on flow rate, speed and diameter. Let:
d = turbine diameter, m
w = peripheral velocity, m/s
Q = flow rate, m^3/s
A = turbine cross-sectional area, m²
v = water velocity, m/s
p = pitch angle, °
a = angle of attack, 5° for flat plates, 3° for curved plates, 4° for NACA
profiles
A = 3.1416 * d² / 4
v = Q / A
w = 3.1416 * d * RPM / 60
p = a + arctan (v / w)
Hope this info helps.
Eilrem
chilechinadollz <[EMAIL PROTECTED]> wrote:
Hi Eilrem,
Have you determined your turbine's efficiency yet? Also, how did you
compute the ideal tip speed ratio? I have material that computes
diameter, speed and power from flow and head, but nothing on blade
pitch. I guess this is buried in the specific speed of the prop, but
I don't have info on how to get from specific speed to blade pitch.
Could you point me towards reference material that would be helpful?
Thanks kindly,
Martin
--- In [email protected], Eilrem Fernandez
<[EMAIL PROTECTED]> wrote:
> Hi,
>
> I've tried using a boat propeller on one of our axial turbines
(15kW, 3m gross head) and it worked. What I did is: first I computed
for the ideal blade profile based on flow rate, gross head, and RPM
requirements. Then, I looked for a propeller that closely matched my
computations. By buying an off-the-shelf component, I ensure that
replacement parts for the turbine will be available without special
fabrication, although you have to sacrifice a bit of efficiency due to
the inappropriate shape of the blade tip. The difficult part is in
looking for a propeller that closely matches what you need.
>
> The blade has only a limited range of tip-speed ratio wherein it
will operate at high efficiency. Increasing the tip speed will
increase the RPM and reduce the pitch for a given head and flow rate.
Increase in head will increase the RPM and flow rate if the turbine
size remains constant.
> Flow rate is proportional to the axial turbine's cross-section and
is related to the square-root of the gross head.
>
> Use of flat plates will reduce your efficiency but if the flow rate
is more than sufficient, I think it is worth it for a bit of sacrifice
in efficiency in excange for simplicity of design, fabrication and
maintenance.
>
> Axial pumps are not so available in the Philippines but there is a
wide variety of propeller sizes, shapes and configurations.
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