# What is reason of low axial velocity at tip and hub of an axial turbine?

I am analyzing results of my turbine simulation. I have created one line from hub to casing at downstream of the flow...please see the following figures. I want to investigate along this line how the axial velocity changes. In one of my results, my new blade(VT in last Figure or blue line in last figure) has the lower axial velocity than to base blade(CT) at hub and tip. (Please see attachment.) What does this mean? I think that this is due to low separation in downstream of flow of new blade(VT). Am I right?

Another question:

According to the following picture, Is axial velocity equal to velocity V in my case?

• The blades don't look constant pitch in your images, they look straight, which would imply an AOA varying with radius? Oct 13, 2016 at 19:20
• Brian Drummond, my English language is not very good. this turbine has symetrical blade(Naca 00XX). Angle of attack changes with radius because tangential velocity increase with radius. it is named Wells turbine. en.wikipedia.org/wiki/Wells_turbine What is your exact meaning of "pitch "? Oct 13, 2016 at 20:17
• Pitch is the axial distance in which the blade would turn one complete revolution. Normally, a turbine blade is twisted along the radius, so that pitch is independent of radius, or alternatively, AOA is approximately independent of radius. With straight blades, the AOA is only optimal at one radius, the tip is stalled and the hub is at too low AOA to be effective. Which might explain your result. However, you can't twist the blade of a Wells turbine... Oct 13, 2016 at 21:47
• Hi Brian Drummond , Why do you say that " the tip is stalled and the hub is at too low AOA"? The tangential velocity is low at hub, so AOA is high at hub than to tip Oct 14, 2016 at 15:02
• In fact, I think that because of unchanging the pitch angle in wells turbine(my case), We should change the axial velocity distribution in order to having better AOAs along the height of blade. Am i right? Oct 14, 2016 at 15:16

Regarding section 4, i designed a blade that thickness changes from hub to tip. In other words, thickness increases from hub to tip along of blade height(variable thickness blade). In high flow coefficients, this design makes lower separation at tip in variable thickness blade than to constant thickness blade because thicker profile has lower separation and names trailing edge stall. therefore, performance of improved blade is better in higher flow coefficients. The turbine tuque is better in variable thickness blade in high flow coefficient....I think that this event is due to more uniform axial velocity distribution along of blade height in variable thickness blade at downstream of the turbine rotor . What is your opinion?

I searched about the pitch and I deserve a detailed description about the pitch because i am new in turbine. I am grateful that tell me that i am right or wrong. Please be careful that performance of Wells turbine is similar to Wind turbine.

Section 1:

According Brian Drummond stated, I think that the pitch is as follows:

In fact(according to above picture), for a given wind speed, to maintain a constant angle of attack along the length of the blade, the pitch of the blade must be increased as it approaches the bade root. Am i right?

Section 2:

In fact, i think that a twisted blade has different pitch along the height of blade in order to maintain a constant angle of attack along the height of the blade . Am i right?

section 3:

Now here, i have one question:

Is there any deference between the pitch and pitch angle in turbines?

Section 4(Major section for me):

Regarding the following figure, VT blade(my new blade designed) has better performance than to CT blade because there is lower axial velocity(lower AOA) at hub and tip in VT blade. Am i right?

In fact, I think that because of unchanging the pitch in wells turbine(my case), We should change the axial velocity distribution in order to having better AOAs along the height of blade. Am i right?

Please be careful that My means is downstream of the turbine rotor not downstream of the flow.

The turbine rotational direction and axial flow are in attachment are as follows:

Flow is bi directional in this turbine. the turbine is named Wells turbine.

I have another question:

I have created one line from hub to casing at upstream of the turbine rotor ..please see attachment. I plotted the axial velocity distribution on the line according to attachment. I think that the axial velocity distribution has not changed in these two blade because of unchanging flow blockage but variation of before axial velocity distribution is due to efferent thickness along of the blade height. Am i right?

I am grateful that guide me more about these sections.

• (0) This is not an answer but a refinement of the question... (1) Yes (2) No. The pitch angle changes because the radius changes but the pitch remains constant. (3) They are different, the pitch angle is the angle required (for a radius) to meet the pitch (axial distance per revolution). (4) Wells turbine makes it difficult. If the blade is more efficient at non-constant axial V, can you arrange the right axial V profile without wasting energy? Alternatively, how does chord affect the AOA? Can you keep a symmetrical airfoil but make chord a function of radius? Oct 14, 2016 at 10:57
• Regarding to section 2, I said (pitch along the) not pitch angle, Brian Drummond. Oct 14, 2016 at 11:12
• I think the problem is that the first diagram illustrates pitch angle, but labels it just "pitch". Oct 14, 2016 at 11:18
• oh, the following figure is about pitch angle or pitch? google.com/… Oct 14, 2016 at 11:36
• pitch angle. Propellers (both air and water) define pitch as a distance, thus encapsulating the variation in pitch angle with radius in a single number. Unless someone comes along to tell us turbine designers use a different convention, I suggest distinguishing between the two. (I suspect this may only achieve constant AOA along the blade at a specific design windspeed, but that's a separate issue). In any case it seems to be irrelevant for a reversible fixed blade (Wells) turbine. Oct 14, 2016 at 11:44