Design of Power Station Turbine Blades

[Jimmy87]

Hi pf,

I know that the stream turbine blades are twisted in design so that when steam is forced across the face there is a torque exerted on the turbine. If you look closely at the blades then they are roughly at a 45 degree angle to the oncoming steam which means a component of the force from the steam will provide torque.

What I don't understand is the problem with the following design. Imagine no twist in the blades at all. The blades are attached so that if you look at the turbine head on you are looking at each blade directly sideways on. You then blast steam right at the very bottom of the turbine from sideways on so that the steam hits the face of the blades exactly perpendicular to their surface. Surely then you get the maximum possible component from the force of the steam? Why don't they do this?

 

[fresh_42]

If I put the question bottom to top it asks: Why aren't ships driven by propellers like those on a steamroller or a water mill but with some very elaborated twisted shaped propellers? I would search there for an answer. Their actual shape is a science on its own.

 

[russ_watters]

Yeah, the simple answer is that that isn't as efficient as you think it is, but the long answer can be very long...

Also, different fluids and loads have different needs. There are cases where something similar to what you describe is used: Google "Pelton turbine".

 

[256bits]

Hi pf,

I know that the stream turbine blades are twisted in design so that when steam is forced across the face there is a torque exerted on the turbine. If you look closely at the blades then they are roughly at a 45 degree angle to the oncoming steam which means a component of the force from the steam will provide torque.

What I don't understand is the problem with the following design. Imagine no twist in the blades at all. The blades are attached so that if you look at the turbine head on you are looking at each blade directly sideways on. You then blast steam right at the very bottom of the turbine from sideways on so that the steam hits the face of the blades exactly perpendicular to their surface. Surely then you get the maximum possible component from the force of the steam? Why don't they do this?

They don't do this since it ends up being very inefficient. Energy costs money so one tries to get the most transfer energy from the fluid to the blades as possible.

There are two basic types of turbine classifications - reaction and impulse. Athough both extract power from a fluid, the priciples of operation diverge in the respect that one uses the pressure of the fluid to convert it into a high velocity jet that strikes the blades ( impulse turbine ); and the other uses the pressure to accelerate the fluid within the blade space causing a reaction against the blade ( reaction turbine ).

One has to note that the runner blades are moving, so relative to the steam jet, the moving flat surface is not perpendicular to the jet so there is not a maximum energy tranfer at the "flat" blade angle.. Secondly, there has to be a method to remove the fluid that has already contacted the runner blade so as not to interfer with the incoming stream and that requires a modification of the design. Thirdly, to utilize the maximum amount of energy of the fluid, the blades are designed so that the flow direction is reversed as much as possible, thus increasing the momentum transfer from the fluid to the runner blade.

In the end, one ends up with curved blades for the runner and the stationary blades as a result of the runner blade velocity and the fluid velocity, and the resultant velocity vectors of the fluid entering and leaving the blade area.

 

[Jimmy87]

Thanks guys, very informative!

 

[SteamKing]

What I don't understand is the problem with the following design. Imagine no twist in the blades at all. The blades are attached so that if you look at the turbine head on you are looking at each blade directly sideways on. You then blast steam right at the very bottom of the turbine from sideways on so that the steam hits the face of the blades exactly perpendicular to their surface. Surely then you get the maximum possible component from the force of the steam? Why don't they do this?

The only problem with that arrangement is that steam turbines use more than one set of blades to turn the energy of the expanding steam into rotary motion. You might blast the first row of blades in this manner, but what about the ones which are located in succeeding stages?

Here is a diagram of the arrangement of a typical steam turbine:

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The moving blades alternate with the sets of fixed blades which serve to direct the flow of steam into the next set of moving blades. In a large turbine, there may be 20 or more sets of moving and stationary blades. What is not shown here is that all of these blades are surrounded by a casing which keeps the steam flowing in one direction:​