# Why are turbines built differently when a change of entropy or temperature is involved?

We are talking about turbines for extraction of energy from a fluid.

Consider a typical wind turbine, or a francis turbin for water - we have one hub with on set of vanes that are driven by the medium.

Now, consider a typical turbine in a steam engine - here we have alternating moving vanes and fixed counter-vanes. The fixed vanes redirect the flow after each stage, something that's apparently not neccessary in a water turbine for a high efficiency.

1 - My intuition tells me that this has something to to with the change of temperature or entropy along the turbine. Is this intuition correst?

2 - Why are the turbines built differently? Assuming it's not possible to extract a reasonable amount of energy with a one-stage turbine from steam, why is that so?

3 - Assuming an appreciable pressure drop without a big change of temperature along the turbine (maybe a pneumatically driven turbine), will I still need a multi vane design to extract most of the energy?

Bonus question - can the same reasoning be applied to pumps or compressors - a multi stage design is neccessary for decent efficiency if a significant temperature change is involved?

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Ultimately, energy addition/extraction is a function of $\dot{m} \Delta V$; gas/steam compressors/turbines have a high $\Delta V$ to work with, but are restricted in $\dot{m}$, whereas wind/water pumps/turbines have a low potential $\Delta V$, so have to increase $\dot{m}$.