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?