Multiple spool compression efficiency gain

[Levi P]

I am a student an I am new to this forum but I am very interested in turbomachinery. This thread has more to do with the aerospace side of things but the same principles apply. With lubrication and other complexities aside, would a fully independent 5 stage axial compressor (by independent I mean each compressor disc has its own corresponding power source Eg. turbine) designed to drive each stage at an optimal frequency. The reason I ask this is because the Rolls royce Trent 1000 aircraft engine has 2 compression spools, a IP (intermediate pressure) and HP (high pressure) spool for IP and HP compressors respectively. The IP compressor has 8 stages and the HP compressor has 6 stages and they are contra-rotating which improves efficiency. So if you could have each stage completely independent from all the others and each stage contra-rotating compressor stages, would you even need stator vanes and would there be an efficiency increase over a single spool compressor with the same number of stages all variables held constant?

 

[zdas04]

Levi,
Are you quite certain that contra-rotation improves efficiency? It does balance forces on an airframe which is always useful, but I'm having trouble finding any term in the efficiency calculation that would be impacted by rotation angle.

Putting each stage in its own frame would be the opposite of improving thermodynamic efficiency of an axial compressor since each stand-alone machine has its own parasitic loads (oil pumps, etc.) that do not increase in a linear manner when you increase load.

I do not know if this post will be here for long since site management does not allow student posts. I responded because your question felt more like something you wanted to know rather than homework. I'm sure someone will see it differently and ask management to delete the thread. We can talk as long as the thread lasts.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[rotw]

I found this
Link

 

[georgeverghese]

Theoretically, yes, but you'd be increasing mechanical complexity and CAPEX considerably-there are many other routes taken by turbine manufacturers to increase themodynamic eff in these turbines at a lower incremental CAPEX - use of higher metallurgy or composite materials to permit higher temperatures on the upstream set of HP power turbine blades; blade cooling; reheat; intercooling on the axial compression side etc. In some of these larger machines ( >500MW or so by from ABB, Siemens etc), such approaches result in overall net thermodynamic ISO eff approaching 40-45% in a turbine without cogen, which was unheard of not many years ago.