# Are compressor maps for multistage compressors drawn for one blade row or the whole compressor?

Apologies if this is a stupid question. I am a little confused about compressor maps. Are compressor maps such as this one, drawn for one blade row only or the whole compressor?

(source: Wikimedia)

If it is for one blade row, can you predict the behavior of the next blade row from the compressor map of the previous row? Like, I am guessing that the blade rows of say the HP compressor have the same speed lines because they are on the same spool, so would they move up or down the speed line?

If those maps are drawn for the whole compressor, then does that mean that all blade rows have the same operating conditions (in my opinion, they should have different operating conditions)?

Any insight would be really appreciated! Thank you!

• @Bianfable Thank you for your insight! Yes, regarding operating conditions in a blade row, I meant N, P, T and also efficiency. I also meant would different blade rows be at the same point on a compressor? Like say you are running at low power settings, I believe that in this case the first stages can stall due to high incidence, and the rear stages can choke because of the annulus area decrease. But the surge line is for me at a different point on the map then choking conditions. So would different "dots" be used to represent that? Sep 4, 2019 at 18:20
• The compressor map is used to account for the performance of a whole compressor by taking various parameters into account- mass flow parameter, shaft speed, pressure ratio, efficiency & Reynolds no. Pressure ratio parameter is essentially the ratio of pressure at the exit and entrance of the compressor, Mass flow parameter and Shaft speed parameter take the Temperature at the entrance into account. So as the characteristic map is based on the entrance & exit parameters of the compressor, the characteristic map is representational of performance of compressor as a whole. Sep 4, 2019 at 18:22
• For low power settings, I would say lower spool speed and mass flow as well as lower pressure ratio i.e. at the leftmost side of the characteristic map. As you mentioned the rear stage is choke as no more mass flow can pass through it, the initial stages will stall. If you try to locate this on the map using numerical values, you will reach a surge line, if I am not wrong. I believe you are trying to relate this with the stage of a compressor, but that isnt the case for the characteristic map which is scaled to starting & end conditions. Also, operation is governed by choking of rear stages. Sep 4, 2019 at 18:42

The image you are referencing is definitely an entire compressor. You can tell because the pressure ratio axis goes up to 17:1. You won't get 17:1 out of a single blade row, ever. One-tenth of that would be more likely for a single stage. i.e. this could be a 10 stage compressor with each stage contributing a 1.3 - 1.4:1 pressure ratio.

I've only ever seen compressor maps like this drawn for entire compressors. Not to say you couldn't do a single stage, but I've not seen it done.

Are compressor maps such as this one, drawn for one blade row only or the whole compressor?

The shown compressor map is indeed for an entire compressor as can be seen from the pressure ratio.

If it is for one blade row, can you predict the behavior of the next blade row from the compressor map of the previous row?

No you cannot predict the behavior of the next row based on a single stage map, single stage maps do exist though. In theory you can stack these maps to create a complete compressor model, but various loss models prevent that you get an accurate representation. But, since these maps are very hard to get, performance engineers are happy to get these data even if it is a stage map. Also, when modelling gas turbines you may want to split a compressor to model the flows or torques at a certain stage more accurately. We've done this in the past to calculate the individual stage torque contribution of a 3 stage fan of a low bypass turbofan fighter engine.

I am guessing that the blade rows of say the HP compressor have the same speed lines because they are on the same spool, so would they move up or down the speed line?

The smaller the blades the more important losses become, e.g. a constant tip gap in the compressor will have a smaller effect on large compressor blades than it will have on smaller blades. This means that efficiency and pressure rise are lower the further down the compressor stages, on the speed line the operating point would go down the speed line.

If those maps are drawn for the whole compressor, then does that mean that all blade rows have the same operating conditions (in my opinion, they should have different operating conditions)?

As the air is compressed, the inlet conditions for each stage are different. The axial speed is the same though.