Air flow off the Turbo compressor map

[Max965]

What happens to the air flow when its not on a turbo compressor map in the choke area? i.e too much air flow when you flow it from low rpms?
Looking at the islands it looks like the pressure ratio just drops off almost vertically near the edges. Is this what happens and the turbo cannot compress the air at all and just passes it through?

Thanks

 

[Lou Scannon]

The key word in your post is choke. Some part of the compressor, perhaps the inlet, is choking the flow. You know what happens when flow is choked.
You would never deliberately want to run a turbocompressor in choke-- the net effect is to create a great deal of exhaust backpressure to heat up the inlet air without any worthwhile density increase. Also, typically the compressor will be approaching or exceeding its design speed, when in choke.

 

[gtsim]

As you are aware, compressor maps are normally plotted with pressure ratio VS non-dimensional air or mass flow (W(RT/Gamma)^0.5))/AP) for a series of non-dimensional speeds ND/((GammaRT)^0.5).

W = mass flow rate
R = gas constant
Gamma = cp/cv (ratio of specific heats)
T = absolute temperature
P = pressure
A = Reference flow area
D = reference diameter

These non-dimensional terms, flows and speed are indeed Mach numbers. At any given speed when the compressor inlet approaches chokes the Mach number approaches unity (note the compressor inlet is not choked and when the inlet chokes all the speed lines will collapse to a single line). Therefore non-dimensional flow is nearly constant and this is why the speed line becomes vertical in the “choked” region of the compressor map. For any given value of P and T, W will be constant. The compressor speed is influenced by the shape of the compressor characteristic and therefore choked condition will influence the compressor speed. However, system characteristics (e.g. losses and restrictions upstream and down stream of the compressor) will also influence the compressor speed. Increasing the compressor speed when operating in the “choked” region will increase the non-dimensional flow and therefore mass flow rate W.

Operating in the choked region of the compressor map is undesirable because we encounter shock stall and this can give rise to vibration and should be avoided. However, gas turbine compressors (axial types) are often designed to run near choke conditions all the way up to surge. This is done to achieve the maximum mass flow rate thus maximising thrust or power output.

Merry Christmas and Happy New Year to Everyone.

Regards,
gtsim

 

[Max965]

Many thanks both and for the very detailed response!

Just to claify in laymans terms if there is 100CFM being compressed to 2:1 pressure ratio (@70% in steady state) but then the inlet flow doubles instantly to 200CFM and the compressor starts to slow down and eventually fall off the map.

What happens to air through the turbo? Is it just uncompressed or does the air start to compresses it self before the inlet if for example this air could not flow back and was being forced in.

 

[gtsim]

The increase in flow through the compressor will result in the expander/turbine imposing an increased back pressure on the compressor. This results in an increase in compressor pressure ratio and speed. Imagine this running line on the compressor map, which is approximately parallel to the surge line but in the stable part of the compressor map. Thus the operating point will not drift towards the choked region on the map but increases in pressure ratio with flow and speed. In other words as the flow increases the compressor ratio cannot be constant (for a fixed geometry compressor turbine arrangement). The increased compressor power demand has to be provided by the expander/turbine.

I trust this help.

Regards,
gtsim