Well, I am not familiar with a Z;duct persay, but I could imagine that this is analogous to a duct with two bends with no straight section in between them. THe reason this is different than a duct with two bends with a straight section inbetween is that the losses are much, ,much greater...
I have some experience with compressor stall on aircraft APU's and ECS's.
First thing you need to do is familiarize yourself with the compressor stall charateristics. Your manufacturer should be able to provide this data. I am used to a plot the has corrected air flow as the x-axis (W *...
