John,
Before I begin, let me underline how such a GREAT guy you are ... No, seriously, .... I have wintness you help shitload of peoples here, young and old, newbies and "Less nembies" (Not to say experts) ... You're such a great person and life has a special place for you buddy ... It will come ..
Okey ... Now back to our chickens, and to answer your question about why there are 2 ejector stages, the answer is "Temperature reduction" ... That's all ... Well dynamically speaking. There is another factor that calls for that second ejector, and it's the "Flexible Hose" that attach to this second ejector tube.
Okeyy ... Here's a bit of additional info for you. This system is a "Heat Generation" unit that uses a 13 foot long "High Temperature Flexible Hose" to bring that heat where it's needed. Now, not only this hose is limited to 600°F, but it also comes in standard diameters such as 2", 3", 4", 5" ... ect ect diameters. Since that turbine's exhaust pipe is already 3.3" ID, using a 4" hose connected to a 4" ejector tube only provide us with 1 ejection stage and the temperature was found to get to high. Using a 5" hose connected to a "Single" 5" ejector tube to again only use 1 ejection stage, then, the diameter diference between the turbine tail pipe (3.3") and the 5" ejector tube is so great, that we would have to extend that "Single" ejector tube longer so exhaust gases can expand to create "Pumping", but again, there was no waranty the "One Stage Ejection" would provide enough hot gases dillution to drop that temperature below the 600°F mark, not mentioning the overall system would be longer then necessary.
So, this is why we went from 3.3" tail pipe to a 4" first ejector tube to a 5" second ejector tube to a 5" hose, so we could keep that hose as small as possible while dropping the temperature to that 600°F mark, and why we went with two ejection stages for "Overall" considerations (i.e. : Hose temperature limitation, system overall length, Flex Hose Standard size, compactness ... Bla bla bla).
Now, your message above kinda made use realize that wanting to stick to "Standard Size" even with the first ejection stage tube although this one IS 100% "Fabricated", was a mistake. In fact, although I always knew that when using ejectors, "More you get away from the source, smaller the Gaps must be", I obviously "Drop" (Forgot) this last one and it came back BITTING me hard ... ((*(%/?%$%%$(&?%$%$/? ... stupid me).
Starting from the turbine's tail pipe which is 3.3", I should have gone maybe 4.5" for the first stage ejector and 5" for the second. And the worst here, is that I have a CFD Analysist software that would have told me better if I would have gone through the effort of using it ... Like I JUST DID (See the attached Pictures) ... Lollllllllll
I have to admit though, that one of the reason we were never tempted to CFD the rear end of our system, is the fact that we can not have data from that flex hose such as resistance, or roughness or back pressure build-up data ... Nothing ... NADA ... So we had no choice of venturing on a "Trial & Error" design using that hose.
Anywayyyyy ... The CFD is done now, thanks for your inputs, and the system is beeig corrected as I'm writing you this message, including dropping the elbow and overall length reduction.
By the way, I have a new SUPER computer here that can run complex thermodynamic analysis that use to take 50 hours +, now in 20 minutes. So, if you ever neeed complex CFD for your Babys ... Just say so and it's free for you Buddy ...
Cheers,