Peter
Q referring specifically to 'is inducing flow and heating it up with maximum afterburner'
I take this to refer to the induced secondary flow which flows over the red-hot afterburner casing and the intimation that this is significant in producing thrust. However due to the high air flow rate (1/3 intake entry flow) and it being a poor design of heat exchanger would the heat transfer really be significant (ie per lb/sec)? My original question basically revolved around the above interpretation of mine, ie the heating up of the induced flow took place before it got to the ejector,ie heating it up with max a/b meant? through heat transfer from the red-hot outside of the a/b duct....... or was the author referring to something else?
That's the way I interpreted it also, and why I initially suggested that lighting up the burner would induce more secondary flow because of the primary jet temperature increase. Now I'm not so sure
Using Peter Law's data and the SR 71 FM I get
Compressor inlet temp 427C (700K) this is also the cooling air entry temp.
Turbine exit temp 795C (1068K) this would be Tj with no afterburner
Tj Primary nozzle temp with A/B 1760C (2033K)
Secondary temp at nozzle A/B on 649C (922K)
Ps/Pj 0.29
I am 'guesstimating' the secondary temp at the nozzle to be around 550C (823K) when the afterburner is switched off.
The nozzle efficiency seems to peak at around a corrected secondary flow ratio of 0.08. Keeping that for the moment those temperatures give secondary (cooling)/primary mass flow ratios of 0.09 without A/B and 0.12 with A/B, which is consistent with my original explanation even though a lot lower than the 1/3 intake flow you mentioned originally. BUT, the primary nozzle area would increase when A/B comes on, which would reduce the secondary flow ratio from 0.08 to I don't know what (yet!). If the original quote is correct then this effect would be lower than the heating effect.
I see in
Quote:
NASA TM X- 67976
FACTORS WHICH INFLUENCE THE ANALYSIS AND DESIGN OF EJECTOR NOZZLES
page 3 'However, it is evident that heating the secondary
inlet flow would result in a decrease in nozzle
efficiency.'
I don't know if I have taken it out of context but understand it to say that any heat transfer into the nacelle secondary flow upstream of the ejector would not be good? theoretically at any rate, but perhaps not significant in practice.
It might be evident to the author, but it sure ain't to me
Part of the problem is that there are so many variables and he doesn't say what he is keeping constant with that remark. It certainly reads as if heating the secondary would be bad news, but everything else being unchanged that would increase the secondary flow ratio so the result would depend on which side of the optimum (mu = 0.08) you started.
Brian
Just a suggestion - when one reduces A/B flow on the SR71 the rpm and engine mass flow stay unchanged, as does the intake entry flow. Following up my argument in an earlier posting that would imply that although the actual engine contribution to thrust was dropping (from 13% towards zero) the intake, and to a lesser extent the nozzle, thrusts would be little changed. Could it be that at minimum afterburner the engine is, in fact, behaving like that apocryphal pump which connects the two?