Turbofan

In the Pegasus turbofan engine used in the Harrier, the fan and core engine exhausts are two totally separate flows.

There is no inherent reason for mixing the flows in a turbofan engine, let alone a requirement to mix the flows in a duct burning high bypass turbofan engine.

 

Now the fool is putting up slides that show EXACTLY what I've been saying and thinks it contradicts what I said, this guy is a piece of work.....

The third slide show the classic relationship between bypass ratio and fan pressure ratio. Big bypass ratios equal lower fan pressure ratios. The fool hasn't put 2+2 together and understood that you need to go down in bypass ratio to get enough pressure ratio to run an afterburner (getting enough PR to choke the exit nozzle and have a C/D nozzle).. What a tool..

 

******

Hey, genius, check out the post before yours. I just explained why there is there no inherent reason to assume the flows have to be mixed.

 

He does appear to be quite handy with a shovel.

Just sayin'...

 

Compared to Yellowjacket, when it comes to shoveling I'm an amateur.

 

Low Bypass Turbojets

(Article published in Bonneville Racing News, February 1997)

Attached is an article by me describing my proposal for how to modify the jet engines in Thrust SSC to improve steering. My proposed modifications took advantage of the fact Thrust SSC used turbofans rather than turbojets.

As it turned out, the Thrust SSC team needed every ounce of thrust and every cent they could muster to reach their goal of a supersonic land speed record. However, the erratic yaw behavior the car later demonstrated did also vindicate the benefits that thrust-vectored steering enhancements would have provided.

The loss in thrust from the reduction in the flow of cold air going into the afterburner could have been recovered with duct burning.

 

God, here we go again.. In that post I was referring to the bold statement in your post concering duct burning turbofans. You clearly aren't taking the time to read and understand anything I'm saying.

Further along in that post I explained why higher bypass engine CAN'T mix the core and fan streams. So as you go to higher bypass ratios the two streams can't be mixed. I already explained that.

What you haven't grasped it that increasing BPR and DECREASING FPR are tied together, even if the streams aren' t mixed. There is only so much energy in the core. If you try to take too much work out of the core stream, you won't generate any core thrust, you will actually have exhaust drag....

What you haven't understood and the point you are totally missing in all of this is that you need pressure to run an afterburner. If you don't have a 2.2 PR you can't use a converging/diverging nozzle and cant generate big thrust from your A/B. This is what limits the bypass ratio of augmented engines. You can, as they are trying to do in ADVENT, run a lower FPR when they aren't in A/B, but then when they go to A/B they alter the cycle TO GET THE PR NEEDED FOR THE A/B.

Once you set the FPR to 2.2 or greater, that effectively limits the bypass ratio to a number a good bit less than any high bypass ratio engine. So now can we please put to bed this absurd idea that you have that you can put an A/B on a high bypass engine.

 

WOW! Truth stranger than fiction, I love it.

<personal attack / insult removed>

Surprise, surprise!

 

The plain fact is the pressure ratio of 1.7 in a high bypass turbofan is PLENTY to make duct burning work, since THERE IS NO REASON whatsoever in a duct burning turbofan for the fan exhaust and core engine exhaust to share the same exhaust duct.

You can either have a plenum chamber and separate nozzles as is done with the Pegasus engine in the Harrier, or you can isolate the core engine exhaust from the fan exhaust by simply having a single large diameter duct for the fan exhaust surrounding a smaller diameter duct for the core engine exhaust.

Why don't you write Dr. Mattingly, Dr. Heiser, and Dr. Pratt to let them know they obviously didn't know what they were talking about?

 

<profanity removed, again>

 

FR,

It appears you've responded twice since my last post.

The beautiful thing is, I can't see your response only that you have posted.

It's what happens when you place someone on your "ignore" list.

What a great feature, thank you forum admin!

 

Well, I guess if you don't want the flow at the exit of the exhaust to choke, then it isn't necessary, but you need 1.875 PR to choke a nozzle in dry air. (look it up), and if you aren't choked, you can't use a C/D nozzle, so you can't get much thrust out of it. In real terms pressure ratios of greater than 2.2 are used for afterburners. So, could you burn in the duct, the answer is yes, but the reality is you won' t get much for all the fuel you dump in there if you don't choke it and follow it up with a nozzle. In practical terms you really can't or no real engineer would try to do something that dumb.

Just keep going, you're slowly hanging yourself.

Don't bother to reply, I've activated the Ignore function too. I'm done.

 

What about the great synergy created here where ADULTS are sharing, teaching, discussing and helping each other, unlike the all-too common drivel found on the majority of Internet forums?



I wish I had Moderator powers.

 

Nozzles

Jet dragsters don't use convergent/divergent nozzles and work fine. In fact, they don't even run variable geometry nozzles.

http://www.youtube.com/watch?v=GHOgZlafcQs

http://www.youtube.com/watch?v=vQeonaZ-9yI

http://www.youtube.com/watch?v=mgY6GbWhHCI

 

The thread has been cleaned twice already of previous insults/personal attacks/profanity. I'm sorry that it didn't work out, but hopefully now the thread can return to civil discussion without further profanity/insults. The forum should be a place where everyone can enjoy coming to share knowledge, and the jabs, insults, and profanity need to be taken to some other site. Thanks to everyone who remained civil and tried to maintain an intelligent discussion despite the repeated problematic posts.