augmentor efficiency

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redneck
Posts: 89
Joined: Tue Dec 26, 2006 10:32 am

augmentor efficiency

Post by redneck » Fri Oct 19, 2007 3:19 am

hey yall, its been awhile i know but i have a question. i need to know how to measure the cfms of a jets exhaust, and how to figure out how many cfms and augmentor can suck into a jet stream flowing through it with a given cfm intering through it and with given augmentor specs. can an augmentor suck in enough air to support a secondary combustion with no oxygen input at all from the exhaust of the primary jet. im about to go on a 7 month cruise on the harry s truman to the gulf. i need something to keep my mind occupied, so when i get back, i hope to have a jet built, and i am trying to make a secondary ram combustor off of a pulsejet exhaust stream, this is not a pulse jet afterburner however. it will be a secondary combustor that surrounds a pulsejet, the pulsejet gaining speed with help of the augmentor, then finally the ramjet ( formerly the augmentor) taking over opperation.

please tell me of a program or formula to find the effeciency of an augmentor to find out how much thrust it can produce. foward airspeed will not be a factor in my figuring right now, as i want it to be able to run at zero airspeed. any help will be appreciated.
hooowee that was loud! do it again!

redneck
Posts: 89
Joined: Tue Dec 26, 2006 10:32 am

Post by redneck » Wed Oct 31, 2007 6:53 am

well, either i caint explain what i want or nobody knows. so i will rephrase the question. is there a way to figure out how much air will be pulled into a duct from the bernolli/venturi principal by an airsource, say exhaust? maybe larry can suggest someway? i was thinking of actually building a chamber, pushing a known amount of air through it then in the back side, hook up a sensor from a car, to tell me the mass air flow coming out, then subracting the air input from the output to tell me how much air an augmentor actually draws in. but im kinda hoping somebody has already done this, or knows where i can find the info. thanks again yall....
hooowee that was loud! do it again!

larry cottrill
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Post by larry cottrill » Wed Oct 31, 2007 12:05 pm

redneck -

All I know is some basic general principles for eductors - but, that might be useful in terms of further experimentation, so here goes. The most important thing to play with, in terms of mass entrainment, is the "gap" between the driving stream exit and the throat of the eductor shell. As you move the driving jet (exhaust nozzle) forward, opening up the gap, the exit velocity from the eductor decreases, but the entrained massflow increases - up to a point. The changes are apparently quite non-linear, so there is some spacing at which the combination of massflow & velocity peaks for the greatest net thrust. According to the designer I talked to, in real life they don't generally use equations for design, but rather, carefully compiled tables of empirical data (which are closely guarded secrets of each design shop). The behaviour will be quite different for very narrow driving streams with large throats vs "tight" designs where the driving nozzle is almost as big as the throat ID. Also, a design that's optimum for pulsing operation will not be the best for a steady driving stream, and vice versa.

Exremely helpful, I'm sure - ha. Good luck!

L Cottrill

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