combustor info

[redneck]

Hey guys, hadnt been here in a while, but I am looking for some way to find out if there should be some optimal pressure ratio between the area right before the combustor and the combustor. what i want to figure out is, how much should the air slow down, or pressure increase, or what is the area difference between the combustor area and the compressor area? is there any major difference between a ram jet combustor section and a turbine cumbustor? thanks, any help would be great.

[racketmotorman]

Hi Redneck

There should be around a 5% pressure drop between outside and inside the flametube .

Once the air is slowed to ~300 ft/sec in the comp diffuser theres not much to be gained by slowing further as theres very little dynamic component of total pressure to be converted into static pressure .

The crossectional area of the flametube needs to be ~3 times the comp wheels inducer area to slow down the air sufficiently for stable combustion .

both a ramjet and gasturbine combustion sections required air that has been slowed down converting dynamic pressure into static pressure when combustion takes place .

Cheers
John

[redneck]

if there is a pressure drop from the outside to the inside of the flame tube, that would then imply that the air speed increases in the flame tube, but i thought that the flame tube was designed to slow the air, build up pressure and then eject it a a higher speed and lower pressure than when the air intered the chamber? So did you mean that the air pressure should be 5 % higher in the flame tube than when it enters?

The crossectional area of the flametube needs to be ~3 times the comp wheels inducer area to slow down the air sufficiently for stable combustion .



where you mention the compressor wheels inducer, that is the area where the air enters the compressor I am guessing? also, you mention a speed of 300ft/sec in the comp diffuser, is that the same as the exducer? once the air has left the exducer, does the increase in area in the combustion chamber also slow the air any further? should the cross sectional area or the combustion chamber match that of the exducer for the compressor section?

thanks for your response, and forgive my questions, I am gearing up to build a turbine ( or a grenade ) and I am trying to understand a few things before i do. I did manage to build a ramjet in " the great ram jet challenge " a while back, and had some success with it, and am now trying to apply what i learned about that to GT's. I am wondering if it is as simple as throwing a properly deigned compressor in front and a turbine in the rear to make a GT.

Thanks again, and if there are any posts or FAQs you can think of that could help me out lemme know.... thanks......

[racketmotorman]

Hi Redneck

The 5% pressure drop across the flametube wall is a loss due to the force required to get the air thru the holes .

May I suggest you get a copy of Thomas Kamps book
Model Jet Engines published by Traplet Publications , .............it has all the answers:-))

Can be purchased online

http://www.traplet.com/

https://shop.traplet.com/search.aspx?q=thomas%20kamp&h=

Cheers
John