Gauge placements?

[cebaero]

I am looking to weld a fitting to my combustion chamber to attach a pressure gauge to. Is there any specific spot or does it not matter? I was just thinking in the middle of the outer wall of my combustion chamber.

I am also looking to weld a compression fitting to attach a thermocouple to. My flame tube is attached to the turbine via a 6 to 3 inch reducer. I was thinking about putting this thermocouple in this reducer, is that okay? My TC is Inconel so I shouldn't be worried about it melting off and flying into the turbine, right? 

Also, this is farther down the line but I am beginning to look a bit into putting an afterburner on once my engine is complete. I was wondering if anyone can share exhaust temps they have experienced? I have a 62mm turbo running off of kerosene, and I am wondering if I can get away with the afterburner autoigniting so i dont have to attach another sparkplug. Any advice or insight is appreciated greatly as always. 

[richardm]

For one thing do not install your thermocouple probe upstream from the turbin wheel. If it breaks ( from possible heat damage and erosion) it will it the turbine and ruin your engine Also exposing people and goods to hot and fast shrapnels. Put it in the exhaust flange just past the turbine.and derate your turbine max temp by 100 -150 degrees to stay on the safe side .

[racket]

P2 pressure pickup in a quiet corner of the combustor where the air speeds are lowest .

Afterburners need an ignition source , but you can run a hot streak system rather than a spark .

Keep your turbine OUTLET temperatures below 650C - 1200F , for either dry or wet

[cebaero]

Oct 22, 2024 15:32:12 GMT -5 racket said:

P2 pressure pickup in a quiet corner of the combustor where the air speeds are lowest .

Afterburners need an ignition source , but you can run a hot streak system rather than a spark .

Keep your turbine OUTLET temperatures below 650C - 1200F , for either dry or wet

What is a hot streak system? I cant find anything clearly on the internet about it

[cebaero]

Oct 22, 2024 14:40:52 GMT -5 richardm said:

For one thing do not install your thermocouple probe upstream from the turbin wheel. If it breaks ( from possible heat damage and erosion) it will it the turbine and ruin your engine Also exposing people and goods to hot and fast shrapnels. Put it in the exhaust flange just past the turbine.and derate your turbine max temp by 100 -150 degrees to stay on the safe side .

I was planning on just having it but up against the end of the compression fitting, so it will still be in the engine but wont be protruding enough to break off (maybe a mm or two). Plus being inconel and rated for 2600F i couldnt see anything happening to it, right?

[racket]

Hot streak is a momentary large squirt of fuel into the flametube that is still burning as it enters the A/B .

Put your thermocouple DOWNSTREAM of the turbine wheel as Richard advised ...................why do you think the full sized aero engines put them there rather than upstream , but if you know better, then fit it where you want to.

[cebaero]

Oct 22, 2024 16:40:20 GMT -5 racket said:

Hot streak is a momentary large squirt of fuel into the flametube that is still burning as it enters the A/B .

Put your thermocouple DOWNSTREAM of the turbine wheel as Richard advised ...................why do you think the full sized aero engines put them there rather than upstream , but if you know better, then fit it where you want to.

Why the hell are you giving me attitude for asking a question? Isn't that what this forum is for? I didn't say he was wrong and didn't say I was going to do it, I gave him more information (the fact that the probe would not protrude in the engine) and asked if that would still be bad. God forbid you want to discuss problems and learn instead of blindly following what others say.

[richardm]

Contrary to your belief a thermocouple is not made of inconel. It is the junction of two different alloy that generates a small voltage in response to a temperature change. Most common is type "K" and it is made of "Alumel" and "Chromel" alloys It can be shielded in a protective tube mostly stainless . All wires and connectors from the probe to the reading device must be made of the same alloys Do not use any other metal ( copper steel aluminum etc) Doing so will induce an error .
You can learn more about thermocouple doing your own search on the net .
And yes they do fail. I got 28 000 hours of flying time mostly on gas turbine / jet engines and despite those thermocouples are probably the best you can get they occasionally fail. Thats why they are placed behind he turbines wheel.
They are quite heat resistant but they get eroded and get much of a beating from hot gas flowing at hundred if not thousand feet per second around them.

[cebaero]

Oct 22, 2024 18:01:40 GMT -5 richardm said:

Contrary to your belief a thermocouple is not made of inconel. It is the junction of two different alloy that generates a small voltage in response to a temperature change. Most common is type "K" and it is made of "Alumel" and "Chromel" alloys It can be shielded in a protective tube mostly stainless . All wires and connectors from the probe to the reading device must be made of the same alloys Do not use any other metal ( copper steel aluminum etc) Doing so will induce an error .
You can learn more about thermocouple doing your own search on the net .
And yes they do fail. I got 28 000 hours of flying time mostly on gas turbine / jet engines and despite those thermocouples are probably the best you can get they occasionally fail. Thats why they are placed behind he turbines wheel.
They are quite heat resistant but they get eroded and get much of a beating from hot gas flowing at hundred if not thousand feet per second around them.

If you look up inconel thermocouples you will see many available for sale. The “protective tube” you stated is actually called the sheath, which is what is sometimes made of inconel. Obviously the wires inside are not but the sheath of mine is all inconel. Mine is specifically rated for 2300F. I understand that the hot gases would erode the probe, but if it’s completed encased in the compression fitting, I just can’t see anything happening to it unless it sucks it out of the compression fitting and melts it off.

[racket]

Its not a case of attitude , its a case of saving you a lot of heartache when you find your turbine wheel damaged ............it ain't a nice feeling :-(

Having it not protruding will mean you'll get incorrect readings as the boundary layer along the duct wall that you'll be reading won't be at the same temperature as the main flow , theres a temperature gradient , have the end of the thermocouple protruding ~15-20mm from your JETPIPE wall and you'll get a more meaningful result .

The combustion within the flametube is very intense with lotsa vibration that can destroy things especially when they're already glowing orange , I've destroyed a number of thermocouples over the years , they ain't indestructible even when placed in the jetpipe .

So take Richards advice and mount it downstream of the turbine

Cheers
John

[richardm]

If your thermocouple probe is encased in the compression fitting your reading will be that of the compression fitting temperature. Since that fitting is attached to ab are metallic mass your readings will lag quite a lot Youi want get instantaneous readings. So the probe must be surrounded by the gases stream, nothing else
Also some thermocouple have an expose junction for a faster response
exmple xhttps://evosensors.com/en-ca/collections/exposed/products/k1x-s304-125-ex-6-mpcx