CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Mar 1, 2019 13:41:48 GMT -5
PS - It looks like you have another sleeve connecting the turbine outlet to a straight section of the tube where you have the thermocouples... If that isn't absolutely sealed, it could be drawing in air at a potential leak path and diluting everything down stream of it.
So the thermocouples and your color observation could both be correct.
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Post by madpatty on Mar 1, 2019 14:29:36 GMT -5
PS - It looks like you have another sleeve connecting the turbine outlet to a straight section of the tube where you have the thermocouples... If that isn't absolutely sealed, it could be drawing in air at a potential leak path and diluting everything down stream of it. So the thermocouples and your color observation could both be correct. Hi Tony, That's a really good point there. I don't think that sleeve can flow enough though because the ID of the sleeve and OD of both tubes(both turbine outlet and extension tube have similar ID and OD) is a sliding fit. Some air film can leak from the small annular gap but I don't think that's enough to cause like 100+ degrees of variation. I am also speculating a bit. Thanks.
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Post by madpatty on Mar 1, 2019 14:40:47 GMT -5
Hi Patty, That's a good question. It seems like you have quality thermocouples giving you good numbers if they are in agreement when tested over a burner. From that, I would tend to trust the thermocouples. But yes, if the turbine is glowing orange like shown in the picture... it looks hotter than 660C. I can't speculate on how it would work, but, could it be that both observations are correct at the same time? Maybe some other factor is at work between the turbine and thermocouples? A big change in pressure at the turbine's exducer?... Thermocouples being heat sinked at their coupler?... Dilution?... Air getting sucked into a leak path at the thermocouple's sleeve?... Without a nozzle to provide back pressure it may be possible. IDK. Just throwing out some speculation. BTW, that's an awesome build. Tony Tony. The turbine exducer is 76mm and the outlet tube ID is also ~76mm so no big step change that can change the pressure. Also thinking about the air being sucked into the tube, I don't think the static pressure inside the tube is below ambient pressure to suck in any air from outside. And also a point worth noting- If the color temperature of my turbine is correct (which seems like 800 degrees C) then according to my calculations the TIT should be about ~ 1000 degrees C and the static pressure ratio at turbine outlet should be about 1.26PR (about 3.77 psi above ambient). Speculating what is going on though. Thanks.
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Mar 1, 2019 18:09:54 GMT -5
Could the exducer throat area be choked so hard that the exiting gas is expanding and cooling just before the thermocouples?
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Post by enginewhisperer on Mar 1, 2019 18:37:42 GMT -5
Does the colour look the same in real life as it does in the video?
Video cameras often have different responses to IR which affect the red colours. In my experience it usually looks more pink in those cases though.
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 399
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Post by monty on Mar 1, 2019 20:20:52 GMT -5
IDK....dark to medium cherry red is around 660C. The gases are probably glowing into the turbine. The NGV and combustion liner could be glowing and altering the color you see too. Plus, at night everything looks brighter. The center of the turbine looks like night time medium cherry to me.
I'd tend to trust the thermocouples.
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Post by finiteparts on Mar 2, 2019 22:38:16 GMT -5
Hi Patty,
Good to see your new project!
One of the things that is tough about determining the rotor temperature from the emission of visible light from the glowing rotor is that due to the rotation, it is hard to see the exducer color because the rotor is glowing hotter further behind it. Think about looking at a spinning fan, you can see right through it. So even if the exducer is colder, the radiant emissions from the hotter portion behind it will be dominant, giving a false sense of hot exhaust temperature.
The next thing that I noticed is that your thermocouples are very large. When you take these temperatures, do sit at one condition for a while till the temperatures stabilize? With a thermocouple that thick, I suspect the response time is very slow. If that was the case and you didn't wait long enough for the system to stabilize, you can get some very confusing readings. I know that you have to sit on point for 30 seconds or more sometimes just to get the rotor speed to stabilize...thus turbine temps will be changing too.
Generally with thermocouples, you try to use the smallest ones possible. Smaller TCs have quick response, low conduction errors, lower radiation errors and low flow induced errors. The large size of your TC will cause the exhaust flow to locally accelerate as it goes by. This increases the disparity between the static temperature and total temperature...and if you further increase the recovery error of the TC due to the large size, you get a large disparity between the metal surface temperature of the TC and the bulk gas temperature...so now you are reading lower than expected due to the convective heat transfer from the gas to the metal surface of the TC...but there's more...
So now we have some amount of heat being transferred to the TC and if that amount of heat was confined to the tip of your TC, the recovery error would be all that you would have to worry about. But, unfortunately, metals conduct really well, so now you have another path for the heat to disperse away and not increase the temperature of the thermocouple junction creating further error. The primary means to reduce conduction error is to use as small of a TC as possible...so when I see your TC, my first thought is that you probably have a large conduction error.
I did a quick calculation based on some assumptions...the length of the TC is 18 inches, 2.5 inches are exposed to the hot exhaust gas, the exhaust gas total temperature is 1600 F, the ambient air temperature is 75 F, the diameter is 0.25 inches... I then assumed a 200 F average stem metal temperature on the cold side of the TC...using Zhukauska's relation for a circular cylinder immersed in cross flow, I estimate that the thermocouple is receiving roughly 0.15 kW. If the upper stem of the TC had this conducted up to it and then convected it out through free convection, the heat loss to the hot gas side of the TC would have a 52 F error. Now in reality, some of the heat will be conducted and convected through other areas of the exhaust system, so this is a very conservative estimate of the error.
There is a radiation error also...but I have covered this in previous post and my stance is that I think you should use radiation shielded TCs when using radial turbines since there is a straight line of sight from the TC to the hot inlet region of the turbine (what is termed "view factor").
Now, the only thing that I can suggest is possibly, insulate the exposed cold side of the thermocouples to reduce the conduction error...or better yet, buy smaller TCs.
Good luck!
Chris
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Post by madpatty on Mar 4, 2019 15:50:52 GMT -5
Could the exducer throat area be choked so hard that the exiting gas is expanding and cooling just before the thermocouples? Hi Tony, The turbine exducer chocking is very less likely as it is almost 54% greater area than the compressor inducer so it can flow substantially more than what compressor can provide and I have designed for the NGVs to choke. Thanks.
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Post by madpatty on Mar 4, 2019 15:52:04 GMT -5
Does the colour look the same in real life as it does in the video? Video cameras often have different responses to IR which affect the red colours. In my experience it usually looks more pink in those cases though. The color appeared "cooler" than what the camera footage shows but still not what i would have expected at those temperatures. Thanks.
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Post by madpatty on Mar 4, 2019 15:55:38 GMT -5
IDK....dark to medium cherry red is around 660C. The gases are probably glowing into the turbine. The NGV and combustion liner could be glowing and altering the color you see too. Plus, at night everything looks brighter. The center of the turbine looks like night time medium cherry to me. I'd tend to trust the thermocouples. Monty, I think that's probably what was happening. Thanks.
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Post by madpatty on Mar 4, 2019 16:03:41 GMT -5
Hi Patty, Good to see your new project! One of the things that is tough about determining the rotor temperature from the emission of visible light from the glowing rotor is that due to the rotation, it is hard to see the exducer color because the rotor is glowing hotter further behind it. Think about looking at a spinning fan, you can see right through it. So even if the exducer is colder, the radiant emissions from the hotter portion behind it will be dominant, giving a false sense of hot exhaust temperature. The next thing that I noticed is that your thermocouples are very large. When you take these temperatures, do sit at one condition for a while till the temperatures stabilize? With a thermocouple that thick, I suspect the response time is very slow. If that was the case and you didn't wait long enough for the system to stabilize, you can get some very confusing readings. I know that you have to sit on point for 30 seconds or more sometimes just to get the rotor speed to stabilize...thus turbine temps will be changing too. Generally with thermocouples, you try to use the smallest ones possible. Smaller TCs have quick response, low conduction errors, lower radiation errors and low flow induced errors. The large size of your TC will cause the exhaust flow to locally accelerate as it goes by. This increases the disparity between the static temperature and total temperature...and if you further increase the recovery error of the TC due to the large size, you get a large disparity between the metal surface temperature of the TC and the bulk gas temperature...so now you are reading lower than expected due to the convective heat transfer from the gas to the metal surface of the TC...but there's more... So now we have some amount of heat being transferred to the TC and if that amount of heat was confined to the tip of your TC, the recovery error would be all that you would have to worry about. But, unfortunately, metals conduct really well, so now you have another path for the heat to disperse away and not increase the temperature of the thermocouple junction creating further error. The primary means to reduce conduction error is to use as small of a TC as possible...so when I see your TC, my first thought is that you probably have a large conduction error. I did a quick calculation based on some assumptions...the length of the TC is 18 inches, 2.5 inches are exposed to the hot exhaust gas, the exhaust gas total temperature is 1600 F, the ambient air temperature is 75 F, the diameter is 0.25 inches... I then assumed a 200 F average stem metal temperature on the cold side of the TC...using Zhukauska's relation for a circular cylinder immersed in cross flow, I estimate that the thermocouple is receiving roughly 0.15 kW. If the upper stem of the TC had this conducted up to it and then convected it out through free convection, the heat loss to the hot gas side of the TC would have a 52 F error. Now in reality, some of the heat will be conducted and convected through other areas of the exhaust system, so this is a very conservative estimate of the error. There is a radiation error also...but I have covered this in previous post and my stance is that I think you should use radiation shielded TCs when using radial turbines since there is a straight line of sight from the TC to the hot inlet region of the turbine (what is termed "view factor"). Now, the only thing that I can suggest is possibly, insulate the exposed cold side of the thermocouples to reduce the conduction error...or better yet, buy smaller TCs. Good luck! Chris Hello Chris, I chose those thicker thermocouples back when I was experimenting with really crude turbine designs and when the turbines were often melting etc. etc. I destroyed many thermocouples also back then, by overheating them. I was more concerned about temperatures in the range of upper 800s (as colour suggested) than close to 700c(if taking into account the conduction losses etc.). Also the turbine was fairy stabilized when i took those readings. Yea I am thinking of getting some thinner and shorter thermocouples now. Thanks.
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akira
New Member
Joined: June 2021
Posts: 2
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Post by akira on Jun 30, 2021 10:08:42 GMT -5
Hi guys, Sorry for digging an old thread, but I think a I may face the same problem of Patty: big compressor/small wheel. It's a knock-off BorgWarner K24: comp inducer 48mm, comp exducer 67.5mm, turbine inducer 59, turbine exducer 49mm. In the post jetandturbineowners.proboards.com/post/12325/thread "racket" point out that a solution could be increase the exducer open area to free the excess of compressor flow, since the compressor is big for the wheel size. My question is: instead of machining the wheel, could the gap between the wheel and the NGV be increased ? Thanks
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Post by racket on Jun 30, 2021 16:53:44 GMT -5
Hi
Do you mean , increasing the radial clearance of the turb wheel exducer to gain flow area rather than clipping back the exducer axially ??
With a 48 mm comp inducer and a 49 mm turb exducer you shouldn't run into a lot of trouble as most modern comp wheels have pretty wide flow capacity
Cheers John
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akira
New Member
Joined: June 2021
Posts: 2
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Post by akira on Jun 30, 2021 17:12:00 GMT -5
Do you mean , increasing the radial clearance of the turb wheel exducer to gain flow area rather than clipping back the exducer axially ?? Hi,
Yes, exactly, and as side affect made the machining of the NGV a tad easier.
Regards,
Nelson
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Post by racket on Jun 30, 2021 20:51:03 GMT -5
Hi Nelson
Be careful about providing too much clearance as your temperatures will climb , a turb wheel of that size will normally have ~0.5mm diametric clearance , 2 mm diametric ( 51 mm exducer bore) would be enough to start with until you get some running temperatures and general parameters.
Cheers John
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