fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 21, 2016 15:29:42 GMT -5
Hello fellow builders. I´ve got an issue with my build and I suspect either flametube or fuel nozzle. I had the engine running with nice results at maybe 70-80% thrust. This was with a flame tube heavily modified from what the usual specs are 30/20/50 % of inducer area. I made some change in hos fuel was distributed, I went from just a plain hoze connector to real piping. When I made this change I removed a small shield I put there from the start to slow incoming fuel to more easily have it combusted in a controlled way. It´s built out of a TB03 turbo with 37.7mm inducer, 76mm diameter, 24cm long flame tube. I´ve made several changes in order to get it running smooth again with no luck. It ignites and spools fine with decent exhaust gas temperatures (except for standard design flame tubes 30/20/50) where it runs too hot. What happens is a bit unclear, but a muffled rythmic sound comes almost as soon I leave idle speed. Sound like the air is bouncing or flame is unstable, more so when I cut down on fuel wich points me in direction of unstable flame. What is there to do about this? I´ve added pictures of the build itself. And linked to a video when it ran ok before I started to make changes. www.youtube.com/watch?v=9C5hZv0uIe4Any ideas of what end I should start with? I can´t seem to make it run ok beyond idle speed.
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Post by smithy1 on Apr 21, 2016 19:53:10 GMT -5
Nice setup you have there, it seems to run pretty well in the video, good temps, pressures etc...
Whereabouts are you..?
As you say, fluttering of the combustion is usually from poor flame front/combustion....we may need to do some more diagnosis....are you able to show a video of the engine running with your latest mods??
Cheers, Smithy.
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Post by finiteparts on Apr 21, 2016 22:33:52 GMT -5
Hi fidel,
Thanks for sharing your project, it is a nice looking little engine.
It sounds to me like you have pushed the compressor over into a slight surge condition...we could tell you better if you had a video of the modified condition so we could hear the sound. But, just that fact that you can hear it tells me that surge is the most likely cause. These types of combustors do not normally make enough combustion instability to actually be audibly noticeable. If you had the combustor really lean or really rich, the flame front does become unstable, but that has it's own set of problems. If it was really rich, you would likely see burning into or after the turbine. If it was really lean, you would probably just blow out.
So let's just say that you are entering compressor surge...what would be the reasons for this?
1. You could be getting a higher combustor temperature (due to late combustion, overfueling, etc) that would cause increased backpressure (due to the lower density) and reduce the engines swallowing capacity. With a reduced mass flow, the compressor operating point shifts over towards the left side of the map (surge line). The physical reason that the left side of the compressor map shows a surge line is because the reduced mass flow causes the relative inlet flow to meet the inducer blade at a increasingly negative incidence angle. This causes the flow to diffuse as it goes through the inducer section of the impeller and become increasingly unstable due to flow separation on the blade suction side.
2. A second reason could be that you have changed the flow area through the combustor? Did you reduce the hole sizes on the combustor? If the combustor liner holes get too small, you will get a backpressure on the compressure that would cause the same surge tendency.
So what I would suggest first is to change your fuel nozzle. You don't have any real flame stabilization devices in your combustor design, you are relying on flow blockage in small local pockets of turbulence formed at the liner inner surface, between the primary holes to anchor the flame up near the top of the combustor. But, with your hole pattern in your fuel nozzle, you're shooting the fuel down towards the secondary zone of the liner. Make a new injector...drill the holes through the flats of the bolt head so the fuel injects radially outward. This will keep the fuel up toward the primary zone where you are trying to do most of the burning.
Remember, the primary zone is were you want the bulk of the combustion to occur, the secondary zone helps burn out some of the slower reaction stuff and finally, the dilution holes should just be cooling the flow of completed combustion gases to the allowable turbine entry temperature prior to it entering the turbine...there should be no reaction still occurring in the dilution zone. If the reaction is still occurring in the dilution zone, it is likely to get "frozen", which means incomplete combustion...which means lower combustion efficiency. The liners you are showing don't appear to have any heat staining in the primary zone...this should be a very hot zone and should have some noticeable heating.
If that doesn't fix it, then drill a few more dilution holes. Since you have already run with larger amount of liner hole area successfully and everything else downstream has stayed the same, then if you did change the liner hole area, you may be undersized.
Good luck!
Chris
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Post by stoffe64 on Apr 22, 2016 5:14:51 GMT -5
Hello Fredrik What kind of oil pressure do you get from that biltema pump? Very Nice machine there, keep up The good work! Cheers /stephan
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 22, 2016 11:19:40 GMT -5
Hello Fredrik What kind of oil pressure do you get from that biltema pump? Very Nice machine there, keep up The good work! Cheers /stephan Hello. I built it with a bypass system so I manually can control the pressure. When I reach 5-6 bar the pump is running a bit rough but this pressures are beyond what is needed. Usually I try to keep it at 3-3.5 bars.
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 22, 2016 11:20:47 GMT -5
Nice setup you have there, it seems to run pretty well in the video, good temps, pressures etc... Whereabouts are you..? As you say, fluttering of the combustion is usually from poor flame front/combustion....we may need to do some more diagnosis....are you able to show a video of the engine running with your latest mods?? Cheers, Smithy. My location is Sweden / Gothenburg. I'll get a video within the next few days when I run it with the sound.
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 22, 2016 11:25:02 GMT -5
Hi fidel, Thanks for sharing your project, it is a nice looking little engine. It sounds to me like you have pushed the compressor over into a slight surge condition...we could tell you better if you had a video of the modified condition so we could hear the sound. But, just that fact that you can hear it tells me that surge is the most likely cause. These types of combustors do not normally make enough combustion instability to actually be audibly noticeable. If you had the combustor really lean or really rich, the flame front does become unstable, but that has it's own set of problems. If it was really rich, you would likely see burning into or after the turbine. If it was really lean, you would probably just blow out. So let's just say that you are entering compressor surge...what would be the reasons for this? 1. You could be getting a higher combustor temperature (due to late combustion, overfueling, etc) that would cause increased backpressure (due to the lower density) and reduce the engines swallowing capacity. With a reduced mass flow, the compressor operating point shifts over towards the left side of the map (surge line). The physical reason that the left side of the compressor map shows a surge line is because the reduced mass flow causes the relative inlet flow to meet the inducer blade at a increasingly negative incidence angle. This causes the flow to diffuse as it goes through the inducer section of the impeller and become increasingly unstable due to flow separation on the blade suction side. 2. A second reason could be that you have changed the flow area through the combustor? Did you reduce the hole sizes on the combustor? If the combustor liner holes get too small, you will get a backpressure on the compressure that would cause the same surge tendency. So what I would suggest first is to change your fuel nozzle. You don't have any real flame stabilization devices in your combustor design, you are relying on flow blockage in small local pockets of turbulence formed at the liner inner surface, between the primary holes to anchor the flame up near the top of the combustor. But, with your hole pattern in your fuel nozzle, you're shooting the fuel down towards the secondary zone of the liner. Make a new injector...drill the holes through the flats of the bolt head so the fuel injects radially outward. This will keep the fuel up toward the primary zone where you are trying to do most of the burning. Remember, the primary zone is were you want the bulk of the combustion to occur, the secondary zone helps burn out some of the slower reaction stuff and finally, the dilution holes should just be cooling the flow of completed combustion gases to the allowable turbine entry temperature prior to it entering the turbine...there should be no reaction still occurring in the dilution zone. If the reaction is still occurring in the dilution zone, it is likely to get "frozen", which means incomplete combustion...which means lower combustion efficiency. The liners you are showing don't appear to have any heat staining in the primary zone...this should be a very hot zone and should have some noticeable heating. If that doesn't fix it, then drill a few more dilution holes. Since you have already run with larger amount of liner hole area successfully and everything else downstream has stayed the same, then if you did change the liner hole area, you may be undersized. Good luck! Chris Thank you for your reply! The only thing I changed from the video is the nozzle. This brings me to try another nozzle which spreads fuel to the sides. As debugging I tried a few other flame tubes with minimal results. I'll get a video of how it sounds and try the nozzle and get back to you. Thanks again for your thorough feedback! /Fredrik
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Post by stoffe64 on Apr 22, 2016 13:30:42 GMT -5
Hello Fredrik What kind of oil pressure do you get from that biltema pump? Very Nice machine there, keep up The good work! Cheers /stephan Hello. I built it with a bypass system so I manually can control the pressure. When I reach 5-6 bar the pump is running a bit rough but this pressures are beyond what is needed. Usually I try to keep it at 3-3.5 bars. Hello Fredrik What kind of flow per minut you get at that pressure?, i had such pump which we used to empty a fueltank filled With wrong fuel, the pump motor got heat damaged!
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 22, 2016 14:24:51 GMT -5
Hello. I built it with a bypass system so I manually can control the pressure. When I reach 5-6 bar the pump is running a bit rough but this pressures are beyond what is needed. Usually I try to keep it at 3-3.5 bars. Hello Fredrik What kind of flow per minut you get at that pressure?, i had such pump which we used to empty a fueltank filled With wrong fuel, the pump motor got heat damaged! The official specs are 14liters per minute at 2 bar with 2 meters pressure height. The pump isn't made for continous runs. However the same pump is all over eBay with a lot of different specs, I took a chance since I didn't have a better idea of a decent oil system
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Post by racket on Apr 22, 2016 17:01:27 GMT -5
Hi Fredrik
If you needed to go from the 30,20,50 division of air to get the engine running then you probably have a case of "two wrongs making a right" , but maybe not a very good "right" :-(
Firstly , you have a highly tangential air delivery to the combustor at the Primary Zone , this can create problems , I've lost count of the number of guys who've made this mistake and ended up with a poorly running engine .
Your propane injector is another concern , with such a small engine you need small injector holes of <1mm dia , and they must all be radially orientated towards the primary air holes , NO axial component , your current nozzle will only produce a fuel rich core in the flametube , exacerbated by the tangential air delivery producing a swirling airflow with minimal radial penetration.
Your flametubes don't appear to have a "funnel" at their outlet end to smoothly transition the gases from the circular flametube to the rectangular scroll inlet , without a funnel there is gross turbulence at the flametube outlet restricting flow which can cause the comp to go into surge.
Hope this helps :-)
Cheers John
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 23, 2016 2:23:03 GMT -5
Hi Fredrik If you needed to go from the 30,20,50 division of air to get the engine running then you probably have a case of "two wrongs making a right" , but maybe not a very good "right" :-( Firstly , you have a highly tangential air delivery to the combustor at the Primary Zone , this can create problems , I've lost count of the number of guys who've made this mistake and ended up with a poorly running engine . Your propane injector is another concern , with such a small engine you need small injector holes of <1mm dia , and they must all be radially orientated towards the primary air holes , NO axial component , your current nozzle will only produce a fuel rich core in the flametube , exacerbated by the tangential air delivery producing a swirling airflow with minimal radial penetration. Your flametubes don't appear to have a "funnel" at their outlet end to smoothly transition the gases from the circular flametube to the rectangular scroll inlet , without a funnel there is gross turbulence at the flametube outlet restricting flow which can cause the comp to go into surge. Hope this helps :-) Cheers John Thank you for your input John. I will try to make another nozzle tomorrow and see if this helps. I hope it makes a big difference. The flametube has no funnel as you say, it is designed like this pic: The flame tube is tightly fitted inside the combustion chamber, so no "leakage" can happen after the flame tube ends and the connector to the turbine begins. Do you suggest that I remake the connector and make it round against the combustor side and let it go into a rectangular shape as it goes closer to turbine? The air delivery, how is this a problem? My first idea was just to put it straight in since that would mean minimal welding and a better looking build according to me, byt a friend convinced me that the swirling and turbulence would be a good thing for fuel mixture. Are you saying this is wring? Moving inlet would be minimal work for me and could well be worth a try if you think this is one of the issues. Again thank you for your input! /Fredrik
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steamn51
Member
Joined: March 2016
Posts: 14
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Post by steamn51 on Apr 23, 2016 2:30:20 GMT -5
Hi Fidel How did you get that oil pump to work? I have one that looks indentical and it overloaded with the hose size outlet and trips out.
Cheers Andy
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 23, 2016 2:42:28 GMT -5
Hi Fidel How did you get that oil pump to work? I have one that looks indentical and it overloaded with the hose size outlet and trips out. Cheers Andy The pump has been working great. It's covered by 1 year warranty so if it breaks they'll have to fix it. It's supposed to be serviced with new cogwheels and coals for the electric motor every 200 working hours. I can highly recommend it since it fairly cheap.
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 23, 2016 3:45:29 GMT -5
Another thing that struck me is the measurements of the tube. Inducer is 37.7mm and tube is 76mm. That makes the usual twice diameter of inducer. However I've seen recommendations of making flame tube 3 times diameter of inducer, in jets specs for example. What are the actual recommendations?
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fidel
Member
Joined: April 2016
Posts: 28
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Post by fidel on Apr 23, 2016 15:55:29 GMT -5
Update! I made another nozzle and did a testrun. It now runs just as smooth as in the video if not even better. Just a slight fluctuation at high loads and only when I throttle down. So much better there! I also tried to run it again with the "standard" amount of holes, but it runs really really hot and I don´t have the guts to bring it up to idle with that flame tube in fear of meltdown. As racket wrote, there´s still some things that don´t add up. See flame tubes bellow. Could moving the air inlet really do all this difference if I was to go ahead and do that change? With the engine running fine again I´m hesitant to put the angle grinder into it.
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