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Post by propellanttech on Aug 17, 2011 22:05:36 GMT -5
Ok, I'm a new guy here.
I just joined the DIY group on Yahoo, and one of the members pointed me here. So I thought I would ask to see if anyone here has a complete understanding of higher math.
Don't get me wrong, I am extremely versed with many aspects of mechanics, electronics ect., but I have encountered an equation I can't not figure out.
The equation comes from Thomas Kamps book (Model Jet Engines) which deals with building/designing model jet engines. If you have the book, this equation starts on page 69 and continues to 70 with respect to the critical rotational speed of the turbine shaft. His ending units are l/s, and I can't find a l/s reference anywhere. Also he doesn't give correlation to rpm from l/s.
I know about turbine and compressor rotational speed, but the shaft speed will need to be calculated, for I am changing it's design.
I am working on a turbine engine, but I wouldn't necessarily consider it a model engine. The compressor is going to be large. One of the largest available on the market. It will also have some unique items incorporated, but I do not want to divulge that information at this point.
I would appreciate anyone who may be able to help.
I will try to do a better introduction for my next thread.
James L
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Post by Johansson on Aug 18, 2011 0:31:37 GMT -5
Hi and welcome to the forum!  A top secret project is no fun at all, drop the curtain and start a build thread dammit! ;D |
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Post by Richard OConnell on Aug 18, 2011 3:06:15 GMT -5
Any way you could give us just a little more information about the equation? I would love to help but from reading the post, it would appear difficult to determine its function in the equation.
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Post by propellanttech on Aug 18, 2011 14:28:36 GMT -5
I will give images from the text, as the information is copyrighted.     These should give an idea of the confusion I'm having. J |
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Post by propellanttech on Aug 18, 2011 14:35:59 GMT -5
Hi and welcome to the forum! A top secret project is no fun at all, drop the curtain and start a build thread dammit! ;D The project is not top secret. There are specifics which I'm not disclosing at the moment. This is due to the nature of the modifications. The project is a large turbine with a large compressor and a custom Iconel turbine (axial) with an Iconel NGV. But without getting the calculations done, the project will not continue. I'm looking for at least 150 lbs of thrust. It will be air start, and have some innovations about the design, if I can have my way. The pressure ratio is looking to be 5-5.25 with a maximum rotational speed of 100k, which is beyond most recommended rpms. But is on the map for the compressor. James |
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Post by Johansson on Aug 18, 2011 15:19:41 GMT -5
Sounds like a very interesting project, I only meant to tease you a little with the comment if you thought othervise. 150lbs of thrust with 5PR and 100.000rpm sounds like living on (or beyond) the edge to me, the engine I am building with an estimated 100+lbs of thrust has a max rpm of 66.000rpm. For shorter durations it might be possible to stretch it to 70.000rpm but certainly not further. Boreless titanium compressor wheel? Regarding the calcs l/s stands for angular velocity of the shaft (rads/s), one radian is aprox. 9.5493rpm. |
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Post by propellanttech on Aug 18, 2011 17:14:09 GMT -5
Sounds like a very interesting project, I only meant to tease you a little with the comment if you thought othervise. 150lbs of thrust with 5PR and 100.000rpm sounds like living on (or beyond) the edge to me, the engine I am building with an estimated 100+lbs of thrust has a max rpm of 66.000rpm. For shorter durations it might be possible to stretch it to 70.000rpm but certainly not further. Boreless titanium compressor wheel? Regarding the calcs l/s stands for angular velocity of the shaft (rads/s), one radian is aprox. 9.5493rpm. You know...you are right. I rechecked the map of the compressor, and my first quote was wrong. It was from a different turbine with a smaller diameter. The larger one is mapped to 87995 rpm. I probably won't push it beyond 82k. 77k is where the graph starts dropping in efficiency (drastically). The diameter of the turbine puts that rpm over 400 m/s. I seriously doubt I get 150 lbs of thrust, but it is a goal. I knew you were teasing.......  I would like to hear more about your project as well. Can I ask if you are using a radial or axial turbine? The specs on the compressor doesn't say the material, but I know some of the new turbines are titanium. It would be nice, but I'm going for the pressure ratio, more than the material. Thanks for the explanation of the l/s. I don't know where they come up with l for radians, but I'll take your word for it. James |
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Post by racket on Aug 18, 2011 20:19:22 GMT -5
Hi James
For 5 -5.25 : 1 PR you'll be needing a comp tip speed of probably 2000 ft/sec ..............heh heh , sounds like a GT5533R if spinning to ~88K , with 4.5 PR at 77K and ~1800 ft/sec at 72% effic and 130lbs/min .
With an efficient axial turb stage , 140 lbs would be possible, 150 might be a stretch unless the temps are pretty high :-(
High PRs might help the fuel burn rates but its the mass flow we need as well
Cheers
John
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Post by propellanttech on Aug 18, 2011 23:15:06 GMT -5
Hi James For 5 -5.25 : 1 PR you'll be needing a comp tip speed of probably 2000 ft/sec ..............heh heh , sounds like a GT5533R if spinning to ~88K , with 4.5 PR at 77K and ~1800 ft/sec at 72% effic and 130lbs/min . With an efficient axial turb stage , 140 lbs would be possible, 150 might be a stretch unless the temps are pretty high :-( High PRs might help the fuel burn rates but its the mass flow we need as well Cheers John Yea.....you read my mind. The GT5533R was the turbine I was quoting. I actually found some ABB's (new technology) which are nice, but I haven't a clue if I can get the Garret at this point. Pressure ratios are important, but they also raise the TIT, which can be a bad thing. The fuel efficiency of the turbine is a critical point for me. I also want to keep the temps in a safe range, so 150 lbs is a stretch. But I have some tricks up my sleeve which may make it possible. I seriously doubt the compressor will take the 88k. That will be the last thing tested if I ever get to that point. I have a lot of other hurdles to get over before getting a turbine up to that RPM. The other problem with that RPM is if the turbine will take it. I also doubt it. Those levels of rpm wouldn't be tested unit the unit had been put through it's paces below 82k or so. I think your (group) help will get me there. Or at least as close as possible. James |
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Post by Johansson on Aug 19, 2011 0:24:37 GMT -5
You know...you are right. I rechecked the map of the compressor, and my first quote was wrong. It was from a different turbine with a smaller diameter. The larger one is mapped to 87995 rpm. I probably won't push it beyond 82k. 77k is where the graph starts dropping in efficiency (drastically). The diameter of the turbine puts that rpm over 400 m/s. I seriously doubt I get 150 lbs of thrust, but it is a goal. I knew you were teasing....... I would like to hear more about your project as well. Can I ask if you are using a radial or axial turbine? The specs on the compressor doesn't say the material, but I know some of the new turbines are titanium. It would be nice, but I'm going for the pressure ratio, more than the material. Thanks for the explanation of the l/s. I don't know where they come up with l for radians, but I'll take your word for it. James Just to get the whole picture, what are you going to do with the engine once it is ready? Start mass producing or just put it in the shed? If you are going for the lowest SFC possible and lots of thrust you might consider building a smaller gas producer and adding a power turbine stage with a high bypass fan, that way you will get much more thrust than with a pure jet engine and at a significantly lower SFC. You can read about my build in the DIY section here in the forum, the thread is named something with landspeed bike. The whole rotary assembly is from a Garrett TV94, 1.2kg/s massflow and relatively cheap. |
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Post by racket on Aug 19, 2011 1:04:05 GMT -5
Hi James The GT55 is a 47 Trim wheel , for your high PRs you probably need a wheel with a Trim closer to 40 ..................the comp wheel will cope with the high rpm , they are extremely strong , but a better option would be one of the cast titanium wheels instead of an alloy one , have you looked at the large Holset turbos , www.holset.co.uk/mainsite/files/2_1_1_7-Holset%20HX82-83.phpAn Inco 713c turb wheel will cope with pretty severe stresses , but you may need to try and keep it as small as possible to keep its tip speeds down to acceptable levels , generally we need a turb wheel ~90% of the comp diameter for the "numbers" to workout , the GT55 turb is only 83.7 % . If you look at high PR engines like the Allison 250 with its single centrif comp , they employ 2 turb stages of modest size to power the comp , the small sizes keeps stresses under control . Cheers John |
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Post by propellanttech on Aug 19, 2011 15:41:37 GMT -5
Just to get the whole picture, what are you going to do with the engine once it is ready? Start mass producing or just put it in the shed? If you are going for the lowest SFC possible and lots of thrust you might consider building a smaller gas producer and adding a power turbine stage with a high bypass fan, that way you will get much more thrust than with a pure jet engine and at a significantly lower SFC. You can read about my build in the DIY section here in the forum, the thread is named something with landspeed bike. The whole rotary assembly is from a Garrett TV94, 1.2kg/s massflow and relatively cheap. My end result is not exactly clear at this point. I want to prove a few things, and could make a few for sale if someone wanted them. A high bypass fan is an option, but it complicates the design quite a bit. I love your build. Very nice....and extremely clean. I hope you have good results when you finally power it up. I'm trying to stick with an axial turbine to get more efficiency out of the unit. Radial reduces the throttle response some but may be an option of I go with a bypass design. James |
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Post by propellanttech on Aug 19, 2011 15:49:37 GMT -5
Hi James The GT55 is a 47 Trim wheel , for your high PRs you probably need a wheel with a Trim closer to 40 ..................the comp wheel will cope with the high rpm , they are extremely strong , but a better option would be one of the cast titanium wheels instead of an alloy one , have you looked at the large Holset turbos , www.holset.co.uk/mainsite/files/2_1_1_7-Holset%20HX82-83.phpAn Inco 713c turb wheel will cope with pretty severe stresses , but you may need to try and keep it as small as possible to keep its tip speeds down to acceptable levels , generally we need a turb wheel ~90% of the comp diameter for the "numbers" to workout , the GT55 turb is only 83.7 % . If you look at high PR engines like the Allison 250 with its single centrif comp , they employ 2 turb stages of modest size to power the comp , the small sizes keeps stresses under control . Cheers John I didn't know about those turbo's. I'm just starting and it is evident you guys know much more about it than I. But that is why this group is so great. I need the high PR for some of the "secret" design elements. I hadn't thought of a dual turbine arrangement. How would you calculate that? I figure the turbines would have a different degree of angle on each wheel (blade angle), but that may not be the case. I couldn't find a complete map for the Holset turbo, but I didn't spend a lot of time looking for it either. I was cruising through Johansson's build (very nice). I'm so glad I found this group. You guys are great. James |
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wolfdragon
Senior Member
Joined: April 2011
Posts: 287
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Post by wolfdragon on Aug 19, 2011 16:01:29 GMT -5
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Post by racket on Aug 19, 2011 17:36:31 GMT -5
Hi James The throttle response does slow a bit as the rotating assembly gets heavier , unfortunately it doesn't appear to be in direct relationship to the mass flow , my bikes TV 84 seemed to accelerate better than my latter bigger builds www.youtube.com/watch?v=P-5PgWqgIJo , it could also be because of the fuel delivery method with the bike having high pressure spray (up to 750psi) vs evaporators for latter builds . You might be getting your "slow acceleration" data from Kamps book , the RC aircraft guys run very low idle power settings due to their need for minimal thrust levels to lower landing speeds , we're not restricted to such low rpm, we normally idle at ~40-45 % N1 which makes it quicker to accelerate the rotor than from say 30% N1 . Heh heh , after 20 years of DIY turbines I'm starting to get a bit of knowledge together :-) ...........there aren't many "secrets" with our hobby , high PRs give better fuel burn and a tad more power but its efficiencies that have the biggest impact and "generally" high PRs for us mean comp efficiencies suffer due to the available components , there aren't many low trim swept back comp wheels out there . Even without a comp map for a wheel we can make pretty close guesstimates of its performance , I never had a map for my bikes TV84 ......................maps help , but aren't strictly necessary as things will change when you fit it to a bespoke diffuser system . Cheers John |
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