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Post by azwood on Jul 2, 2018 0:27:09 GMT -5
What do you guys think the minimum air gap between the ft an can would be if i have a decent plenum volume and defuser can i get away with 10mm
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Post by racket on Jul 2, 2018 1:44:53 GMT -5
Depends on the diameters
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Post by azwood on Jul 2, 2018 1:52:46 GMT -5
200mm can with a 180mm ft 350mm long
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Post by azwood on Jul 2, 2018 1:54:01 GMT -5
Or would i better off just going a 170mm ft and have more air gap
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Post by azwood on Jul 2, 2018 4:12:22 GMT -5
Thorght id do a drawing for a change feel free to say what you think. but thats pretty much what im thinking id try
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Post by racket on Jul 2, 2018 4:32:23 GMT -5
Yep , 170 inside 200 .
You could "lower" the Secondary line by another 50 mm to lengthen the Primary Zone .
What fuel injection method are you using , this will determine placement of the Primary hole line , which needs to be displaced further down , roughly where that top line of cooling holes is at about half the FT diameter ( 85 mm ) , from the end
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Post by azwood on Jul 2, 2018 4:49:30 GMT -5
Thanks i was thinking evap tubes say 12mm and about 10 of them but id like to have them enter from the side of the ft then go up from there maybe 200 long and about 40mm off the wall Yep , 170 inside 200 . You could "lower" the Secondary line by another 50 mm to lengthen the Primary Zone . What fuel injection method are you using , this will determine placement of the Primary hole line , which needs to be displaced further down , roughly where that top line of cooling holes is at about half the FT diameter ( 85 mm ) , from the end
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Post by azwood on Jul 2, 2018 4:56:41 GMT -5
Something like this
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Jul 2, 2018 11:57:51 GMT -5
Hi Aaron, For liquids or gasses, I have a general rule of thumb when trying to determine the flow area required to minimize pressure drop leading up to a flow orifice(s), I try to keep the flow area 3-4 times greater than the flow area of the final orifice(s). This rule of thumb generally holds true for liquids and gasses in relatively short flow passages. In answer to your question about the annulus flow area around your flame can, try to make the annulus flow area ~4X greater than the flow area of the holes in the flame can. This will keep the air speed at a reasonable level (not too fast) and it will maximize the pressure drop of air going into the flame can holes. Where you want the pressure drop to happen. You can always make the annulus flow area greater than 4X but after 4X there are rapidly diminishing returns. In some cases beyond 4X, the flow area around the flamecan can be too large. If it becomes too large it can create excessive annulus turbulence and tumbling of air going up around one side of the annulus and then back down the other side where you get a big recirculation zone going topsy-turvy around the flame can. Not good. When this happens it can offset the air flow from one side to the other resulting in a tilt of the recirculation bubble (toroidal vortex) INSIDE the flame can... potentially reducing combustion stability. Below is an example of annulus turbulence disrupting the flow symmetry inside the flame can. As can be seen, it's a mess. This would likely result in combustion instability and reduced flame out limits. Notice how the flow on the right side of the annulus is shooting up so fast that when it hits the top of the dome it is turned around and shots back down the other side of the annulus. The resulting flow inside of the flame can is anything but symmetrical and it probably wouldn't perform very well. There are many radial holes that dont show up in the cut-away images but everything is there. Primary, Secondary and Tertiary. All of them. Also see this flow pipe image showing how excessive annulus turbulence can have undesirable effects. Its showing the wrong kind of turbulence in the wrong place. This is what you want to avoid. Below is another image showing balanced flow. In the example below I used different turning vanes (not shown) to reduce the turbulence leading up to the annulus. I also implemented the use of orifice plates inside the annulus to cut down on the large scale, topsy-turvy annulus turbulence. By using the orifice plates I sacrificed a small amount of dynamic pressure to better stabilize the primary zones toroidal vortex. Take a good look at the primary zone to see how the toroidal vortex is recirculating flow back into itself. This recirculation pattern is what stabilizes and anchors the flame. Try to position your radial holes such that it creates a circular motion. If you position them right, you will get a toroidal vortex much like this. You will notice I have a swirler at the top of these flame cans. A swirler can help with the torroidal vortex but they are not a requirement. You can still get a good toroid recirculating with properly placed radial holes. See the location of my Primary Zone radial holes to get an idea of what you are targeting. When the Primary radial holes of of flame can are all pointing to the center, the jets of incoming air all converge into one spot. When the air jets collide, half of the air will be splashed down the middle of the flame can and the other half of the air will splash back up toward the fuel nozzle. This upward flow is what forms the recirculation which draws in more fire and burning fuel back into the fan of the fuel spray nozzle. If you can get a good recirculation bubble curling up inside the primary zone, it will help to keep your flame cans burning process well stabilized. If you can position your vaporizer sticks to shoot into the vortex's upward flow it will help to increase mixing and combustion efficiency. Tony
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Post by finiteparts on Jul 2, 2018 14:23:05 GMT -5
Tony,
In your examples, it appears that you only have at most a 1.03% pressure drop across the liner. I think you have something wrong with your CFD and as such, I am not sure it should be used as an example, since the flow field has no real pressure to drive it. I am assuming that you are plotting static pressures, and the total pressure is what we quote when we talk combustor pressure drop, but at the low Mach numbers that should be in this region, the difference in static and total should not be too significant. With that in mind, you might try to plot contours of total pressure and see how far out you are. If the pressure fields don't seem correct, I would caution that believing the flow field results is likely not a wise choice.
I would actually disagree with you on the over sizing the annulus area. The goal is to provide the highest static pressure feed to the dilution holes and unless you have some form of total pressure recovery device on the holes, such as a scoop, then you need to minimize the flow speed. Even if the flow is not steady or slightly biased to one side, if it is slower, there is a smaller difference between the static and total pressures. I don't think it is wise to suggest that the homebuilder would produce a more stable flow with a smaller annulus. Since the flow stability is very dependent on what it is being fed from upstream. Homebuilt diffusers are likely not controlling the flow well enough to inhibit it from experiencing large flow separations, that feed downstream turbulence.
Now I do agree with you rule of thumb on the flow areas, but I would say it refers to it as a minimum. It is generally agreed that 4X the flow area is where you can begin assuming the upstream flow is essentially a plenum feed (thus the total and static pressure are close enough to be considered equal), but this also assumes that the initial flow speed is at a low Mach number. If you think in terms of the mass conservation equation, it is obvious that all you are really saying with the 4X rule is that for a constant mass flow, you are reducing the flow speed by a fourth. So if the flow speed was higher than ideal, you might not have enough flow speed reduction with only 4X area increase and still have a larger than desirable dynamic pressure loss.
- Chris
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Post by azwood on Jul 2, 2018 16:40:56 GMT -5
Tony, In your examples, it appears that you only have at most a 1.03% pressure drop across the liner. I think you have something wrong with your CFD and as such, I am not sure it should be used as an example, since the flow field has no real pressure to drive it. I am assuming that you are plotting static pressures, and the total pressure is what we quote when we talk combustor pressure drop, but at the low Mach numbers that should be in this region, the difference in static and total should not be too significant. With that in mind, you might try to plot contours of total pressure and see how far out you are. If the pressure fields don't seem correct, I would caution that believing the flow field results is likely not a wise choice. I would actually disagree with you on the over sizing the annulus area. The goal is to provide the highest static pressure feed to the dilution holes and unless you have some form of total pressure recovery device on the holes, such as a scoop, then you need to minimize the flow speed. Even if the flow is not steady or slightly biased to one side, if it is slower, there is a smaller difference between the static and total pressures. I don't think it is wise to suggest that the homebuilder would produce a more stable flow with a smaller annulus. Since the flow stability is very dependent on what it is being fed from upstream. Homebuilt diffusers are likely not controlling the flow well enough to inhibit it from experiencing large flow separations, that feed downstream turbulence. Now I do agree with you rule of thumb on the flow areas, but I would say it refers to it as a minimum. It is generally agreed that 4X the flow area is where you can begin assuming the upstream flow is essentially a plenum feed (thus the total and static pressure are close enough to be considered equal), but this also assumes that the initial flow speed is at a low Mach number. If you think in terms of the mass conservation equation, it is obvious that all you are really saying with the 4X rule is that for a constant mass flow, you are reducing the flow speed by a fourth. So if the flow speed was higher than ideal, you might not have enough flow speed reduction with only 4X area increase and still have a larger than desirable dynamic pressure loss. - Chris Wow thanks for takeing the time to wright that ill have a good think about it and draw something up and post a pic when i get my head around all that
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Post by azwood on Jul 2, 2018 16:42:07 GMT -5
Hi Aaron, For liquids or gasses, I have a general rule of thumb when trying to determine the flow area required to minimize pressure drop leading up to a flow orifice(s), I try to keep the flow area 3-4 times greater than the flow area of the final orifice(s). This rule of thumb generally holds true for liquids and gasses in relatively short flow passages. In answer to your question about the annulus flow area around your flame can, try to make the annulus flow area ~4X greater than the flow area of the holes in the flame can. This will keep the air speed at a reasonable level (not too fast) and it will maximize the pressure drop of air going into the flame can holes. Where you want the pressure drop to happen. You can always make the annulus flow area greater than 4X but after 4X there are rapidly diminishing returns. In some cases beyond 4X, the flow area around the flamecan can be too large. If it becomes too large it can create excessive annulus turbulence and tumbling of air going up around one side of the annulus and then back down the other side where you get a big recirculation zone going topsy-turvy around the flame can. Not good. When this happens it can offset the air flow from one side to the other resulting in a tilt of the recirculation bubble (toroidal vortex) INSIDE the flame can... potentially reducing combustion stability. Below is an example of annulus turbulence disrupting the flow symmetry inside the flame can. As can be seen, it's a mess. This would likely result in combustion instability and reduced flame out limits. Notice how the flow on the right side of the annulus is shooting up so fast that when it hits the top of the dome it is turned around and shots back down the other side of the annulus. The resulting flow inside of the flame can is anything but symmetrical and it probably wouldn't perform very well. There are many radial holes that dont show up in the cut-away images but everything is there. Primary, Secondary and Tertiary. All of them. Also see this flow pipe image showing how excessive annulus turbulence can have undesirable effects. Its showing the wrong kind of turbulence in the wrong place. This is what you want to avoid. Below is another image showing balanced flow. In the example below I used different turning vanes (not shown) to reduce the turbulence leading up to the annulus. I also implemented the use of orifice plates inside the annulus to cut down on the large scale, topsy-turvy annulus turbulence. By using the orifice plates I sacrificed a small amount of dynamic pressure to better stabilize the primary zones toroidal vortex. Take a good look at the primary zone to see how the toroidal vortex is recirculating flow back into itself. This recirculation pattern is what stabilizes and anchors the flame. Try to position your radial holes such that it creates a circular motion. If you position them right, you will get a toroidal vortex much like this. You will notice I have a swirler at the top of these flame cans. A swirler can help with the torroidal vortex but they are not a requirement. You can still get a good toroid recirculating with properly placed radial holes. See the location of my Primary Zone radial holes to get an idea of what you are targeting. When the Primary radial holes of of flame can are all pointing to the center, the jets of incoming air all converge into one spot. When the air jets collide, half of the air will be splashed down the middle of the flame can and the other half of the air will splash back up toward the fuel nozzle. This upward flow is what forms the recirculation which draws in more fire and burning fuel back into the fan of the fuel spray nozzle. If you can get a good recirculation bubble curling up inside the primary zone, it will help to keep your flame cans burning process well stabilized. If you can position your vaporizer sticks to shoot into the vortex's upward flow it will help to increase mixing and combustion efficiency. Tony Thanks so much for the in depth reply
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Jul 2, 2018 17:22:03 GMT -5
Hi Chris, Never mind the numbers shown in the rainbow colored legend. I dont know why it always shows the low numbers but I do know they are not correct. So I dont bother with the legend numbers anymore. I know the legend isn't correct because in simulation, I vented the flame can into a hard vacuum and the legend still shows impossibly low deltas. So the legend is useless. The CFD is however useful in illustrating the flow field directions, patterns and it gives a good breakdown of mass flow partitioning when needed. Everything in Solidworks CFD seems to work great... except for the legend. Using the Lefebvre equations I sized the total orifice flow area to create a ~2.5 PSI delta P with an assumed 0.57 Cd at a 3.5 pressure ratio. And yes, ideally, it's best to have near zero velocity just before the orifice, but in my experience the majority of dynamic pressure is conserved at 4X due to V^2/2g. 4X is my "safe" practical, minimum ratio. I try not to go less than 4X unless there is some other demanding compromise forcing it. Going up from 4X is always good until it comes at the cost of other compromises. The ratio is of course flexible depending on the circumstances. When I start my own build thread I'll put in all the design and calculation details while eagerly looking forward to some good peer review. Rip it up all the way. The more the better! Tony
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Post by azwood on Jul 2, 2018 19:59:35 GMT -5
Hi Chris, Never mind the numbers shown in the rainbow colored legend. I dont know why it always shows the low numbers but I do know they are not correct. So I dont bother with the legend numbers anymore. I know the legend isn't correct because in simulation, I vented the flame can into a hard vacuum and the legend still shows impossibly low deltas. So the legend is useless. The CFD is however useful in illustrating the flow field directions, patterns and it gives a good breakdown of mass flow partitioning when needed. Everything in Solidworks CFD seems to work great... except for the legend. Using the Lefebvre equations I sized the total orifice flow area to create a ~2.5 PSI delta P with an assumed 0.57 Cd at a 3.5 pressure ratio. And yes, ideally, it's best to have near zero velocity just before the orifice, but in my experience the majority of dynamic pressure is conserved at 4X due to V^2/2g. 4X is my "safe" practical, minimum ratio. I try not to go less than 4X unless there is some other demanding compromise forcing it. Going up from 4X is always good until it comes at the cost of other compromises. The ratio is of course flexible depending on the circumstances. When I start my own build thread I'll put in all the design and calculation details while eagerly looking forward to some good peer review. Rip it up all the way. The more the better! Tony So what your saying is the 3.5in exit of the ft is too large for the 170mm ft size? Does the turbine ar ratio and turbine exducer size efect this and also the fact its a twinn scroll housing
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Post by racket on Jul 2, 2018 20:42:13 GMT -5
With evap tubes , the total flow area of the tubes needs to be ~10-12% of inducer area , your 81 mm inducer has 5152 sq mms , so you'll need between 515 to 620 sq mms total area , with say 6 tubes that'll amount to ~100 sq mms/tube for their bore size , so ~11.28 mm ID .
Length needs to be long enough to produce ~6 times inducer area for their total heating surface , so ~5152 sq mms/tube if 6 of , if say 1/2"-12.5 mm OD tube with a circumference of ~39 mm , the length needs to be ~132 mm to achieve that 5152 sq mms/tube
Your 3.5" exit is OK , as long as theres a smooth transition of flow area between the 170 mm FT and the scroll inlet/s
Cheers John
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