monty
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Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Sept 18, 2019 9:07:22 GMT -5
Hi Monty I've had a look at the numbers I did for Andy's engine with 3 lbs/sec flow , with 900C for the T I T but with a tad lower pressure going into the stage , and I ended up with 18 vanes set at ~26-28 degrees , with a max of 30 degrees , dependant on the actual vane "thickness" alongside the throat, with a throat of ~10-10.5 mm for the 18 throats at 20 mm wide axially , the 20 mm made allowance for clearance between the wheel and NGV flange, and the blade shroud , ~0.5 mm each side , which allowed "lowering" of the vane angle a tad compared with using the 19 mm wheel tip height ,........... for a total of ~5.6 sq ins of throat area I have to always draw the NGV up on paper with the correct vane shape/dimensions etc so as to get the exact throat dimension vs angle , Andy was able to do it in CAD which gave a much more accurate number , but still needed "juggling" to get things right . Is your 18.8 psia a static pressure or a total pressuure ??................., with the best part of 1,000 ft/sec exiting the exducer theres a fair bit of dynamic pressure component in any total absolute pressure pitot readout. Cheers John John,
all my numbers are stagnation (total) temp and pressure.
My turbine design book has a range of angles the flow needs to be to minimize loss into the inducer. I've got to find time to crawl through all these numbers. Thanks to you and Chris, I will know if I'm totally out in left field. I'll let you know how my numbers come out.
Monty
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Post by racket on Sept 18, 2019 18:31:12 GMT -5
Hi Monty
Thats good as it gives me a couple more psi drop across the stage, but I think you might still be having difficulties getting that 3 lbs/s out of the wheel at such low static pressure, you might find theres a bit extra static.
I'd go along with Chris' recommendation of 5.2 sq ins of throat area , my numbers came out a tad bigger at 5.4 sq ins but I assumed a slightly less efficient NGV ...........LOL, I always go conservative with my flow areas , a tad bigger either gives a bit more flow or we can use a bit more temperature .
At 5.4 sq ins , 3484 sq mms and 18 throats that gave me 193.5 sq mms/throat , if 20 mm wide then the throat will be ~9.7 mm , you'll need to juggle the angles to get the required throat
Cheers John
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Post by finiteparts on Sept 18, 2019 20:08:13 GMT -5
Monty, I just thought that I might point you to some older posts that I put up with some good radial turbine/NGV stuff, in case it might help. You may have already seen it, but if not, it starts here: jetandturbineowners.proboards.com/post/10742/threadGood luck, Chris
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monty
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Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Sept 18, 2019 22:14:41 GMT -5
Monty, I just thought that I might point you to some older posts that I put up with some good radial turbine/NGV stuff, in case it might help. You may have already seen it, but if not, it starts here: jetandturbineowners.proboards.com/post/10742/threadGood luck, Chris Chris,
I had not seen that. Been so busy I haven't been able to keep up with things on here. Thanks for the link. That pic from Wooley and Hatton is in my turbine design book by concepts NREC. I was planning to use the method outlined therein. Basically set the angle to give proper flow into the turbine and then adjust the thickness at the nozzle to get the correct area. I'll then put it through the CFD ringer to make sure I don't have any bad separation issues. Getting closer, but wish I had more time to spend on it. I keep having to set it aside, and then it takes me a day just to get caught back up and remember where I was. I need a solid week to concentrate on things. Not going to happen until Christmas at the earliest.
Monty
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monty
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Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Sept 18, 2019 22:18:01 GMT -5
Hi Monty Thats good as it gives me a couple more psi drop across the stage, but I think you might still be having difficulties getting that 3 lbs/s out of the wheel at such low static pressure, you might find theres a bit extra static. I'd go along with Chris' recommendation of 5.2 sq ins of throat area , my numbers came out a tad bigger at 5.4 sq ins but I assumed a slightly less efficient NGV ...........LOL, I also go conservative with my flow areas , a tad bigger either gives a bit more flow or we can use a bit more temperature . At 5.4 sq ins , 3484 sq mms and 18 throats that gave me 193.5 sq mms/throat , if 20 mm wide then the throat will be ~9.7 mm , you'll need to juggle the angles to get the required throat Cheers John John,
It's all spread sheet numbers now. Won't know for sure till I light it up and measure things!
Monty
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monty
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Currently being spanked by mother nature.......
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Posts: 400
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Post by monty on Dec 24, 2019 20:46:59 GMT -5
One man Turbofan Winter Solstice extravaganza, Merry Christmas, happy Kwanzaa, Hanukkah....whatever your flavor.... may you have a happy Festivus!
The 3D printer was employed with the utmost care.
The parts were made.
No glue was spared.
The divots and blemishes removed from the parts.
Use Vaseline to thin the wax, not the kind for babies, your nose it will tax!!!
After mixing, and measuring twice.
your mold will come out really nice!
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monty
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Joined: September 2018
Posts: 400
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Post by monty on Dec 26, 2019 18:32:12 GMT -5
Poured the other side of the mold today.
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monty
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Post by monty on Dec 26, 2019 18:34:09 GMT -5
Was a huge relief and cause for an adult beverage when the mold parted properly!! Unless you have suffered a stuck mold, you cannot fathom the relief!!! Now all I have to do is dry it without cracking, burn out the PLA and pour the aluminum.....
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monty
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Post by monty on Dec 29, 2019 18:48:29 GMT -5
Burned out the mold over the weekend. It took a while!
You can see the grey powder left behind from the grey PLA. I know better, but I was out of natural. If you are going to try lost PLA, make sure to use natural with NO coloring AT ALL. It will vaporize completely at 550-600F. Amazing really. Doesn't leave a trace. It doesn't even soak into the plaster like wax does. But if you use colored PLA, the colorants are usually a mixture that contains mineral fill and dyes. These leave ash behind, and no matter how hot you get things there will be residue in the mold. It also melts at higher temps and will not run as fluidly from the mold. The edge broke off because the mold was so heavy the oven grate bent and put pressure on the edge. I will just put it in place when I assemble the mold. It's at the top, on a sacrificial surface worse case I machine away a chunk of metal there. Just glad it mostly came out.
Procedure is as follows:
Do this outside in a dedicated oven....unless you want a divorce. In the case where divorce is desired perform this operation in the kitchen, with all the doors and windows closed....you will soon be back on the market.
1.) Heat to about 170F slowly. Let the molds come up to temp.
2.) Gradually raise the temp to 230F. Leave it here with the oven cracked for a loooooong time. Until all the steam stops coming off the parts. It took about a day for these large molds. If you skip this step, the molds will crack and fall apart.
3.) gradually ramp up to about 500F. Leave it here until all the PLA has melted out of the mold. This will take a while with a mold this big.
4.) Raise the temp to 550F to vaporize all of the remaining PLA residue. Hit it with torch to set the stubborn deposits on fire. (If you do something stupid like hit it with a torch while massive amounts of PLA are still vaporizing.....(A, you won't ever do it again, B, I warned you and I'm not responsible for burning down your house or loss of hair/eyebrows.)
5.) SLOWLY cool the molds back to room temp. 6.) Clean the ash and residue out with a shop vac and compressed air.
7.) Assemble mold while treating the parts as a combination of delicate mm thick glass and sweaty dynamite...
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lofi
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Posts: 47
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Post by lofi on Dec 30, 2019 13:22:18 GMT -5
Incredible. Thanks for taking the time to document the process.
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Post by Johansson on Dec 31, 2019 11:01:57 GMT -5
Wonderful work!
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monty
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Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Jan 1, 2020 16:52:31 GMT -5
Well guys,
The foundry gods were not so pleased with my efforts. I knew it was bad when I first started the pour. It was going too fast and the sprue didn't fill up quickly. Too much mixing of the metal with air as it went into the mold. I should have done a more careful job with my sprue, runner and gate sizing. Even if I had though, it don't think it would have turned out. When the Mold filled up the riser and vents had too much dross and they were bubbling. Which should not happen. When you heat the mold to 230F it drives off the bulk water in the mold. Once that is done, the chemically bonded water must be driven of. The 500 degree stage, melts the PLA, but it also drives off the bound water. This helps keep the PLA from being absorbed into the mold.....I know the mold was dry....hmmm
When I opened the mold I had a moment where I thought just maybe my impressions were wrong.
Unfortunately I was correct. There were a large number of gas defects. Part of the problem was the mold filling too quickly. But I also think there was something going on with all of the surfaces I smoothed with wax. Either the wax or the vaseline soaked into the investment. The plaster was all discolored around those areas. I've used this technique before, but I burned it out in a gas fired burnout oven at much higher temps. Looks like that doesn't work in a regular oven. Whichever culprit it was outgassed when the metal was poured into the mold.
Never fear however, foundry as a hobby is not for the instant gratification crowd! I have a new plan based on what I learned from this episode. The new mold design has a tapered sprue, a pour basin, a gate and runner 2X the area of the sprue, a dross trap at the end of the runner, and a large riser. The riser is fed directly from the gate so it will be the hottest part of the casting. There are trash traps at the bottom and open risers at the top. Mostly this will allow me to blow out the mold after burnout and assist with drying the plaster. I'm also going to vent the snot out of it, even if I don't think it needs a vent, it gets a vent!! This will be a much smaller one piece mold. That last one was way too heavy. I almost couldn't move the thing. Having to bend over and gently maneuver a delicate 50 lb mass is not my idea of fun..
I went and got two 3kg spools of natural PLA, and I'm not going to use anything to smooth it. I'll glue it together with acetone. I've changed how I print the model, so that all the rough areas will be on surfaces that will be machined away. No smoothing other than with sandpaper!. Unfortunately I have to cut the model up into a lot of pieces that will be glued together. Already printed the first piece last night and waiting on the second now.
The weather just might give me another window for casting this weekend....
Monty
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Jan 5, 2020 20:59:14 GMT -5
New mold form with vents. I used printer filament vented to the risers and attached with a hot glue gun.
I put a "flask" around the investment made from hardware cloth. I was worried that the investment would crack and fall apart. This was a good idea. It did crack in multiple places, but the flask held it together.
I also brushed the model with food grade propylene glycol. This is supposed to prevent bubbles from forming on the part. It seemed to work OK. But I really need to have some method of sealing the print. There were bubbles coming up in the plaster when I poured it. I knew what that meant....and it wasn't good. It meant plaster was seeping into the part, which is bad....really bad. It forms these things:
They form inclusions in your casting. You can use methelyne chloride to seal and smooth the print....but that stuff is nasty! Much worse than acetone. No thanks....I want a non-toxic something to both smooth and seal the prints. I have some ideas. Need to try them out.
The burn out oven turned off in the middle of the night and the mold was full of these things when I looked at it. I poked all of it out with a dowel and a bent welding rod that I could reach. Then I put the investment back in the oven....now time was getting short! After burn out was finally done, I used compressed air to blow out a bunch more inclusions and dust. I kept at it until I thought I had gone too far, and then I went on some more! I had no way of knowing whether I got everything out or not. I thought, well.....I'll see how the pour cup, gate and runner work....but I had no hopes for a good outcome. The pour went relatively well. THIS is what the risers should look like!! No bubbling, nice and shiny:
I was rewarded with a good casting!! Totally unexpected!
This calls for more than a beer or gin and tonic!! I have been saving this Christmas present until I got a good casting. CHEERS!!!
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Post by racket on Jan 5, 2020 23:33:49 GMT -5
Hi Monty
Perseverance pays off ..............enjoy your reward :-)
Nice casting .
Cheers John
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Post by Johansson on Jan 6, 2020 17:01:17 GMT -5
Awesome, top notch home casting! Hats off to you and cheers from Mobacken Racing!
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