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Post by apophys on Sept 10, 2017 16:11:04 GMT
I like this new idea a lot. In this case, we could very easily use tungsten-encapsulated boron for 2500K outlet usage (and boron only boils at 4200 K, by which point tungsten would have much too high vapor pressure already). Tungsten "has the lowest vaporization rate of any known material" at high temperature, better than TaHfC (which is tested and compared in this source link): ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650001401.pdfMaybe we should call it the encapsulated fusion radiator. "Fusion" may confuse people to think about nuclear fusion rather than heat of fusion. "Encapsulated liquid radiator," or "pebble bed radiator," is better.
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Post by Enderminion on Sept 10, 2017 16:14:53 GMT
you could run the tungsten at 3000k, right?
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Post by Kerr on Sept 10, 2017 16:15:58 GMT
you could run the tungsten at 3000k, right? Yes, if you want to keep it relatively strong.
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Post by apophys on Sept 10, 2017 16:33:54 GMT
you could run the tungsten at 3000k, right? That might also work with a boron carbide filling (melting point 3036 K, specific heat 1288 J/(kg*K)). It would be close to the limit allowable for tungsten's vaporization rate, and might actually be too high (I do not have data for this).
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Post by n2maniac on Sept 10, 2017 18:40:02 GMT
If by "sand" you mean something like graphite or tungsten and by "solid particle" you mean that stuff that doesn't flow without vibration, clogs easily, does not wet surfaces (important for heat transfer!), and refuses to work with nice valves... wait, what was I saying? (In all seriousness, I would have some serious implementation follow-up questions if this was a proposal in a design review. Got any precedents for this in a non-slurry form?) Larger particles can be shaped like ball bearings and simply roll their way around. Smaller particles would need something like a conveyor belt or a bucket rain to move from heat exchanger to radiator and back. Tiny particles can be manipulated by electrostatic forces: www.sciencedirect.com/science/article/pii/S0921883110001640 or electromagnetic fields: www.google.sr/patents/WO2005069311A2?cl=enRight, spheres, that at least eases transport. And for heat transfer? Dry surfaces don't transport heat terribly well, and high heat fluxes (500K/mm temperature gradient in tungsten/osmium thermocouples) require an exceptionally good thermal interface? My gut feeling would be to wet it with a liquid metal, but that will dry out over time in the semi-open system. Distill it off of the spheres before they exit?
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Post by newageofpower on Sept 10, 2017 19:35:46 GMT
Liquid niobium has a really high Critical point, and is pretty good for ludicrously high temperature reactors as a working fluid. Does anyone have data on liquid niobium vapor pressures?
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Post by treptoplax on Sept 11, 2017 16:52:16 GMT
you could run the tungsten at 3000k, right? That might also work with a boron carbide filling (melting point 3036 K, specific heat 1288 J/(kg*K)). It would be close to the limit allowable for tungsten's vaporization rate, and might actually be too high (I do not have data for this). Note, though, the real prize here is that heat of fusion. For straight boron it's 4.6 MJ (that's mega!) per kg if I'm doing the math correctly, to go from 2350K to 2348K. Boron Carbide would run much hotter but the heat of fusion is 'only' about 1-2 MJ/kg. It seems like heat of fusion is very roughly inversely proportional to molecular mass... (with some other factors being important too, of course).
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Post by newageofpower on Sept 11, 2017 20:23:17 GMT
That might also work with a boron carbide filling (melting point 3036 K, specific heat 1288 J/(kg*K)). It would be close to the limit allowable for tungsten's vaporization rate, and might actually be too high (I do not have data for this). Note, though, the real prize here is that heat of fusion. For straight boron it's 4.6 MJ (that's mega!) per kg if I'm doing the math correctly, to go from 2350K to 2348K. Boron Carbide would run much hotter but the heat of fusion is 'only' about 1-2 MJ/kg. It seems like heat of fusion is very roughly inversely proportional to molecular mass... (with some other factors being important too, of course). Hail Boron.
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Post by apophys on Sept 11, 2017 23:05:46 GMT
Note, though, the real prize here is that heat of fusion. For straight boron it's 4.6 MJ (that's mega!) per kg if I'm doing the math correctly, to go from 2350K to 2348K. Boron Carbide would run much hotter but the heat of fusion is 'only' about 1-2 MJ/kg. I found conflicting numbers. This source gives it as 4.65 MJ/kg for boron, but a massive 7.96 MJ/kg for boron carbide (B 4C). It also gives 4.58 MJ/kg for Al 2O 3 , which is notable due to the ease of obtaining this material. www.dtic.mil/dtic/tr/fulltext/u2/a546871.pdfWikipedia gives it as 50.2 kJ/mol for boron, which works out to 4.64 MJ/kg, so this one value is about right. en.wikipedia.org/wiki/Heats_of_fusion_of_the_elements_(data_page)And then there's this, which is probably your source, listing boron carbide's heat of fusion at 1.35-2.03 MJ/kg. www.azom.com/properties.aspx?ArticleID=75Not sure which is the right number for boron carbide, because I am literally unable to find any other source.
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Post by matterbeam on Sept 12, 2017 0:57:50 GMT
apophys : Look at this interactive chart: www.periodictable.com/Properties/A/FusionHeat.htmlI personally believe that for most spacecraft, the mass optimzation within the radiator capsules will have an insignificant effect on overall dry mass. This is especially striking when we consider how many megawatts even a tiny radiator will be removing. It would be much more interesting to find an element that melts at a decent temperature, is cheap to replace and easy to handle. Boron is quite rare and therefore expensive.
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Post by apophys on Sept 12, 2017 2:18:08 GMT
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Post by apophys on Sept 13, 2017 0:43:31 GMT
Here are a few other interesting materials I found. Calcium oxide (CaO) - melting point 2980 K, heat of fusion 0.913 MJ/kg (source [1]) Magnesium oxide (MgO) - melting point 2915 K, heat of fusion 1.92 MJ/kg (source [1] and [2] agree) 50% CaO, 50% MgO - melting point 2573 K, heat of fusion 1.42 MJ/kg (source [2]) Beryllium oxide (BeO) - melting point 2823 K, heat of fusion 2.84 MJ/kg (source [2]). Expensive. Thorium dioxide (ThO 2) - melting point 3225 K, heat of fusion 4.61 MJ/kg (source [2]). This feels weird somehow. >.> [1] - link in my previous post [2] - "CRC Handbook of Tables for Applied Engineering Science" by Ray E. Bolz. 1973. - link(Which by the way has a value for boron that does not agree with other sources, though it does label it as unreliable. Probably old data.) Also duplicated in "CRC Handbook of Engineering Tables" by Richard C. Dorf. 2003. - link This source agrees with my previously found value for Al 2O 3. There are various aluminates and silicates of magnesium and calcium (which are interesting, since alumina, silica, magnesium oxide, and calcium oxide are all decent/good), but data is sparse. In particular, I want heat of fusion data for MgAl 2O 4 and BeAl 2O 4. I can't find any.
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Post by matterbeam on Sept 13, 2017 1:22:27 GMT
apophys : Very good work and nice finds! Try contacting Ian Mallet.
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Post by Enderminion on Sept 13, 2017 3:00:10 GMT
Thorium has to radiate it's OWN heat, and only stays thorium for a few decades multipules of the age of the earth (so I didn't get to make the radioactive decay point)
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Post by newageofpower on Sept 13, 2017 4:22:11 GMT
Thorium dioxide (ThO 2) - melting point 3225 K, heat of fusion 4.61 MJ/kg (source [2]). This feels weird somehow. >.> Thorium is very dense and likely to make massive (read: bad) LDRs. ... I also realized my personal pick, Niobium is fairly (75% of Lead) dense as well. It has an extreme critical temperature (and thus likely a low vapor pressure) but is probably unsuitable for LDR...Herp. Wikipedia has the vapor pressure of Niobium listed below: Pressure (Pascals) | 1 | 10 | 100 | 1 kPa | 10 kPa | 100 kPa | Temperature (Kelvin) | 2942 | 3207 | 3524 | 3910 | 4393 | 5013 |
For reference, 100kPa = 0.986 atm.
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