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Post by matterbeam on Nov 27, 2017 14:46:10 GMT
Hello! I'm doing some research and I'd like to call upon some of your expertise, especially newageofpower and @rocket witch. The object of my search is high power density turbogenerators and turbocoolers. The turbine in a turbogenerator typically uses high temperature steam and runs a 600K or so gradient between inlet and outlet. It is usually made of dense ceramics (silicon carbide) or even denser superalloys. From what I have gathered, the power density of the turbine stage alone is about 1kW/kg to 7kW/kg. Turbocoolers are not something I have not found a lot of information on, but in short, they are a heat pump where a cold gas is compressed by a turbine to a higher temperature, then a warm heatsink absorbs the heat along the now reversed heat gradient. At the compressor's outlet, the coolant expands back to the original pressure but at an even lower temperature. From what I've seen, this is called a charge-air-intercooler in cars, or a turbo-Brayton refrigerator in industrial applications. Efficiency ~80% of the maximal Carnot limit, power density from 0.1 to 150kW/kg, depending on designs I've seen. If we wanted to significantly increase both the operating temperature and the power density of this technology, what materials will we use? Assume that the coolant fluid is helium, so there are no concerns with oxidation or chemical erosion. I know that high-heat-tolerance carbides such as hafnium carbide would have the temperature resistance and strength to survive in 2000K+ temperatures, but the design of such designs is incredibly high (over 10000kg/m^3) and I am certain it plays a role in reducing the overall power density. What about carbon fibre? Or graphite epoxy composite? Might it be possible to have usable strength at 3000K? What examples of power densities for these components have you stumbled upon? I would love to have references I can extrapolate from. Regards, MB
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Post by n2maniac on Nov 27, 2017 17:53:47 GMT
Helium is going to permit/ask for/require much higher blade tip speeds (and thus stronger blade materials) than something that is denser, but makes the heat exchanger part easier. It will also cause the blade reynolds number to be lower, potentially increasing the lower size limit or hurting efficiency. Carbon does keep usable strength at high temperatures (hence carbon-carbon being used on the space shuttle leading edges), but epoxy is right out. You would want something with maximal high-temperature tensile strength to weight ratio (that is feasible for continuous use, often creep limited!). Ceramics are crack sensitive, metals creep and are heavy. Maybe a monocrystalline high strength and high breaking length ceramic? CNT/C composite? SiC? Too bad diamond doesn't last. Article on carbon-carbon composite strength at high temperature: www.iccm-central.org/Proceedings/ICCM17proceedings/Themes/Materials/CARBON%20-%20CARBON%20COMPOSITES/D3.6%20Kobayashi.pdfNotable risk is it creeping. Breaking length to get mach ~ 0.8 for blade tips is brutal for helium at 3000K (80% of 3.2km/s -> 2.6km/s -> 334km!). If the materials aren't up to that task, increasing gas density (Ne?) to alleviate this would probably help (up to the point where it makes the heat exchangers too big).
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Post by matterbeam on Nov 27, 2017 18:20:57 GMT
Helium is going to permit/ask for/require much higher blade tip speeds (and thus stronger blade materials) than something that is denser, but makes the heat exchanger part easier. It will also cause the blade reynolds number to be lower, potentially increasing the lower size limit or hurting efficiency. Carbon does keep usable strength at high temperatures (hence carbon-carbon being used on the space shuttle leading edges), but epoxy is right out. You would want something with maximal high-temperature tensile strength to weight ratio (that is feasible for continuous use, often creep limited!). Ceramics are crack sensitive, metals creep and are heavy. Maybe a monocrystalline high strength and high breaking length ceramic? CNT/C composite? SiC? Too bad diamond doesn't last. Article on carbon-carbon composite strength at high temperature: www.iccm-central.org/Proceedings/ICCM17proceedings/Themes/Materials/CARBON%20-%20CARBON%20COMPOSITES/D3.6%20Kobayashi.pdfNotable risk is it creeping. Breaking length to get mach ~ 0.8 for blade tips is brutal for helium at 3000K (80% of 3.2km/s -> 2.6km/s -> 334km!). If the materials aren't up to that task, increasing gas density (Ne?) to alleviate this would probably help (up to the point where it makes the heat exchangers too big). Thank you. I believe heat exchangers can be 'solved' by just using very large and thin surfaces. A noble gas might be the way to go about it. How did you work out that 334km figure? OT: I've found it impossible to '@' Rocket Witch.
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Post by Kerr on Nov 27, 2017 19:02:12 GMT
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Post by n2maniac on Nov 27, 2017 19:04:32 GMT
matterbeamIf that is your standpoint, I would honestly not put something like Xenon out of the question if blade strength (and then stage compression ratio / device efficiency) is the biggest limitation. Have to go through actual design to determine the sweet spot though. 1/2 m v^2 / m = K (stress / density), assume K=1, 3.3MJ/kg, divide by gravity (9.8m/s2) -> 334km breaking length. Useful metric for space tethers and energy density. We can argue about what K should be and the ratio between structural strength keeping the blades from ripping apart versus capturing lift, but it gives a good starting ballpark without needing to dive into full turbine design. Lower mach numbers can be used, but given the compression ratio of each stage is related to the square of that, going too low is painful. If you have a good reference on internal speeds of turbine parts I would love to see it to sanity-check my assumptions.
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Post by Rocket Witch on Nov 28, 2017 14:39:20 GMT
OT: I've found it impossible to '@' Rocket Witch. My display name is Rocket Witch but my username to @ with is stringwitch. Nitrided steels and nickel superalloys are used in high temperature gas turbines. In the context of their use 'high temperature' means up to 1250K; they are dense materials; and their use is often dictated by a need to maintain corrosion resistance at these temps. Removing corrosion issues, maraging and tungsten steels could be usable at somewhat higher temps, maybe up to 1500K. Titanium alloys may match this strength and temperature range while being lighter. Beyond 1500K, and without going into heavy metals like molybdenum and tungsten, it's ceramic time. NASA demonstrated an ostensibly-uncooled monolithic silicon nitride hydrolox rocket engine which reached 1600K. Silicon nitride is notable due to its thermal stability over large gradients; that is, it will actually withstand the device being turned on and powered/heated up from a cold resting state. Spinning silicon nitride into a fibre might make one of the best solutions for a turbine. And on the subject of fibres, of course, carbon is a no-brainer. It may also be worth looking into the industrial applications of sapphire; its mechanical properties are somewhat diamond-like and it can be built up as high purity monocrystals.
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Post by The Astronomer on Nov 28, 2017 14:45:48 GMT
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Post by newageofpower on Nov 28, 2017 16:06:45 GMT
Before we start, let's consider the design differences between powerplant turbines and aircraft turbines. Civil power plants attempt to minimize cost, downtime and maintenance demands while maximizing reliability and efficiency. A relatively new GE power station gas turbine weighs 330t while generating 293 MW; this is less than 1kw/kg. Meanwhile, a similarly modern GE civil aircraft turbine develops over 10kw/kg, and military aircraft engines have far higher power/weight ratios.
Military spacecraft will almost certainly seek to optimize around power to weight ratio over other concerns.
Like RocketWitch has said, high temperature turbine materials - or even the turbine itself - are unlikely to be functional at lower temperatures. As a general rule, ceramics have a reduced (read: superior) thermal expansion coefficient compared to metals and alloys; but the expected operating temperature will be so high that designers will be forced to sacrifice low-temperature usability. Using an well-known real life example, consider the SR-71 Blackbird. It's thermal expansion at operating temperature was so significant the designers were forced to build it with gaps that only sealed when the airframe had been heated sufficiently.
Working fluids in very high temperature reactors (which I assume are the heat sources for your turbomachinery) are likely to be some sort of liquefied metal, meaning that a safe reactor design must be sufficiently hot to keep the metal molten even in 'off' state. Luckily, liquid metals are usually less corrosive than high temperature steam - though care must be taken to select a coolant that will not absorb & dissolve the selected ceramic at expected temperature ranges - for example, I recall reading about liquid Tungsten having a tendency to absorb Carbon, leaching it out of multitude of carbide-based ceramics, greatly reducing their lifespan.
Though significant weight savings over civil power turbines are plausible with compromises (reduced operational lifespan, narrow bands of operational temperature or even narrow bands of 'safe' temperature) in design, I'd expect a decrease in power/weight compared to a similarly designed lower temperature turbine, simply because the known very high temperature ceramics have a lower strength-to-weight ratio than known high-strength alloys.
TLDR Conclusion: As such, actual spacecraft turbomachine based reactors are unlikely to approach CDE thermocouple based reactor power density until the advent of large-scale Graphene fabrication; CDE thermocouple-based powerplants can pass 100 kw/kg (over 50 kw/kg even with armored radiators).
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Post by Enderminion on Nov 28, 2017 16:58:54 GMT
Graphene wont change anything because you can't use it for anything, yes it has a very high specific strength but that doesn't change the fact that you can only have a single sheet of Graphene before it becomes graphite
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Post by newageofpower on Nov 28, 2017 17:47:21 GMT
Graphene wont change anything because you can't use it for anything, yes it has a very high specific strength but that doesn't change the fact that you can only have a single sheet of Graphene before it becomes graphite By Graphene, I should have clarified - I meant nanostructured Carbon. And it's unlikely you'd use such substance by itself. For example, you could thread a turbine blade with pre-tensioned CNTs, massively enhancing tensile strength along the desired axis. Another advantage would be adding Graphene flakes ( which have been shown to significantly enhance mechanical properties in many materials) to the base material.
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Post by Enderminion on Nov 28, 2017 19:15:32 GMT
yes you should have clarified
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Post by matterbeam on Nov 28, 2017 19:32:43 GMT
Looking at the BNTR concept described here (), it seems much more reasonable to use a Xenon/Helium mix as coolant. n2maniac: What is the 3.3MJ/kg figure? I have started looking at how turbine performance is determined, and high compression ratios are ideal. The higher the better. This is because the higher the pressure gradient, the better the temperature drop, and a large temperature drop across the turbine allows for greater Carnot efficiency. I used the following equation : Inlet Temperature/Outlet Temperature= Pressure Ratio ^ (1 - 1/y) y is the adiabatic gas constant. For helium, a monoatomic gas, it is equal to 1.6. So if you have a pressure ratio of 100, you can get a temperature drop of 5.62. If your inlet temperature is 2000K, it can be lowered to 355K at the exit. The highest compression ratio I've come across is the Boeing 777's latest jet engines, with 61:1. I am unsure how compression ratio relates to the stresses on the turbine blades, which determines how strong they'll have to be, and more importantly, how much the thing will mass. Only the latter figure really matters. Rocket Witch: Thank for your input. If I'm getting this correctly, I can take the figures for an actual turbine, replace its turbine blades with a ceramic, calculate the ratio of specific strengths, and reduce/increase the mass of new turbine by that ratio, assuming the actual and new turbine are operating under identical conditions. How useful is this calculation? What do you think about protected turbine blades? A very high specific strength material, such as zylon fibres, covered by a ceramic 'shield' so that it is not in direct contact with the gasses. Active cooling would run in the gap between the shield and the blade core, to prevent heat from being carried across. The Astronomer: Thanks! newageofpower: The kW/kg ratings of turbofans are very promising, but they are lower than it should be for our purposes. For example, they have a massive fan in front, a compression stage, combustors, an expansion stage and a nozzle, plus or minus structural support supposed to withstand 10g+, starter engines and so on. We only want the expansion stage alone. Based on that, the kW/kg figures of turbocompressors, such as the 150kW/kg Brayton pumps in the SSME's rocket engines, are somewhat relevant. One aspect of turbine design is that they operate in multiple stages. Each stage has a compression/expansion ratio, and they add up across all stages to achieve a total pressure ratio of 30-100+. Only the first stage would have to deal with very high temperatures. If we have a 2 stage turbine, with each stage having a compression ratio of 10 and the inlet temperature starting at 2000K, then the first stage operates across a temperature range of 2000K to 843K (assuming y=1.6). The second stage operates at much more moderate temperatures, but the overall pressure ratio is 100.
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Post by newageofpower on Nov 28, 2017 20:48:36 GMT
matterbeamNotice my last line includes the entire reactor system - the comparison of a reactor's ancillary equipment has to be figured in; for example, a liquid metal/ceramic turbine would need a 'warming engine' that would likely be more complex and heavier than a starter engine, although I suppose that could be an externalized system that would perform the same function would let us save weight at the expense of making powerplant restarts without support facilities implausible. Furthermore, aircraft/rocket propulsion systems do not include apparatuses for translating mechanical energy into electrical power; though this should be reasonably light. From literature I've read, the most critical limitations in modern turbomachinery is that temperature rises as compression ratio increases; maximal temperature (and thus compression) is limited by the heat tolerance of the turbine blade alloy. The other limitation is that most alloys suffer increased creep fatigue when operating near maximal safe temperature and under high stress. Ceramics are far more resistant to high heat load and many exhibit superior creep fatigue mechanics. For these reasons, significant research is already taking place in ceramic composite turbines.
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Post by matterbeam on Nov 28, 2017 23:50:02 GMT
matterbeam Notice my last line includes the entire reactor system - the comparison of a reactor's ancillary equipment has to be figured in; for example, a liquid metal/ceramic turbine would need a 'warming engine' that would likely be more complex and heavier than a starter engine, although I suppose that could be an externalized system that would perform the same function would let us save weight at the expense of making powerplant restarts without support facilities implausible. Furthermore, aircraft/rocket propulsion systems do not include apparatuses for translating mechanical energy into electrical power; though this should be reasonably light. From literature I've read, the most critical limitations in modern turbomachinery is that temperature rises as compression ratio increases; maximal temperature (and thus compression) is limited by the heat tolerance of the turbine blade alloy. The other limitation is that most alloys suffer increased creep fatigue when operating near maximal safe temperature and under high stress. Ceramics are far more resistant to high heat load and many exhibit superior creep fatigue mechanics. For these reasons, significant research is already taking place in ceramic composite turbines. To put this into context: I am trying to estimate the mass of the four main components (heat source, turbine, generator, radiator) of a turbogenerator system to add to my Advanced Solar Power Part II post. The source of heat is a heat exchanger, such as tungsten, absorbing sunlight until it reaches a high temperature. The heat is then passed onto a gaseous coolant, which reaches high temperatures and pressure. This coolant is what enters the turbine. The turbine expands the gas and converts the pressure and temperature difference between the inlet and outlet into kinetic motion. Therefore, the highest temperatures and pressure are encountered by the first set of blades. The temperature within the turbine is strictly lower inside than the initial temperature of the coolant. I also with the calculate the turbine's mass and performance separately, so that I can use a different set of numbers to describe the electric generator. The reason I came to COADE forums is because I simply could not find data on high temperature turbines that was neatly separated into masses and pressure ratios for individual turbine stages, so I could not comfortably extrapolate from them using higher temperatures or different materials. I still think using ceramics is a worthwhile change. While the specific strength of the material is lower than an alloy with a lower temperature limit, requiring a more massive turbine, I believe the loss in power density is lower than the increase in overall system power density that comes from a higher operating temperature. Do you think looking up supersonic turbojets is a reasonable avenue of research?
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Post by newageofpower on Nov 29, 2017 0:37:57 GMT
matterbeam Personally, I think we need to look up literature in existing liquid-metal turbines; the properties of a compressible gas coolant are extremely different from a liquid coolant. Liquid metal turbines have been built before, but I'm having trouble finding scientific papers on the subject. IIRC, the Soviet Alfa-class submarines used a lead-bismuth design. Once you can extrapolate the differences between, say, a naval gas turbine and a naval liquid metal turbine, we can then use that to extrapolate a hypothetical min-maxed Very High Temperature reactor turbine that would be mounted on a spacecraft.
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