mitch
Senior Member
Joined: August 2014
Posts: 285 
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Post by mitch on May 10, 2015 1:26:02 GMT -5
Just a quick thought I had, I was doing a little reading on a mitsubishi turbine www.mhi-global.com/company/technology/review/pdf/e483/e483001.pdf and thinking about some other high temp turbines If we could create a material that could withstand higher temperatures, we could use less air drawn in from the compressor for cooling, and more for burning fuel, increasing both efficiency and power output. Has any big manufacturer ever tried to create an internally water cooled turbine blade? Would centrifugal forces be too great for a pump to reliably push cooling water through passages in the turbine blades? I think of it like comparing water cooled piston engines to air cooled piston engines. Anyone else ever thought about this? |
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Post by racket on May 10, 2015 2:31:14 GMT -5
Hi Mitch
Water cooling would require passageways in the blading , something beyond us DIY'ers ..............LOL, at some time in the past, everythings been tried by a manufacturer somewhere.
What I would think a DIY'er could try is air cooling the blades by having half hot gases, half cold air, passing through the turbine wheel , it'd have to be an axial wheel , it was tried on the early aircraft turbochargers which had to cope with leaned out exhaust temperatures whilst using far less heat resistant metals than we have available to us today , just split the inlet annulus into two 180 degree segments and feed extra hot gases through one whilst cooling air through the other , its the mean temperature that the wheel metal is at thats important .
Cheers
John
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Post by finiteparts on May 22, 2015 19:41:05 GMT -5
Just a quick thought I had, I was doing a little reading on a mitsubishi turbine www.mhi-global.com/company/technology/review/pdf/e483/e483001.pdf and thinking about some other high temp turbines If we could create a material that could withstand higher temperatures, we could use less air drawn in from the compressor for cooling, and more for burning fuel, increasing both efficiency and power output. Has any big manufacturer ever tried to create an internally water cooled turbine blade? Would centrifugal forces be too great for a pump to reliably push cooling water through passages in the turbine blades? I think of it like comparing water cooled piston engines to air cooled piston engines. Anyone else ever thought about this? Hey Mitch, There has been a lot of work done in advanced cooling schemes over the decades, but the only manufacturer that I know of using steam is GE. Their 7H and 9H turbines used steam cooled, single crystal turbine components to achieve ultra-high efficiencies in combined cycle operation. site.ge-energy.com/prod_serv/products/tech_docs/en/downloads/ger3935b.pdfYou don't want to use liquid water because the through-thickness temperature gradient would be HUGE! Thermal stresses are created by temperature gradients and the stronger the gradient, the higher the local peak stresses. The potential also exists when using water to actually insulate the hot surface due to the presence of the vapor layer. If you had a region of metal which lost it's backside cooling due to the vapor layer keeping the cool water off the surface, it would heat up quickly. Then if the vapor layer moved and water suddenly hit the hot metal, you could thermally shock that metal and crack it. But, you can use steam to cool the hot gas path parts as shown in the GE paper. The use of higher temperature materials for turbine parts is a constant struggle in the industry. Improved creep resistant materials have allowed turbine firing temperatures to climb...see the plot in their work... www.virginia.edu/ms/research/wadley/high-temp.htmlYou can see that the advent of single crystal (S-X) materials have helped the metal capability. This is because at high temperatures and loads, the material "creeps"...this means that it plastically deforms over time due to the stress, even when the stresses are well below the yield stress of the material. By removing the boundaries that exist between metallic crystalline structures and going to a single crystal, you reduce the slip mechanism that happens at those boundaries. But even that isn't enough. Advanced air cooling helps keep the metal parts alive in gas flows that are hundreds of degrees above the metals melting temperature. Thermal barrier coatings (TBC) provide an insulation blanket so that the heat doesn't transfer through as fast, but TBC still requires backside cooling, just less than what is required without a TBC. I have seen some sales gimmicks saying that just applying their coating will somehow keep your parts cool and live longer. Think of it like this...you have your cold drink in a thermos. It will eventually reach the ambient temperature, it just takes longer. If you wanted to keep it cool, you would have to provide a means of removing any heat that got through the thermos wall...plop some ice in there and it's change of phase can absorb that extra heat for a while. So just putting on a coating doesn't mean that you will have a cooler part...you have to balance the heat loads. So moving past TBC, the capabilities of metallic components is quickly reaching a limit. Higher temperature materials like ceramics off the next best step in firing temperature increases. They are hard to make...hard to mount because they usually have low thermal expansion coefficients and have to mount to metal parts with high thermal expansions...they are usually brittle...but the industry is getting there. www.netl.doe.gov/publications/proceedings/99/99ats/4-8.pdfThe key advantages of ceramics are multiple...they are lighter than nickle or cobalt alloys...the higher temperature capabilities mean that you potentially can run them without cooling air, air which has to be compressed and thus cost fuel burn... The interesting thing is that the water and ceramics are not new ideas! Water... ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930089053.pdfntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930089208.pdfntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090452.pdfetc... A final interesting one...they use the water to cool the air prior to entering the blade cooling passages for contingency power. Remember, contingency power reduces engine or part life, but it may be needed to avoid a crash or other worst case scenario. ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870010847.pdfCeramic... ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082046.pdfIt is appealing....since water's thermal conductivity is 24 time that of air and it's heat capacity is 4 times that of air, it does make a better coolant! The heat capacity means it takes more heat to raise the waters temperature and the thermal conductivity means it can move heat faster. So jumping into 70F water shouldn't be a surprise that it feels colder than standing in 70F air...it pulls the heat out of your body faster (24 times faster!!!) and it doesn't heat up as fast... ~ Chris |
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