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Low-pressure turbine system heat xchanger?

  1. Jan 9, 2016 #1
    Hi Guys,

    A question that ive been mulling in my mind over the last few days, is it is practically possible to make a low pressure 'turbine' system at atmospheric pressures?

    I was looking at a low-pressure stirling engine, and wondering if the same thermodynamic principles would equally be able to applied to build a turbine system that could produce a model that is able to at least keep itself 'turning', even if it had little practical output that could be taken advantage of?

    As a specific example, if I were to have a say:-

    - A meter of steel pipe (which is 100mm diameter for argument sake)
    - Had a fan mounted at each of the pipe which both are directly connected with a drive shaft between them (i.e. one fan forces air into the tube, the other extracts it)
    - The steel tube is 'heated' with either a water jacket or exposed to some other form of heat source (say a chimney stack)

    My idea with using the ideal gas law is that if air is forced into the steel tube, it is heated, it should logically increase its pressure and apply more pressure on the outlet fans, so there is theoretically overall a net gain in the system. Obviously, the fans would need to started by a hand spin or something equivalent, but once they started, is the system able to self sustain?

    If the gas in the tube could have its temperature increased by say 80 degrees, then gas going in at one atmosphere at 293k would have a pressure increase of 27% above atmosphere.

    Im not saying its a super amazing system that is able to yield any material practical use, but just curious if it is physically feasible at a theoretical level where if the heat exchanger kept applying heat to the gas in the tube, where the fans will keep spinning?

    Thanks for your help!

    Cheers,
    P
     
  2. jcsd
  3. Jan 10, 2016 #2

    Nidum

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    A conventional gas turbine will not work as you have described .

    There are possibilities though . Long before actual gas turbines were thought of there were lots of basic turbines in use - windmill , watermill and chimney jack being best examples .

    The chimney jack consisted of a very simple turbine located in the chimney of a domestic kitchen or a bakehouse . Thermal currents in the chimney could drive the turbine quite effectively and provide enough power to turn things like roasting spits . There is sketchy evidence that attempts were made to build industrial versions with proper furnace , larger size turbine and high chimney .

    The turbines used were little more than few large area fan blades on a vertical axis spindle .

    I have actually seen a chimney jack working . When I was at Mtu I travelled all over Bavaria and surrounding areas . There were many delightful old restaurants out in the sticks and one of these had a traditional large open hearth still in use for cooking and complete with working chimney jack . (The meal I had will remain in my cherished memories for the rest of my life) .

    Anyway back to the question -

    I mention the above because the chimney jack concept would still be a viable way of getting small amounts of useful power from heat sources which could not easily drive anything else .

    You mention Stirling engines . These are most commonly piston based . These can be made to work from remarkably feeble heat sources like candle flames and sunshine but getting a Stirling engine to produce significant power levels reliably is more difficult .

    If you are interested in how real gas turbines work ask some more questions .
     
    Last edited: Jan 10, 2016
  4. Jan 10, 2016 #3

    Nidum

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  5. Jan 10, 2016 #4

    JBA

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    Contrary to the above response, in principal, what you are proposing is essentially the equivalent of a gas turbine engine with the compressor fan, intervening burner and turbine. On the other hand, actually making a working unit could be a more of an issue and require more than a simple straight pipe between the compressor and the turbine because you want the heating to accelerate the air flow rate going to drive the turbine. Further, the performance of your unit will likely be very inefficient due the fact that heating of the air through direct in-stream combustion, as in a standard turbine engine, is much more efficient than trying to heat the air by conduction through the pipe wall from an external heat source.
     
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