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STIRLING ENGINES WITH ROTARY ANNULAR TRILOBIC PISTONS (SPRATL)

Presentation Stirling's cycles State of the present art Specifications The SPRATL's answer Technical details Thermal study of the regenerator

Principle and advantages of the regenerator

Stirling's cycle in (P,V) and (T,S) diagrams

    The regenerator (RGN) is fundamental to recycle inside the device (1) the thermal transfers of the isochoric stages (2-3 et 4-1) and
thus to stretch  toward the limit of Carnot. The technical difficulties it currently raises are a major brake to the improvement of Stirling's engines. The current regenerators are often victims of the alternate flux of the fluid, not encouraging any good thermal exchanges, unless to use fine grids that pose two new problems then : their cost and especially the losses of loads by lamination of the fluid. Otherwise, they are often cumbersome and difficult to isolate.


    Thus, the present invention makes the choice of unidirectional out-flows of fluid between every couple of PRATL machines, one hot (2C) and the other cold (2F), what permits to constitute a thermal exchanger with 4 pipes,
of preference rolled up in helical. These pipes are browsed by the fluid: two among them constantly bring the fluid from (2F) to (2C), and the two other in the inverse sense: from (2C) to (2F). Besides, they are mutually in thermal contact and constitute an optimal exchanger of temperatures because :
-    the length of the pipes can be strongly raised while keeping a good compactness because they are rolled up in helical,
-    the section of the pipes can be sufficiently big to make negligible the load losses by lamination, of preference gaseous,
-    it is easy to isolate such a regenerator by using a cylindrical muff (ISO) with reflecting faces (to prevent the radiative losses) and having at least one empty cylindrical cavity (to block the thermal conduction from the regenerator toward the outstide).






Optimization of Stirling's cycle with SPRATL engines

    That the cycle is motor or receptor, one sees that the working is optimized with an effective regenerator, but also when its area is maximized. So that the area is maximized, the transformations must perfectly follow the previously described thermodynamic trajectories; the figures 1A to 1F describe 
indeed the normalized cycles, with a perfect regenerator with ideal thermodynamic evolutions of the fluid. Actually, the cycle followed by the fluid moves away meaningfully of the one of Stirling as drawn on figures 1G and 1H. Four shortcomings are in general present :
-    DTC : defect of homogenization of the fluid to the hot temperature,
-    DTF : defect of homogenization of the fluid to the cold temperature
-    DVMAX : volumic defect at maximum volume Vmax,
-    DVMIN : volumic defect at minimum volumet Vmin.
thin feature : ideal Stirling's cycle ; hatched feature : current cycle ; strong feature : improved cycle of SPRATL engines
    When these shortcomings accumulate themselves, the mechanical work of every cycle decreases (smaller area) and the thermodynamic output of the cycle moves away strongly of the optimum of Carnot (because of imperfect thermal exchanges). It is why the present invention fights especially against these difficulties :
-    defect of homogeneization of temperature due to :
    o    the imperfect recovering of calories or frigories in the regenerator,
    o    the slowness of the thermal diffusion in the fluid when it is put to the contact of the hot or cold sources,
-    Volumic defect due to :
    o    imperfect kinematics of Stirling's machines,
    o    possible flights of fluid.

    The present invention settles
completely the volumic problems thanks to a kinematics which perfectly respects the isochoric stages. It also limits the flights of fluid well thanks to all surfacic contacts and the possible use of many segments of tightness. Moreover, it allow intense thermal transfers :
-    by convection :
    o    in the cold and hot machines where the fluid is injected in the rooms, then transported within the cold and hot machines  
          , while being
entirely surrounded by metal walls transmitting it their temperature (cold or hot),
    o    in the regenerator allowing a setting in temperature much better
just before the isochoric stages.
-    and by diffusion nearly the contact walls/fluid.

    The approach by convection is essential because it is a mean of a lot faster homogenization than the only diffusion. Thus, as shown 
in strong features on figures 1I and 1J, thanks to the present invention, the cycles are a lot nearer of the ideal Stirling's cycle (in thin features) and bigger than the present cycles (hatched feature). The isotherms will be as much better respected that :
-     the machine will turn at low speed : the strong volumes by round of piston (PRA) with the machines (2,2F,2C) is in itself an asset, permitting to make work a lot of fluid in spite of a relatively weak rotation speed,
-    the fluid will have an elevated thermal conductivity : one will be able to use some fluids under more elevated pressure and/or specific gases
(Hydrogen, Helium) already met with success in the industry of the Stirling's motors.




Thermal working of the SPRATL regenerator

        The regenerator (RGN) is fundamental to recycle within the device (1) the thermal exchanges of the isochoric stages and
thus to stretch toward the limit of Carnot. In the device (1), the regenerators assure unidirectional out-flows of fluid (inside a given pipe) between the hot machines (2C,2C1,2C2.) and the cold machines (2F,2F1,2F2.), what permits to create thermal exchangers with pipes, of preference rolled up in helical. The half of these pipes constantly drives the fluid from (2F,2F1,2F2.) to (2C,2C1,2C2.), and the other half in the inverse sense: from (2C,2C1,2C2.) to (2F,2F1,2F2.).
    Besides, they are mutually in thermal contact and constitute an almost-perfect exchanger of
temperatures. As it has already been evoked, it is possible to pair some couples of pipes in only one : then the out-flow becomes unidirectional and continuous. Indeed, the fluxes of a category of rooms (big: GC) (small: PC) of a machine (2, 2F,2F1,2F2, 2C,2C1,2C2.) are intermittent, identical and in opposition of phase ; so that while connecting only one pipe on the 2 exits of identical rooms of a same machine, the flux, in addition to be unidirectional, becomes continuous.

   The thermal aspect of the regenerator is entirely explained in SPRATL patent (in french) or in this extract from the patent (in french) by using the figures 5A, 5B et 5C and by calling the
equations of the thermal diffusion. Some regenerating outputs of more than 99% are reachable with very acceptable sizing and usual material like steel..
Modelling of the regeneratorn, sizes and results of the SPRATL engines


Presentation Stirling's cycles State of the present art Specifications The SPRATL's answer Technical details Thermal study of the regenerator
SYstems for COnversion of MOtions and REnewable ENergies Motors
& Pumps
MPRBC Concept Concept POGDC
Special
STIRLING's engines

Back to the main menu