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Wave shape heat exchanger
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IPS-Micro-CHP Comparison: Wave form heat exchangers to tube bundle heat exchangers
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Internal wave form heat exchangers
(International Patent: PCT/AT01/00169,2001)
Paralell gap wave form heater head

One of the major obtacles in the development of high efficient Stirling engines is the design of the heater head, which is needed to transfer the driving heat flow from the outer combustion gases to the working gas on the inside. This is due to strongly differing heat transfer rates on both sides. While the working gas, in most cases Helium, is pressurized and passing the heat transfer area with a velocity higher than 10 m/s, flow of the combustion gas on the outside is laminar and near ambient pressure. Thus, heat transfer coefficients for the working gas are often much higher than 1000 W/m²K whereas heat transfer coefficients on the side of the combustion gases stay below 100 W/m²K.

Article In order to overcome this barrier, Stirling-Engineering developed a new type of heat exchanger for Stirling engines. This type is located inside the machine by forming piston domes an cylinder heads like combs, which fit into each other as the piston approaches the cylinder head in upper dead centre. In 3-dimensions the heat exchanger surface looks like a wave form profile, which was the source for naming these type of heat exchanger. By this means, a great number of narrow channels can be created between piston dome and cylinder head, which is prevented when using standard heat exchangers mainly due to manufacturing reasons, as outlined above. Evidently, dead volume and pressure loss is smaller in that way resulting in higher power output and efficiency of the machine. It should be noted, that an optimisation of Stirling heat exchangers without any restriction in terms of manufacturing results in geometry providing an infinite number of channels with zero length and zero cross section area.

Article This is excatly the path, Stirling-Engineering follows with this new and innovative type of heat exchanger offering the opportunity to overcome the limitations for power output and efficiency of Stirling engines and approach the ideal values much better than standard heat exchangers nowadays do. Cycle evalutions for a Stirling engine designed by Stirling-Engineering show the advantage in numbers. While the engine based on wave form head exchangers reaches a cycle power output of 9,25 kW and a cycle efficiency of 48,4 %, for the same engine equipped with tube bundle heat exchangers, however adapted and optimised for the specific design, an efficiency of 42,5 % only is calculated maintaining constant power output of 9,25 kW. The latter configuration does not contain any additional dead volume for manifolds in order to achieve a proper flow in and out of the tube bundle heat exchangers.

If an appropriate non active tube length for heat transfer is taken into account as a result of the need for leak tight bonding as well as non sufficient flow of the outer heat transfer fluid, calculated values for power output and efficiency drop further to 8,80 kW and 41,8 %, respectively.

Information Literature: "Application of internal wave form heat exchangers to Stirling engines"
Prof. Dr.-Ing. B. Thomas, Reutlingen, 22.7.05

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