In this paper a method to evaluate the expansion of condensing steam in presence of incondensable gases is presented. The expansion of such a mixture involves thermodynamic processes very different from those related to the expansion of pure steam (as it happens in conventional steam power plants) or to the expansion of a gaseous mixture without steam condensation (as it happens in gas turbine power plants, combined and mixed cycles). In this paper these thermodynamic processes are investigated and an evaluating method is developed. In particular using this method it is possible to evaluate the excess turbine back pressure, that is the pressure increase with respect to the steam saturation pressure, and the specific expansion work of the mixture. The proposed method is then applied in order to evaluate the performance of an hydrogen/oxygen (fuel/oxidiser) cycle, where the presence of incondensable gases into condensing steam is unavoidable owing to the processes that give H2 from fossil fuels and O2 from air. The results of the developed investigation confirm that the presence of incondensable gases can not be neglected for a realistic evaluation of H2/O2 cycle performance.

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