The influence from the working medium on the profile loss in compressor and turbine airfoils

A number of CCS-technologies are currently being developed for the reduction of CO2 emissions from thermal power stations. One such technology is the oxyfuel process, in which a mixture of CO2 and steam is used as the working medium. The semi-closed oxyfuel combustion combined cycle (SCOC-CC) is an oxyfuel cycle where the working medium mainly consists of CO2 (85-95%). Current practice is to design turbomachinery using 1D and 2D flow tools, which primarily rely on loss models derived from experiments with air. For the oxyfuel case, the losses are hence extrapolated from air to a CO2/steam mixture, which can have adverse effects on the accuracy of the loss model. Therefore, the applicability and accuracy in using profile loss correlations derived with air when changing the working medium to the oxyfuel like environment of pure CO2 was investigated. The reason that 100% CO2 was chosen as the working medium and not a CO2/H2O mixture is that the water content present is relatively low and varies from case-to-case. Hence, a general water content could not be specified that was relevant for all cases. The study was done with typical compressor and turbine airfoils using a steady-state Navier-Stokes' 3D flow solver. This solver type can resolve the boundary layer (y(+) of about unity) rather than relying upon a boundary layer equation, - hence eliminating the latter as a source of error. The hypothesis was that the profile loss depended on the viscosity, and that amendments to the viscosity would affect the profile loss. This trend was observed, e.g. when changing the working medium from air to CO2, the profile loss coefficient (Y-p) for the compressor was reduced with 25% and for the turbine with 6%, respectively. A slight difference in profile loss for an individual cascade was found when changing the working medium from air to CO2. Theoretically, this difference leads to an increased mismatch of the stages downstream even at design point, and thus increases the losses and reduces the stability. However, the difference in profile loss is relatively small at the design point, and thus it is the authors' opinion that the practical effect will be quite small. Therefore, it is considered safe to use loss correlations derived from air for design point calculations even when the working medium is CO2. However, there is a certain risk involved that air- based loss models are not capable of predicting the behavior over the full operating range, as the boundary layers risk to behave in a different manner. Another aspect that was considered is how the wet surface area (physical size) of a turbomachine performing the same work (mho) will change between the two gases. This is important as the total profile loss in a whole compressor or turbine is directly proportional to this change. The conclusion was that the total wet area would increase by some 20% for CO2 compared to air.