## 4 Results and Discussion  
### Turbine Performances  
The axial micro-turbine has been designed to work with pure NovecTM649 fluid. In order to study the impact generated by the use of the mixture NovecTM649/HFE7000 on the operation of the expansion element, the dimensionless parameter A was calculated in both cases (mixture and pure fluid) as a function of the pressure ratio. This value corresponds to the normalized blade throat area (A*/A0), which represents an intrinsic value of the expansion component (see [81] for theoretical details on the compressible and isentropic flows of real gases).  
Due to the conception of the expansion element, the dimensionless parameter A is expected to be a constant value for the pure fluid that the turbine was initially designed for. The behavior of the pure fluid on the turbine, represented with purple data in Figure 3, is found, as expected, to be constant, which first confirms the accurate functioning of this element.  
In the case of the mixture, the value of \(\tilde{\text{A}}\) is identical to the one for pure fluid. This result, therefore, experimentally confirms that the operation of the turbine is not affected by the change in working fluid, provided that the physical dimensionless parameter (\(\tilde{\text{A}}\)) is considered to be relevant and generalized to real gases.  
The overall electrical efficiency of the turbine, shown in Figure 4 on the right ordinate, varies in the same way as the mixture and with the pure fluid, confirming the suitable operation of the expansion component with the mixture. There is a slight decrease of 10% in the value of the efficiency in the case of the zeotropic mixture: this difference can be attributed to the evolution of the mixture's thermodynamic properties at the turbine inlet compared to those in pure fluid.  
During the experiment, the turbine reached, respectively for the pure NovecTM649 and the zeotropic mixture NovecTM649/HFE7000, a maximum electrical gross production of 0.715 kW \(\pm\) 2 W and 0.703 kW \(\pm\) 2 W. At these specific points, the isentropic efficiencies of the expansion component are found to be respectively 61.4% and 61.5%. With these isentropic efficiencies, the expansion component is ranked at the average level of the expansion devices in the 1-10 kW power production range, in comparison to the available experimental literature [23], to the best of our knowledge.  
For the overall electrical efficiency of the turbine, the values are found to be 58.2% for the pure fluid and 56% for the mixture. These values, replaced within the work of [PERSON] et al. [82] and described in Figure 5, rank the present turbine among the most efficient for this type of technology. Compared to other types of technologies, for a power production close to 1 kW, the turbine is very interesting because its performances are comparable to that of the Scroll technology, which is the most widespread technology in small power ORCs as observed in Figure 5. The energy conversion at the  
Figure 4: The turbine’s overall electrical efficiency according to the pressure ratio.  
Figure 3: The turbine’s dimensionless parameter according to the pressure ratio.  
generator level is very efficient; therefore, the best way to further increase the overall efficiency of the turbine is to improve the expansion element itself and, thus, increase its isentropic efficiency.