Organic Rankine cycle (ORC) has gained an increasing worldwide attention due to its high efficiency in converting low-grade thermal energy into electricity. The expander is the most critical component in the ORC system. Among the influential factors that define the performance of the expander, the velocity coefficient of the nozzle is crucial. This work numerically investigates the effects of the nozzle height, length, surface roughness, outlet geometric angle, and expansion ratio, on the velocity coefficient of the nozzle in the ORC turbine with hexamethyldisiloxane (MM) as working fluid. In the 3-D viscous numerical analysis, the shear stress transports k-ω turbulence model is employed and the numerical method is verified by the experimental data of the nozzle with pressured air based on hotwire technology. The numerical results show that the velocity coefficient is almost independent of expansion ratio compared to other factors due to the relatively small flow boundary layer and high Reynolds number. Since the existing correlations for the gas nozzle cannot well predict the velocity coefficient of the organic nozzle, an empirical equation is proposed according to the numerical results with the maximum deviation of 3.0%.

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