Organic Rankine Cycle (ORC) Power Plants can be of greatimportance in the energy transition as they are suitable for converting wasteheat to power and can utilize renewable energy for their operation. To improvethe efficiency of ORC Power Plants, the physical phenomena inside thesemachines must be understood. Understanding the boundary layer of complexorganic fluid flows in these systems is crucial, as it is estimated to beresponsible for one-third of the losses in turbomachinery. n this thesis, two-dimensional steady state boundarylayer flows of nonideal gas have been investigated numerically. The objectivewas to find the influence of complex fluid nonideality, characterized by idealgas departure, on boundary layer flows. In particular, a high-speed densevapour expansion of organic fluid Hexamethyldisiloxane (MM) inside a de Lavalnozzle test section has been studied. The nozzle is part of a measurementcampaign to collect experimental data with the purpose of validation andcalibration of Non-Ideal Compressible Fluid Dynamics (NICFD) software.  MATLAB program was developed for solving thetwo-dimensional steady state boundary layer equations including generalthermophysical properties. Transition prediction methods, the algebraicCebeci-Smith turbulence model (CS-model), and state-of-the-art thermophysicalmodels were implemented. The program was verified and validated for air withliterature. The turbulence model was validated with experimental data oflarge-scale zero pressure gradient adiabatic flows. The results match for theentire Mach-number range from 0.2 up to 2.8. The program also proved to becapable of predicting the turbulent boundary layer along a flat wall inside ade Laval nozzle expanding air. Deterministic simulations of the boundary layer along thecurved wall surface of the aforementioned nozzle expanding MM were performed.The results showed a larger decrease in the newly defined property C_e inthe core flow along expansion compared to air. In contrast, the propertygradients; namely density ratio c and Chapman-Rubesin parameter C,inside the boundary layer were found to be negligible. Furthermore, the resultsshow that the influence of the pressure history upstream of the nozzle throatis relatively small or even negligible in the diverging nozzle section. Theboundary layer displacement thickness for both laminar and turbulent flow wasfound to be negligible compared to the nozzle cross section, which results in anegligible effect on the nozzle core flow. The program needs further validation for flows departingfrom ideal gas. First, the flow condition in de Laval nozzles, laminar orturbulent, needs to be obtained by conducting experiments. Then, sensitivitystudies need to prove if the inviscid nozzle design is a robust design forviscous flows too; namely, being insensitive to changes in total inputconditions, uncertainties in closure coefficients, and variations in upstreampressure history.