This thesis investigates the performance of vaneless radial and conical diffusers operating with supercritical carbon dioxide (SC-CO2) under various geometric configurations. The motivation arises from the growing interest in compact, high-efficiency turbomachinery suitable for supercritical fluid cycles. The study employs a combination of one-dimensional modelling (1D) and computational fluid dynamics (CFD) to analyse pressure recovery in view of frictional effects and flow separation. All the CFD simulations were conducted using commercial software Ansys CFX 2024 and 2025 integrated into Ansys Workbench and all the coding, including 1D modelling and post-processing was done with Matlab 2024. The performance of SC-CO2 operating diffusers was compared against air simulations at ambient conditions at matched Mach regime. The results show the advantageous pressure recovery of SC-CO2. Furthermore, the comparison between 1D and CFD modelling shows acceptable agreement for low expanding diffusers but reveals significant deviations in outcomes close to separated flow conditions.