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Dynamic non-contacting seals have been identified as the most appropriate sealing technology for reduced-scale and high-speed turbomachinery applications since they provide a clearance gap preventing rotor-to-seal contact during operation. Yet, any clearance, however small, between rotor and seal permits the passage to fluid flow across regions of unequal pressure as a leakage which penalize the overall system efficiency. A thorough theoretical and experimental investigation of these types of gas seal at a reduced scale, however, is missing today. Furthermore, in thermodynamic cycles, fluids often operate within the vicinity of the saturation line where the well-known ideal gas law is not valid. For reduced scale turbomachinery seals operating close to the fluid saturation line, validated models to capture the real gas effects are not available yet. The aim of this thesis is to address these shortcomings. Initially, a qualitative research approach has been used for the selection of the ideal seals for reduced-scale applications. Using the qualitative data available from the literature of large-scale seals, a weighted decision matrix has been implemented. For assessing the robustness of the selection process, the sensitivity analysis has been carried out by using different weight distributions. The top two ranked seals, namely, Hole Pattern Seal (HPS) and Pocket Damper Seal (PDS), have been identified as the most promising seals for reduced-scale turbomachinery applications due to their low leakage rates and their low influence on the rotordynamic system. In order to predict the seal performance, the commonly used bulk-flow approach has been used for the numerical modeling approach. Furthermore, for the comparison purpose, three different state-of-the-art models have been implemented. As a benchmark, the predicted results from the different models are compared with the experimental data of well-established literature. A good agreement between the different seal models and the experimental data has been observed. A dedicated hermetic test-rig for the experimental investigations on the reduced-scale seal has been designed and built. A modular test section design has been adopted to test different types and sizes of seals. Six different reduced-scale seals, both on cylindrical and conical cross-section, have been investigated experimentally at different inlet boundary conditions with R134a and air. The experimental data for different seals were compared between the predicted results from the developed models as a validation process. Compared to the-state-of the-art models, the author’s model demonstrates a superior prediction quality in terms of leakage, cavity pressure, and temperature distribution in all test cases. The Hole Pattern Seal (HPS) has been identified as the most effective seal for reduced-scale applications. The experimental data suggests a strong impact of the real gas effects on leakage. Compared to a high degree of superheat the leakage increases by up to 10% for an operation close to the saturation line. The author’s model shows better prediction quality at different degrees of superheat compared to state-of-the-art models and demonstrates its ability to capture the real gas effect.