Preliminary design of Axial-flow compressors

This project aims to provide the preliminary design of axial-flow compressors with a variable mean diameter in order to create a more realistic model. To achieve this, the developed model assumes a constant tip radius, with the hub and mean radii varying accordingly. Each stage is composed of three sections: the rotor inlet, the rotor outlet or stator inlet, the stator outlet. The stator outlet coincides witch the rotor inlet of the following stage. The model is versatile and applicable to any fluid, as it uses input data such as inlet pressure and temperature, mass flow rate, and the desired outlet pressure. The fluid type can be selected, and the model will treat it as a real gas. To adapt the compressor to different fluids, the rotational speed and the number of stages can be specified. Other input parameters include blade geometry, such as aspect ratio, thickness-to-height ratio, and blade family type. Additionally, the degree of reaction, flow coefficient, and work coefficient for the first stage need to be specified, while the same quantities are calculated for subsequent stages. It’s important to observe that also the isentropic enthalpy variation can be selected for each section. Some assumptions are made to calculate the required quantities: a constant meridional velocity and a repeating stage design. The process of maintaining a constant tip radius is iterative, as certain quantities need to be adjusted to achieve the desired values. The program outputs fully characterize the compressor, providing results at the hub radius, mean radius, and tip radius. These outputs include thermodynamic properties, velocity triangles with fluid angles, blade height, and the number of blades. The free vortex model is adopted to calculate the quantities at hub and tip radius, starting from the mean radius. The next step involves calculating the design incidence angle and deviation angle at each radius (hub, mean, and tip) using Aungier's criteria with an iterative process. These angles are essential for defining the blade geometry, including the metal and camber angles. These results are based on an initial assumption about the isentropic efficiency of each stage. To achieve the correct value of this efficiency, it is recalculated using loss coefficients. The new isentropic efficiency is then substituted into the calculations and the quantities are recalculated. This process is iterative and continues until the two values of isentropic efficiency converge. The loss coefficients are calculated using established literature correlations, which account for profile losses, secondary losses, end-wall losses, shock losses, and tip clearance losses. Once the entire stage is defined, including velocity triangles and blade geometry, the onset of stall is analyzed. Three criteria are used and compared: the De Haller criterion, the Koch criterion, and the Aungier criterion. This analysis is useful during the preliminary design phase, as it can detect the onset of stall and help avoid it by adjusting some of the input parameters discussed earlier. The fluid used in this analysis is CO2, chosen due to recent advancements in technologies such as carbon capture and storage (CCS) and power cycles. At the conclusion of the study, an air compressor is also analyzed, and the results for CO2 and air compressors are compared. Since CO2 is denser than air, the input properties of the models must account for this difference, affecting factors such as rotational speed, blade aspect ratio, and thickness-to-chord ratio. The results for both configurations will be analyzed and compared, with a particular focus on the characteristics of the blades. Further developments of this work could involve assuming a specific blade type as an input and conducting a CFD analysis to refine the design further.