Study On Loss Mechanism And Clocking Effect In Centrifugal Compressors And Steam Turbines

Two main turbomachines including a centrifugal compressor and an industrial turbine were discussed in this thesis. Numerical simulation and analysis were conducted for these two cases by steady and unsteady CFD methods. The loss mechanism in different flows was focused and then clocking effect was studied for different turbo machines. Finally improved assembly design projects were proposed for each case based on the clocking effect research. The result provides an advanced reference for the industrial applications. The research work includes two parts:The first part was focused on the numerical analysis on the flow in the semi-open impeller stage in a centrifugal compressor. First, under design condition, the unsteady flow field was analyzed based on a full channel computational model. And a diffuser improvement method was proposed to improve the aerodynamic performance and to widen the stable operational range on the basis of the flow separation phenomenon. The unsteady full channel flow field under two off-design conditions was then analyzed to study in depth the vortex shedding frequencies near the stall operating condition. The results can provide reference for the exploration of impeller fracture cause in the actual operations. Second, the clocking effect was focused on in the centrifugal compressor. Based on a simplified reduced model, the compressor efficiency under 8 clocking indexes was obtained to find out the best clocking position under different operating conditions. The variation due to the vane indexing in overall compressor performance was 0.4% for design load and 0.992% for higher load. Based on the unsteady simulating results under the peak efficiency condition, the loss mechanism and transient blade loading characteristics were interpreted so as to study the influence of clocking effect on the semi-open impeller stage. Finally, according to the basic clocking theory found in the centrifugal compressor, a model with real blade number was discussed, and the results will provide the evidence for the design and assembly for the same type of compressors.The second part was focused on the numerical analysis on the flow in the last three stages of an industrial steam turbine. First, the tip leakage flows for the last three stages were investigated. The loss due to tip leakage for the antepenultimate and penultimate stage mainly presents in the form of the tip leakage vortex and the tip separation vortex, while the loss due to tip leakage for the last stage merely presents in the form of the tip leakage vortex. The size and starting position of the tip leakage vortex captured under different conditions were different, but the transient leakage flows showed periodic patterns for each condition. When the mass flow rate was lower than 92% of the design mass flow rate, the tip leakage vortex could hardly been identified. Then the loss model for the last three stages was established and the loss due to tip leakage was shown to account for 22%-27% in the periodic variations. In the loss model, the development of the tip leakage vortex was qualified in terms of its position and size. Then the curve of relation between the tip leakage vortex development and mass flow rate was achieved. Finally, the clocking effect in the steam turbine was also studied based on a simplified model. The efficiency benefit due to the combination of vane indexing and rotor blade indexing was 0.413% in overall performance for the last three stages. The result provides a beneficial guidance for the future design and assembly for the same type steam turbines.