Prediction of losses due to the tip clearance presence in axial flow machines

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Theoretical modelling of relative wall motion effects in tip leakage flow

Summarization: abstract The influence of relative wall motion in modifying the leakage flow through the tip clearance is investigated. A theoretical model is developed, in order to calculate the mass flow rate through the gap. The physical mechanism by which relative wall motion affects the leakage flow is analyzed and the differences between the turbine and compressor case are identified. This model, being an extension of an already existing one, not taking into account relative wall motion, is incorporated into the tip clearance calculation procedure, already developed by the authors. Theoretical results of the complete calculation procedure (secondary flow plus tip clearance model) are compared with experimental data, for the case of compressor and turbine cascades, as well as for the case of a single rotor. Good agreement between theory and experiment is obtained.Παρουσιάστηκε στο: ASME 1995 International Gas Turbine and Aeroengine Congress and Expositio

Prediction of the tip clearance vortex circulation and its induced flow field in axial flow machines

Summarization: A model for the prediction of the leakage vortex circulation was developed, based on the assumption that the leakage jet flow enters as a whole the vortex core, increasing its radius and its moment of momentum in the direction of the vortex axis. Using the assumption that the leakage vortex has a solid body rotation, an expression was derived for the vortex circulation, which demonstrates that this circulation is proportional to the square root of the corresponding tip clearance height. This theoretical result is supported by the available experimental data for both compressors and turbines. A simple model was developed, which demonstrates the ability of the proposed theory to calculate the leakage vortex circulation, provided that the vortex trace is known.Παρουσιάστηκε στο: ASME paper 96-GT-7

A method for the calculation of the tip clearance flow effects in axial flow compressors. Part I: Description of basic models

Summarization: Theoretical models for the investigation of the phenomena connected with the tip clearance are presented. The mass flow rate through the gap, the formation and evolution of the leakage vortex and the losses occurring inside and downstream of the gap are considered. Firstly, a model was developed for the description of the flow through the gap, which uses different simple velocity profiles at the gap exit. The model recognizes the basic flow characteristics inside the gap. A new method is proposed for the calculation of the shed vorticity and the formation of the leakage vortex. The moment of momentum equation is used along with the conservation of mass, in order to provide the circulation of the leakage vortex. A diffusion model for the vorticity distribution is used for the calculation of the pressure deficit field, so that the total pressure losses due to the presence of the leakage vortex, are derived. Theoretical results are compared to experimental ones for compressor and turbine cascades as well as for single rotors. The agreement between theory and experiment is good.Παρουσιάστηκε στο: ASME 1993 International Gas Turbine and Aeroengine Congress and Expositio

Modelling of the tip clearance losses in axial flow machines

Summarization: The possibility of predicting the total pressure loss radial distribution, due to the tip clearance presence, is examined in this paper. Models advanced for the diffusion of a line vortex are used for the simulation of the leakage vortex induced velocity and pressure fields, with sufficient success. The leakage vortex strength seems to control directly only a small part of the total pressure loss distribution, the one connected with the pressure deficit and the rotating flow. The remaining profiles result as functions of a free parameter — the constant of integration — and an assumption is needed to close the problem. The widely proposed observation for lost secondary jet kinetic energy is considered as a method of predicting the total amount of tip clearance loss in successive planes inside and downstream the blade passage. A calculation procedure for predicting the tip clearance effects in the flow field inside and downstream the tip clearance, has been developed. The method, being compatible with a meridional flow calculation procedure, accounts for the calculation of the peripherally mean deficit profiles of the various flow quantities. The predictive capability of the calculation procedure is established in a wide range of test cases, including axial flow compressor cascades, isolated rotors and multi-row machines. The radial variation of tip clearance pressure loss is calculated with sufficient accuracy for engineering purposes.Παρουσιάστηκε στο: ASME 1996 International Gas Turbine and Aeroengine Congress and Exhibitio