The starting point of this paper is an established turbine tip leakage loss model based on energy considerations. The model requires a discharge coefficient as an empirical input. The discharge coefficient is the ratio of the actual to the theoretical tip gap mass flow rate, The nondimensional parameters influencing the discharge coefficient are determined by a dimensional analysis. These parameters are: gap width to length ratio, end wall speed to gap flow velocity ratio and gap Reynolds number. Ranges for these parameters, valid for typical turbine tip gap situations, are presented. The numerical investigation of the turbulent flow in a plane perpendicular to the blade chord line supplies the discharge coefficient versus the nondimensional gap width. Depending on the gap width to length ratio, various degrees of mixing of the flow downstream of the vena contracta can be detected. Based on these observations, a simple tip gap flow model is presented. The discharge coefficients computed by this model are compared with the numerical results as well as with experimental values from the literature. Finally, the model is used to calculate the discharge coefficients of improved tip gap geometries (squealers, winglets).

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