AN APPROXIMATE ANALYSIS AND PREDICTION METHOD FOR TIP CLEARANCE LOSS IN AXIAL COMPRESSORS

A simple model for loss created by the rip clearance flow in axial com pressors is presented, based on an experimental program performed in c onjunction with the Dawes three-dimensional Navier-Stokes calculation method. The principal mechanism of loss (entropy creation) caused by t ip leakage flow has been established to be the mixing of flows of simi lar speeds but different direction. Calculations show that relative mo tion of the endwall relative to the rip has a small effect on clearanc e flow. The simple model correctly predicts the magnitude of tip clear ance loss and the trend with changes of tip clearance for the cascade tested. For a given geometry the loss is almost exactly proportional t o the ratio of tip clearance to blade span; the loss directly associat ed with the clearance is smaller than often assumed. The simple model for tip clearance loss has been expressed in terms of conventional non dimensional design variables (for example: solidify, aspect ratio, flo w coefficient, loading coefficient) and from these the contribution to the overall loss of efficiency caused by rip leakage flow is convenie ntly represented. The trends are illustrated far a number of possible compressor design choices. Blade row loss increases more slowly than b lade loading (for example, diffusion factor). As a result the decremen t in stage efficiency associated with clearance flow decreases as the stage loading is raised in the practical range of flow and loading coe fficients.