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Estimation of Profile Losses. P M V Subbarao Professor Mechanical Engineering Department. Irreversible flow Past A modified blade profile……. Correlation For Profile Losses. The correlation for profile loss is developed using test data on linear cascades.
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Estimation of Profile Losses P M V Subbarao Professor Mechanical Engineering Department Irreversible flow Past A modified blade profile……
Correlation For Profile Losses • The correlation for profile loss is developed using test data on linear cascades. • One major issue in development of a correlation is the choice of independent variables. • Cascade tests can not be carried out with a variation of one parameter. • For instance, if the effect of Reynolds number is being measured, almost invariably the Mach number or the aspect ratio of the cascade is being simultaneously altered.
Need for Identification of Loss Parameters Power law relationship Cascade tests
Need for Identification of Generalized Loss Parameters Cascade 3 Cascade 1 Cascade 2
Normalized Loss Parameter Cascade 3 Cascade 1 Cascade 2
Definition of New Profile Loss Ratio Independent Parameters: Reynolds Number based on Opening Roughness factor
Non-dimensional Variables affecting Profile Losses • The correlation evolved is based on the analysis of over 100 specific cascade tests and on comparisons with a wide variety of published information. • All losses in are related on a basis of velocity coefficients and are dependent on the following parameters : (1) Reynolds number (Re) (based on outlet velocity) (2) Aspect ratio (blade height/chord length ratio); (3) Blade angles and passage geometry; (4) Pitch to chord length ratio; (5) Mach number (Mu); (6) Incidence.
General Practice • The profile loss correlation is presented in the form of a basic loss correlation for incompressible flow conditions. • The basic correlation itself are derived originally from low speed tests where it could be assumed that the Ma effects could be ignored. • The correlation mainly involves a variation in Blade angles, passage geometry andPitch to chord length ratioonly • Multiplying correction factors are presented to consider the variation of other parameters.
Effect of Reynolds number • Re will have a pronounced effect on profile loss. • Typically in the range of Re between 2 x l04 and 2 x l05 the loss will be halved. • A general prediction method for use in steam turbine analysis requires that the effect of Re should be predictable up to values of Re, equal to about 4 x l06. • This range is obtained at the inlet of modern high pressure (h.p.) cylinders. • Thus any correlation of cascade data which neglects the Reynolds number of test is of little value.
The Most Important Dimensionless Parameter The Re based on the blade opening ks : Equivalent grain size
Class – 1: Guided Flow : Selection of Profile Parameters • Relative profile thickness, tmax/b, • Position of maximum thickness, xtmax/b • Relative radius of curvature, Ru/Rd Depending on the distribution of Design loading, decisions are made; on the position of maximum thickness, the relative value of curvature upstream and downstream of the throat, Ru/Rd.
Preferred Ranges of Key design parameters 0.20<xtmax/b<0.35, 0.15<tmax/b<0.30 and 4.0 < Ru/Rd < 10 The lower range of Ru/Rd corresponds to a lower degree of aft loading.
More fundamental Study of Roughness Effect • It is customary when evaluating friction losses to divide the guided flow regime into three regions: • leading edge, • suction side trailing edge, and • Pressure side trailing edge.
Nature of Losses in Nozzle Blade Passage Flow • Nozzle suction side roughness affects stage efficiency approximately three times more than pressure side roughness. • In a nozzle of impulse stage, because of the higher pressure drop through the nozzles, approximately 75% of a stage efficiency loss caused by surface roughness is attributed to the nozzles.
Nature of Losses in Rotor Blade Passage Flow • The leading edge roughness will have the greatest contribution to stage efficiency loss occurring on the bucket. • Rotor Blade (Bucket) leading edge suction side roughness affects stage efficiency approximately two times more than pressure side roughness. • This information is affected by turbine section. • In general Higher Reynolds Numbers are found in the High-Pressure section (smaller boundary layer), the smaller the projections have to be in order to avoid an increase in friction loss.
Overall profile loss Vs Lift Impulse Blade >80% reaction Blade
The Seriousness of Unguided Flow Attached flow Flow stall on Suction side
Class – 2 : Unguided zone design parameters: • One major aspect of profile design is the behaviour of flow in the unguided zone. • In this zone, flow diffusion takes place. • The flow behaviour in the unguided zone highly influenced by • curvature variation and turning downstream of the throat, • the wedge angle and • the trailing edge thickness.
Clues to Design Unguided Zone • These parameters determine the resulting dissipation in the dead steam region. • The deviation angle—and must be carefully optimized during the profile design process. • The trailing edge thickness, having the predominant influence on trailing edge loss generation, has to be reduced to a minimum thickness compatible with structural and manufacturing constraints
Class – 3 : Leading Edge Parameters • Leading edge radius, LER, • Wedge angle, eLE
Clues to Design Leading Edge • The sizing of the leading edge radius and leading edge wedge angle has to be compatible with incidence range considerations. • Higher values of eLE are desirable. • Quasi 3D and 3D design aspects also important to control (reduce) loading in the leading edge region.
