Classification of Flow Regimes : Blade Profile Losses P M V Subbarao Professor Mechanical Engineering Department Industrial (Closer) study of Cascade Flows .……

Another Classification of cascade Flow Regimes More closer study of the flow through cascade is essential for deeper insight into the viscous flow. Demands a further division of flow regimes. These regimes will help in comparison of behaviour of base line profile with the new profile. Ru Sub-parts of A Cascade Flow Guided Flow Unguided Flow Rd Leading Edge flow

Classification of cascade Flow Regimes b

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.

Measure of Irreversibility due to Blade roughness

Need for Identification of Loss Parameters Cascade tests Power law relationship

Need for Identification of Generalized Loss Parameters Cascade 3 Cascade 1 Cascade 2

Normalized Loss Parameter Cascade 1 Cascade 3 Cascade 2

Definition of New Profile Loss Ratio Independent Parameters: Reynolds Number based on Opening Roughness factor

Trending of Overall Losses Vs Reynolds Number Cascade tests on profile loss Due to roughness Power law relationship Tests on a three stage low pressure model steam turbine working into the wet region. Tests on the Rolls-Royce Conway gas turbine Range of a number of tests on single stage impulse turbines.

Identification of Generalized Variables Model Turbine Test Curves cover a wide range of axial flow turbines, but they are all notably less steep than those based on cascade tests. The only factor which will cause the slopes of the turbine curves to differ significantly from the slopes of the cascade curves is the presence of other turbine losses, such as tip leakage. These other losses do not change with Re. The presence of a bulk of Re insensitive losses reduces the steepness of the curves.

Profile loss ratio Vs Reynolds number effect

Discussion of Profile Losses due to Roughness The effect of Re on blade cascade performance (due to roughness) is pronounced. Thus any correlation of cascade data (profile loss) which neglects the Reynolds number of test is of little value. Typically in the range of Re between 2  l04 and 2  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  l06 which are now obtained at the inlet of modern high pressure (h.p.) cylinders.

Overall profile loss Vs Lift Impulse Blade >80% reaction Blade