An examination of the flow and pressure losses in blade rows of axial-flow turbines

Abstract

The design of axial-flow turbines has been hampered in the past by a lack of comprehensive data regarding pressure losses and gas deflections through rows of turbine blades. In the present report much of the available information relating to this subject is studied and analysed to determine magnitudes of gas pressure losses and deflections in a wide variety of blade rows and also to determine the separate influences of variables such as blade shape, blade spacing, gas Mach number, Reynolds number, incidence, etc. Of particular importance are the effects of secondary flows on the aerodynamic performance of a blade row and special attention is paid to 'secondary losses', which form the difference between the total losses occurring in an actual turbine blade row and the smaller two-dimensional flow losses which are usually measured in a blade cascade tunnel. Effects of blade tip clearance are also studied. Resulting from this analysis a number of empirical guiding rules and charts have been derived from which approximate values of the overall pressure losses and gas deflections in a range of blade rows can be deduced. A particularly significant feature brought to light is that the secondary losses can in many instances be large, the loss being generally found to be great when the blading has low reaction.