On Reliable Performance Prediction of Axial Turbomachines

There is a need to reliably predict the performance (efficiency and total pressure rise) of axial turbomachines from model tests for different load ranges. The commonly used scale-up formulas are not able to reliably predict the performance, especially beyond the design point. Furthermore these formulas do not regard changes in relative roughness as they usually occur in practice. An improved scale-up formula is proposed which achieves not only the reliable scale-up of efficiency, but also the scale-up of the pressure coefficient. It is motivated from measurements on two geometric similar axial model fans with a diameter of 1000 mm respectively 250 mm at different rotational speeds, hence Reynolds numbers. By bonding grains of sand to the impeller the influence of relative roughness was investigated. For applying the formula to different load ranges a factor V is introduced that depends on the quotient of effective flow coefficient to optimal flow coefficient.