Large exit flanges in diffuser-augmented turbines lead to sub-optimal performance

Abstract

The use of a diffuser enhances turbine performance by augmenting the mass flow rate through the rotor. The use of large annular flanges at the exit of the diffuser has been studied extensively, but the present study shows that large exit flanges lead to sub-optimal performance when considering the power coefficient as defined by the maximum frontal area of the combined diffuser-rotor system (CPtot). Viscous actuator disk simulations are employed to investigate the effect of variable diffuser geometry on performance, and the investigation is divided into two sub-studies. In the first, it is shown that the mass flow rate exhibits a consistent linear relationship to the sectional diffuser lift coefficient, across geometries and rotor thrust coefficients. Since large flanges tend to enhance diffuser thrust more so than sectional lift, their utility is called into question. In the second sub-study, flange height is varied for an otherwise fixed geometry, and it is shown that a flange height of h/Di ​= ​0.04 yields the best performance of CPtot ​= ​0.576. This flange height is much smaller than the flanges typically considered in the literature; it is effectively a Gurney flap, a small trailing edge device known to enhance lift on airfoils.