Computational Investigation of the Three-Dimensional Flow Structure and Losses in a Low Reynolds Number Microturbine

In this work, three dimensional numerical studies of the aerodynamics in laminar subsonic cascades at relatively low Reynolds numbers (Re < 2500) are presented. The stator and rotor blade designs are those for a MEMS-based Rankine microturbine power-plant-on-a-chip with 100 micron chord blades. Blade passage calculations in 2D and 3D were done for different Reynolds numbers, four different tip clearances (0%, 5%, 10% and 20%) and four incidences (0°, 5°, 10° and 15°) to determine the flow patterns and compute losses. These conditions are applied to a blade passage without rotation (stator) and with rotation (rotor), both for a stationary and moving outer casing. The 3D blade passage (without tip clearance) indicates the presence of two large symmetric vortices due to the interaction between flow curvature and hub/casing boundary layers. With tip clearance, a secondary vortex appears due to tip flow. This so-called tip vortex becomes dominant in the case of tip clearance above 10%. Relative wall motion also impacts the 3D flow patterns due to the important tangential drag at these low Reynolds numbers. Two dimensional calculations characterize well the flow at the mid-height plane, but are not sufficient for loss predictions due to the omission of the 3D flow structures. The 3D total losses increase dramatically for Re<500, which is similar to 2D studies. This suggests an operating Reynolds number greater than this to obtain efficiency levels necessary to operate a heat engine. The losses also increased monotonically with increasing tip clearance and incidence.