A study on turbine unsteady flow via PIV and hotwire

Unsteady wake represents blade pneumatic load and affects turbine characteristic greatly, while secondary flow loss is the main part of turbine endwall loss. The appearance, attenuatiori of unsteady wake and their interaction with downstream secondary flow have significant effect on turbine design and characteristic. Experiment measurement and numerical simulation are adopted to study secondary flow and wake. Upstream rotating blade can be replaced by cylinder with the same wake velocity and stagnant pressure loss. In experiment, two inlet velocities are taken to study the effect of inlet flux on blade characteristic. At the same time, upstream cylinders move quarter of circumferential pitch discretely and the influence of upstream cylinder wake to downstream blade is researched. Afterwards, two hollow boards are used to cover part of blade height to show tip and bottom secondary vortexes alternation under different relative blade height. First, the system integrating method of hotwire measurement is developed with a rotating slanted hotwire in order to measure three-dimensions flow field at blade outlet. The procedure for solving the mean velocity vector involving the least-squares technique can be accomplished via the LSQNONLIN optimization function of Matlab. Then the whole velocity field of horizontal-flow and perpendicular plane has been measured with a Particle Image Velocimetry (PIV). Finally, numerical calculation is carried out. The result shows coincidence between experiment and calculation. It is found that outlet secondary flow is obvious and severe at higher inlet velocity and smallest relative blade height. When the upstream rotor wake impinges upon the leading edge of the down stream stator, the corresponding stage efficiency will be higher, when the upstream wake is transferred into the mid passage of the downstream stator, the corresponding stage efficiency will be lower. As a result, the optimal blade circumferential layout is an effective way to increase turbine pneumatic efficiency. The numerical calculation shows that pressure pulse intensity on pressure side weakens rapidly along blade profile, while pressure pulse intensity on suction side weakens along profile first and then strengthen due to unsteady separation on trail edge.