Experimental and numerical investigation of unsteady impingement cooling within a blade leading edge passage

The cooling passage within the leading edge of a turbine blade is simulated using a cylindrical target channel supplied by 10 impinging jets, with exit flow in the axial direction, at one end of the passage. Values of the impingement Reynolds number, based on the jet diameter, are 10,000, 15,000, and 20,000. Thermochromic liquid crystals (TLC) are employed to measure transient, spatially-resolved surface Nusselt numbers, and a FLUENT solver, with a green-Gauss cells gradient method and SIMPLEC for pressure-velocity coupling, is employed for steady, half-steady, and unsteady predictions. Distributions of target surface Nusselt numbers, and predictions of flow characteristics, along with instantaneous and local time-averaged magnitudes of root-mean-square temperature fluctuations show that the most important factors which influence local, instantaneous target surface Nusselt number distributions are the target passage cross-flows, Kelvin-Helmholtz vortex structures, and the unsteadiness which is associated with these phenomena. Of particular importance are: (i) vortex structure skewness, as affected by greater bending of the impingement jet trajectories, which increases as passage cross-flows become more non-uniform, and also increase in intensity and unsteadiness, and (ii) augmented shear which develops from the accumulated cross-flow which intensifies and augments the development of the Kelvin-Helmholtz vortices, which causes the unsteadiness associated with these vortices to increase, which further increases local time-averaged magnitudes of root-mean-square temperature fluctuations.