Heat Transfer Coefficients

The heat transfer on the blade surfaces is expressed by the Nusselt number

Similar to the blade pressure distribution the unsteady effects are less obvi­ous in the NGV. There, the most significant phenomena are taking place on the suction side close to the leading edge. The ejecting cooling ft>w interacts with the main flow, triggering time dependent separations of the main flow immedi­ately behind the NGV leading edge. The obvious discontinuity at around 50% normalized axial distance (x/lax = 0.5) on the NGV suction surface is caused by the connection of a very fine grid to the relatively coarse surrounding grid. The high gradients of the quite sensitive Nusselt number are smeared out on the coarser grid, causing a discontinuity if plotted along the blade surface.

The overall level of the Nusselt number along the uncooled rotor blade sur­face is by far smaller compared to the cooled NGV. Unsteady effects are domi­nant throughout the entire blade passage (Fig. 7). The range of the time depen­dent Nusselt number can reach more than three times the level of the steady or time averaged calculation questioning the reliability of steady heat transfer calculations in multistage configurations.

The hot streaks of uncooled flow and the cooling jets emerging from the NGV enter the rotor passage in an alternating way (Fig. 8). In cases where relatively cool air from the jets impinges on the rotor blade surface the Nusselt number changes its sign, indicating a heat flux from the rotor into the flow (Fig.7).