Study of effects of axisymmetric endwall contouring on film cooling/heat transfer and secondary losses in a cascade of first stage nozzle guide vane
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The film cooling and heat transfer on the endwall especially in the junction region of a vane/blade which encounters strong vortical flow is of great significance in turbomachinery. Achievement of excellent film cooling and aerodynamic performance by controlling the secondary flow in that region can lead to increased gas turbine efficiency. This study investigates axisymmetric endwall contouring promising to design the assembly of combustor and gas turbine part (first nozzle guide vane) with film cooling in the upstream junction region of nozzle guide vane (NGV). Moreover, being first stage NGV, a slot between them was also modeled with a leakage flow. The study investigated the effect of contouring on the film cooling, heat transfer and secondary flow. Moreover, two contouring angles were studied along with a flat endwall for comparison. The results showed that the film cooling effectiveness from discrete holes provided more coolant coverage in the stagnation region with the large contouring angle. Similarly, cooling flow from the slot was improved significantly with contoured endwall, particularly in the middle section of the passage at higher endwall contouring. Moreover, coolant coverage on the pressure side of the NGV was improved significantly in the passage region. The heat transfer study showed that the contouring angle of 25° reduced Nusselt number in the junction region compared to 15° contoured endwall and planner endwall both with and without film cooling. The heat transfer raised with the presence of flow from the film cooling holes and slot compared to the case without film cooling also suppressed with the contoured endwall with large contouring angle being more effective. The secondary flow generated in the stagnation plane was reduced substantially with the contoured endwall illustrated the phenomenon of improved film cooling effectiveness. The size of the horseshoe vortex was suppressed and stepped up to the stagnation point. The mitigated strength of the secondary flow, improved film cooling effectiveness and reduction of heat transfer greatly depended on the endwall contouring angle.