Effects of mid-passage gap with a variable surface angle on a turbine vane endwall’s aerothermal and film cooling performances

The mid-passage gap is an inevitable structure in a vane passage due to turbine vanes being manufactured individually. The coolant from this gap is able to prevent hot mainstream ingression and provide cooling protection for the endwall. A novel idea of enlarging the endwall’s coverage area and reducing the endwall’s thermal load by applying the mid-passage gap with variable surface angles is carried out in this paper. The endwall’s aerothermal and film cooling performances under four mid-passage gap modes at three typical mass flow ratio conditions are numerically investigated. Results indicate that under the traditional mid-passage mode, the coolant flows into the mainstream with a perpendicular incidence angle and can’t stick to the endwall. Thus, cooling failure occurs, and the endwall’s thermal load is badly increased. The film cooling level at the suction-side endwall is improved when applying the mid-passage gap of a 45° surface angle due to the secondary vortex being suppressed. In addition, when applying the mid-passage gap of a 135° surface angle, the horseshoe vortex is pushed away, and the coverage area at the pressure-side endwall is enlarged significantly. The best film cooling performance is achieved when the upstream surface angle is 135° and the downstream surface angle is 45° due to the adiabatic film cooling effectiveness being increased at both the pressure- and suction-side endwall. When the mass flow ratio is 1.5%, the coverage area is enlarged by 43%, and the area-averaged adiabatic film cooling effectiveness is increased by 37%, when compared with those under the traditional mid-passage mode.