Imran Mahbub Saleem
National University of Sciences and Technology, Islamabad, Pakistan

The low-pressure turbine blades of aero-engines play a vital role at cruising altitudes, where the density of the atmosphere becomes low. This phenomenon is observed particularly at the boundary layer of low-pressure turbine blades in the range of low Reynolds number. At this stage, flow reversal occurs in the boundary layer on the suction side of low-pressure turbine blades. It is because of increasing pressure gradient, and so, the efficiency of a turbofan engine is lost by consumption of more fuel. Therefore, the design of the blades should be optimized for controlling flow separation to minimize pressure losses. One of the methods to control flow separation is by using dimples. Six different geometrical shapes were engraved by 85% Tax on the suction of the blade. They were rectangular, triangular, oval, half-hexagonal, circular and square in shapes. T-106A low-pressure turbine blade profile has been used for analyzing boundary layer behavior. SST transition model was used to control boundary layer separation at Reynolds number 91000 with low turbulent intensity. Square dimple with diameter d=2.00 mm produced the least amount of pressure losses, about 8.00 % in controlling flow separation behavior at low Reynolds number. Rectangular dimple (D=10.00 mm; d=1.00 mm) showed an interval of the laminar shear layer, and an interval of transition, to be [0, 0.90] and [0.90, 1] respectively. Specific transition patterns from laminar to the turbulent phase of the suction side in the boundary layer have
been observed.

Dimple acts a vortex generator and causes an earlier transition in the boundary layer on the suction side.
However, various dimple shapes have different impacts in getting transition results. Since, 85% T
ax is the optimum location for dimple geometry [2], various dimples (rectangular, triangular, oval, half-hexagonal, circular and square) were made at 85% Tax for getting best results in getting transition pattern from laminar to turbulent boundary layer and pressure gain at Re=91,000.
Square dimple (d=2.00 mm) at 85% C
ax produced the least amount of pressure loss about 8.00 %. Moreover, the coefficient of pressure graph for different dimple geometries suggested the optimum location for rectangular dimple (D=10.00 mm; d=1.00 mm) to be 0.90% Cox, as the level of transition starts from there. Specific transition patterns were observed in dimple geometries for a rectangular and square dimple. Circular dimple did not show any vortex production. Moreover, triangular, oval and half-hexagonal dimples produce a single vortex pattern for all cases (with changing diameter and depth) of the dimples.
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