| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 91 === Computations have been conducted on a flat, three-dimensional discrete-hole film cooling geometries that included the mainflow, injection tubes, impingement chamber, and supply plenum regions. The geometrical shapes of the vent of the cooling holes are cylindrical round, simple angle (CYSA), forward-diffused, simple angle (FDSA) and laterally diffused, simple angle (LDSA). The effects of blowing ratio and hole’s shape on the distributions of flow field and adiabatic film cooling effectiveness over a flat-plate collocated with two rows of injection holes in staggered-hole arrangement were studied. The blowing ratio is varied from 0.5 to 1.5. Diameter of different shape of cooling holes in entrance surface are 5.0mm and the injection angle with the main stream in streamwise and spanwise are 35o and 0o respectively. Ratio of the length of the cooling holes and the diameter in the entrance surface is 3.5. The distance between the holes in the same row as well as to the next row is three times the diameter of hole in the entrance surface. Based on the diameter of cooling hole in entrance surface as the characteristic length, the mainstream Reynolds number is fixed at 3400. Meantime, the author also simulate the complex flow field on a rotor blade with film cooling holes and coolant plenum.
The governing equation is the fully elliptic, three-dimensional Reynolds-averaged Navier-Stoke’s equations. The mesh used in the finite-volume numerical computation is the multi-block and body-fitted grid system. Four other two-equation turbulence models, i.e. standard k-ε, low Reynolds number k-ε, RNG k-ε, and low Reynolds number k-ω, have been tested. The simulated streamwise distribution of spanwise-averaged film cooling effectiveness exhibited that low Reynolds number k-εmodel gives the best agree to the experimental data of the previous investigators. Present study discovered that the geometrical shape of the cooling holes has significant effect to the adiabatic film cooling efficiency especially in the area near to the cooling holes. Results reveal that the thermal-flow field over the surface of the film-cooled tested plate is dominated by strength of the counter-rotating vortex pairs (CRVP) that are generated by the interaction of individual cooling jet and the mainstream. For LDSA shape of hole, the CRVP are almost disappeared. The FDSA shape has shown a higher value in distribution of spanwise-averaged film cooling effectiveness when the blowing ratio is equal to 0.5. However, when the blowing ratio is increased to 1.0 and 1.5, the LDSA has shown a better cooling performance than other shapes. It is due to the structure of the FDSA and LDSA is capable of reducing the momentum of the cooling flow at the vent of the cooling holes, thus reduced the penetration of the main stream. Moreover, the structure of the LDSA can increase the lateral spread of the cooling flow, thus improves the span wise average film cooled efficiency.
To the simulation of the rotor blade with showerhead film cooling, present results indicate that the blowing of the coolant flow in the leading edge of the blade will obvious reduce the surface temperature as compared to those without cooling. A higher distribution of adiabatic film cooling effectiveness value can be observed on the suction side of blade, especially near by the holes. In addition, the holes in different position of blade have distinguish features in developing of coolant flow structure due to various freestream boundary-layer pressure gradients around the leading edge. Neither uniform nor parabolic distribution of velocity in the exit plane of coolant hole is observed in present study. For more practice, a complete simulation model should include the coolant plenum and rows of injection hole. Present study also tests the influence of mainstream inlet angle on the passage flow structure and temperature distributions over blade’s surfaces.
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