| As an important part of the aircraft engine, gas turbines are operating at high temperature and high pressure conditions. In order to protect gas turbines from the erosion of the hot external mainstream and ensure the safety of gas turbines, internal cooling channels are often designed inside turbine blades. When the cooling air, extracted from the compressor, flows through the cooling channel, it takes extra heat from the turbine blades through convective heat transfer. Then the cooling air is exhausted from the internal cooling channel though film cooling holes designed in the surface of the turbine blade, and forms a thin film layer with low temperature there. In order to help people develop new effective cooling schemes, it is important to investigate the fluid flow and heat transfer in turbine blade’s internal cooling and film cooling methods. Furthermore, with the development of the computer technology and turbulence models, numerical simulations become more widely used in the investigations of turbine blade. This thesis uses numerical simulations to investigate the internal cooling and film cooling of the turbine blades, its main contents are as followings:Part I:Investigations of internal cooling channel inside a real gas turbine blade.In this part, the characteristics of fluid flow and heat transfer within a smooth three-pass channel of a real low pressure (LP) turbine blade have been investigated through experimental and numerical approaches. The serpentine channel consists of two inlet passes, two dividing walls, two 180 degree bends, twenty-five exits at the trailing edge, and two exits at the blade tip. In the experiments, purified water was used as working medium, the secondary flow patterns at five cross-sections were captured by a particle image velocimetry (PIV) system, the inlet Reynolds number was controlled by a turbine flow meter, and the mass flow rate ejected from each exit was measured by rotameters. Using the commercial software ANSYS CFX 13.0, numerical investigations were carried out. The practicability of four turbulence models, the SSG RSM, SST k-ω, RNG k-ε and standard k-ε models, were estimated. Through qualitative and quantitative comparisons of the secondary flow patterns, local velocity variation trends and mass flow rates between the experimental data and numerical results, the SSG RSM was selected as the most appropriate model in the following numerical investigations. Using ideal gas as working medium, the impacts of Reynolds numbers and rotation numbers on the heat transfer performances were numerically investigated. The numerical results predicted three interesting phenomena: 1) The locally averaged Nusselt number increases generally with the inlet Reynolds numbers. However, the increasing amplitude is significantly different from the correlation suggested by Dittus-Boelter, Nuo=0.023Re0-8Pr0.4. The effect of the Reynolds number on the Nusselt number is substantially enhanced due to the serpentine channel design with two 180 degree-bends. The enhancement amplitude is described by two fitted coefficients based on Dittus-Boelter correlation.2) Under a rotation condition, in the 1st and 3rd passes, the enhancement amplitude of the average Nusselt number on the pressure side (PS) is more significant than that on the suction side (SS), whereas in the 2nd pass, the enhancement amplitude on the PS is lower than that on the SS.3) In the 3rd pass, a higher rotation number leads to a more uniform distribution of the local Nusselt number along the streamwise direction on both the PS and SS.Part II:Investigations of pulsed film cooling on a turbine vane.As a new potential cooling scheme, pulsed film cooling can not only decrease the consumption of the cooling air, but also maintain or even enhance the cooling effectiveness of the turbine vanes. The second part of this thesis uses numerical simulations to investigate the effects of pulsed film cooling at both adiabatic and conjugated heat transfer conditions. At first, a numerical investigation of adiabatic pulsed film cooling on a modified NASA C3X vane has been presented. This vane has five rows of film cooling hole:three rows at leading edge, and the other two at pressure side and suction side, respectively. Square and sinusoidal waves are considered to pulse cooling air injection. The performances of film cooling effectiveness are discussed at three blowing ratios (0.5,0.75 and 1.0) and four Strouhal numbers (0.0027,0.0054,0.0108 and 0.0216). Previous investigations of pulsed film cooling were limited within flat plates or semicircular cylinders, therefore it is necessary to provide a comprehensive reference regarding pulsed film cooling on an entire turbine vane. Through the present numerical simulations conducted on the entire turbine vane, the following interesting phenomena are discovered:1) at leading edge and pressure side, film cooling effectiveness decreases when blowing ratio or Strouhal number increases, but at suction side, the trend reverses; 2) sinusoidal wave pulsed flow leads to less mainstream ingestion into film hole than square wave pulsed flow; 3) in the leading edge region, sinusoidal wave pulsed flow exhibits higher film cooling effectiveness than steady flow at blowing ratios of 0.75 and 1.0. Then a conjugate heat transfer simulation of pulsed film cooling on a complete NASA C3X vane has been presented. Both square and sinusoidal waves are considered to pulse cooling air injection. The Nusselt number distributions are discussed at three blowing ratios (0.78,1.17 and 1.56) and four Strouhal numbers (0.0029,0.0058,0.0116 and 0.0232). The results show that 1) at suction side, when blowing ratio increases, the Nusselt number decreases downstream of Row 1 in the steady flow case. However, the pulsed flow cases still have large Nusselt number in the same region, which indicates the pulsed flow is not suitable at suction side when blowing ratio is high. On the contrary, at pressure side, when blowing ratio increases, the Nusselt number of the pulsed flow becomes smaller than that of the steady flow, which indicates that pulsed film cooling is more suitable at pressure side when the blowing ratio is large.2) at suction side and leading edge, when Strouhal number increases from 0.0029 to 0.0116, the Nusselt number of the same flow type decreases, but when Strouhal number further increases to 0.0232, the Nusselt number increases, which indicates that it is critical to select an appropriate pulsing frequency at suction side and leading edge. However, at pressure side, Nusselt number increases with Strouhal number for all flow types. |
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