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Computational Simulation of an Incompressible/ Compressible Turbulent Jet-into-crossflow – An Innovation in Film Cooling

Javadi, Khodayar | 2007

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39896 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Taeibi-Rahni, Mohammad; Darbandi, Masoud
  7. Abstract:
  8. This work deals with the computational investigation of film cooling technique, which is one the best practical way to protect gas turbine components form high thermal loads. In this regards, previous works are extensively reviewed and most important effective parameters are classified into three general categories, as geometrical parameters, flow characteristics, and physical surface factors. Each of these categories is then divided into subcategories and more details studies of each are performed. Then, a novel near-wall flow control technique of using staggered arrangement of small injection ports near a film cooling hole (combined-triple-jet; CTJ) is introduced. The fluid injected from small ports changes the flow dynamics downstream, resulting in a considerable enhancement of cooling efficiency. This approach provides considerable improvement in: 1) film cooling efficiency, 2) uniform distribution of the coolant film, 3) reduction of the mixing strength between free stream and coolant jets, and 4) reduction of skin friction drag. In addition, this work introduces a cooling uniformity coefficient (CUC) to evaluate how well a coolant film spreads over a surface being cooled. Another goal is the evaluation of different turbulence models in predicting three-dimensional jet-into-crossflow (JICF) interactions. For this purpose, at first comprehensive discussions on the near wall flow complexities due to discharge of a jet into a crossflow are presented. In this regards, large scale coherent structures, such as: counter rotating vortex pairs (CRVP’s), near wall secondary motions, horseshoe vortices, and wall-jets-like flow are discussed. Second, the abilities of different turbulence models in predicting such flows (JICF) are evaluated. Our evaluation is based on three points of view, including: 1) JICF characteristics, 2) location computed, and 3) sensitivity to different flow variables. In this regard, the turbulence models such as k-ε, k-ω, shear stress transport (SST), and Reynolds stress model (RSM) are employed. The related results are compared to benchmark experimental/numerical data as well themselves. Since the same basic code with the same grid density (as well as numerical discretization scheme) is used, it is safe to conclude that any differences in the results are due to different abilities of turbulence models. On the other hand, we developed a pressure-based incompressible algorithm to solve three-dimensional compressible and incompressible turbulent flow regimes. To achieve a unified algorithm suitable for treating a wide range of regimes, the mass flux components, i.e., , were chosen (instead of the primitive velocity components ) as our primary dependent variables. This choice of dependent variables provides a number of benefits. For example, the non-linearities within the system of conservative equations are definitely decreased. On the other hand, from the perspective of compressible flow treatment, we extended a new Favre-like averaging technique, which is very helpful to reduce the non-linearities in the formulation. Additionally, there is less requirement to interpolate the fluxes at the cell faces. The current experience showed that this choice of dependent variables helps us to achieve a robust algorithm which performs great capabilities in treating both flow regimes (incompressible/compressible), with a wide range of Reynolds and Mach number applications. To impose the hyperbolic behavior in compressible regime we introduce an artificial hyperbolicity in pressure correction equation. We used turbulence model and incorporated the compressibility effects as a correction to model the turbulent part. To evaluate the ability of the new extended unified algorithm, three test cases were targeted and the results were compared with previous credential experimental data and numerical solutions. These cases were an incompressible turbulent backward-facing step problem, a turbulent flow over a wide range of open to closed cavities, using 0.2≤M≤0.8, and a three-dimensional compressible turbulent channel flow at M=0.3. The results indicate that there are reliable agreements with those of reliable experimental and numerical works for a wide range of applications.

  9. Keywords:
  10. Film Cooling ; Jet into Cross Flow Interaction ; Turbulent Flow ; Combined Jets ; Compressible/Incompressible Algorithm

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