An optimal design of film cooling is a key factor in the effort of producing high-efficiency gas turbine. Understanding of the fluid dynamics interaction between cooling holes can help engineers to improve overall thermal effectiveness. Correct prediction through modeling is a very complex problem since multiple phenomena are involved such as mixing, turbulence, and heat transfer. The present work performs an investigation of different cooling configurations ranging from single hole up to two rows. The main objective is to evaluate the double-rows interaction and the effect on film cooling. Strong nonlinear effects are underlined by different simulations, while varying blowing ratio (BR) and geometrical configuration of cooling holes. Meanwhile an initial analysis is performed using flat plate geometry, verification and validation is then extended to realistic stage of high pressure (HP) turbine. Multiple cooling holes configurations are embedded on the pressure side (PS) and suction side (SS) of the single stage. The main outcome is the verification of the thermal effectiveness improvement obtained by cooling jets interaction of multiple rows design. The effects of curvature surface and frame of reference rotation are also evaluated, underlying the differences with standard flat plate test cases.
Numerical modelling of internal cooling passages in gas turbine blades is a challenging task due to their physical characteristics, such as rounded duct corners, the presence of rib turbulators and their staggered locations between surfaces. This results in complex fluid dynamic phenomenon such as counter-rotating vortices and other secondary flow structures that can drive the heat transfer. Heat transfer mechanisms in such passages are inherently coupled with momentum transport and diffusion. Current industry practices for numerical modelling of such passages use unstructured mesh generation tools, steady Reynolds-averaged Navier-Stokes (RANS) equations and two-equation turbulence models such as k-ε and k-ω SST. This paper investigates two generic, engine-representative rib geometries using current numerical practices to determine their limitations. Three mesh generation tools and two turbulence models are compared across two rib geometries. The results are qualitatively and quantitatively compared to detailed experimental Nusselt numbers on the passage walls. It was found that as long as the rib geometry results in a secondary flow that directly impinges onto the wall, the meshing tools and turbulence models agree reasonably well with experiments. When the passage includes wall-wrapped ribs resulting in more complex secondary flows, this decreases the validity of the numerical tools, suggesting that more sophisticated modelling techniques are required as rib geometries continue to evolve.
In the present study, a numerical method was developed to simulate the presence of spacer grids with mixing vanes in nuclear reactors fuel assemblies. These mixing devices usually have a complex morphology that results in difficult mesh procedures and a high computational cost. Spacers and vanes were simulated using momentum sources to reduce the computational power requirements and to enable the feasibility of a quarter of the full reactor core simulations. Several approaches were tested using RANS models. The starting point is calculation with bodyfitted mesh of one grid span of a fuel assembly with spacer grids featuring dimples, springs and mixing vanes. Velocities and Reynolds stresses are extracted from the solution, then converted to source terms. A new computational domain is created with a coarser mesh and without the presence of dimples, springs, and vanes, but source terms are added to the momentum and Reynolds stress transport equations forcing the solution as the detailed geometry computation. The forcing is imposed only in few mesh elements of the domain corresponding to the original location of dimples, spring and vanes. The approach was numerically stable and relatively implemented in the open source code, Code_Saturne (EDF) and in the commercial code Star-ccm+ (Cd-Adapco). Best practice was defined and grid sensitivity analysis on different quantities was performed. The robustness of the new numerical method was demonstrated. This new methodology creates an intermediate approach that provides higher spatial resolution than subchannel codes and reduced computational cost compared to detailed CFD simulation. Development of the method was based on anisotropic second order closure models, since lower order models are not able to capture swirling and secondary flow generated by the mixing devices; nevertheless, this novel approach applies also to one and two equations turbulence models, without lack of generality.
The flow and heat transfer over a three-dimensional axisymmetric hill and rectangular ribbed duct is computed in order to evaluate the Shear Stress Transport - Scale Adaptive Simulation (SST-SAS) turbulence model. The study presented here is relevant to turbine blade internal cooling passages and the aim is to establish whether SAS-SST is a viable alternative to other turbulence models for computations of such flows. The model investigated is based on Menter’s modification to Rotta’s k-kL model and comparison is made against experimental data as well as other models including some with scale resolving capability, such as LES, DES & hybrid LES-RANS. For the hill case the SAS model dramatically overpredicts the size of the separation bubble. The LES on the other hand proved to be more accurate even though the mesh is courser by LES standards. There is little improvement of SST-SAS compared with RANS. Broadly speaking all models predict streamwise velocity profiles for the ribbed channel with reasonable accuracy. The cross-stream velocity is underpredicted by all models. Heat transfer prediction is more accurately predicted by LES than RANS, DES & SST-SAS on a mesh that is slightly coarser than required by LES standard, however it still exhibits significant error. It is concluded that more investigation of the SST-SAS model is required to more broadly assess its viability for industrial computation.
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