Resumen de Grid generation and fluid-solid coupling methods for the investigation of gas turbine rotor blade internal cooling
With the rapid growth of computer power, numerical methods are available and showing their advantages in identifying the complex underlying physics of flows. However, it hasthey have encountered a number of challenges in internal cooling problems of gas turbine rotor blade due to the complexity of some characteristics of rotating passages, such as geometries, flow conditions and heat transfer behaviours under rotations. This thesis focuses on the challenges in the numerical grid generation, coupling fluid/solid heat transfer method and understanding the rotational effect on flow and heat transfers.
Mesh generation is a critical process in Computational Fluid Dynamics (CFD) since mesh quality can not only have a direct impact on the final solution, but become a bottleneck of the whole analysis process. Grid spacing should be smoothly and sufficiently refined in certain regions to properly resolve steep solution gradients, whereas it should be coarser in other regions not compromised by mesh resolution considerations. A key element to achieve good quality meshes in viscous flow simulations at high Reynolds numbers is a high enough mesh resolution near solid walls to properly capture boundary layer gradients in the wall-normal direction. These regions account only for a small fraction of the computational domain but are directly linked with the quality of aerodynamic simulations. However, significant difficulties arise in the presence of complex geometric features such as multiple concave/convex corners, micro-surfaces, etc., which may result in mesh generation failure, or considerable elapsed time of the mesh generation process with the subsequent associated labour cost. Therefore, in an industrial environment, the generation of viscous mesh still remains a challenginge task. The present work proposed a robust and efficient viscous mesh generation method to construct the computational grid in a hybrid manner in order to mesh the complex 3D configurations efficiently for CFD analysis. The method first focuses on providing a robust criteria for the validity checking procedure for complex problems, and then improves the efficiency by implementing a pre-processing scheme to reduce the number of nodes that requires to be checked. Therefore, the methods substantially reduces the generation time. The presented algorithm is proved to be much more efficient than common commercial or open source codes within equal or higher robustness.
The accuracy of temperature prediction is of paramount importance for turbine analysis, since an accurate prediction can prevent local hot spots and increase turbine blade life, avoiding at the same time the use of excessive cooling air and therefore allowing higher turbine inlet temperature. Traditionally, CFD and solid thermal conduction analyses have been performed separately at different stages of the design loop. Several hypotheses are implicit in this process and the accuracy of this kind of methods inevitably relies on empirical database and engineering judgement. Therefore, in highly complex cases, traditional methods may lead to considerable errors in the estimate of the temperature field, and to severe logistic problems to handle the exchange of information among different codes. Greater accuracy is offered by coupled fluid-solid heat transfer methods, in which fluid and solid heat transfer problems are solved simultaneously without the use of further hypotheses, since the heat flux and temperature continuity across fluid-metal interface is ensured by construction. Because of their relevance, coupled heat transfer algorithms have received considerable attention from different engineering perspectives in the last decades. However, the main disadvantage of coupled heat transfer methods is the relatively large computing times compared to that of an uncoupled stand-alone CFD simulation. Therefore, it is difficult to apply in practical engineering applications. This thesis proposes a fast and robust steady state loosely coupled fluid-solid heat transfer method based on the improvement of a loosely Dirichlet-Robin coupling approach to reduce the computational time cost. This method has been validated and applied with representative turbomachinery cases. The results have shown that the computational time has been substantially reduced compared to the baseline method, and it only spends around 35% more than a steady state CFD simulation, which is attractive for industrial applications Rotating flows can be met in aero-engines due to system rotation. The rotating flows under Coriolis and centrifugal forces affecting the flow patterns drastically compared to stationary cases. It is difficult to carriyed out real design without a deep understanding of the physics of rotating flows. The last part of this work provides the understanding of the rotational effect in the internal cooling channels of turbine rotor blade. The flow and heat transfer behaviour of an engine representative rotating channel has also been investigated and presented as an example of the numerical method in real applications.
RESUMEN Con el rápido crecimiento de la potencia de computación, los métodos numéricos están disponibles mostrando ventajas en la identificación de la compleja física subyacente de los flujos, sin embargo, se han encontrado una serie de desafíos en los problemas de refrigeración interna en los álabes del rotor en turbinas de gas debido a la complejidad de algunas de las características de los pasajes giratorios, tales como geometrías, condiciones de flujo y comportamientos de transferencia de calor en movimeintos rotatorios.
Esta tesis se centra en los desafíos de la generación de mallas numéricas, el método de transferencia de calor acoplando modelos fluido/sólido y comprendiendo el efecto de rotación en el flujo y las transferencias de calor.
En el presente trabajo se desarrolló un robusto método de generación automática de malla viscosa 3D para crear las mallas computacionales de forma híbrida con el fin de acoplar las complejas configuraciones 3D de manera eficiente para el Análisis de Fluidos Dinámicos Computacionales. El método se enfoca primero en proporcionar un criterio robusto para el procedimiento de comprobación de validez de problemas complejos y, a continuación, mejora la eficiencia implementando un esquema de pre-comprobación para reducir el número de nodos que se requiere comprobar.
Además, esta tesis también propone un nuevo, rápido y robusto método de transferencia de fluido-sólido de acoplamiento suave basado en la mejora de un enfoque de acoplamiento de Dirichlet-Robin para aumentar la precisión del campo de temperatura. Este método ha sido validado y aplicado a casos representativos de turbomáquinas.
Por último, este trabajo proporciona la comprensión del efecto de rotación en los canales de refrigeración internos de la pala de rotor de la turbina. También se ha estudiado el comportamiento de flujo y transferencia de calor de un canal giratorio representativo del motor.
Palabras clave: generación de malla viscosa, malla híbrida, método de transferencia de calor acoplado, efecto de rotación, refrigeración interna, transferencia de calor
