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Owing to the limited supply and pressure margin in the air system, a cooling technique providing efficient heat transfer with lower flow loss is highly desirable for gas turbine blades. Microscale ribs have promised to be a potential cooling candidate. In this work, large eddy simulations are implemented to reveal the scale effect of micro ribs on the near-wall turbulent transport in a cooling channel. Considering a mechanistic study and practical applications, both single-rib and rib-array arrangements are studied with a wide range of dimensionless viscous-scaled rib heights involving the entire boundary layer. The results indicate that the rib-induced destruction and regeneration of coherent structures are, respectively, responsible for the weakened momentum transport and enhanced heat transport in the near-wall region. Using tiny ribs, regenerated quasi-streamwise vortices are mainly located in the buffer layer. The resulting turbulence burst greatly enhances wall heat transfer while keeping a lower flow loss due to the weak form drag. Regenerated hairpin vortices using tall ribs are activated in the log-law layer and intensively interact with mainstream. Along with improved wall heat transfer, the significant form drag results in a remarkably high flow loss. Accordingly, heat transfer and flow loss show different dependencies on the rib height, which contributes to an optimum height interval of ribs (e+ = 20–40) located in the high buffer and low log-law layer for maximizing the overall performance. Furthermore, for the rib-array scheme, adequate inter-rib spacing is essential to achieve turbulence regeneration for enhancing near-wall heat transport. [ABSTRACT FROM AUTHOR]

• The effect of wall curvature on the cooling features in a ribbed cooling channel is explored; • Various walls with distinct curvatures are comprehensively evaluated in a unified ribbed channel; • Counter-rotating vortex pairs on the spanwise plane are induced by the curved wall; • The heat transfer and flow performance are synthetically evaluated for an optimal curved wall; Simplified rectangular ribbed cooling passages with a flat wall are extensively considered in exploring the internal cooling features of turbine blades, but the realistic blade has a twisted shape inherently. The effects induced by the curved wall have not been clarified in detail. In this work, adopting a verified v 2 f turbulence model, numerical investigations are completed to evaluate the effects of the curved wall on the internal cooling characteristics of a ribbed channel. Adopting the unified ribbed channel, flat, convex, and concave walls with distinct curvatures are comprehensively evaluated and compared in a wide Re range for the turbulent flow and heat transfer features as well as the flow and thermal performance. It is found that using the flat wall, ribs can typically induce recirculation vortices having a two-dimensional nature. In contrast, the curved wall significantly contributes to the counter-rotating vortex pairs on the spanwise plane. Combined with recirculation vortices offered by the ribs, the turbulent flow of the cooling channel with the curved wall has a remarkable three-dimensional feature. Hence, the turbulent activity and fluid mixing are enhanced greatly along with the raised heat transfer enhancement and friction loss. Particularly, the convex wall with a curvature of K = 4 provides 28.6 % higher heat transfer performance (Nu / Nu 0) but 88.4 % higher resistance (f / f 0) than the flat wall. Considering the overall cooling performance, the concave wall with a relatively small curvature is suggested with an improvement of up to 32.8 % concerning the factor (Nu / Nu 0)/(f / f 0) and 9.5 % on (Nu / Nu 0)/(f / f 0)1/3. Finally, it is highlighted that considering the effect of the wall curvature, the current study stimulates the mechanistic understanding and provides a design guideline for high-performance blade internal cooling. [ABSTRACT FROM AUTHOR]

• A multi-parameter optimization method is developed based on the simplified conjugate-gradient algorithm. • Heights of perturbation ribs in turbine blade internal cooling are optimized at various conditions. • All of the optimal designs possess a hierarchically nonuniform characteristic. • The design direction is provided for perturbation elements involving turbulent flow heat transfer. Owing to the limited cold-air amount and pressure in supply systems, high-efficient heat transfer with low-level friction loss is highly desired for cooling units of a turbine blade. To exploit the potential improvement of hierarchically arranged ribs in cooling passages proposed previously, multi-parameter optimizations for rib arrangements are implemented by integrating the simplified conjugate-gradient algorithm with the turbulent flow and heat transfer model. Rib heights as design variables are optimized with various performance indices as objective functions at a fixed Re. The optimizations confirm that using the wall temperature difference and Nu as the objective function, respectively, a limited heat transfer improvement is achieved with a greatly increased friction loss. Taking the overall performance factor as the objective function, different optimal designs at different constraint conditions possess hierarchical characteristics. A significant friction loss reduction of 52.1%, 54.7%, and 54.8%, is achieved with a moderate heat transfer loss of 10.9%, 7.0%, and 2.3%. Despite different thermal and friction performances, their overall performances are consistent with a remarkable increase of 13.9%, 21.2%, and 27.3%. Finally, the optimization strategy coupling the multi-parameter optimization and hierarchical scheme is confirmed as effective for enhancing the thermohydraulic performance of convective heat transfer systems with perturbation elements. [ABSTRACT FROM AUTHOR]

• A novel hierarchical scheme of rib arrangements is proposed for turbine blade internal cooling. • Various representative rib configurations are considered to implement the hierarchical scheme. • The improvement achieves greatly-reduced friction loss and low-level heat transfer deterioration. • Heat transfer details and turbulent flow structures are analyzed to reveal the underlying physics. • The feasibility and generality of the hierarchical scheme are verified comprehensively. Turbulators, such as ribs, dimples, and pin-fins, play a vital role in the internal cooling efficiency of turbine blades. As a typical turbulator, various rib configurations using a uniform arrangement scheme have indicated high heat transfer enhancement but the friction loss is simultaneously subject to a great increase. In this work, a novel hierarchical arrangement scheme of ribs is developed aiming to improve the cooling efficiency. Adopting the uniform scheme as a baseline, the hierarchical scheme is implemented for six representative rib configurations (including transverse ribs, angled ribs, V-shaped ribs, inverted V-shaped ribs, M-shaped ribs, and inverted M-shaped ribs) and evaluated for its feasibility and generality. For different cooling designs, turbulent flow and heat transfer of the ribbed cooling channel are studied by three-dimensional numerical simulations based on the finite volume method with a constructed turbulence model. It is found that for all rib configurations, the hierarchical scheme can remarkably reduce the friction loss as desired, especially for the inverted V-shaped rib with a reduction of up to 50%. Due to the occurrence of flow separation, secondary flows offered by transverse ribs are characterized by a two-dimensional recirculation vortex behind the rib. For other rib configurations, secondary flows present a typical three-dimensional characteristic including the downwash flows along the inclined rib leg and the longitudinal vortices. The usage of the hierarchical scheme with small ribs strongly suppresses these secondary flows, which contributes to the significant decrease in form drag loss. Meanwhile, using the hierarchical scheme produces a slight heat transfer deterioration commonly, which is because the constrained secondary vortices weaken the turbulent mixing and convection heat transfer. Significantly, for the two W-shaped ribs, the limited secondary vortices but fully developed under the hierarchical scheme achieve a higher heat transfer enhancement. Finally, for all considered ribs, the hierarchical scheme can improve the overall performance factor of (Nu / Nu 0)/(f / f 0)1/3 by more than 10%, and up to 21.15% for the V-shaped rib. Adjusting design variables, including the decreasing ratio of the rib size and the initial rib size, the hierarchical scheme still provides even higher performance enhancement. [ABSTRACT FROM AUTHOR]