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In film cooling for thermal protection of turbine airfoils, the interaction of the cooling jet with the mainstream creates a counter-rotating vortex pair, causing the coolant to detach from the wall, particularly at high blowing ratios (BRs). Eight bionic composite cooling structures inspired by the shark skin structure are proposed to improve the film cooling effectiveness (FCE). The flow characteristics, adiabatic FCE and flow loss are numerically analyzed in detail by using RANS method and Realizable k-ε model, emphasizing the flow loss in the BRs range of 0.5–1.5. Because the cooling jets are separated towards both sides, the coolant near the centerline of the sine-wave and V-shaped trenches is relatively reduced, but the CRVP formed on both sides pushes the cooling fluid back to the cooled wall surface. The mechanism of improved FCE is revealed by the gas convergence to the centerline of the V-shaped, fan-shaped, and wave-shaped surface structures. The variation of blow ratio has a slight influence on the FCE of different cooling configurations. At 25 < X/D < 30, the spanwise-averaged FCE of the VT-WaveS increases significantly, (11.66 %–12.20 % when the BR is 1.0 and by 10.74 %–12.42 % when the BR is 1.5) compared to the sine-wave trench with V-shaped surface. The blade-shaped surface forms more uniform temperature distribution on the flat plate. The discharge coefficient of the VT-BladeS performs best at the BRs of 1.0 and 1.5. The total pressure loss coefficients of the cases are very close. This study revealed the mechanism of the composite structure to improve the FCE of the blade, and the proposed surface structure laid a foundation for the application of shark-skin-inspired surfaces in blade cooling. [ABSTRACT FROM AUTHOR]

Mist-assisted film cooling has exhibited great potential for efficient cooling of a turbine vane due to the vaporization of water droplets flowing with the air. To further improve the film coverage and cooling effectiveness, it is necessary to find a structure that allows the coolant flow to expand laterally and inhibits the separation of the coolant jet from the wall. Therefore, we propose in this work to study the influence of serrate structure on the cooling performance of trenched holes by examining the flow and heat transfer characteristics of the mist/air mixture. As water evaporation is involved in this process, the dry-bulb temperature may not be suitable for evaluating the film cooling performance. Therefore, a wet-bulb-temperature-based film cooling effectiveness (WFCE) is proposed. The results show that the included angle of serrate plays a significant role in WFCE. In addition, the area-average WFCE of the trenched holes increases first and then decreases as the blowing ratio increases. The concentration of mist and the diameter of droplets are found to be crucial for significantly affecting the WFCE. The relationship between the film cooling effectiveness and the contribution of mist is explicated by empirical correlation with the mist concentration and droplet diameter. The empirical correlation, especially the exponential increasing trend with the square root of mist concentration at a constant droplet diameter, provides the basis for further theoretical analysis of the mechanism of cooling effectiveness improvement. Under the practical condition, it is also proved that the serrated transverse trench structure and the addition of mist to the coolant air can improve significantly the film cooling performance for the turbine vane. • Influence of serrate structure on mist/air film cooling performance of trenched holes is studied. • Wet-bulb-temperature-based film cooling effectiveness (WFCE) is proposed and evaluated. • WFCE increases linearly with droplet diameter when mist concentration is given. • WFCE increases exponentially with square root of mist concentration at constant droplet diameter. • The cooling strategy is proved for turbine vane under practical condition. [ABSTRACT FROM AUTHOR]

In this work, the influence of several lip structures on the effectiveness of film cooling and the flow fields of transverse trenched holes at different blowing ratios (BRs) ranging from 0.5 to 2.0 is numerically investigated, using the Reynolds averaged Navier Stokes method with the realizable k - ε turbulence model. The trenches include the traditional transverse trench, the bevel trenches, and the fillet trenches. It shows that the trenched holes produce weaker counter-rotating vortex pairs, which can enhance the effectiveness of film cooling compared with the standard cylindrical film hole. It demonstrates that the lip structures can improve the effectiveness of film cooling of the trenches at low BRs, but it has a negative effect at high BRs. It is found that the lower lip structures have less influence on the effectiveness of film cooling at different BRs than the upper lip structures. It is also found that the upper part of the fillet structure employs the Coanda effect to make the coolant flow close to the wall, thereby improving the effectiveness of film cooling; while the lower part of the fillet structure is conducive to converting the coolant flow into a vertical jet, thereby improving the effectiveness of film cooling at low BRs. [ABSTRACT FROM AUTHOR]