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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]

Film cooling is a commonly used thermal protection technology for turbine airfoils. However, due to the counter-rotating vortex pair (CVP) generated by the interaction between the cooling jet and the mainstream, the coolant gradually breaks away from the wall, which is particularly evident at high blowing ratios. For the sake of improving the film cooling effectiveness (FCE) under high inlet temperature conditions, a novel film cooling structure (SerrTrench-VSS) that combines serrated trench film holes with shark skin-inspired V-shaped surface (VSS) is proposed. The RANS method and realizable k - ε model are used for the simulations. The flow characteristics, adiabatic FCE, and resistance loss are analyzed in detail when the blowing ratio (BR) is 0.5–1.5 and then compared with the traditional cylindrical film hole and transverse trench film hole. In addition, the impact of VSS height on the cooling performance is investigated and the best height of the structure is determined. The results show that the SerrTrench-VSS has better FCE. The serrated trench can destroy the CVP and improve the spanwise spreading ability of the coolant. Also, the VSS can converge the cooling jet downstream of the film hole toward the centerline, thereby improving its extension ability along the flow direction. In the studied range of blowing ratio, the SerrTrench-VSS has the best FCE, and the spanwise-averaged FCE increases by up to 10.50% when BR = 0.5. The empirical correlations between the globally-averaged FCE and the height ratio within the range of the present study are also given. For the serrated trench, the cooling jet has the best spanwise spreading and flow extension ability when the VSS height is 0.5 D. The proposed novel film cooling structure (SerrTrench-VSS) can effectively improve the FCE and provide feasible means for the efficient cooling of a turbine blade under practical conditions. [Display omitted] • A V-shaped surface structure inspired by shark skin was proposed for film hole cooling. • It can concentrate the downstream cooling jet and inhibit the counter-rotating vortex pairs. • The best height of the V-shaped surface structure was determined. • The best cooling effectiveness can be obtained by combining a serrated trench with this structure. • The globally-averaged film cooling effectiveness and the height ratio were empirically correlated. [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]

• A vane model including double-wall structure and air/mist mixture. • Average temperature of blade surface drops substantially by 156∼174 k. • Distribution of Nusselt number at internal wall of chamber. • Total pressure loss coefficient of vane under real working conditions. • Thermal performance factor is introduced to evaluate the cooling characteristics. This paper establishes a comprehensive vane model which includes a double-wall cooling structure, with a C3X vane as the prototype and the air added with droplets as the coolant. The temperature of the vane, the trajectory of the evaporating droplets, the water vapor mass fraction on the vane surface, the distribution of the Nusselt number at the internal wall of the chamber, and the total pressure loss coefficient of the vane are calculated subsequently under real working conditions. The thermal performance factor is also introduced to evaluate comprehensively the cooling characteristics. It shows that when the coolant is air, the average surface temperature of the vane with double-wall cooling structure drops by 35 K; when a 10% mass fraction of air is replaced by droplets, the average temperature of the vane surface drops substantially by 156∼174 K. At the same time, it is found that a smaller droplet is conducive to the performance improvement. This work reveals a promising method of vane cooling design by using the proposed structure with impingement/film cooling and air/mist mixture, although the double-wall can relatively increase the total pressure loss. The total pressure loss of a vane with a double-wall structure is increased by 10.54% compared to the traditional vane. It is proved that the double-wall cooling structure and the addition of mist to the coolant air can improve significantly the thermal performance factor. As the mist concentration varies, the area-average temperature, the globally-averaged Nusselt number, and the thermal performance factor vary significantly, while the total pressure loss changes slightly. The relationship between the dimensionless area-average temperature and the contribution of mist (in terms of mist concentration and droplet diameter) is explicated. [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]

• A new method of defining the velocity field in the coolant inlet is used to quantitatively investigate the effects of the coolant swirl intensity on the film cooling performance in the operating conditions of h series turbine vanes. • The critical value of the coolant vorticity to the transformation of film coverage is 20,000 /s. • When the coolant vorticity is 40,000 /s, the area-averaged film cooling effectiveness at the near field downstream of the film hole is 54 % higher than the non-swirling case. • With high blowing ratios and large vorticities, a stable flow field is formed, where the additional coolant vortex and induced vortex appear. • For high blowing ratios, with the increase of the vorticity, the momentum advantage of the coolant gradually transforms into the effectiveness advantage. Advanced H/J series heavy-duty gas turbines are widely equipped with film cooling to protect turbine blades. Previous studies have shown that coolant swirl has an important influence on film cooling performance. However, there are few studies on the quantitative analysis of the coolant swirl intensity because of the swirl generated by the specific structure, and most of the research was carried out at relatively low temperatures. In this paper, a new method of defining the velocity field in the coolant inlet is used to quantitatively investigate the coolant swirl intensity. A numerical study of the film cooling performance for the coolant jet with various swirl intensities is conducted in the operating conditions of H series turbine vanes. The effects of the blowing ratio and coolant vorticity on the vortex structure and mixing process of the flow field are investigated emphatically, and the mechanisms for the variation of the film cooling effectiveness are analyzed. The results show that the film cooling effectiveness is greatly improved with the swirling coolant jet at high blowing ratios. When Ω n =40,000 /s, the area-averaged (L = 5 D) film cooling effectiveness is 54 % higher than the non-swirling case. Ω n =20,000 /s is a critical value of the coolant vorticity. When Ω n >20,000 /s, the film coverage transforms significantly. Furthermore, the main vortical structures in a typical SJICF flow field with high blowing ratios and large vorticities are proposed. Compared with the non-swirling JICF, the additional coolant vortex and induced vortex appear in the SJICF. Under the influence of the two vortices, the coolant momentum is sacrificed for enhanced attachment to the wall resulting in the momentum advantage gradually transforming into the effectiveness advantage with the increase of the vorticity. [ABSTRACT FROM AUTHOR]

• A novel double-wall with composite swirl/film cooling structure was proposed. • The structure exhibited outstanding overall cooling effectiveness (OCE) in the simulated range. • The cooling performance at high blowing ratios were obtained and the OCE was sensitive to bi. • The empirical correlations obtained by nonlinear fitting were in good agreement with the simulations. • It provided a guidance for developing advanced gas turbine blades double-wall cooling structures. A novel double-wall with composite swirl/film cooling structure is proposed in this work to enhance the cooling effectiveness of contemporary gas turbine blades. The influence of different shaped chambers and Biot numbers on the overall cooling effectiveness (OCE) are investigated by Reynolds time-averaged Navier-Stokes simulations and conjugate heat transfer analysis. The results suggest that the proposed composite cooling configuration exhibits outstanding OCE in the simulated range. When the R m is 1.00 and the Biot number is 0.11, the OCE of the composite structure is improved by 8.92 % over the conventional double-wall structure, and the distribution is more uniform. In addition, the optimal flow and cooling performance of the composite swirl/film cooling structure at high R m conditions are obtained. The OCE is very sensitive to the variation of the Biot number. Finally, the empirical correlations obtained by the multivariate nonlinear regression fitting method are in good agreement with the simulations, with errors within 10 %. The results also provide important guidance for the development of advanced gas turbine blades double-wall cooling structures. [ABSTRACT FROM AUTHOR]

• Impact of double rows on air/mist film cooling is studied for the first time. • Sellers prediction model is appropriately modified for double rows. • Modified sellers method is proved to be more suitable for air/mist film cooling. • Air/mist film cooling effectiveness is accurately correlated by the modified model. Improving the multi-row film cooling effectiveness of turbine vanes and blades is becoming increasingly important, and the configuration design of the multi-row holes is becoming increasingly complex. This paper presents a simulation study of the air/mist film cooling characteristics on double-row-hole flat plates and makes predictions about spanwise-averaged cooling effectiveness using a modified Sellers superposition method. The effects of blowing ratio, row spacing, row arrangement, droplet diameter, and mist mass ratio on film cooling are systematically investigated. It is found that the injected mist improves the film cooling effectiveness under all operating conditions for the coolant air. The air/mist cooling is particularly effective at a blowing ratio of 1.25 or a row spacing of 5 D with the staggered two-row holes. The improvement in cooling effectiveness increases with increasing mass ratio or decreasing droplet diameter of the mist. Furthermore, the spanwise-averaged cooling effectiveness can be accurately predicted by modifying the classical Sellers superposition model. This study lays the foundation for accurately predicting the distribution of air/mist film cooling effectiveness of turbine vanes and blades, and helps to promote the development of practical air/mist film cooling technology. [ABSTRACT FROM AUTHOR]

The gas turbine blade tip might face substantial heat loads because of leakage flow between the blades and the casing. For blade tip cooling, a composite cooling structure with film holes and broken ribs is first used on GE-E3 blade in this work. The flow and cooling characteristics of the innovative structure are studied by numerical simulation under various blowing ratio (BR) conditions. Meanwhile, the impact of modifying both the rib angle and the rib height on the adiabatic film cooling effectiveness (AFCE) at the tip of the squealer is analyzed. According to the results, adding rib structures to the squealer tip can effectively regulate the paths of cavity vortices and kidney-shaped vortex pairs (KVP) at the tip. As a result, the averaged AFCE at the blade tip is improved. The notch pressure-side broken rib structure has good aerothermal performance, and the highest AFCE at BRs of 0.50, 1.00, and 1.50 basically occur under the "R60-100 %" condition (R60 refers to the rib structure of 60°, and 100 % is the ratio of rib height to notch depth), and the corresponding AFCE are 27.71 %, 26.00 %, and 32.47 % higher than those of the no-rib case, respectively. The corresponding AFCE increased by 27.71 %, 26.52 %, and 32.47 %, respectively, compared to the no-rib condition. The highest AFCE at a BR of 1.50 occurs at "R75-70 %", which is a 38.20 % increase in AFCE compared to the no rib case. The improvement in AFCE is due to the difference in the flow of the cooling jets, which are subject to cavity vortices at different BRs. The analysis shows that the addition of ribs disrupts the formation of KVPs and weakens the influence of the cavity vortex, thus reducing the low AFCE region at the lower end of the tip groove and increasing the AFCE. However, due to the blocking effect of the ribs, the pressure loss at the blade tip is elevated. The proposed blade tip cooling structure is expected to provide new ideas for the next generation of advanced gas turbine cooling designs. • A composite tip cooling structure with film holes and broken ribs is proposed. • The flow and cooling characteristics under various blowing ratio conditions are studied. • Impact of modifying rib structure dimensions on effectiveness at squealer tip is analyzed. • Squealer tip ribs can regulate cavity vortices and kidney-shaped vortex pairs, thus improving cooling effectiveness. • The proposed tip cooling structure provides new ideas for next generation blade cooling designs. [ABSTRACT FROM AUTHOR]

Modern gas turbines use high-performance ribs to improve internal cooling performance. In this study, the aerothermal performance of a bio-inspired truncated rib arrangement with stepped length and height variations, proposed in our previous work, is systematically studied using the numerical simulation method and the rib structures are optimized using the response surface methodology. A square channel model is employed to simulate the flow of actual turbine blade, and the rhombus-patterned ribs are placed on the two channel sidewalls. The studied structural parameters include the length variation rate, height variation rate, and pitch distance. It is found that as the rib height decreases, heat transfer is enhanced while the pressure drop is decreased. In general, however, as the variation rates of height and length increase, both the heat transfer coefficient and flow loss increase. The correlations between the thermal performance factor and the three structural parameters are obtained from thirteen cases of aerothermal performance analysis, and the optimal structure of rhombus-patterned ribs is further obtained: Δ l / l 1 = 0.333, Δ e / e 1 = 0.167, and p / e 1 = 2.000. The geometric layout optimization of the novel ribs is important for providing higher thermal performance of the internal channel and hence efficient cooling of the turbine blades. • Rhombus-patterned rib structures are optimized to enhance turbine blade aerothermal performance. • The response surface methodology is used for optimizing the structural parameters. • The optimized parameters include length variation rate, height variation rate, and pitch distance. • The relationship between the thermal performance factor and these parameters are correlated. • The rhombus-patterned ribs are also suitable for application in many heat exchange devices. [ABSTRACT FROM AUTHOR]