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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]