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• 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]