## 5 Conclusions  
In this work, The LES numerical calculation method is employed for the swirl nozzle with a spiral hole. WALEM is selected to study the relationship between structure and heat transfer characteristics of the 0\({}^{\circ}\) straight hole and 45\({}^{\circ}\) spiral hole under the conditions of \(Re=\) 16,000 and \(H/D=2\) for jet impingement cooling. The flow characteristics of the vortex structure and evolution of the nozzle in the jet space with two kinds of flow guiding elements are analyzed.  
From the dynamics of the coherent structure of the jet, the differences of the coherent structure with two kinds of flow guiding elements are compared. The coherent structure is analyzed, which affects the spectrum characteristics of the turbulence in the jet space. On the other hand, the transient heat transfer characteristics of the impact target surface are studied. The transient heat transfer characteristics of the target surface and the causes are analyzed. The mutual coupling relationship between the airflow and heat transfer on the adherent surface is shown. The main conclusions are as follows:  
Figure 18: Spectrum of temperature fluctuations at different positions on the impact target surface.  
1. Compared with experimental data, the LES numerical simulation method can accurately simulate the flow and heat transfer change of the rotating impinging jet of the flow guiding element. With the same grid scale and time step, the WALEM can better capture and predict the flow results near the wall than the DSM subgrid model.
2. For the jet with a 0\({}^{\circ}\) straight hole, it forms a \"round table\" region with a relatively small diffusion area around the center of the jet. The jet from the 45\({}^{\circ}\) spiral groove has a large tangential velocity component, and the blended airflow forms a \"skirt-type\" region with a larger diffusion area on the periphery of the central potential flow region.
3. In the area near the wall, due to the velocity deflection, the fluctuating velocity of the 45\({}^{\circ}\) spiral hole is larger than that of the 0\({}^{\circ}\) straight hole; that is, the turbulence degree of the jet of the wall with the 45\({}^{\circ}\) spiral hole is higher, and the ability of airflow transportation is higher. Compared with the MCIJ of the 0\({}^{\circ}\) straight angle, the heat transfer intensity of the target surface for the SIJ with a 45\({}^{\circ}\) spiral hole slightly increases, and the enhanced amplitude is within 5-10%. Excited swirl can enhance heat transfer on the concave surface at a small impact distance of \(H\)/\(D\) = 2.
4. The vortex distribution of the boundary layer of the jet region of the wall affects the fluctuation of the temperature boundary layer, and the fluctuation law of the temperature boundary layer is consistent with the change of the \(Nu\) number. The scale of temperature fluctuation on the wall is larger than the scale of radial velocity fluctuation near the wall, and there is no correspondence between them both.  
Conceptualization, L.X. ([PERSON]). and J.G.; methodology, L.X. ([PERSON]); validation, L.X. ([PERSON]), J.G. and L.X. ([PERSON]); formal analysis, Y.M.; investigation, X.Z.; resources, J.G.; data curation, X.Z.; writing--original draft preparation, Y.M.; writing--review and editing, X.Z.; visualization, Y.M.; supervision, Y.L. and L.X. ([PERSON]); project administration, Y.L. and L.X. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
The research work is supported by Guangdong Basic and Applied Basic Research Foundation (2018A030313183), National Natural Science Foundation of China (51876157) and National Natural Science Foundation of Shannxi Province in China (2019 JM-096).  
No conflict of interest exists in the submission of this manuscript, and the manuscript is approved by