Investigation of Film Cooling Vortical Structures Behind a Cylindrical Hole Fed by an Internal Crossflow

The film cooling vortical structure behind a cylindrical hole fed by internal crossflow was investigated by large eddy simulation (LES). The simulation was performed on a single simple cylindrical hole (i.e., a spacing of 3D, an entry length of 6.5D, and an injection angle of 35). Two blowing ratios (M = 0.40 and 0.80) were simulated with a density ratio of 0.97. The results were validated in comparison with the measured data. The vortical structures were identified by the Q-criterion coherent isosurface and their unsteady signatures were identified by the standard deviation (SD). The vortical structures were demonstrated to play the main role in cooling performance and its instability. The internal flow was significantly produced a vortex tube structure, which responsible for the shear vortex (Kelvin–Helmholtz instabilities) between the coolant and the mainstream at the hole exit. As a result, it forced the legs of the counter-rotating vortex pair (CRVP) to widely spread and enhance the coolant effectiveness downstream. INTRODUCTION Nowadays, increasing the operating temperature of the gas turbine has been required to achieve its high performance. Such temperature exceeded the blade melting temperature, and thus the cooling technique was introduced. Film cooling is the widest cooling technique applied, where the coolant is ejected from discrete holes to form a thin layer over the blade surface to protect it. However, film cooling is known as a jetin-cross-flow (JICF) problem, in which a complex threedimensional flow mixing occurs at the hole exit (Andreopoulos & Rodi, 1984; Karagozian, 2014). It produces a large number of vortices, which forms their vortical coherent structures (Fric & Roshko, 1994; Mahesh, 2011) that force the coolant to be unstable over the blade surface (Kohli & Bogard, 2011). Consequently, the coolant surface may be partially or completely exposed to the mainstream hot gases, which lead to the blade failure with the duty operation of the gas turbine. Therefore, understanding the film cooling flow structure is highly desirable. Recently, Acharya and Kanani (Acharya & Kanani, 2017) summarized the film cooling structures behind the simple inclined cylindrical holes. It characterized by different vortical structures including counter-rotating vortex pair (CRVP), horseshoe vortex, Kelvin–Helmholtz (K-H) vortices, hairpin vortices, and wake vortices. These structures significantly changed with coolant to mainstream scaling parameters (i.e., density ratio (Eberly & Thole, 2013) (DR = ρc ρm ⁄ ) , velocity ratio (Dai et al., 2016) (VR = Uc Um ⁄ ), and blowing ratio (Sakai et al., 2014) (M = DR ∙ VR) ) and the mainstream boundary layer (Zhong et al., 2016). The literatures have been successfully investigated those structures by considering vertical plenum internal flow. However, the coolant in the realistic gas turbine flows perpendicular to the mainstream along the blade axial direction in internal coolant passages. This configuration is known as internal crossflow (Gritsch et al., 2003; Peng & Jiang, 2012a; Peng & Jiang, 2012b; Saumweber & Schulz, 2009). It significantly changed the jet exit profile, coolant trajectory, and coolant flow field. Furthermore, Peng and Jiang (Peng & Jiang, 2012b) and Acharya and Leedom (Acharya & Leedom, 2012) did large eddy simulation (LES) and found significant change in the instantaneous flow behaviour as well. Although these studies reported the asymmetric of the CRVP, they have not investigated, presented, or even deeply discussed those structures. Consequently, the vortical structures associated with the flow field could change with the internal flow effects, and thus this work investigates this behavior. The main objective is to perform a three-dimensional compressible LES approach to provide a detailed evaluation of the identified unsteady flow structures. The simulation was performed on a physical problem taken from the experimental work of the authors (Qenawy et al., 2019). The numerical results were