Search Results (3)

The spatiotemporal evolution of the flow structures and coolant coverage of double-row film cooling with upstream forward jets and downstream backward jets, having a significant impact on film-cooling performance, is studied using the simplified thermal lattice Boltzmann method (STLBM). Moreover, the effect of the inclination angle of downstream backward jets is considered. The high-performance simulations of film cooling have been conducted by using our verified in-house solver. Results show that special flow structures, such as a sand dune-shaped protrusion, appear in double-row film cooling with upstream forward jets and downstream backward jets, which is mainly because of the blockage effect resulting from the coolant jet with backward injection. The interaction among structures results in the generation of an anti-counterrotating vortex pair (anti-CVP). The anti-CVP with the downwash motion can result in the attachment of coolant to the bottom wall, which promotes the stability and lateral coverage of coolant film. The momentum and heat transport are strengthened as the backward jet is injected into the boundary layer of the mainstream. Although the downstream evolution of the backward jet is not very smooth, its core attaches closely to the bottom wall due to the downwash motion of anti-CVP. Moreover, there is an obvious backflow zone shown in the trailing edge of the downstream backward jet with a large inclination angle. The obvious backflow makes the coolant attach to the bottom wall well. Therefore, the film cooling effectiveness is improved as the inclination angle of the downstream backward jet varies from ${\alpha }_{down}=135°$ to ${\alpha }_{down}=155°$, with a constant blowing ratio of $BR=0.5$. In addition, the fluctuation of the bottom wall’s temperature is weak due to the stable coverage of the coolant layer under ${\alpha }_{down}=155°$. The film-cooling performance with an inclination angle of ${\alpha }_{down}=155°$ is the best among all the cases studied in this work. This work provides essential insights into film cooling with backward coolant injection and contributes to obtaining a complete understanding of film cooling with backward coolant injection. Full article

Film cooling has been widely applied to the highly efficient thermal protection of gas turbines. By using the simplified thermal lattice Boltzmann method (STLBM), a series of large-scale simulations of film cooling are performed to dig up the mixing mechanism of double rows film cooling with the combination of forward and backward jets at the first attempt. The combination of an upstream row with forward jet and a downstream row with backward jet is considered. The Reynolds number is 4000. The blowing ratio of the upstream coolant jet is fixed as $B{R}_1=0.5$. For the downstream coolant jet ($B{R}_2$), five values ranging from 0.2–0.8 are considered. The inclination angles of forward jet and backward jet are 35° and 145°, respectively. The numerical results reveal that the performance of film cooling is greatly improved by backward downstream jet due to the suppression of counterrotating vortex pair (CVP). Moreover, the flow structure is changed with the blowing ratio of backward jet. An anti-CVP having the opposite rotational direction to CVP appears as the blowing ratio of backward jet is large. The special flow structure weakens the adverse effect of CVP and transports much coolant jet to the cooled wall. Correspondingly, the time-averaged film cooling effectiveness is increased and the fluctuation of film cooling effectiveness is decreased. All of these indicate that a backward downstream jet with a large blowing ratio improves film cooling performance. The results obtained in this work help to the optimization of film cooling scheme, which also benefit the promotion and application of STLBM in gas turbine engineering. Full article

Single-row double-jet film cooling (DJFC) of a turbine guide vane is numerically investigated in the present study, under a realistic aero-thermal condition. The double-jet units are positioned at specific locations, with 57% axial chord length (C_x) on the suction side or 28% C_x on the pressure side with respect to the leading edge of the guide vane. Three spanwise spacings (Z) in double-jet unit (Z = 0, 0.5d, and 1.0d, here d is the film hole diameter) and four spanwise injection angles (β = 11°, 17°, 23°, and 29°) are considered in the layout design of double jets. The results show that the layout of double jets affects the coupling of adjacent jets and thus subsequently changes the jet-in-crossflow dynamics. Relative to the spanwise injection angle, the spanwise spacing in a double-jet unit is a more important geometric parameter that affects the jet-in-crossflow dynamics in the downstream flowfield. With the increase in the spanwise injection angle and spanwise spacing in the double-jet unit, the film cooling effectiveness is generally improved. On the suction surface, DJFC does not show any benefit on film cooling improvement under smaller blowing ratios. Only under larger blowing ratios does its positive potential for film cooling enhancement start to show. Compared to the suction surface, the positive potential of the DJFC on enhancing film cooling effectiveness behaves more obviously on the pressure surface. In particular, under large blowing ratios, the DJFC plays dual roles in suppressing jet detachment and broadening the coolant jet spread in a spanwise direction. With regard to the DJFC on the suction surface, its main role in film cooling enhancement relies on the improvement of the spanwise film layer coverage on the film-cooled surface. Full article