Effects of Density and Blowing Ratios on the Turbulent Structure and Effectiveness of Film-Cooling

Abstract

Large eddy simulations (LES) were performed to investigate film cooling of a flat plate, where the cooling jets issued from a plenum through one row of circular holes of diameter D and length 4.7D that are inclined at 35° relative to the plate. The focus is on understanding the turbulent structure of the film-cooling jet and the film-cooling effectiveness. Parameters studied include blowing ratio (BR = 0.5 and 1.0) and density ratio (DR = 1.1 and 1.6). Also, two different boundary layers (BL) upstream of the film-cooling hole were investigated — one in which a laminar BL was tripped to become turbulent from near the leading edge of the flat plate, and another in which a mean turbulent BL is prescribed directly. The wall-resolved LES solutions generated were validated by comparing its time-averaged values with data from PIV and thermal measurements. Results obtained show that having an upstream BL that does not have turbulent fluctuations enhances the cooling effectiveness significantly at low velocity ratios (VR) when compared to an upstream BL that resolved the turbulent fluctuations. However, these differences diminish at higher VRs. Instantaneous flow reveals a bifurcation in the jet vorticity as it exits the hole at low VRs, one branch forming the shear-layer vortex, while the other forms the counter-rotating vortex pair. At higher VRs, the shear layer vorticity is found to reverse direction, changing the nature of the turbulence and the heat transfer. Results obtained also show the strength and structure of the turbulence in the film-cooling jet to be strongly correlated to VR.

Hybrid large‐eddy simulation with adaptive downstream anisotropic eddy viscosity model

“…Figure 11 shows the time‐averaged velocity profiles at several locations downstream of the film‐cooling hole along z / D = 0. In this figure, LES from this study and LES from Reference 37 can be seen to yield similar results. Also, results from the hybrid method can be seen to match the LES results more closely than the $$$ k-\varepsilon $$$ results from Reference 37.…”

“…In this figure, LES from this study and LES from Reference 37 can be seen to yield similar results. Also, results from the hybrid method can be seen to match the LES results more closely than the $$$ k-\varepsilon $$$ results from Reference 37. The mismatch between the results from the hybrid method and LES from this study is the error incurred by the two‐way coupling and the inadequacies in the adaptive downstream RANS model.…”

“…With these geometric and operating conditions, the cooling flow once exiting the hole will separate from the plate and then reattach at a downstream location, physical processes that RANS models cannot predict accurately. The problem just described is identical to the one studied in Reference 37, and the LES data generated in that study was used to validate and assess the method developed in this study.…”

“…In this section, the performance of the hybrid method with the adaptive downstream RANS model is examined in both the LES and the RANS regions. The performance is assessed by comparing results obtained by the hybrid method developed with results obtained by the realizable $$$ k-\varepsilon $$$ model and by LES from Reference 37. In Reference 37, both the $$$ k-\varepsilon $$$ model and the LES were applied to the entire flow domain.…”

“…The performance is assessed by comparing results obtained by the hybrid method developed with results obtained by the realizable $$$ k-\varepsilon $$$ model and by LES from Reference 37. In Reference 37, both the $$$ k-\varepsilon $$$ model and the LES were applied to the entire flow domain. The performance of the hybrid method is also assessed by comparing with results obtained by an LES simulation for the entire flow domain performed in this study.…”