EXPERIMENTAL CHARACTERIZATION OF THE UNSTEADY INTERACTION BETWEEN EFFUSION COOLING AND SWIRLING FLOWS IN A GAS TURBINE COMBUSTOR MODEL

The analysis of the interaction between the swirling and cooling flows, promoted by the liner film cooling system, is a fundamental task for the design of turbine combustion chambers since it influences different aspects such as emissions and cooling capability. The requirements for improving the modern gas turbine combustors are: swirler injectors for flame stabiliza- tion, increasingly higher temperature and pressure values, and an increased amount of air dedicated to the combustion process. All these aspects make the design of even more efficient cooling systems, and the correct estimation of liners heat load, a hard task. Experimental works in the literature have addressed the problem with dedicated test rigs using steady-state measure- ment techniques to analyze the interaction between swirling main flow and effusion cooling flow. However, the fluid dynamic mechanisms, which govern turbulent mixing between main and coolant, are the flow field instabilities. In particular high turbulence oscillations, eddies, and tangential velocity com- ponents induced by the swirling flow deeply affect the behavior of effusion cooling jets demanding for dedicated unsteady flow field (near-wall coolant sub-layer) and adiabatic effectiveness experimental analysis. For this reason the present research activity is aimed at an unsteady characterization of the turbulent interaction between effusion cooling and swirling flows in a gas turbine combustor model. The experimental setup of this work consists of a non-reactive single-sector linear combustor test rig scaled up with respect to engine dimensions to increase spatial resolution and reduce the frequencies of the unsteadiness. The test section was equipped with an effusion plate with standard inclined cylindrical holes to simulate the liner cooling system. The degree of swirl for a swirling flow is usually characterized by the swirl number, Sn, defined as the ratio of the tangential momentum flux to axial momentum flux. To assess the impact of such parameter on the near-wall effusion behavior, a set of three different axial swirlers with swirl number equal to Sn = 0.6 - 0.8 - 1.0 were designed and tested in the experimental apparatus. The tests were also carried out by varying the feeding pressure drop of the effusion plate to evaluate the effect of this parameter. During the first phase of the research, the rig was instrumented with a 2D Time-Resolved Particle Image Velocimetry system, focussing on different field of views. An analysis of the main flow field by varying the Sn was first performed in terms of average velocity, Root Mean Square, and turbulence related quantities like kinetic energy spectra and length scale information. In a second step, the analysis was focussed on the near-wall regions: the impact of Sn on the coolant jets was quantified in terms of vorticity analysis and jet oscillation, highlighting a strong effect of the swirl number on film behav- ior. Subsequently, film effectiveness was acquired using the Fast Response Pressure Sensitive Paint technique; the scale of the model and the acquisi- tion frequency allowed to track the effusion jets unsteadiness. With both the measurement techniques, the collected results show the importance of using an unsteady analysis to perform an in-depth characterization of the mixing phenomena between the main flow and the coolant, which in significantly affected by the Sn value.