Effusion Cooling 3D Simulations to Establish a Discharge Coefficient Correlation

Abstract

Effusion cooling is one of the most widespread system used to cool combustion chamber liners nowadays: it is efficient, cheap and light. Effusion cooling consists of drilling thousands of submillimetric holes into the combustor wall in order to cool it down from inside the holes and create a cooling film inside the combustor protecting it from the hot gases. Effusion cooling has long time been very challenging for combustor simulations because it involves lengthscales ranging from ½ millimeter (about the size of the effusion holes) to ½ meter (about the diameter of the combustor). That is one of the main reasons for which 3D simulations of effusion cooling has long been inaccessible, and has generated the studies presented in this paper. This study will focus on the effusion cooling holes discharge coefficient evaluation as a function of numerous aerothermal and design parameters. Many attempts to describe aerodynamically effusion have occurred, mainly based on experiments exploring few of the parameters mentioned above. But none of these studies tried to elaborate a model able to handle most of them. This is the purpose of this study which will set up a 3D model able to describe finely the physical phenomena involved in combustor effusion cooling holes and the influence of the design parameters available to combustor engineers on these phenomena. The strategy which prevails in the setup of the numerical 3D detailed model is to find a compromise between the reliability and the CPU cost of the simulation. Indeed the objective is to study the influence of a wide range of effusion cooling design parameters such as hole diameter, orientation, shape, etc. . . on the effective cross section. In addition, for a better understanding of the physical phenomena, all the simulations are performed at the same aerothermal conditions. These aerothermal conditions as blowing ratio, cooling temperature, pressure are not design parameters of effusion cooled walls. They are usually imposed by the gas turbine thermodynamic cycle very early in the development of a new engine. A preliminary study allowed to select the parameters which were both the least known and the most influential on the effusion hole mass flowrate according to literature, preexisting numerical and experimental databases. More than 60 new CFD simulations have been performed and show the influence of each effusion cooling design parameter taken separately: effusion holes inclination, orientation and taper angle. A mesh sensitivity study has been performed in order to validate the numerical approach. Then, the analysis of both the preexisting data and this new numerical database showed that some of the design parameters have strong effects and coupled influences on the mass flow rate through the holes. On the other hand, some other parameters could be easily described by simple models or even neglected. This study concludes by quantifying the improvement of a proprietary effective cross section correlation of effusion cooled walls, based on the analysis mentioned in this study.