Turbomachinery

Abstract

Combustion modelling methods are implemented into the Nektar++ framework to create a reactive flow solver capable of accurately modelling combustion flow regimes at low Mach numbers. The study investigates how the high-order resolution of a mixing flow field translates to modelling of turbulent diffusion flames, in regard to both cost and accuracy. First, a non-reacting pseudo-compressible flow solver has been developed based on the Incompressible Navier-Stokes code, using a low Mach variable density approach able to simulate mixing between fuel and oxidizer streams. For modelling of reactive flow, results of a 1D flamelet generated manifold (FGM) formulation dependant on mixture fraction, Z, are tabulated and coupled into the solving process. Validations are presented with turbulent cold flow mixing and canonical reacting flow cases, including combustor relevant geometry such as Sandia Propane-Jet flow and Sandia Piloted CH4/Air Flames. Implicit Large Eddy Simulations are nowadays becoming possible in an industrial setting, particularly by deploying more efficient high-order codes such as Nektar++. Turbomachinery companies are one of the realms where this is of particular interest to promote the understanding of complicated and varied turbulent phenomena appearing throughout aircraft engines. One of the components of interest are the high pressure turbines sitting just downstream of the combustor. In this work, the LS89 geometry was considered: this case was studied in a linear cascade configuration in the experimental campaign carried out in the early 1990s at the von Karman institute [1]. In this talk, two flow conditions will be presented: a low and a high pressure ratio. Only in the latter case a shock will appear, with slightly different meshing requirements being necessary. A complete 2D study will be shown to demonstrate the know-how gained in setting-up and running these cases. In particular, a considerable effort was made to match the experimental boundary conditions by using different strategies. A comparison of the final results with polynomial order 4 with those provided by the Reynolds-Averaged-Navier-Stokes solver SU2 and the experiments will show the benefit of using Nektar++. Considerations in terms of meshing requirements to reach Direct Numerical Simulations will also be discussed. Finally, three-dimensional results and analysis will be presented for the low Mach number case with a more in-depth discussion of the flow physics. Advances in computer architecture, parallel programming and high-order methods in computational fluid dynamics have enabled Direct Numerical Simulation (DNS) in turbomachinery components in the last decade. High-order frameworks such as Nektar++ has become very attractive since the industrial sector strives to increase fidelity and accuracy during the design process at low computation cost. In this talk, Implicit Large Eddy Simulations (iLES) of two turbomachinery components were investigated using Nektar++ framework. First, compressibility effects are studied in an engine intake pressure distribution at high Reynolds numbers in absence of transonic effects and under strong adverse pressure gradients on a flat plate. After that, preliminary results of the well known T106A Low Pressure Turbine (LPT) will also be discussed. In addition, a new functionality was implemented in the nektar++ framework to deal with local regions where additional mesh refinement is required, e.g. flow separation at the boundary layer, without needing to redesign the mesh. Thus, with this functionality, it is possible to locally change the polynomial order (expansion) of the selected elements for each composite in the mesh. This functionality was designed not only to deal with the flow physics and avoid re-meshing but also to save computational resources in expensive numerical simulations as found in industrial applications. So that, it is not necessary to change the expansion in the whole composite anymore if only a small part of the domain needs refinement.