Computational Modeling of DLR Micro Gas Turbine Spray Burner

Abstract

The current automobile industry is undergoing a paradigm shift from the conventional internal combustion engines to electric vehicles. Electro mobility is a greener alternative compared to the conventional engines. But one of the negative factors for the demand of electric cars is the maximum range which they can offer in a single battery charge. To extend the maximum range of electric cars Micro Gas Turbines are used due to their small size, better performance and lower emission compared to their internal engine counterparts. The automobile industry as any other industry is governed by stringent emission norms and to adhere to these emission levels, Flameless Oxidation (FLOX) combustion technology is employed. In this kind of combustors FLOX type of combustion is achieved based on high internal flue gas recirculation that is driven by high momentum of the air and fuel jet entering the combustion chamber. By doing so the chemical reactions are delayed and the combustion is rather distributed over a large volume leading to lower peak temperatures and NOx emissions. This concept has been applied on conventional gas turbines however the application of this to a liquid fueled Micro Gas Turbine offers additional challenges which affects the physical processes of atomization, evaporation, fuel/air mixing etc. The objective of this thesis is to investigate these effects computationally and validate it against the experimental data measured at aDLR (German Aerospace Center) test facility to gain better understanding of the underlying physics. Experimental data are available for a model combustor at DLR Stuttgart on light heating oil. Spray characteristics have been measured which in this study are used for validation of the combination of CFD models for turbulence, combustion and dispersed liquid phase flow. CFD results for droplet size, velocity, volume flux are compared against the measurements and additionally temperature and other flow characteristics are computed that are not measured which helps in developing further insight into the combustion process inside the burner. Further additional CFD simulation is done with higher air preheat temperature to study its effect on the combustion process. For the validation study in this thesis, 3D steady state Reynolds-Averaged-Navier-Stokes (RANS) Eulerian- Lagrangian simulations are performed where gas phase computations are performed within Eulerian framework and dispersed phase is treated in Lagrangian framework. Two different combustion models are used: combined Eddy Dissipation and Finite Rate Chemistry (ED/FR) and Eddy Dissipation Concept (EDC). These combustion models are used with two different turbulence models of Reynolds StressModel and Shear Stress Transport turbulence models with various other models for modeling dispersed phase. It was observed that the combined ED/FRmodel predicted early spatial heat release leading to inaccurate droplet velocity predictions while EDCmodel predicted proper heat release leading to better prediction of velocity profiles. Further, the applicability of Flamelet approachwas investigated through FlameletGeneratedModel (FGM) and Steady FlameletModel (SFM). While SFM predicted combustion much upstream of the combustion chamber, FGM model predicted proper reaction zone. However, the FGM model predicted early spatial heat release due to the prediction of near equilibrium chemical time scales as the Damköhler number is close to unity. Further effects of turbulent two-way coupling are investigated and are found to be important. Radiation is found to have only a minor role in this case.