Stefano Puggelli

Multi-coupled numerical simulations of the DLR Generic Single Sector Combustor

ABSTRACT This paper presents a set of numerical analyses carried out on a model combustor equipped with a prefilming airblast injection system using a multi-coupled approach that includes the solution of the liquid film over the prefilming surface. The main objective of this study is to perform a systematic investigation of all the relevant aspects involved in the liquid fuel preparation of airblast atomizers, ranging from the interaction between the gas phase and the liquid film to the effect of velocity fluctuations on the dispersion of droplets downstream of the injector exit. Measurements at high pressure and reacting conditions are available for the case considered here, therefore, allowing to perform such investigation at engine-relevant conditions. The solution of the liquid film evolution over the prefilming surface suggests that the interaction between the gas phase and the liquid film is an important aspect to be considered for a reliable simulation of prefilming airblast systems since it has a strong impact on both velocity and fuel temperature at the atomizing edge. The role of primary breakup has been investigated by performing a sensitivity analysis to different theoretical and correlation-based models. Results obtained from this analysis, performed using Reynolds averaged Navier–Stokes simulations, show that the various formulations predict a quite different diameter, affecting the mixing field in the downstream region and therefore pointing out the necessity of more advanced and robust formulations. A comparison between experimental measurements and a scale-adaptive simulation of the combustor, performed using the spray setup determined in the sensitivity analysis, demonstrates the necessity of including in the simulation time-resolved velocity fluctuations to improve the prediction of the dispersion of droplets and therefore give a reliable prediction of fuel location and mixing.

Subgrid Liquid Flux and interface modelling for LES of Atomization

Traditional Discrete Particle Methods (DPM) such as the Euler-Lagrange approaches for modelling atomization, even if widely used in technical literature, are not suitable in the near injector region. Indeed, the first step of atomization process is to separate the continuous liquid phase in a set of individual liquid parcels, the so-called primary break-up. Describing two-phase flow by DPM is to define a carrier phase and a discrete phase, hence they cannot be used for primary breakup. On the other hand, full scale simulations (direct simulation of the dynamic DNS, and interface capturing method ICM) are powerful numerical tools to study atomization, however, computational costs limit their application to academic cases for understanding and complementing partial experimental data. In an industrial environment, models that are computationally cheap and still accurate enough are required to meet new challenges of fuel consumption and pollutant reduction. Application of DNS-ICM methods without fairly enough resolution to solve all length scales are currently used for industrial purpose. Nevertheless, effects of unresolved scales are generally cast aside. The Euler-Lagrange Spray Atomization model family (namely, ELSA, also call, Σ−𝑌 or Ω−𝑌) developed by Vallet and Borghi pioneering work [1], and [2], at the contrary aims to model those unresolved terms. This approach is actually complementary to DNS-ICM method since the importance of the unresolved term depends directly on mesh resolution. For full interface resolution the unclosed terms are negligible, except in the far-field spray when the unresolved terms become dominant. Depending on the complexity of the flow and the available computational resources, a Large Eddy Simulation (LES) formalism could be employed as modelling approach. This work focus on the two main terms that drive these different modelling approaches namely the subgrid turbulent liquid flux and the resolved interface. Thanks to the open source library OpenFoam® this work is an attempt to review and to release an adapted modelling strategy depending on the available mesh resolution. For validation, these solvers are tested against realistic experimental data to see the overall effect of each model proposal. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4694

Development of an evaporation model for the dense spray region in Eulerian-Eulerian multiphase flow simulations

In the present study, a novel implicit numerical model to describe evaporation phenomena in the dense spray region is proposed. The main aim is to go beyond the limits of standard vaporization models, which are normally based on a dilute spray assumption, to deal with high liquid volume fractions. The proposed method is based on an a priori computation of steady state equilibrium conditions reached by a system composed by liquid, vapour and air at constant pressure combined with a modelled characteristic time of evaporation. Such equilibrium composition and temperature is then used inside numerical calculations to compute evaporation source terms. The new for-mulation allows to simulate evaporation process in the dense zone of the spray, where, due to the extremely low thermal relaxation time, classical explicit method can lead to unphysical results. Such innovative approach has been implemented in a multiphase solver in the framework of the CFD suite OpenFOAM. An Eulerian-Eulerian solver, de-rived from the Eulerian Lagrangian Spray Atomization (ELSA) model, has been used, in order to correctly describe the liquid-gas flow without assumptions on the topology of the liquid phase. Evaporation source terms have been modelled as function of the amount of surface available for mass and heat transfer. An analysis of the solver has been carried out in RANS framework in order to highlight the capabilities of the approach in dealing with high liquid volume fraction regions with a physically consistent representation of evaporation phenomena. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4652