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

Abstract

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 formulation 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, derived 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. KeywordsSpray modelling, ELSA, Eulerian-Eulerian methods, Evaporation modelling. IntroductionThe future standards on pollutants emissions expected by ICAO-CAEP [1] for the next generation of civil aeroengines have pushed the attention towards the introduction of lean burn technology in aeronautical framework. Here, a drastic reduction of NOx levels can be achieved working on a narrow range of temperature and equivalence ratio. Therefore, all the issues related to liquid fuel atomization and air-fuel mixing have to be carefully investigated and Computational Fluid Dynamics (CFD) has been gaining strong attention for the design process. In this framework, the numerical method chosen for the modelling of the liquid phase can have a strong impact on both simulation accuracy and computational effort. Standard Eulerian-Lagrangian (E-L) approaches, which are based on tracking single liquid discrete entities (i.e. parcels), are characterized by a straightforward introduction of the main interactions between the gas and the liquid phase, even if they are not theoretically suitable in the near injection region where the spray is really dense. An extensive use of experimental correlations in order to introduce the effects of primary breakup inside numerical calculations is therefore required. However, this strategy is not general since a strong spreading between the huge number of available experimental correlations can be determined for the same configuration and operating conditions. Be...