Coupled/decoupled spray simulation comparison of the ECN spray a condition with the Sigma-Y Eulerian atomization model

This work evaluates the performance of the Σ-Y Eulerian atomization model at reproducing the internal structure of a diesel spray in the near- field. In the study, three different computational domains have been used in order to perform 3D and 2D coupled simulations, where the internal nozzle flow and external spray are modeled in one continuous domain, and 2D decoupled simulations, where only the external spray is modeled. While the 3D simulation did the best job of capturing the dense zone of the spray, the 2D simulations also performed well, with the coupled 2D simulation slightly outperforming the decoupled simulation. The similarity in results between the coupled and the decoupled simulation show that internal and external flow calculations can be performed independently. In addition, the use of spatially averaged nozzle outlet conditions, in the case of an axisymmetric (single-hole) convergent nozzle, leads to a slightly worse near-field spray predictions but to an accurate far-field ones. Finally, a novel constraint on turbulent driven mixing multiphase flows is introduced which prevents the slip velocity from exceeding the magnitude of the turbulent fluctuations through a realizable Schmidt number. This constraint increased model stability, allowing for a 4x increase in Courant number.Authors acknowledge that part of this work was possible thanks to the Programa de Ayudas de Investigacion y Desarrollo (PAID-2013 3198) of the Universitat Politecnica de Valencia. Also this study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF(TRA2014-59483-R) project.Desantes Fernández, JM.; García Oliver, JM.; Pastor Enguídanos, JM.; Pandal-Blanco, A.; Baldwin, E.; Schmidt, DP. (2016). Coupled/decoupled spray simulation comparison of the ECN spray a condition with the Sigma-Y Eulerian atomization model. International Journal of Multiphase Flow. 80:89-99. https://doi.org/10.1016/j.ijmultiphaseflow.2015.12.002S89998

A consistent, scalable model for Eulerian spray modeling

Despite great practical interest in how sprays emanate from fuel injectors, the near-nozzle region has remained a challenge for spray modelers. Recently, Eulerian models have shown promise in capturing the fast gas-liquid interactions in the near field. However, with the inclusion of compressibility, it can be difficult to maintain consistency between the hydrodynamic and thermodynamic variables. In order to resolve numerical inconsistencies that occur in segregated solutions of Eulerian spray model equations as well as to provide good scalability and stability, a new construction of a -Y model is introduced. This construction is built around an IMEX-RK3 algorithm which offers accuracy and efficiency. The new algorithm is compared to an existing implementation for speed and is validated against experimental measurements of spray evolution in order to test the accuracy. The predictions of the new construction are slightly more accurate and, when tested on 256 processors, are 34 times faster.Also this research used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. The authors gratefully acknowledge the computing resources provided on the Texas Advanced Computing Center (TACC) at The University of Texas at Austin that have contributed to the research results reported within this paper URL: http://www.tacc.utexas.edu.Pandal-Blanco, A.; Pastor Enguídanos, JM.; García Oliver, JM.; Baldwin, E.; Schmidt, D. (2016). A consistent, scalable model for Eulerian spray modeling. International Journal of Multiphase Flow. 83:162-171. doi:10.1016/j.ijmultiphaseflow.2016.04.003S1621718

A new model for turbulent flows with large density fluctuations : application to liquid atomization

International audienceBased on the original modeling proposal of Borghi and coworkers (A. Vallet, A. A. Burluka, and R. Borghi, Atomization and Sprays, vol. 11, pp. 619-642, 2001), a new model for atomization, focused on the description of the primary breakup, has been developed. For high injection velocities, the dense zone located just at the injector exit is generally not well described because of the strong interactions between complex phenomena. The classical Lagrangian approach that considers individual liquid droplets or blobs is not a satisfactory representation for such complex liquid flow topologies with liquid core ligaments and eventually droplets. Here, a more global approach is used, with liquid and gas considered as two species of a unique turbulent flow. Therefore, the initial liquid dispersion is given by a turbulent liquid mass flux. The purpose of this work is first to evaluate the usual models for this turbulent liquid flux and then to modify them for the specific turbulent flow with a very high density ratio considered here

Modelling collision outcome in moderately dense sprays

To simulate primary atomization, the dense zone of sprays has to be addressed and new atomization models have been developed as the ELSA model [4]. A transport equation for the liquid/gas interface density is stated and extends the concept of droplet diameter. Several related source terms require modelling attention. This work describes the contribution of collision and coalescence processes. Several questions arise: Is it possible to represent collision/coalescence from an Eulerian description of the flow? What are the key parameters? What are the particular features of collision in dense spray? To answer these questions, a Lagrangian test case, carefully resolved statistically, is used as a basis to evaluate Eulerian models. It is shown that a significant parameter is the equilibrium Weber number: If it is known, Eulerian models are able to reproduce the main features of Lagrangian simulations. To overcome the Lagrangian collision model simplification that mostly considers collisions between spherical droplets, a new test case has been designed to focus on collision process in dense spray. The numerical code, Archer, which is developed to handle interface behaviours in two-phase flow by the way of direct numerical simulation (DNS) [19] is used. Thanks to DNS simulations and experimental observations, the importance of non spherical collisions is demonstrated. Despite some classical drawbacks of DNS, we observed that an equilibrium Weber number can be determined in the considered test case. This work emphasizes the ability of interface DNS simulations to describe complex turbulent two phase flows with interfaces and to stand as a complement to new experiments

MODELING COLLISION OUTCOME IN MODERATELY DENSE SPRAYS

Luret, Gautier Menard, Thibaut Berlemont, Alain Reveillon, Julien Demoulin, Francois-Xavier Blokkeel, GregoryTo simulate primary atomization, the dense zone of sprays has to be addressed and new atomization models have been developed as the Eulerian Lagrangian spray atomization (ELSA) model [Blaisot, J. B., and Yon, I., (2005) Exp. Fluids, 39(6), pp. 977-994]. A transport equation for the liquid/gas interface density is stated and extends the concept of droplet diameter. Several related source terms require modeling attention. This work describes the contribution of collision and coalescence processes. Several questions arise: Is it possible to represent collision/coalescence from an Eulerian description of the flow? What are the key parameters? What are the particular features of collision in dense spray? To answer these questions, a Lagrangian test case, carefully resolved statistically, is used as a basis to evaluate Eulerian models. It is shown that a significant parameter is the equilibrium Weber number. If it is known, Eulerian models are able to reproduce the main features of Lagrangian simulations. To overcome the Lagrangian collision model simplification that mostly considers collisions between spherical droplets, a new test case was designed to focus on the collision process in dense spray. The numerical code, Archer, which is developed to handle interface behaviors in two-phase flow by way of direct numerical simulation (DNS) [Iyer, V, and Abraham, J., (2005), Atomization Sprays, 15(3), pp. 249-269], was used. Thanks to DNS simulations and experimental observations, the importance of nonspherical collisions is demonstrated. Despite some classical drawbacks of DNS, we observed that an equilibrium Weber number can be determined in the considered test case. This work emphasizes the ability of interface DNS simulations to describe complex turbulent two-phase flows with interfaces and to stand as a complement to new experiments