Shevchuck,Illya

Numerical Investigation of Droplet Evaporation at Near-, Trans- and Supercritical Environmental Conditions

Motivation

Due to continuous efficiency enhancement of modern technical combustion systems operating with liquid fuels in carrier gas environment, such as combustion chambers of aircraft engines and diesel engines, temperature, pressure or both these state quantities reach the vicinity of critical temperature and pressure of used liquids or even exceed them.

Understanding, modeling and simulative reproduction using the Computational Fluid Dynamics (CFD) techniques of droplet processes under these extreme conditions is essential for further improvement of the technical systems. Therefore, CFD models and tools for have to be developed and implemented, what is essential scope of this project.

Method and Theory

The characteristics of the liquid and gas behavior in the vicinity of the critical point have strong dependence of their properties on temperature, pressure and composition. Thus, assumptions of constant phaseintensive density, viscosity, heat conductivity etc. typically made in most technical multiphase simulations cannot be made for these problems.

In this study focus is put on low Mach number flows with constant mean pressure and steep temperature gradients. This allows several assumptions to reduce the complexity of the problem. Hence, the dependency of fluid properties on the local instantaneous pressure is neglected while their dependency on temperature and composition is taken into account.

Proceeding

In the first part of the present work an approach to extend an existing incompressible solver from the OpenFOAM toolbox in order to treat multicomponent multiphase flows with variable material properties is proposed and implemented. For that purpose, the governing equations for mass, momentum, enthalpy and species mass fractions are considered. Thereby severe further assumptions are made to reduce the complexity, e.g. heaproduction by viscous friction, heat transfer by radiation as well as second order transport effects for heat and mass(Soret and Dufour effects) are neglected first, but can be considered in the further steps of the work if necessary. Additionally, a Volume of Fluid (VOF) based method, extended to deal with density variations, is used to capture the position of the gas-liquid interface. The governing equations are discretized using finite volume method on unstructured meshes and solved using segregated solution procedure.

The final scope of the endeavor is to create a solver capable to capture major near critical phenomena for investigation of single droplets and droplet groups in the context of Direct Numerical Simulations (DNS) and Large Eddy Simalations (LES) as well.