Optical investigation and numerical simulation to influence the mixture formation at variable injection conditionsCopyright: © WSA
The stability of homogeneous combustion processes for internal combustion engines is driven by the stat of mixture at the beginning of combustion. In direct injection engines the state of mixture can be influenced directly by the injection process, i.e. by variation of point in time, duration, amount and division of injection. Therefore the injection strategy is capable for control of mixture formation and combustion process. Because the control of mixture formation and combustion process is not yet possible within one motor cycle mainly due to limitations of the measurement devices, a model predictive control shall be developed within the framework of the collaborative research center. Within the sub-project B4 reduced models of turbulent mixture processes are prepared.
In the first two periods the focus of investigations was on jet dynamics, which appears due to interaction of droplet and gaseous phase. In the third period the focus will be shifted to modeling of turbulent mixture processes in the gaseous phase, which define the starting conditions for the combustion process. The modeling of turbulent mixing will be promoted by measurement of the macrostructure in the gaseous phase und velocity field of gaseous and droplet phase. Validation will be performed by measurement and calculation of the state of mixture, i.e. mixture fraction and scalar dissipation rate, which have been identified as suitable quantity to describe the state of mixture in the second phase.
The modeling will be performerd for a turbulent full cone spray, whose fluid dynamics is dominated by turbulent mixing and turbulent dispersion. Starting point is a two phase fundamental fluid dynamics model, which is able to describe both phases completely and was developed in the previous funding periods. To reduce the model complexity the droplets are assumed to follow the gaseous phase ideally which means that they can be treated as a gaseous phase component. Therefore droplet and gaseous phase can be simulated as only one phase. By further model reduction the full con spray can be approximated by a free jet. For this reduction approach quasi-steady model exist, which can build a base for the model reduction of unsteady sprays and variable injection strategies. The developed mixture formation model must be able to predict the scalar dissipation rate and mixture fraction, which will be handed over as initial conditions for the combustion models of the “virtual test rig” of the collaborative research center. On basis of this prediction a suitable injection strategy can be set to reach a stable combustion process.