Slide 1

Role of detailed chemical kinetics on CFD diesel spray ignition and combustion modellingCoupling CFD and Chemical kinetics

Optimization of existing engine performance

Development of advanced combustors

Process affected by kinetics

Pollutant emissions

Flame stability

Aim

Evaluating the performance of reaction mechanisms of different degrees of complexity applied to diesel spray combustion CFD simulations.

Effects of chemical kinetics on diesel flame ignition and combustion

For the purpose n-heptane sprays are injected into a constant volume vessel.

CFD modelling results are compared against those obtained experimentally available at the well-validated public database for n-heptane fuel

The additional complexity related to modelling real engines is avoided by simulating a single n-heptane spray injected into a constant volume vessel.Uncertainty of fuel oxidationReasons

There is no complete mechanism available for describing the oxidation process of multi-component diesel fuels, so that surrogate fuels have to be employed. Kinetic mechanisms are available in the literature with different levels of detail.Surrogate Fuel

CFD with detailed chemical kinetics requires a simplified fuel model.

Surrogate fuel must emulate combustion behaviour and physical properties of target fuel.

Diesel Spray IgnitionComputational models and numerical setup

OpenFOAM

Pre-implemented capabilities such as discretisation, numerical and physical models, mesh management.

Different models on the same platform.

Gas phase is solved in an Eulerian framework where equations are considered to be continuous in space and time.

Liquid spray is treated by a standard discrete droplet method (DDM).

For numerical setup

Set of differential equations along the trajectory of each particle that is solved in physical space in a Lagrangian manner according to the mass, momentum and energy exchangeTurbulent flow is modelled via RANS, a time-averaged version of the NavierStokes equations(A) Turbulence model

Spray atomization, breakup and dispersion is necessary while using a DDM

The atomisation process consists of the liquid core breakup into tiny droplets at the nozzle exit, and together with the secondary droplet breakup, they are one of the most complex phenomena taking place in diesel sprays.

At the same time, they have a primary effect on spray behaviour and then on airfuel mixture formation.(B) Spray submodels The fundamental mechanisms that govern such spray breakup in the model are the KelvinHelmholtz and RayleighTaylor disturbances.

whereKelvinHelmholtzBased on stability analysis provides a dispersion equation.Rayleigh-TaylorBased on a second type of instability associated with the rapid deceleration of the droplets.

If the wavelength of the faster growing wave is smaller than the droplet diameter, the RayleighTaylor waves start to grow on droplets surface.

The life time of the growing RayleighTaylor waves is then tracked from then on

when the life time exceeds the characteristic breakup time defined in equation after a catastrophic break-up occurs and the radii of the new droplet is given by

REFERENCES

[1] D. Veynante, L. Vervisch, Turbulent combustion modeling, Progress in Energy and Combustion Science 28 (3) (2002) 193266.

[2] R.D. Reitz, C.J. Rutland, Development and testing of diesel engine CFD models, Progress in Energy and Combustion Science 21 (2) (1995) 173196.

[3] R. Payri, F. Salvador, J. Gimeno, L. Zapata, Diesel nozzle geometry influence on spray liquid-phase fuel penetration in evaporative conditions, Fuel 87(7) (2008) 11651176.

[4] J. Desantes, R. Payri, F. Salvador, J. de la Morena, Influence of cavitation phenomenon on primary break-up and spray behavior at stationary conditions, Fuel 89 (10) (2010) 30333041.

The following model equation for k is commonly used.

The Prandtl number k connects the diffusivity of k to the eddy