Abstract
The reduction of diesel engines emissions is a challenging task. Reducing both soot and NOx at the same time is quite difficult, especially with no fuel consumption penalty. Reducing them both is possible by enhancing the air-fuel mixing process in order to improve combustion. Research is directed towards smaller orifice injectors and higher fuel injection pressure, in order to favour the atomization and evaporation of the spray. This is particularly important in new combustion concepts engines, such as HCCI (Homogeneous Charge Compression Ignition). Experimental research still does not give us enough information about the very complex thermo-and fluid dynamic processes that are occurring in a diesel engine. In addition, experimental research is still quite expensive and time consuming. The Diesel spray creates a very turbulent flow field, with strong gradients. The spray simulations in the CFD codes are based on the Lagrangian approach, because this way the nozzle region does not have to be fully resolved by the mesh. In the Lagrangian description, the spray is represented by points, also called parcels or droplets. These points have zero dimension and do not occupy any space in the domain. They only act as markers. These points have properties assigned, such as: location, velocity, diameter, mass, temperature, fuel composition, etc. They have to be tracked through the domain in which they move from cell to cell and distribute mass, momentum and energy. The objective of this work is to model constant-volume Diesel combustion using Computational Fluid Dynamics (CFD) modelling with the OpenFOAM code. Constant-volume combustion is a first step, used to validate CFD models for modelling real in-cylinder Diesel combustion, where things are even more complicated. Constant-volume combustion geometry and a wide range of injection parameters, pressure, temperature, fuel type, experimentally measured data, was freely available from Sandia National Laboratories. They provide this data for the use of CFD modellers. The different CFD models can be adjusted to match the same, well documented experimental data. For this parametric study, N-heptane was used as diesel surrogate fuel and a range of initial conditions were modelled, such as ambient oxygen concentration, temperature, density, injection pressure, nozzle diameter, fuel temperature, injection duration. The studied output parameters were liquid penetration, vapour penetration, lift-off length and ignition delay. A mesh dependency study was also conducted to obtain a good mesh for this kind of simulations and to exclude the mesh influence on the results. Lots of CFD model parameters had to be studied and adjusted so the simulation output data matches quite good the mentioned experimental results.
Keywords: Diesel, Combustion, CFD, Simulation, OpenFOAM