Spray Combustion

This paper has made a comparative study between two reference fuels such as n-heptane and  n-dodecane,  both  widely  used  as  surrogates  for  diesel  fuel  in  several  studies concerning  on  injection,  combustion  and  emissions  formation  in  CI  engines.  In  order  to achieve  this  it  has  been  simulated  the  injection,  evaporation  and  auto  ignition  of  both fuels, using a Computational Fluid Dynamics (CFD) package. Results have been validated with experimental data provided by SANDIA laboratories, conducted at different operating conditions and with different fuels. The comparison is made in terms of typical parameters such as liquid penetration length, the flame lift-off and the ignition delay time.Results  indicated  that  there  are  a  good  concordance  between  CFD  simulation  and experimentation, when each surrogate fuel is compared. Nevertheless, when they were compared  to  D2  fuel,  the  n-heptane  was  the  surrogate  that  best  matched  the  D2  fuel behavior, in terms of Lift-Off length and ignition delay time.

Numerical simulations of soot formation were performed for n-dodecane spray using the transported probability density function (TPDF) method. Liquid n-dodecane was injected with 1500 bar fuel pressure into a constant-volume vessel with an ambient temperature, oxygen volume fraction and density of 900 K, 15% and 22.8 kg/m 3 , respectively. The interaction by exchange with the mean (IEM) model was employed to close the micro-mixing term. The unsteady Reynolds-averaged Navier-Stokes (RANS) equations coupled with the realizable k-ϵ turbulence model were used to provide turbulence information to the TPDF solver. A 53-species reduced n-dodecane chemical mechanism was employed to evaluate the reaction rates. Soot formation was modelled with an acetylene-based two-equation model which accounts for simultaneous soot particle inception, surface growth, coagulation and oxidation by O 2 and OH. The modelling results for ignition delay, lift-off length, flame length evolution and distribution of soot volume fraction () are compared with the corresponding experimental data. Good predictions of the temporal evolution and spatial extent of the soot volume fraction have been observed. The findings suggest that the transported probability density function approach for soot modelling is a promising framework.

The main role in air/fuel mixture formation at the IC diesel engines has the energy introduced by fuel into the IC engine that is the characteristics of spraying fuel into the combustion chamber. The characteristic can be defined by the spray length, the spray cone angle, the physical and the chemical structure of fuel spray by different sections. Having in mind very complex experimental setups for researching in this field, the mentioned characteristics are mostly analyzed by calculations. There are two methods in the literature, the first based on use of the semi-empirical expressions (correlations) and the second, the calculations of spray characteristics by use of very complex mathematical methods. The second method is dominant in the modern literature.
The main disadvantage of the calculation methods is a correct definition of real state at the end of the nozzle orifice (real boundary conditions). The majority of the researchers in this field use most frequently the coefficient of total losses inside the injector. This coefficient depends on injector design, as well as depends on the level of fuel energy and fuel energy transformation along the injector. Having in mind the importance of the real boundary conditions, the complex methods for calculation of the fuel spray characteristics should have the calculation of fuel flows inside the injector and the calculation of spray characteristics together. This approach is a very complex numerical problem and there are no existing computer programs with satisfactory calculation results.
Analysis of spray characteristics by use of the semi-empirical expressions (correlations) is presented in this paper. The special attention is dedicated to the analysis of the constant in the semi-empirical expressions and influence parameters on this constant. Also, the method for definition of realistic boundary condition at the end of the nozzle orifice is presented in the paper. By use of this method completely avoid a use of the coefficient of total losses inside the injector. At the same time, semi-empirical expressions have the universal constant that does not depend on the injector design.