Potential heterogeneous and homogeneous flow boiling conditions in a high-pressure diesel fuel injector

The development of direct injection (DI) Diesel engines over the last 20 years has been quite remarkable. The trend in technology is towards diesel fuel pressurisation in the excess of 250 to 300 MPa thus leading to simultaneous reduction of soot and NOx emissions. At high diesel fuel pressures, the excessively high velocity values occurring during the discharge of the liquid through the fuel injector nozzles (up to 660 m/s with a maximum Reynolds number of 1.3Ÿ10 5 observed in this study) induce high wall friction, which in turn leads to significant fuel heating with Brinkman number values ranging from 20 to 60. The combination of significant pressure reduction within the sac volume (during low needle lifts) and the nozzles, together with high fuel injector wall temperatures occurring by both heat conduction from the cylinder head and heat convection from the hot gases within the engine cylinder, may potentially lead to heterogeneous flow boiling of the diesel liquid close to the fuel injector walls. In this paper a non-isothermal study of diesel flow within a fuel injector is carried out. The extensive engine cylinder data available from both test results and combustion simulations are characterized to set up the appropriate thermal boundary conditions for the conjugate heat transfer (CHT) simulations carried out in this study. The thermal boundary conditions for the fuel injector simulations are affected by both the thermal state within the engine cylinder (which in turn is primarily affected by combustion) and hence the temperature within the cylinder head. Due to the transient nature of the engine cylinder temperature and heat transfer variations, a sensitivity analysis is performed in order to better understand the effects of such variations on the nozzle wall temperature. The steady state CHT simulations of the fuel injector are carried out, using ANSYS Fluent®, based on the assumption of single phase flow and constant physical properties of the diesel liquid. From the results of CHT simulations, a comparison between the wall nozzle temperature and the saturation temperature of the diesel liquid inside the nozzle, are presented. The second set of simulations presented here use a typical constant wall temperature of 180C for the fuel injector as the boundary conditions for the non-isothermal two phase cavitating flow simulations based on the assumption of variable properties of diesel liquid with respect to both temperature and pressure. These simulations were carried out using City University’s CFD code (GFS). Overall, the simulations reveal that at certain locations within the fuel injector geometry (mainly close to the fuel injector walls), the diesel liquid temperature reaches values well above the saturation temperature of the flowing diesel liquid, thus implying the occurrence of heterogeneous flow boiling regions within the fuel injector used for this study.