Simulation of Heating Effects Caused by Extreme Fuel Pressurisation in Cavitating Flows through Diesel Fuel Injectors

Abstract

Pressurization of Diesel fuel in modern common-rail injectors in excess of 2000bar can result to increased temperatures and significant variation of the fuel physical properties (density, viscosity, heat capacity and thermal conductivity) relative to those under atmospheric pressure and room temperature conditions. Moreover, due to the sharp de-pressurization experienced by the fuel at the inlet of the injection holes, significant gradients of the above properties are established. The subsequent fuel acceleration at velocities reaching 700m/s is also inducing further wall friction and thus heating. Consequently, the characteristics of cavitation taking place at the entrance to the injection holes are altered while the volumetric efficiency of the nozzle is significantly affected. The present study quantifies the role of these effects in mini sac-type Diesel injectors operating at pressures up to 2400bar through use of a RANS cavitation CFD model. The flow solver is accordingly modified to account for such effects during the solution of the flow conservation equations. Two different injector designs have been considered, both based on the same mini sac-type nozzle body; one with sharp-inlet cylindrical holes and one with tapered holes with inlet rounding. The results indicate significant changes in terms of the details of the flow development but also to bulk flow characteristics such as the volumetric efficiency of the injectors and the mean fuel injection temperature relative to the isothermal/constant properties case.

Friction-induced heating in nozzle hole micro-channels under extreme fuel pressurisation

“…On the contrary, the increase of thermal conductivity with pressure indicates that heat conduction within the flowing liquid will be enhanced as pressure increases. A includes the relations used and the trends are plotted in [33]. With regards to the implementation of variable flow properties in the flow solver, the following procedure has been followed.…”

“…Diesel temperature increase as function of the discharge coefficient of the flow passage. Reproduced from[33].…”

Numerical simulation of fuel dribbling and nozzle wall wetting

“…(3) Finally, the present study assumes isothermal flow and adiabatic nozzle walls; thus, it neglects the dependency of fuel properties on pressure and temperature and heat transfer on the nozzle walls. This is justified by the fact that the injection pressure of ;160 MPa utilised in pilot injection events, as those tested here, result to temperature difference of less that ;20°C; see selectively [72][73][74][75][76][77] for fixed needle lift and transient needle lift studies in Salemi et al, 78 Strotos et al 79 At the same time, recent measurements for the diesel fuel properties have been reported and modelled using advanced equations of state, such as the PC-SAFT. 60,[80][81][82] For the pressure range investigated here, it can be estimated that the variation of fuel properties is practically negligible and renders the additional complexity of such approaches out of the scope of the present work.…”

Optimization of Design and Technology of Injector Nozzles in Terms of Minimizing Energy Losses on Friction in Compression Ignition Engines

“…Fuel pressurization up to 300 MPa, as required by modern common rail self-ignition engines, resulted in significant variation of the physical fuel properties (density, viscosity, heat capacity and thermal conductivity) relative to those at atmospheric pressure and room temperature conditions [33,34]. The high acceleration of the fuel at velocities reaching 700 m/s as it is flowed through the nozzle hole orifices to induce cavitation was found.…”