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

•CFD simulation of cavitation in Diesel fuel injectors up to 3000 bar.•The energy conservation equation is solved for temperature variations.•Simulations include variable fuel properties and induced heating due to wall friction.•Discharge through a nozzle causes internal cooling and strong temp. gradients.•Flow found dependent on ρ,μ, thermal conductivity and heat capacity gradients. Fuel pressurisation up to 3000bar, as required by modern Diesel engines, can result in significant variation of the fuel physical properties relative to those at atmospheric pressure and room temperature conditions. The huge acceleration of the fuel as it is pushed through the nozzle hole orifices is known to induce cavitation, which is typically considered as an iso-thermal process. However, discharge of this pressurised liquid fuel through the micro-channel holes can result in severe wall velocity gradients which induce friction and thus heating of the liquid. Simulations assuming variable properties reveal two opposing processes strongly affecting the fuel injection quantity and its temperature. The first one is related to the de-pressurisation of the fuel; the strong pressure and density gradients at the central part of the injection hole induce fuel temperatures even lower than that of the inlet fuel temperature. On the other hand, the strong heating produced by wall friction increases significantly the fuel temperature; local values can exceed the liquid’s boiling point and even induce reverse heat transfer from the liquid to the nozzle’s metal body. Local values of the thermal conductivity and heat capacity affect the transfer of heat produced at the nozzle surface to the flowing liquid. That creates strong temperature gradients within the flowing liquid which cannot be ignored for accurate predictions of the flow through such nozzles.