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

Abstract

Fuel pressurisation up to 3000 bar, 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.

A model for droplet heating and its implementation into ANSYS Fluent

“…the maximum heating of the fuel due to friction inside the nozzle orifice is estimated at 20 K [11], hence the actual fuel temperature at the nozzle exit is expected to be 370 ± 10 K.…”

Transient heating effects in high pressure Diesel injector nozzles

“…Large vortical structures are observed in the sac volume with a low velocity magnitude. Inside the injector hole, the flow accelerates substantially reaching velocities of the order of 600m/s when the full lift is considered; the present model accounts for compressibility effects (in subsonic flows) as described in (Theodorakakos et al, 2014). At the inlet of the hole the flow turns direction and aligns with the axis of the hole.…”

“…The present study focuses on the thermal effects occurring in high pressure diesel nozzles by solving the energy equation and including the friction induced heating. The CFD model used is an Eulerian-Lagrangian model which has been built upon the in-house CFD cavitation model reported in (Giannadakis et al, 2008); this work is an extension of that presented recently in (Strotos et al, 2014a;Strotos et al, 2014b;Theodorakakos et al, 2014) which additionally examines the effect of needle motion. In the absence of relevant experimental data, the present work aims to quantify the numerical effects of using constant or variable properties, the effect of two-phase flow, the effect of inlet pressure increase and the effect of initial and boundary conditions on temperature distribution within the injector.…”

Experimental assessment of the fuel heating and the validity of the assumption of adiabatic flow through the internal orifices of a diesel injector

“…Some of these works assume that the flow is isothermal [11] [12][13] [14]. Nevertheless, the raising injection pressures may also induce relevant fuel temperature changes due to friction heating or due to important fuel depressurization across the injector control orifices or the nozzle [15] [16]. These fuel temperature changes, in turn, affect the fuel properties, which are strongly dependant on temperature and pressure [19][20] [21][22] [23].…”