Effects of micro-hole nozzle and ultra-high injection pressure on air entrainment,liquid penetration,flame lift-off and soot formation of diesel spray flame

Abstract

Increasing the injection pressure and downsizing the nozzle orifice diameter have been major measures for diesel engines to facilitate fuel–ambient gas mixture formation and combustion processes. The objective of this investigation is to carry out a quantitative analysis on the effects of micro-hole nozzle and ultra-high injection pressure on the mixing and combustion characteristics of diesel spray flame. Hence, laser-induced fluorescence and particle image velocimetry technique was employed to quantitatively access the gas entrainment of diesel spray emerging from nozzle with orifice diameter down to 80 µm under injection pressure up to 300 MPa, together with OH* chemiluminescence imaging and two-color pyrometry techniques to resolve the combustion and soot formation processes. Additionally, numerical simulation on the multi-phase flow inside injector nozzle was conducted to obtain information on internal flow dynamics. Experimental results show that over 80% of the ambient gas entrained into a spray plume is through the capturing effect at its tip, followed by the entraining effects at its peripheral boundary. Moreover, both a decrease in orifice diameter and an increase in injection pressure result in enhancement of the instantaneous gas to fuel mass flow rate ratio, shortening of liquid length of spray under evaporating conditions. The lift-off length of a diesel spray flame is substantially extended by the increase in injection pressure, and slightly shortened by the decrease in nozzle orifice diameter. Additionally, the numerically acquired velocity and turbulence data at the nozzle exit plane provide interpretation on the variations of liquid length and lift-off length under different injection conditions. Finally, the combination use of micro-holes and ultra-high injection pressure greatly accelerate the mixing of fuel and ambient gas, avoiding the interference of liquid length and lift-off length, and drastically decreasing the soot formation.