Effect of Spray-Wall Interaction on Air Entrainment in a Transient Diesel Spray 930920

The influence of spray-wall interaction on air entrainment in an unsteady non-evaporating diesel spray was studied using laser Doppler anemometry. The spray was injected into confined quiescent air at ambient pressure and temperature and made to impact on a flat wall. The air velocity component normal to a cylindrical surface surrounding the spray was measured during the entire injection period, allowing to evaluate the time history of the entrained air mass flow rate. The influence of wall distance and spray impingement angle on air entrainment characteristics has been investigated and the results indicate that the presence of a wall increases the entrained mass flow rate in the region close to the surface, during the main injection period. Normal impingement appears to produce stronger effects than oblique incidence at 30 and 45 deg. A qualitative explanation of the results is also proposed, based on the drop-gas momentum exchange mechanism.

Air entrainment in steady and unsteady full cone sprays has been subject of experimental research only quite recently and few works have been published [1, 2, 3, 4, 5, 6, 7, 8 and 9] up to date. The reason of an increasing interest on the subject can be explained by the relevance that this phenomenon has in applied fields like internal combustion engines.

The importance of the phenomenon can be easily understood when considering that the interaction between fuel spray and surrounding gas is of capital importance for the combustion efficiency of DI diesel engines, and is, to a great extent, controlled by the fuel injection characteristics and flow mixing mechanisms, such as swirl and air entrainment. The last process is responsible of the first stage of the mixture formation, and controls spray combustion during early and main diffusion periods [10].

In a previous work [5] the air entrainment into an unsteady diesel spray injected into quiescent atmosphere at room temperature and pressure was measured by laser Doppler anemometry (LDA) and characterised in terms of entrained mass flow rate as suggested in [4], in order to compare the results to those obtained in steady turbulent gaseous jets [11]. In the near field zone investigated, up to 160 nozzle diameters (40 mm), the air entrainment into transient liquid jets resulted to be lower than that in steady gaseous jets, but larger than that in steady liquid jets [4]. The momentum exchange between liquid and gaseous phase was also evaluated and a dependence on the injection mass flow rate was deduced [5].

Recently, it has been found experimental evidence that increasing fuel injection pressure, coupled with retarded injection timing, reduces soot concentration [12], black smoke emission [13] and improves fuel consumption [13, 14]. This result has been interpreted as a consequence of augmented mixing and air entrainment, which makes mixture formation less dependent on air swirl. This is beneficial in the lower engine speed range, where swirl is always low.

In small DI diesel engines, when high fuel injection pressures are used, the spray is found to impact on the piston bowl before ignition starts [15, 16 and 17]. In fact, in the very earliest portion of the injection event, up to the point of maximum spray-tip velocity, the penetration process is dominated by the fuel delivery characteristics of the injection system [18], the air motion having very little effect. After the fuel is injected into the cylinder, sufficient time elapses (about 1 ms) before the formation of a combustible mixture and ignition occur, well enough to allow the dense core of the liquid jet to reach the piston bowl.

The spray impingement on the piston surface may be detrimental because liquid splashed above the bowl can increase hydrocarbon emissions. However, jet impingement produces strong turbulence, enhancing mixing of rich mixture in the core of the spray, and provides an additional mechanism for droplet break-up [14, 19].

Those findings prompted the idea of studying the influence of spray-wall interaction on air entrainment in an unsteady diesel spray, which is the subject of the present work. The influence of wall distance and spray impingement angle on air entrainment characteristics have been investigated and the results will be presented and discussed.