Dispersion of a high-pressure water-fuel emulsion jet

Abstract

A way of radically increasing fuel efficiency in diesel engines and improving the ecological characteristics of their operation is the use of water-fuel emulsion [1]. However, in spike of the large number of computational an experimental studies, the physical-chemical mechanism of the effect of the water emulsified in the fuel on the processes of carburetion, ignition, and combustion under typical conditions for diesel engines remains unclear, for the most part. In particular, it is assumed that the leading mechanism of water liberation in the fuel and the cause of the additional dispersion of hot water-fuel emulsion droplets is thermal micro-explosions of minute water droplets [2, 3]. In the opinion of many researchers, this effect also explains the mechanical dispersion of the centers of fuel-air jets [1, 4]. Hence emulsification of the fuel leads not only to changes in the kinetics of the ignition and combustion of the mixture, but also affects the pre-ignition stage, i.e., the process of carburetion. But estimates show that the micro-explosion mechanism is possible only at sufficiently low static pressures in the medium [3], and that micro-explosions occur with large induction times for heating of the water-fuel emulsion droplets such that a large fraction of the fuel has been vaporized [5]. Therefore the assumption that thermal explosions of water droplets are responsible for the evolution of the jet of emulsified fuel is questionable, at least in the initial stages of the process.An alternative model of the evolution of a jet of water-fuel emulsion in air was given in [6]. Emulsification is considered as one of the physical factors that leads to an increase in the dispersion angle of the jet immediately after discharge from the sprayer nozzle. It is known that for high-pressure injection of a liquid into a gas the liquid is stretched over the nozzle cross section and the liquid cavitates. In the initial stages the flow resembles the motion of closely packed particles [7]. Because of the cavitation breakdown of the jet, the particles acquire a radial velocity component u r together with the longitudinal velocity u x. The radial velocity determines the dispersion angle of the gas-liquid jet (Fig. la).The cavitation breakdown of a liquid under high tension is well known in the physics of shock waves [8]. Here the velocity of the surface layers (the phenomenon of "peeling") is determined by the difference between the amplitude of the relieved pressure pf and the dynamical tensile strength of the liquid trf. It is important to note that for the condition considered here af is comparable to the tension and strongly depends on the presence of cavitation centers in the liquid. For example, tyf can be as large as 30 MPa for purified and degassified water [9], but it decreases to 0.8 MPa for ordinary water [8]. Therefore the emulsification of the fuel, i.e., the artificial introduction of interfacial boundaries in the fuel in the form of water droplets, leads to enhanced breakdown of the jet and an increase in the ra...