Experimental investigation of atomization and droplet turbulence characteristics of a twin-fluid nozzle with different self-excited vibrating cavity structures

Abstract

The spray atomization properties and droplet turbulence characteristics of a twin-fluid nozzle with and without a self-excited vibrating cavity are investigated and compared using a phase Doppler particle analyzer (PDPA). In the presence of a self-excited vibrating cavity, the spray cone angle and the droplet number concentration increase by nearly 50% and 55%, respectively, and the droplet Sauter mean diameter (SMD) and the mean axial velocity decrease by approximately 40% and 33.8%, respectively. However, the root mean squared (RMS) velocity fluctuation and the droplet turbulence simultaneously worsen. By combining the comparative analysis of the dimensionless parameters of the Reynolds number, Weber number and Ohnesorge number, the results indicate the significant role of the self-excited vibrating cavity in promoting twin-fluid atomization. Quantitative measurements using PDPA with the twin-fluid nozzle in the absence and presence of a self-excited vibrating cavity reveal the effects of the gas to liquid mass flow rate ratio (GLR) on the spray cone angle and the axial distribution of droplet SMD, mean axial velocity and number concentration. The fixed gas flow rate of 0.0003 kg·s−1 and the liquid flow rate of 0.0077 kg·s−1 are selected as representative, and the influence of the structural parameters of a self-excited vibrating cavity on the droplet turbulence characteristics are further investigated. The mean axial velocity along the radial direction first increases as a result of extrusion and diffusion and then decreases, possibly due to the drag effect caused by the surrounding air and droplets; it decreases along the axial direction due to the air resistance. The droplet mean velocity increases with an increase in orifice diameter or orifice depth, and a superior velocity characteristic is obtained when the orifice diameter is 2.5 mm or when the orifice depth is 2.5 mm. Meanwhile, the droplet mean velocity first increases and then decreases with the increase in the distance between the nozzle outlet and the self-excited vibrating cavity, and an appropriate distance value of 4.0 mm is obtained. The variation of RMS velocity fluctuation along the axial direction and radial direction is consistent with the change in mean axial velocity, whereas the droplet turbulence shows an opposite trend. There is a peak of RMS velocity fluctuation and a valley of droplet turbulence at approximately 50 mm from the z-axis. Furthermore, the RMS velocity fluctuation and droplet turbulence increase with the increase in orifice diameter or orifice depth whereas that first increase and then decrease with the increase in the distance between the nozzle outlet and the self-excited vibrating cavity. Thus, the self-excited vibrating cavity profoundly influences the droplet turbulence characteristics by affecting the variation of droplet oscillation.