Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers

The present study compares two twin-fluid atomizer concepts based on the airflow (shock waves) pattern obtained through shadowgraph imaging for atomization of water with a low air/water pressure supply. The research work was conducted using the backlight imaging technique for converging (sonic) and converging–diverging (supersonic) air-assist atomizers with a 3.0 mm (throat) diameter. An annular sheet of thicknesses 70 µm and 280 µm with a high-speed air-core was employed to study the breakup dynamics for different water mass flow rates (100–350 kg/h) and air mass flow rates (5–35 kg/h). Different sheet breakup patterns were identified as the function of the ALR ratio (air-to-liquid mass flow), liquid Weber number (WeL), and Reynolds number (Reg). Different breakup modes extend from canonical Rayleigh bubble breakup, ligament-type breakup, to the pure pulsating breakup via annular sheet disintegration. The sheet breakup dynamics were studied in terms of spray angle and breakup length. With higher ALR values, breakup length showed a decreasing trend, while spray angle showed an increasing trend in the converging and converging–diverging (CD) air-assist atomizers, respectively, owing to the drastic difference in the jet flow dynamics.

Symmetric and Asymmetric Disturbances in the Rayleigh Zone of an Air-Assisted Liquid Sheet: Theoretical and Experimental Analysis

The temporal analysis of symmetric (dilatational) and asymmetric (sinusoidal) perturbations at the interface of a water sheet in a coflowing air stream focuses on low gas Weber number region (Weg < 0.4), namely, Rayleigh breakup zone. The motive for this investigation is to acquire a better insight of breakup phenomena involved, rather than technical relevance, by utilizing Kelvin–Helmholtz instability. Accordingly, perturbations are introduced on the basic flow whose stability is to be examined by the method of normal (Fourier) modes. The temporal growth-rate of perturbations is traced to extract the wavenumber associated with maximum growth-rate. Thus, the critical wavelength, in conjunction with the phase velocity of the disturbance will facilitate to obtain the corresponding breakup frequency of the liquid sheet. The analytical findings on liquid sheet breakup frequency with increasing Weber number ratio exhibit the dominance of symmetric wave over asymmetric wave. It also shows independent evolution of breakup frequency with respect to Weber number ratio for the respective perturbation modes, which appears to be a pointed profile. That is, the frequency contour for dilatational mode dips, whereas it rises for the sinusoidal mode and at the Weber number ratio of 0.518 the crossover occur. The theoretical results were substantiated by high-speed flow visualization studies that discern the coexistence of low-frequency (primary) and high-frequency (intermediate) breakup events. Furthermore, the empirical average frequency data track reasonably well with the dilatational instability.