TRANSITION FROM HETEROGENEOUS TO HOMOGENEOUS NUCLEATION IN A SIMPLE STRUCTURE FLASH-BOILING ATOMIZER

Abstract

The flash-boiling method is currently one of the most common atomization methods in household, painting, and pharmaceutical applications. In most of these spaying systems, the atomization unit consists of an inlet orifice, an expansion chamber, and a discharge orifice. Nucleation occurs at the inlet orifice on the walls (heterogeneous nucleation), the bubbles develop in the expansion chamber, and a rapid bubble growth (flash-boiling) occurs when the mixture leaves the discharge orifice. The nucleation process is a major leading phenomenon that dominates the flash-boiling process and thus, the spray characteristics. Former studies (Sher and Elata, 1977; Park and Lee, 1994; Rashkovan et al., 2003; Sher et al., 2008; Bar-Kohany et al., 2004; Bar-Kohany et al., 2007) showed that the flash-boiling method results in high-quality fine sprays, and it is anticipated that homogeneous (rather than heterogeneous) nucleation may result in a much finer spray. Since the timescale of the bubble growth in homogeneous nucleation is much shorter than that in heterogeneous nucleation, using homogeneous nucleation requires a simpler atomization unit construction and thus may be suitable for fuel-injection systems in combustors and engines. In the present work attempted to reach the desired conditions for homogeneous nucleation and propose a simple visualization technique to determine the border between heterogeneous and homogeneous nucleation regimes in a simple single-orifice atomizer.