Quantitative analysis on surface roughness effects on flash boiling internal flow and external breakup

Flash boiling atomization performances highly rely on the internal phase change, which is sensitive to the nozzle wall surface roughness. Existing investigations have confirmed the significance of surface roughness on the external spray macroscopic characteristics, but the in-depth mechanism lacks experimental proof. In the present study, we quantify the elevated surface roughness of two-dimensional transparent nozzles and achieve direct observation of the development of the internal flash boiling phase change. It is proposed that high levels of surface roughness facilitate the nucleation rate of the bubbles, while suppressing the size of individual bubbles. The global vapor fraction, as well as the exit occupation rate, is increased, which benefits the generation of fine flash boiling droplets. The simultaneous characterization of external spray development confirms that internal bubbles are the key media between internal surface roughness and external spray performance, and that by smoothing the surface, the liquid core structure during the transitional flash boiling stages can be stabilized. This investigation provides important experimental data on the surface roughness impacts on flash boiling internal phase change and corresponding spray behaviour, and offers new prospects to achieve active control of phase change-induced atomization systems.