The effect of a recessed liquid jet on the performance of coaxial air-assist atomisers

Abstract

Air-assist atomisers are used in a wide range of applications. From the fuel injectors on the Rocketdyne J-2 that powered the Saturn V launch vehicle to more recently being used in novel water-saving shower systems. This work analyses the effect of recessing the liquid jet within a coaxial air-assist atomisers. Experimental work was conducted in order to outline the effect of recess on jet breakup and atomisation. Initial experiments were conducted using imaging systems which found that increasing the recess ratio influenced spray behaviour but did not affect the underlying atomisation regimes. Instead, recess enhanced the jet breakup. In order to quantify the effect further, a set of subsequent experiments were developed. A laser imaging system using a particle-droplet image analysis method (PDIA) was utilised to quantify the effect of recess on drop size and velocity correlations. In general, it was found that increasing the amount of recess led to a decrease in arithmetic drop diameter. The effect was most pronounced at lower Weber numbers. However, for specific recess ratios, there was an increase in drop diameter. Additionally, recess was found to reduce drop spreading and dispersion downstream for smaller droplets. To understand the physical mechanism that led to this behaviour, a set of experiments were conducted that used a high-speed camera. Utilising a Fourier Transform method, the growth rates of the disturbances on the liquid jet were measured directly. At low Reynolds and Weber numbers, it was found that increasing the recess led to an increase in axisymmetric instability growth rates, hence leading to earlier jet breakup and smaller drops. However, for specific recess ratios, the growth rates were found to decrease. These are the same recess ratios where the arithmetic drop diameter increased. There was a stabilising effect on the asymmetric disturbances when recess increased. As Reynolds and Weber numbers increased, differing trends were observed where recess had a diminishing effect as turbulent behaviour began to dominate.

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ORCID for Samvir Thandi: orcid.org/0000-0002-5327-7543

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