Experimental Investigation on Influence of Liquid Properties Variation on Swirl Spray Atomization of Light Alcohols

Abstract

Light alcohols such as methanol and ethanol, produced sustainably, hold potential as sustainable fuels due to their low emissions. The process of fuel atomization significantly influences combustion stability and emissions. The proposed research focused on experimental investigations to quantify the correlation between liquid properties and atomization characteristics, providing specific guidelines for the introduction of light alcohols into existing powertrains. The pressure-swirl atomizer, commonly used in gas turbines and combustion furnaces, was chosen for in-depth atomization behavior analysis in this study. The liquid properties viz surface tension, density and viscosity were varied using ethanol and surfactant (Na-Olefin). The experimental approaches presented in this study are (a) Measurement of the liquid properties with different concentrations of ethanol and surfactant, (b) Study of spray structure and instabilities affecting the atomization behavior with high-speed imaging technique and (c) Detailed characterization of the fully atomized droplet size, Sauter Mean Diameter (SMD) and mean velocity (z-direction) with variation in liquid properties using PDPA (Phase Doppler Particle Analyzer) system.

The initial phase of this research involved measuring changes in liquid properties resulting from varying concentrations of ethanol and surfactants. The study delved deeper into test liquids, specifically water-ethanol solutions (10%-ethanol, 20%-ethanol, 50%-ethanol v/v), and water-surfactant solutions (2.5%-Na olefin v/v), to examine their spray formation and atomization characteristics using a pressure-swirl atomizer. The introduction of ethanol (up to 50% v/v) into water decreased surface tension while simultaneously elevating the overall viscosity of the mixtures. In contrast, the surfactant was employed to exclusively reduce surface tension, allowing for a focused examination of its impact on atomization.

The morphology of the spray structure and macroscopic parameters such as spray angles (θ) and breakup length (𝐿𝑏) were studied with the help of MATLAB image processing tool. The experimental setup was initially validated by examining spray morphology and measuring film thickness across a range of pressure drop (∆P = 0.32 MPa to ∆P = 1.10 MPa) across the atomizer. The dimensionless numbers, liquid Weber number (𝑊𝑒𝑙 ), liquid Reynolds number (𝑊𝑒𝑙 ) and Ohnesorge number (Oh) helped in understanding the spray dynamics due to variation in liquid properties. Water served as the reference liquid, and subsequent investigations compared spray parameters to those of 10%-ethanol, 20%-ethanol, 50%-ethanol, and 2.5%-surfactant sprays. It was observed that the spray angles of all test liquids initially increased and then stabilized above ∆P =0.78 MPa. The variation in liquid properties had a very insignificant effect on spray angles. The 𝐿𝑏 decreased as ∆P increased, with ethanol's reduced surface tension resulting in a shorter 𝐿𝑏. However, it is worth noting that the 50% ethanol solution exhibited a slightly longer 𝐿𝑏 likely due to its relatively higher viscosity compared to the other ethanol solutions.

PDPA analysis was conducted along the spray axis downstream, covering the range from z = 30 mm to z = 100 mm for all the test liquids. A sample of 10000 particles was analyzed by the PDPA for each case. The SMD of the test liquids at constant ∆P =0.78 MPa and z=60mm were found to be 12.14 µm (water), 11.12 µm (2.5%-Surfactant), 10.42 µm (10%-ethanol), 11.22 µm (20%-ethanol) and 12.11 µm (50%-ethanol). At the same condition, the mean velocities were 7.12 m/s (water), 8.90 m/s (2.5%-Surfactant), 7.58 m/s (10%-ethanol), 6.56 m/s (20%-ethanol),, and 4.12 m/s (50%-ethanol). PDPA analysis revealed that lower surface tension, as seen in 10%-ethanol and 2.5%-surfactant sprays, produces finer, faster droplets during atomization. Conversely, higher viscosity as in 50%-ethanol, and higher surface tension as in water, leads to the formation of larger droplets due to increased resistance to shear forces. The axial measurement of the droplet size distribution showed an increase in several larger-sized droplets (> 12 𝜇𝑚) for water and 50%-ethanol beyond z = 30mm. Whereas, a very minor increase in the numbers of larger droplets (> 12 𝜇𝑚) was observed for 2.5%-surfactant and 10%-ethanol sprays. The increase in number of larger droplets was substantiated by the coalescence of the liquid droplets to form larger-sized droplets. The coalescence effect was less prominent in low surface tension liquids. However, for the water-ethanol solutions (20%- ethanol and 50%-ethanol) the high viscosity dominates the effect of low surface tension promoting the coalescence effect downstream the spray axis.

Date of Award	19 Dec 2023
Original language	American English
Supervisor	Dimitrios Kyritsis (Supervisor)