Given the large number of biomass-derived fuel molecules, development of screening techniques is essential to down-select candidate fuels having desirable physical properties to ensure good engine performance. This work documents approaches for rapid screening of candidate fuels using relatively small sample sizes (O(µl)) targeting four key physical properties of liquid fuels relevant to spray atomization, evaporation, and mixing – surface tension, viscosity, heat of vaporization, and vapor pressure. Droplet-based approaches are utilized to ensure small sample size and high throughput. To measure surface tension and viscosity, the decay process of droplets set into oscillation by a piezoelectric droplet generator is tracked using strobed imaging. Oscillation frequency and decay time constant extracted from shape oscillation decay are used to measure fuel surface tension and viscosity using relationships derived by a small-amplitude droplet oscillation theory. Measurements are conducted at ambient and heated conditions up to 55 deg C. Surface tension and viscosity are measured for primary reference fuel components (iso-octane and <i>n</i>-heptane) as well as four bio-derived fuel molecules. Measurement results obtained using an average of 5 μl of fuel within 20 s show on average less than 10% deviation from reference data. To measure heat of vaporization and vapor pressure, evaporation-driven droplet size change is measured using a temperature-controlled chamber. Droplet size change data is used with an analytical model for droplet evaporation along with the Clausius-Clapeyron equation to determine heat of vaporization and vapor pressure. The family of droplet-based approaches shows the ability to develop screening tools to aid in down-selection of biomass-derived fuel molecules using micro-liter quantities, while ensuring high-throughput.

Droplet-based fuel property measurement techniques

ACS Fall 2021

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