Fuel vaporization: Effect of droplet size and turbulence at elevated temperature and pressure

Abstract

This paper presents an extensive experimental study regarding the effect of turbulence and droplet size on the evaporation rate of suspended monocomponent alkane droplets at elevated temperature and pressure conditions of up to 100 °C and 10 bar, respectively. Individual droplets of n-heptane and n-decane were suspended at the intersection of two crossed micro-fibers in the center of a fan-stirred spherical vessel. The droplet size was varied in the range between 110 and 730 µm. Eight axial fans generated a controlled turbulent flow field with quasi-zero mean velocity and turbulence intensity up to 1.5 m/s. The results reveal a linear relationship between the initial droplet size and its turbulent steady-state evaporation rate, where larger droplets evaporate at a faster rate than their smaller counterparts at all elevated temperature and pressure conditions. The normalized turbulent evaporation rate increases with pressure, whereas elevated temperature produces the opposite effect. The ratio of the Kolmogorov length scale to initial droplet diameter is shown to be of paramount importance for interpreting the effect of turbulence, as the normalized evaporation rate increases dramatically at lower values of this ratio under all test conditions. However, the ability of turbulence to enhance the vaporization rate vanishes when this ratio approaches unity, suggesting that only droplets which are initially larger than the smallest turbulent eddies experience enhanced evaporation. In addition, the widespread belief that turbulence enhances less volatile fuels more than their high volatility counterparts also depends on the ambient pressure and initial droplet size. The ability of turbulence to generate small-scale structures and, subsequently, the interaction of these eddies with the available fuel concentration gradient at the surface of the droplet governs the relationship between fuel type, initial droplet size, and ambient temperature and pressure. A turbulent Reynolds number or a vaporization Damköhler number is used to correlate turbulent droplet evaporation rates at all explored test conditions.