Experimental investigation of the evaporation of suspended mono-sized heptane droplets in turbulence intensities approaching unity

Abstract

For decades, experimental investigations of turbulent droplet evaporation have generally followed one of two distinct paths: wind tunnel experiments, which generate significant mean flow but low turbulence intensity, and stirred chamber configurations, which yield the opposite characteristics. The present study bridges the gap between these two established techniques by modifying a fan-stirred spherical chamber to incorporate a controllable mean flow while retaining the attributes of highly-energetic homogeneous and isotropic turbulence. Heptane droplets with a fixed initial diameter, d0, of 500 ± 10 µm are suspended on the intersection of two microfibers and evaporated in pockets of high-intensity turbulence (27% < Tu < 103%) at small mean flow droplet Reynolds numbers, Red, of 10 and 50. A rigorous series of particle image velocimetry (PIV) tests mapped the homogeneous and isotropic turbulent flow regions for various combinations of fan speed, and a two-dimensional translating fiber support system allowed the precise spatial placement of the droplet for subsequent evaporation studies. PIV results indicate the generation of isotropic flow regions that satisfy 0.9 ≤ urms/vrms ≤ 1.1 with a minimum diameter of 20d0 at each test condition. The steady-state droplet evaporation rate, K, increases quasi-linearly with turbulence intensity at both Reynolds numbers, with steeper improvement associated with higher Red. A comparison with zero-mean flow data reveals that the mean velocity component remains a significant contributor to the droplet evaporation rate at both tested Reynolds numbers over the explored range of turbulence intensity. Subsequent analysis indicates that the mean flow attenuates the effect of turbulence. The findings are of fundamental and practical interest, as vaporization in high turbulence intensity is a plausible scenario for small droplets subject to a moderate slip velocity.