Evaporation of Nearly Monosized Droplets of Hexane, Heptane, Decane and Their Mixtures in Hot Air and an Air/Steam Mixture

Abstract

This paper describes experiments on evaporating initially nearly monosized droplet chains with diameter 230 ± 15 µm under atmospheric conditions in a grid generated upward turbulent flow with initial liquid temperatures in the range 320 to 350 K and air temperatures in the range 365 to 455 K. The mean air velocity was 5 m/s and the initial droplet velocities were 17 m/s, resulting in a Reynolds number for the flow around the droplets of 100. The turbulent Stokes number was around 10. The liquids used were n-hexane, n-heptane, n-decane and mixtures thereof. A single chain and, for a reduced data set, a spray of 5 parallel chains, to study the influence of increased mass loading, have been investigated. Additionally, the influence of steam with mass fraction 15% on the evaporation rates has been investigated. The size of the droplets was measured by back light illumination and recording by a digital camera with macro optics. Digital image processing was applied to deduce the size of the droplets and their positions. Laser Doppler measurements delivered the droplet velocities. Besides the generation of a data set for validation of evaporation models the main findings were that turbulence of the surrounding air could increase evaporation for decane and that mixtures evaporated preferentially, indicating mixing inside the droplets being larger than pure diffusion limited. For heptane as evaporating liquid it was found that for elevated droplet loading at air temperature below the boiling point the evaporation rate decreased, while at higher temperatures the droplet loading hardly showed any influence on the evaporation rates. Adding steam led to increased evaporation rates for all liquids, most likely due to condensation on the droplets surfaces. NOMENCLATURE D Droplet diameter [µm] D 0 Exit droplet diameter [µm]

Turbulent flow downstream of a large solidity perforated plate: near-field characteristics of interacting jets

“…On the centre line of the jet (y = 14 mm), T L decreases with streamwise development, which was also found for a grid with smaller solidity in Horender and Sommerfeld (2012), see their figure 2(a), which shows that the integral length scale first decreases and then increases with streamwise development. The current flow is not self-preserving, first due to the initial flow development, and further downstream the width of the jet cannot grow any more due to the presence of the neighbouring jets, in contrast to a free jet.…”

“…Fresh air is entrained and heated up in a heat exchanger using the heat from the air leaving the test section, which is exhausted to the environment. This non-closed loop setup is necessary, since the test facility was initially designed for droplet evaporation studies (Horender and Sommerfeld 2012), and therefore it is necessary to dispose of the used air to the environment, so that no evaporated vapour re-enters the test section. For the present experiment, this has not any influence or relevance.…”

“…Since the temperature in the laboratory went down to 8 • C in winter, the inflow temperature of 100 • C avoided condensation of the seeding aerosols on the windows of the test section. Symmetry of the flow could not be checked due to limited optical access, but the same test facility was used by Horender and Sommerfeld (2012) for droplet evaporation in turbulence generated by a 65% blockage plate and symmetric particle dispersion and fluid velocity profiles were reported.…”