The dynamics of liquid-vapor mass transfer largely determines the performance of internal and gas turbine spray combustors. The key mechanisms however typically take place on small spatial scales of less than 100 μm which have been difficult to measure. The present work thus aims at the development and application of an experimental technique for the characterization of droplet evaporation with high spatial resolution. Single chains of monodisperse acetone droplets with diameters of 125 and 225 μm are injected into a channel with a cross-section of 60x60 mm² and quartz glass side walls for optical access. The droplet chains are surrounded by a laminar air flow with velocity and temperature of about 0.1 m/s and 300 K, respectively. The distribution of acetone vapor around the droplets is measured using planar laser-induced fluorescence (PLIF) excited by the 4th harmonic of a Nd:YAG laser at 266 nm. The measurements are performed in thin transversal sections between the droplets in order to avoid signal corruption by halation effects that occur when the laser directly hits the droplets as reported in previous studies. In addition, the spatial resolution of the PLIF setup was enhanced by using proper sheetforming and imaging optics. The resulting in-plane resolution and out-plane-resolution (i.e. thickness of the laser sheet) are both determined to about 20 μm, which thus allows an accurate characterization of the small-scale vapor distribution near the droplets. Using a separate calibration measurement, quantitative acetone concentrations are obtained for non-reacting conditions. As a complementary technique, the droplet evaporation is measured using shadowgraphy droplet sizing. Both non-reacting and reacting droplet chains are studied. The results for the non-reacting cases show that the droplet chains are surrounded by a column of nearly-saturated acetone vapor with a concentration maximum at the centerline. For increasing radial distances, the vapor concentration decays quickly with a half width of 0.5 mm and reaches almost zero for r>1 mm. It is further seen that the width of the vapor column increases with streamwise distance. For the experiment with a reacting droplet chain, which is continuously ignited by a heating wire at the channel inlet, a cylindrical reaction zone around the chain with a radius of about 1.5 mm is observed. The shadowgraphy measurements show that the rate of droplet evaporation is significantly enhanced for the reacting conditions. This is attributed to the high rate of heat transfer from the flame to the droplets and the resulting enhanced acetone mass transfer to the sink at the reaction zone. KeywordsDroplet evaporation, Laser-induced fluorescence, Monodisperse droplet chain, Liquid-vapor mass transfer Introduction Several important energy conversion devices such as internal combustion engines and gas turbines are driven by the combustion of liquid fuels. The liquid fuels are typically first atomized into a spray of small droplets, which are then dispersed in a turbulent flo...

Concentration measurements of evaporated hydrocarbon species by infrared laser absorption spectroscopy at a monodisperse droplet chain are presented. A droplet generator was installed within a ow channel and operated with cyclohexane, iso-octane, n-heptane, n-pentane and 1-butanol. The ow channel was ushed with a laminar ow of air at dierent temperatures. The absorption of a HeNe laser beam at λ = 3.39 µm traversing through the ow channel at varying distances from the droplet chain was exploited to determine the vapor concentrations of the hydrocarbons. Measurements of the absorption cross sections in a heated gas cell (T = 300 to 773 K) enabled the quantication of the absorption signals from the droplet chain. Vapor concentrations were determined in planes perpendicular to the droplet chain. From the increase of vapor concentration between the planes, the evaporation rate could be determined. The evaporation rates were measured in dependence of co-ow temperature, droplet velocity, droplet generation frequency and droplet spacing. In the investigated temperature range of the air (313-430 K) the evaporation rates increased linearly with temperature. The order of the fuels with respect to evaporation rates corresponded with the boiling points of the individual fuels. In addition to the presentation of the results, the paper discusses the performance of vapor concentration measurements by laser absorption spectroscopy at droplet chains which has not been tested before in such a conguration. Particular attention was paid to the spatial resolution of the measurement. The results are well suited to validate models and numerical simulations.