Measurement of evaporation of hydrocarbon droplets by laser absorption spectroscopy

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 flow channel and operated with cyclohexane, isooctane, n-heptane, n-pentane and 1-butanol. The flow channel was flushed with a laminar flow of air at different temperatures. The absorption of a HeNe laser beam at lambda = 3.39 mu m traversing through the flow 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 = 300-773 K) enabled the quantification 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-flow 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 configuration. Particular attention was paid to the spatial resolution of the measurement. The results are well suited to validate models and numerical simulations.