One-Dimensional Rainbow Technique to Characterize the Evaporation at Ambient Temperature and Evaporation in a Flame of Monodispersed Droplets

This paper represents the ability of the newly introduced One-dimensional Rainbow Technique (ORT) to characterize the evaporation of monodispersed droplet in the large range of environment temperature. At the low temperature (room temperature), the small change of drop size is measured from the phase shift of ripple structure. The size change, as small as 10 nm for 80 μm droplet, can be measured. For the evaporation under high temperature of the combustion flame, the gradient of refractive index can be realized from the transition of extracted refractive index during the time. The non-monotonic function of refractive index time variation is observed due to the change of refractive index profile inside the droplet. Introduction Global rainbow technique (GRT) is one attractive technique for simultaneously measuring size and temperature (or composition). Because of the simple, inexpensive setup and fully predictable configurations, it gives us the chances to develop and adapt for various applications. Classically, this technique measures size distribution and averaged refractive index of the droplets in a spray at a point, which is spatially selected by the system of lenses and pinhole [1,2]. In the system of the monodispersed droplet, the measurement of size and refractive index can be as accurate as 0.01 μm for diameter and the forth digit for refractive index by combining with the forward scattering pettern [3]. Since GRT has the point measurement probe, it does not provide the spatial evolution of droplet, i.e. the change of diameter, temperature and composition. To be able to follow those data from the moving droplets, the extension of GRT using slit apertures and laser sheet has been introduced by X.C. Wu., et. al. [4] and called as One-dimensional Rainbow Technique (ORT). The technique gives the possibility to follow the evolution (refractive index and size) of the droplet along the line. This paper applies the recently developed technique, ORT, to characterize the evaporation of the monodispersed droplet under both cold condition and in combustion flame. The configuration of the monodispersed droplet gives a possibility to measure the change of droplet diameter (hence the evaporation rate). Since the change of droplet diameter along the line measurement probe is very small and the change of shape of the main rainbow is not sufficient for the measurement, then the phase shift of the ripple structure is observed alternatively. The refractive index is extracted without taking into account the radial gradient