Temperature characteristics of a combusting droplet stream

This study presents coherent-anti Stokes Raman spectroscopy (CARS) temperature measurements in a combusting droplet stream. The experiment uses a 50 μm diameter nozzle vibrated by a piezoelectric crystal to generate a monodisperse droplet stream with a droplet diameter of about 150 μm and droplet-to-droplet spacing of 10 droplet diameters. The objectives of this study are: (1) to demonstrate CARS thermometry for droplet combustion applications, (2) to describe the flame structure around interacting droplets using the CARS measured temperature field, and (3) to provide experimental data for numerical model comparison. The results show that there is a small thermal wake behind each droplet in the stream. The temperature profile measured radially outward from a droplet has a local minimum near the droplet surface, rises to a maximum about 6.5 droplet diameters away, and then falls to room temperature at a radial distance of 15 mm (100 droplet diameters). The temperature profile measured between two adjacent droplets on the stream axis reveals a thermal boundary layer of 1.5 droplet diameters near the droplet surface. The profile is nearly flat, suggesting that individual flames do not surround each droplet. The local effects due to the presence of individual droplets completely disappear about 15 droplet diameters away from the droplet stream centerline. Temperature contours constructed from the measurements of several radial profiles around two adjacent droplets show a cylindrical flame structure. The local heat transfer Nusselt numbers calculated from CARS measured temperatures agree qualitatively with numerical model predictions.