TOLAS spectrometer for spatially resolved absolute water vapour detection around acoustic levitated single droplets

Abstract

In industrial processes spray drying is one of the most energy consuming tasks which motivates a detailed understanding of the evaporation processes on the scale of single H2O droplets. For the investigation of the mass transport processes between liquid droplet and the surrounding gas phase we combined acoustic levitation and tunable diode laser absorption spectroscopy (TOLAS) at a wavelength of 1369.97 nm to measure the absolute 2D H2O concentration field around a single levitated droplet. Using this techniques, we were able to determine the shape and volume of a levitated droplet within an accuracy of 7% as well as absolute 2D concentration fields within an area of 3×4cm. With an absorption length of only 4cm we achieved a concentration resolution of 67ppm at a spatial resolution of 500μm. Furthermore, the vertical profiles of the measured H2O concentration showed excellent accordance with predictions from theoretical diffusion models.