Development of a Particle Generator for Combined Particle Image Thermo- and Velocimetry in Air

Abstract

Due to the Prandtl number dependence of thermally driven flows there is a strong demand for simultaneous acquisition of instantaneous temperature and velocity fields in air or gas flows. We report on combined Particle Image Thermo- and Velocimetry of air flows, which provides the requested properties. A feasibility study was performed successfully in the past showing the applicability of this promising measurement technique (Bosbach et al. 2007 and Schmeling et al. 2010b). The present paper is focused on the experimental advancement in developing an improved particle generator, which is needed to continuously produce tiny unencapsulated thermochromic liquid crystals. Phase Doppler Anemometry is applied in order to characterize the produced droplets. The particle generator, which utilizes the principle of Rayleigh breakup (Rayleigh 1878), is presented and its functionality is demonstrated. A strong dependency of the diameter of the produced droplets on the Reynolds number and the oscillation frequency of the installed piezoelectric oscillator is observed. With switched off piezoelectric oscillator or low extruding velocities broad size distributions are generated. Within a certain parameter range, which still has to be studied in more detail, the production of monodisperse droplets has been achieved. However, the diameters of the particles are larger by a factor of three in diameter as expected by pure Rayleigh breakup. A reason might be the straight upwards oriented extruding direction, which leads to a thin fluid layer on the aperture plate. Thermochromic liquid crystals forming agglomerates on a black plate, which was moved through the stream of droplets, show a brilliant, temperature depending play of colours.