Analysis of the spray forming process using backscattering phase-Doppler anemometry

Abstract

Abstract A phase-Doppler anemometer (PDA) was extended for the analysis of metal melt gas-atomization. Because of the optical properties of liquid or partly solidified spherical particles with a high value of the imaginary part of the refractive index (which means that measurements can be executed, using the reflected light from the particles), the instrument was arranged in a 180° backscattering position, with a combined transmitting and receiving system. This compact assembly described in principle already by Bauckhage in 1988 and 1989 can easily be moved to realize different measuring locations in the spray cone, needing only one window. Operating from one side of the spray cone scanning PDA-measuring positions from inside the two phase flow could be carried out within milliseconds per measuring position, thus allowing the analysis even for short atomization duration. Due to high particle rates in the spray cone and a difficult optical behavior of the solidified metal particles (rough surface), a new signal processing had to be developed, which can be subdivided into a signal recording and a post processing. During the first part a signal is completely recorded for a definite duration without using any trigger criteria. This results in a kind of signal band, that contains bursts as well as noise. In the second part the bursts are removed from the signal band by using the signals power spectrum after a fast-Fourier transformation (FFT).