Experimental investigations of evolution and velocity of dynamic structures identified in plasma jets

Summary form only given. Dynamic phenomena in thermal plasma jets were investigated using both the records of local plasma jet radiation recorded by photodiode arrays and CCD camera pictures of the jet showing the global jet characteristics. The plasma torch was operated with arc currents 125-200 A and argon gas flow rates 15-80 slpm. Plasma jet optical emission was detected at two cross sections perpendicular to the plasma flow by photodiode arrays and their signals were recorded as long time series. The evaluation of the records was based on the fast Fourier transform (FFT). FFT yielded together with the frequency spectra also relations between phase angles related to signals of photodiode pairs situated along the plasma flow as functions of frequency and the results of the phase analysis were used for evaluations of the plasma velocity. CCD camera system was used to acquire simultaneously with the photodiode records series of images of the plasma jet. Each picture series contained 8 pictures of the plasma jet taken with the interval 12 /spl mu/s and exposure time 2 /spl mu/s. The evaluation procedure was based on the correlation analysis of vectors the components of which were defined either as intensities of the same pixels in the succeeding pictures or the intensities recorded by the pixels in the neighbourhood of each point of the plasma jet image. The results show that plasma jets include various types of structures characterised by different values of correlation between the succeeding images of the plasma jet. The evolution and velocity of the detected structures depend on the operating parameters of the plasma torch. The results of both the experimental approaches (photodiode and CCD camera application) are analysed in the discussion.