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Analysis of thermal plasma radiation by Fourier transform, wavelet analysis and phase portraits
Gruber, Jan ; Šonský, Jiří ; Hlína, Jan
Combination of various methods for signal analysis, including Fourier transform, wavelet analysis and time-embeddings provide alternate views on thermal plasma dynamics. We have recorded and analyzed radiation of cutting plasma torch during the whole cathode life-cycle (from the new unworn cathode to the cathode failure). Changes in dynamics during cathode lifecycle lead to regular oscillations expressed in signal spectra, in sudden discontinuities caused by released molten particles detected by wavelet analysis and overall dynamics change expressed in reconstructed phase portraits by time-embeddings. We present all these various views on changing dynamics to illustrate complexity of this dynamics.
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Influence of Cathode and Nozzle Wear on Distributions of Fluctuations in Cutting Arc
Gruber, Jan ; Šonský, Jiří ; Hlína, Jan
Electric arc produced by cutting plasma torch was observed by CCD camera. Fluctuations in arc plasma intensity were examined in several frequency intervals up to 50 kHz and distributions of their intensities were compared. Plasma torch was operated repeatedly in fixed-time cycles resulting in progressive wear of cathode and nozzle. Changes in distribution of overall arc intensity are rather small, showing compression and expansion zones. However, distributions of fluctuations intensities differ significantly during the nozzle lifecycle, exhibiting formation of double-layer structure and other noticeable changes. The fluctuation distributions are also influenced by frequency interval chosen for the analysis.
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Phase Coupling Between Core and Boundary Regions of a Thermal Plasma Jet
Hlína, Jan ; Šonský, Jiří
Time series of plasma jet radiation distributions in planes perpendicular to the jet axis were analyzed by the fast Fourier transform (FFT). Complex output yielded by FFT was used to study amplitudes and phase angles belonging to various frequencies occurring in the frequency spectra. The distributions of amplitudes and phase angles have shown the existence of characteristic core and boundary regions for various types of oscillations. Phase angle relations give evidence of coupling between the core and boundary of the jet at some frequencies.
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Dependence of Thermal Plasma Flow Velocity on Arc Current Determined by Correlation Analysis of Optical Records
Gruber, Jan ; Šonský, Jiří ; Hlína, Jan
Optical radiation of thermal plasma jet was recorded in two parallel cross-sections perpendicular to the jet axis and separated by 3.5 mm. Sufficient sampling rate was a prerequisite to obtain detectable shift between the time-series recorded in closer and farther cross-sections. We have determined time lags between the recorded signals both in the core and boundary regions by correlation analysis and subsequently determined associated velocities of plasma flow. Arc current was gradually increased in respective measurements, gas flow was fixed. Determined flow velocities are in the range 140 m/s - 280 m/s in the jet core and 110 m/s – 220 m/s in the boundary region. This is in agreement with the results obtained for similar plasma torches by other methods in the past. Velocity increases with arc current almost linearly and this dependence and velocity profile in the plasma jet cross-section are presented in this paper.
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Vliv nestabilit na určení střední teploty ve volném proudu termického plazmatu
Hlína, Jan ; Šonský, Jiří ; Šlechta, Jiří
We have used results acquired by a multi-directional optical system for temperature measurements with a high temporal resolution to compare mean temperatures in the jet obtained by averaging the instantaneous temperature fields with the temperature values acquired in the usual way from averaged radiation intensities. The comparison shows differences in results yielded by both methods which are caused by the nonlinear dependence of radiation intensity on temperature.
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3D diagnostika koherentních struktur ve volném proudu plazmatu
Hlína, Jan ; Sekerešová, Zuzana ; Šonský, Jiří
Optical radiation of a thermal plasma jet was recorded by a fast - shutter CCD camera through an optical system providing two perpendicular and simultaneous views of the jet. The measuring system yielded series of plasma jet images containing 8 pictures with exposure times 400 ns and intervals 7 us. The evaluation of the records was based on reconstructing the distribution of plasma optical radiation in planes perpendicular to the plasma jet axis by the inverse Radon transform. A repetitive application of this transform along the axis of the jet yielded the complete 3D distribution of the optical emission in the plasma jet. The differences between the resulting 3D matrices representing the optical intensities derived from adjacent pictures in the recorded series was used for the identification of coherent structures as regions characterized by the temporal increase or decrease of the optical emission.
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