| 1. |
- Barmann, P, et al.
(författare)
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Spatially and temporally resolved electron density measurements in streamers in dielectric liquids
- 1997
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 30:5, s. 856-863
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Tidskriftsartikel (refereegranskat)abstract
- Spatially and temporally resolved spectroscopic measurements of light emitted from positive streamers in transformer oil are presented. Analyses of the measurements performed with a DC needle-plane gap yield electron densities and indications of the atomic excitation temperatures in the streamers. The hydrogen emission reveals an electron density below 10(16) cm(-3) during the main part of the streamer propagation time (80-90%). Later the light is also characterized by emission from a high-density plasma with electron densities in the range 10(18)-10(19) cm(-3). The electron density during this time increases approximately linearly with distance from the initiation point and a density factor of four higher has been measured at the streamer tip than at the root. Measurements with high spectral resolution detect both high and low electron densities simultaneously. A tentative model of the interior of the streamer plasma, spatially resolved, is presented.
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| 2. |
- Barmann, P, et al.
(författare)
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Spectroscopic measurements of streamer filaments in electric breakdown in a dielectric liquid
- 1996
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 29:5, s. 1188-1196
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Tidskriftsartikel (refereegranskat)abstract
- Emission spectroscopy has been utilized to provide information about the electron density and temperature in streamers and breakdown arcs in transformer oil. Recorded spectra include strongly broadened hydrogen Balmer-alpha lines and vibration/rotation band profiles of the C-2 molecule. The origin of the observed broadening of hydrogen lines is discussed and it is concluded that it arises mainly from collisions with charged particles, so-called dynamic Stark broadening. By assuming that the broadening is due solely to dynamic Stark broadening, electron densities between 1 x 10(18) and 1 x 10(19) cm(-3) were obtained for the rear of positive streamer filaments during the later stages of propagation. For negative streamers we obtained an upper limit of 3 x 10(16) cm(-3) and for breakdown arcs electron densities up to 4 x 10(18) cm(-3). The temperature information in the C-2 emission profiles and the intensity ratio of the hydrogen Balmer lines are discussed. Rough estimations of the temperature are presented both for positive and for negative streamers.
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| 3. |
- Kaplan, Alexander, et al.
(författare)
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An analysis of the heat transfer from a moving elliptical cylinder
- 1997
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 30, s. 1190-1196
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical analysis of the heat transfer from a moving elliptical cylinder is carried out and general solutions in elliptical coordinates are obtained for aspect ratios β, both β and β. The heat-flux distribution and total heat-flow rate are calculated, investigated for a wide range of parameters and presented graphically. The results obtained are useful for applications concerning thermal processes (laser cutting, welding, heat treatment and so on). A comparison of a moving circular cylinder with a numerical model of laser cutting is considered and discussed.
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| 4. |
- Lampa, Conny, et al.
(författare)
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An analytical thermodynamic model of laser welding
- 1997
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 30:9, s. 1293-1299
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Tidskriftsartikel (refereegranskat)abstract
- An earlier model of deep-penetration laser welding has been simplified in order to provide a useful model of process analysis. This work involves the modelling of the various energy-absorption mechanisms which determine the keyhole shape and thus the dimensions of the melt pool. The penetration depth and weld width (top and bottom) predicted by the model are shown to be in close agreement with experimental results. The widening of the top of the weld seam as a result of Marangoni flow is accurately modelled by introducing an artificially enhanced value for the workpiece's thermal conductivity towards the top of the weld. The model allows analysis of the dependence of the weld profile on the process parameters.
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| 5. |
- Tokarev, Vladimir, et al.
(författare)
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Suppression of melt flows in laser ablation: application to clean laser processing
- 1999
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 32:13, s. 1526-1538
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Tidskriftsartikel (refereegranskat)abstract
- It is shown that in laser ablation of materials with large Prandtl numbers (mainly ceramics and polymers) a motion of the melt along the surface caused by the vapour plume pressure is essentially retarded for thin enough melt layers due to the onset of viscous friction. For polymers in nanosecond laser ablation this melt displacement can be obtained to be less than the ablation depth per pulse (which is typically 0.2-1 µm for nanosecond irradiation), when the absorption coefficient, α, and the kinematic viscosity, v, satisfy the condition α2v > 108 s-1. Thus, clean precise laser ablation for such polymers can be explained simply in terms of the thermal mechanism, without invoking the concept of photochemical decomposition, in terms of absorption coefficient, melt viscosity and pressure of the ablation plume on the irradiated surface. From this point of view, several factors facilitating clean laser ablation in multipulse irradiation are discussed. However, for metals (usually having a very small Prandtl number) the viscous friction has no significant effect on the retardation lateral melt flow to the periphery. The quality of the laser spot border can still be improved by using laser pulses shorter than 1 ps. In this case, in a shallow spot, the alternative (explosive) melt expulsion mechanism becomes predominant, producing material removal mainly transverse to the spot surface. Thus, the lateral (along the surface) component of melt expulsion appears to be strongly suppressed, having no chance to spoil the border of the spot.
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| 6. |
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| 7. |
- Sisman, Altug, et al.
(författare)
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On The Power Cycles Working with The Ideal Quantum Gases-I: The Ericson Cycle
- 1999
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Ingår i: Journal of Physics D. - 0022-3727 .- 1361-6463. ; 32, s. 664-670
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Tidskriftsartikel (refereegranskat)abstract
- The Ericsson power cycles working with ideal Bose and Fermi monoatomic gasesare examined. They are conveniently called the Bose and Fermi cycles. Efficiencies of Bose and Fermi cycles are derived (etaB and etaF respectively). Variations of them with the temperature ratio (tau) and pressure ratio of the cycle are examined. A comparison of theefficiencies with each other and that of the classical Ericsson cycle (etaCl) is made. In thedegenerate gas state it is seen that etaB < etaF < etaCl, although etaB = etaF = etaCl in the classical gas state. In a Bose cycle, it is shown that there is an optimum value for the lowest temperature (TL) at which the efficiency reaches its maximum value for a given pressureratio. Furthermore, Bose–Einstein condensation restricts the value of TL of a Bose cycle for agiven value of PH . In a Fermi cycle, there is no an optimum value for TL. However, etaF goesto a finite value of less than unity when tau goes to zero.
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| 8. |
- Larsson, A
(författare)
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An experimental study of inhibited electrical discharges in air
- 1998
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Ingår i: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : IOP PUBLISHING LTD. - 0022-3727. ; 31:15, s. 1823-1830
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Modern society demands a stable electricity supply, the reliability of which is partly determined by the dielectric strength of the outdoor high-voltage insulation systems. This paper concentrates upon the experimental study of one particular discharge ph
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| 9. |
- Larsson, A, et al.
(författare)
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Numerical modelling of inhibited electrical discharges in air
- 1998
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Ingår i: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : IOP PUBLISHING LTD. - 0022-3727. ; 31:15, s. 1831-1840
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The inhibited discharge is a discharge in which the input of energy into the discharge channel is limited by the presence of a large series resistance in the external circuit supplying the discharge current. One situation in which such a discharge occurs
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| 10. |
- Larsson, A
(författare)
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The effect of a large series resistance on the streamer-to-spark transition in dry air
- 1998
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Ingår i: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : IOP PUBLISHING LTD. - 0022-3727. ; 31:9, s. 1100-1108
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The streamer-to-spark transition is an important sub-process of the flashover of high-voltage apparatus. In this paper an investigation of this transition in an electrode gap of the order of 1 cm in dry air with a non-uniform field distribution is present
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