SUBSISTEMA ECONÓMICO

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Figure 4.2 Temperature and pressure distribution of SF6 arcs with a current of 600 A in a supersonic

nozzle, the upper part: temperature; the bottom part: pressure. Exit pressure: (a), 0.680MPa; (b), 0.453MPa; (c),0.272MPa.

Figure 4.3 SF6 arc column broadening in a supersonic nozzle with a current of 600 A and an exit

Previous study has shown that the presence of arc can significantly alter the flow and pressure field especially when a shock exists in the arcing area [28]. The pressure and temperature contours for a current of 600 A respectively with the exit pressures of 0.680MPa, 0.453MPa and 0.272MPa are given in Fig. 4.2.

We can find that the highest temperature occurs in front of the downstream tip and its position moves upstream as the exit pressure increases. The arc temperature in the hollow electrode passage decays gradually as the axial position increases because no Omic heating is taken into account. The largest pressure drop takes place mainly in the arcing area between the electrodes.

If the hollow downstream electrode under the condition of the exit pressure of 0.272MPa is not taken into account in the computation as it was usually treated before, it is found that the shock is generated not before but after the downstream tip as we can find the second shock existing in Fig. 4.2 and its influence on the arcing area is relatively low. Although there exists a hollow passage, flow on the axis is decelerated as a result of the downstream electrode’s blocking effect through a shock whose center is slightly moved upstream with increasing exit pressure.

For the cases in which the exit pressure is lower than 0.272MPa, the field before the downstream tip is hardly affected by the decreasing pressure penetration bringing a similar arcing behaviour during fault current interruption and not presented here. With the exit pressures of 0.680MPa and 0.453MPa, a high pressure penetrates into the arcing area in front of the downstream tip and the space after the downstream electrode tip poses a similar pressure value with the exit. The adverse pressure gradient brings a subsonic external flow with relatively low velocity in the thermal layer which corresponds to an equivalent boundary layer between the arc core and the cold gas. Because the arc core has a quite high temperature and velocity, the high pressure penetration influences the thermal layer more intensely and leads to a vortex where a flow separation takes place. In the thermal layer, the hot arc core and the cold gas interacts strongly and this further modifies the pressure field distribution because the shock cannot penetrate into subsonic flow[29] as we can find in Fig. 4.3.

The high pressure penetration into the thermal layer along with the downstream electrode’s blocking effect both contribute to a low flow entrainment into the arc core to form a shock and hence a broadening effect of the arc column.

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Figure 4.4 Mach number distribution of SF6 arcs with a current of 600 A in a supersonic nozzle, Exit

pressure: (a), 0.680MPa, (b), 0.453MPa; (c),0.272MPa.

Fig. 4.4 gives the Mach number distribution of SF6 arcs in the supersonic nozzle. The flow becomes subsonic in the diverging part after the pressure rise caused by the shock which brings the flow deceleration for all three cases. It also clearly shows the inhibition of a high exit pressure penetration to gas flow. For example, with the exit pressure of 0.680MPa, the gas flow along the central axis does not show supersonic characteristics before the shock. As presented, the highest Mach number in the nozzle mainly occurs along the wall surface in its diverging part. When the exit pressure is increased, its position gradually moves towards upstream with a decreasing value.

We can obtain the influence of high exit pressure penetration by comparing the axial variations of temperature, arc radius and pressure for a current of 800A under different exit pressures as presented in Figs. 4.5-4.6.

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Figure 4.5 Axial variation of temperatures at r=0 (a) and arc radius (b) for a current of 800A. Exit pressure: 1, 0.680MPa; 2, 0.453MPa; 3,0.272MPa; 4, 0.136MPa.

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Figure 4.6 Axial variations of pressure along the central axis (a) and the wall surface (b) for a current

of 800A. Exit pressure: 1, 0.680MPa; 2, 0.453MPa; 3,0.272MPa; 4, 0.136MPa. Two lines correspond to the positions both left and right tip of the downstream electrode.

There exists a location before which the arc and its surrounding flow are hardly affected by the penetrated pressure and poses the same field distribution. Near and after the interaction zone, the arc shape is deformed by the arc-shock interaction with the arc radius

firstly enlarged and then decreased. Along the central axis, the pressure experiences a drop after the shock before the downstream electrode tip due to the influence of a shock. Because the pressure in the electrode hollow passage is lower than that outside at the same axial position in the nozzle diverging part, a rapid pressure rise along the central axis takes place near the right tip of the downstream electrode.

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Figure 4.7 Axial variations of pressure along the central axis (a) and the wall surface (b) with exit pressure of 0.136MPa. 1, cold gas; 2, 200A; 3, 400A; 4, 600A; 5, 800A; 6, 1000A.

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Figure 4.8 Axial variations of axial velocity along the central axis (a) and the wall surface (b) with exit pressure of 0.136MPa. 1, cold gas; 2, 200A; 3, 400A; 4, 600A; 5, 800A; 6, 1000A.

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Figure 4.9 Axial variations of Mach number along the central axis (a) and the wall surface (b) with

exit pressure of 0.136MPa. 1, cold gas; 2, 200A; 3, 400A; 4, 600A; 5, 800A; 6, 1000A.

The influences of different currents on axial variation of pressure, axial velocity and Mach number along the central axis and the wall surface with the exit pressure of 0.136MPa are presented in Figs. 4.7-4.9, respectively. Under this condition, the pressure penetration has negligible effect in the regions before the left tip of downstream electrode. Beside the shock

caused by the electrode blocking effect, a second shock is generated after the left tip of downstream electrode both in and outside the electrode hollow passage. Both shocks caused by the electrode blocking effect and the pressure penetration present a low dependence of its center location on the current value which slightly moves towards downstream as current increase showing increasingly significant influence of the arc. The axial velocity along the central axis increases as current increases in most regions with the exception near the exit. The flow velocity in the cold gas without the arc presents the lowest value along the axis. However, a different condition exists for the variation of axial velocity along the wall surface where the cold gas shows higher axial velocity because its shock center is significantly moved to downstream comparing the condition where an arc exists. The gas flow after the shock along the central axis for the cold gas and for the arc with a current of 1000A together with all cases along the wall surface is still supersonic. The flow Mach number along the central axis increases dramatically after the right tip of the hollow downstream electrode regardless of the increasing pressure and decreasing velocity. This is mainly attributed to the decreasing gas sonic velocity with a rapidly decaying temperature caused by the strong cooling effect that the cold gas outside the hollow electrode passage takes away the heat quite swiftly.