USO POTENCIAL DEL SUELO

Chapter 3- Computer Simulation of a Carbon Arc Confined in Water

As the arc changes its shape and composition, the arc voltage also undergoes a rapid change at the beginning of the arcing process (figure 3.26). The sudden increase in the first 5 µs can be explained by the rapid drop of arc temperature (figure 3.20). The arc voltage then starts to drop. This voltage drop is associated with a number of factors, including pressure, arcing gas composition and arc size. By comparing figures 3.16 and 3.23 it can be seen that there is a significant change of gaseous composition at soon after the arc initiation. The water vapour concentration decreases while the carbon vapour increases. From figure 3.6 it is clear that the electrical conductivity of carbon vapour is much higher than that of the water vapour in the temperature range from 5000 K to 10,000 K. With an increased electrical conductivity the arc voltage decreases. It then settles down with a value of 7 V.

In the arc in water case, the electrodes are cooled by the surrounding water. The conduction energy loss to the cathode surface is calculated using an effective surface temperature of 6,000 K (equation (3-3)). The sensitivity of the arc voltage to this temperature has been studied by using a value of 4,000 K. An arc voltage of 7.9 V was obtained, representing a change of 12.8%. The arc voltage is increased as a result of arc cooling leading to a lower electrical conductivity near the cathode.

Arc voltage 0.0E+00 5.0E+00 1.0E+01 1.5E+01 2.0E+01 2.5E+01 3.0E+01 3.5E+01 4.0E+01 4.5E+01

0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 5.0E-04 Time(s)

Arc voltage(V)

Chapter 3- Computer Simulation of a Carbon Arc Confined in Water

The measured arc voltage for a current of 30 A is 17 - 19 V [San02]. Our prediction gives a voltage of only 7 V. There is a difference of 13 V. This difference is mainly because in the present work we only simulates the arc column and the non-LTE electrode layer is not considered. The voltage drop near the cathode corresponds approximately to the first ionisation energy of the atomic species at the arc root, which is 11.26 eV for carbon. This explains the major difference. The inclusion of a sheath model for the cathode and anode should therefore be part of the improvement in future.

An accurate comparison of the predicted and measured arc temperature is not possible because the experimental uncertainty is not known, especially in the presence of water surrounding the arc. The axis temperature given in [Lange] is 6500 K which is lower than the predicted value of 10,000 K in the current work.

An overall mass balance for carbon vapour was carried out, as shown in figure 3.27. The vertical axis represents the rate of each process as a percentage of the total injected carbon vapour mass (kg) at the anode. The curve labelled DIFF (broken red) reflects the overall numerical accuracy of the mass balance calculation. It is generally better than 10%. It can be seen that in the first half millisecond most of the injected carbon vapour is consumed by chemical reaction. This high rate is a result of the carbon vapour encountering the water vapour species inside the arc column at the beginning of the arcing period. After 0.5 ms, the arc burns in a carbon vapour dominated environment and the reaction yields decreases. About 60% of the carbon vapour is dumped in the bubble and only 30% is used in the reaction. Despite this, the concentration of H2 and CO is low, only 0.2% and 2.8% for their maxima as shown in figure 3.24 and 3.25. Carbon loss at the arc edge and near the cathode is low, only 2% of the injected carbon. The dominant water environment in the arc surrounding region can be explained by figure 3.28 which shows that the water evaporation rate (4×10-5 kg/s) I much higher than the carbon vapour injection rate at the anode (2×10-6 kg/s).

Chapter 3- Computer Simulation of a Carbon Arc Confined in Water -10.0% 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

0.00E+00 5.00E-04 1.00E-03 1.50E-03 2.00E-03

DIFF mass stored reaction

edge cathode channel

Figure 3.27 Overall mass balance for carbon vapour in terms of percentage of injected mass from the anode. The horizontal axis is time (s).

Water evaporation rate

0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 5.0E-04

0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 5.0E-04 Time(s)

Evaporation rate(kg/s)

Figure 3.28 Water evaporation rate as function of time

Chapter 3- Computer Simulation of a Carbon Arc Confined in Water

The arc in water case is simulated in this chapter. The transport properties were calculated based on interpolation between those of the pure carbon vapour and water vapour. The reason for doing this was also given. The reaction rate coefficient was

estimated to be 1.054×106 based on published data at 300 K. Results have shown

that the growth time for the bubble is in the order of millisecond and the electrode gap is rapidly filled by carbon vapour from the anode surface. The growth of the bubble is mainly due to the pressurisation of the bubble by carbon vapour injection. Although about 30% of the injected carbon vapour is consumed by chemical reaction, the concentration of H2 and CO is rather low, less than 2% and 2.8% respectively. The predicted arc voltage is within a reasonable range if the voltage drop in the cathode sheath layer is considered. An improved model for the arc in water case must include the sheath layer in future.

3.7 References

[Hsi01] Hsin Y. L., Hwang K. C., Chen F. R. and Kai J. J., “Production and in-situ Metal Filling of Carbon Nanotubes in Water”, Adv. Mater. Vol. 13, No. 11, 2001

[Gleizes] Gleizes, A., Private communication.

[Zhu02] Zhu H. W., Li X. S., Jiang B., Xu C. L., Zhu Y. F., Wu D. H. and Chen X. H., “Formation of carbon nanotubes in water by the electric-arc technique”, Chem.

Phys. Lett., 366, pp. 664-669, 2002

[Hus75] Husain, D., and A. N. Young , Kinetic investigation of ground state carbon atoms C(2 3Pj ), J. Chem. Soc., Faraday Trans. 2, 71, 525–531. 1975

[San02] Sano N., Wang H., Alexandrou I., Chhowalla M., Teo K. B. K. and Amaratunga G. A. J., “Properties of carbon onions produced by an arc discharge in water”, Journal of Applied Physics, Vol. 92, No. 5, pp. 2783-2788, 2002

Chapter 4 Computer Simulation of the Arcing Process in a