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Logo: Institut für Mehrphasenprozesse/Leibniz Universität Hannover
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Exhaust of multiphase flows under water

 

Dr.-Ing. M. Creutz

 

Financed by the "Deutsche Forschungsgemeinschaft", SFB 264

 

In underwater thermal cutting processes of metal structures, such as underwater plasma arc cutting, gaseous and solid contaminants are emitted. The melted material of the joint remains as a suspension of metalloid particles with particle sizes ranging from some millimetres down to sub micron particles [1-5]. Plasma and cutting gases lead to high vortex flows and flotation of the small particles. The contaminants reduce the optic transparency of the water and thus have to be avoided in processes controlled by systems based on visual observation. Under water cutting processes are already being used for the demolition of nuclear power plants [6] and [7]. Especially for that purpose, contaminants have to be completely removed. For the process of sucking off the contaminants and separating them into gas, clean water and slurry, a tool is developed that is shown in figure 1. The tool is intended to form an integral part of the plasma arc cutting system. Investigations on the sucking process of multiphase suspensions have been done [8]. This text focuses on the sucking process only.

 

 

Figure 1: Underwater cleaning and separation process.

 

 

For more details of the sucking process see Three phase gas-liquid-solid separation.

 

The quality of the sucking process depends on the flow pattern of the multiphase flow that is influenced by the negative pressure in the sucking torch. In figure 2 a sucking of gas against gravity is shown.

 

 

Figure 2: Sucking gas against the gravity.

 

 

Experimental investigations have been carried out on the efficiency of the sucking process. The efficiency is defined as the relation between sucked gas and gas input. In figure 3, the results of the experimental investigations are plotted as a function of the vacuum and the distance between gas inlet and sucking torch. In figure 4 these experimental results are shown as a function of the sucking force related to the buoyancy of the gas bubble. The sucking force is the pressure gradient due to the acceleration of water towards the torch.

Figure 3: Experimental investigations on the efficiency of the sucking process.

Figure 4: Efficiency of the sucking process as a function of the "sucking constant".

 

 

These results just serve as examples. Several investigations are being carried out on two phase liquid solid and three phase gas-liquid solid sucking and on the influence of jet impulse on the efficiency of the sucking process [8].

 

[1] H. Steiner, Partikelmeßtechnik beim Plasmaschmelzschneiden, Fortschrittberichte VDI- Reihe 2, Nr. 245 Düsseldorf, VDI-Verlag 1992.

[2] A. Gruchow, Beitrag zum automatisierten Einsatz thermischer Trennverfahren in der Offshore-Industrie, PhD. University of Hannover, 1993.

[3] H. Stoiber, G. Hammer and H. Schultz, Emissionsreduzierung bei der Anwendung thermischer Trennverfahren zur Zerlegung kerntechnischer Anlagen, Autogen- und Plasmatrennen, 4 th Stillegungskolloquium, Bad Dürkheim, Germany, Nov. 1995. [

4] G. Pilot, R. Leautier, J.P. Noel, H. Steiner, G. Tarroni and B. Waldie, Measurements of secondary emission from plasma arc and laser cutting in standard experiments, Roport EUR 14065, Luxembourg, Commission of the European Communities, Nuclear Science and Technology Series, 1992. [

5] B. Waldie and W. K. Harris, Dross and ultrafine particulate formation in underwater plasma-arc cutting, Report EUR 13798, Luxembourg, Commission of the European Communities, Nuclear Science and Technology Series, 1991. [

6] H. Steiner and N. Eickelpasch, Techniken und Erfahrungen bei der Stillegung des Kernkraftwerkes Gundremmingen Block A, 4 th Stillegungskolloquium, Bad Dürkheim, Germany, Nov. 1995. [

7] N. Adorni, G. Germani, A. Parmeggiani ..., Underwater Plasma-Arc Cutting as a Powerful Dismantling Tool for Water Reactors, Energie Nucleare 17 12, 695-706, 1970.

[8] D. Mewes and M. Creutz, Reinhaltung des Wassers, in annual report of "Sonderforschungsbereich 264", Project B6, DFG, Germany, 1994