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  and . 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 . This text focuses on the sucking process only.
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.
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 .
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