A ROTATING 2-PHASE GAS-LIQUID FLOW REGIME FOR PRESSURE REDUCTION IN UNDERWATER PLASMA-ARC WELDING

Plasma arc welding processes are used in the off-shore industry for co nstruction and maintenance of underwater structures and pipelines in a wet environment. At greater water depths the density of the plasma ga s increases because of the greater hydrostatic pressure. This causes c onductive heat losses to the wet environment to increase. To maintain the energy flux to the workpiece to be welded, the plasma arc has to b urn in a local dry area with an inside pressure of 1 bar. This require ment can be fulfilled by a rotating cylinder with a liquid film flowin g down the inner wall. The flow around the rotating cylinder is experi mentally investigated. The rotating cylinder is placed above the work surface which is simulated by a flat plate. Because of the centrifugal forces of the rotating flow inside the gap between the lower end of t he cylinder and the flat plate the water is forced out of the cylinder . The velocity distribution in the flow is measured by laser Doppler a nemometry. The phase distribution in the two-phase flow in the gap is measured by local electrical probes. The static pressure inside the ga seous atmosphere. is reduced in comparison to the hydrostatic pressure of the surrounding water. The pressure reduction is given by the void fraction, the phase distribution and the volume flow rates of both ph ases in the gap as well as by the speed of revolution and the design o f the cylinder and the work surface. The influence of these parameters on heat transfer from the workpiece to the two-phase flow regime is a lso investigated.