Melt removal

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

Christiano, J. P. and Thompson, M. R., A New Barrier Screw Design Utilizing Solid Bed Deformation with Forced Melt Removal, SPE ANTEC Tech. Papers, 46, 78 (2000)... 

Boil the water only until the ice cube melts. As it melts, remove the meltwater with the sponge. 

During the pressure flow melt removal scheme a force is applied on the melting solid resulting in a pressure build-up within the melt film, causing the melt film to be squeezed... 

Schematic diagram of the melting process with pressure flow melt removal.

Melting with Pressure Flow Melt Removal... 

Cream TLC/ HPLC CHXL (1 2) Add MeOH, heat to 60°C until sample melts, remove and shake vigorously Silica CHCI,-acetone (7 1) 350 nm ... 

Conduction Melting with Forced Melt Removal, 201... 

Fig. 5.3 Schematic representation of drag-induced melt removal and pressure-induced melt removal mechanisms.

We shall see later that this combination of the key variables is also characteristic of conduction heating with phase transfer. The heat flux is infinite at t = 0, but quickly drops with the inverse of t1/2. Thus after 10 s it is only 30% of the flux at 1 s, and after 60 s, it is only 13% of the heat flux at 1 s The obvious conclusion is that conduction melting without melt removal becomes inefficient for anything but short times. 

The preceding examples discuss the heat-conduction problem without melt removal in a semi-infinite solid, using different assumptions in each case regarding the thermophysical properties of the solid. These solutions form useful approximations to problems encountered in everyday engineering practice. A vast collection of analytical solutions on such problems can be found in classic texts on heat transfer in solids (10,11). Table 5.1 lists a few well-known and commonly applied solutions, and Figs. 5.5-5.8 graphically illustrate some of these and other solutions. 

CONDUCTION MELTING WITH FORCED MELT REMOVAL... 

Removal of the melt, also discussed in Section 5.1, is made possible, in principle, by two mechanisms drag-induced flow and pressure-induced flow (Fig. 5.4). In both cases, the molten layer must be sheared, leading to viscous dissipation. The latter provides an additional, important source of thermal energy for melting, the rate of which can be controlled externally either by the velocity of the moving boundary in drag-induced melt removal or the external force applied to squeeze the solid onto the hot surface, in pressure-induced melt removal. 

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