Application of Unsteady State Heat Transfer in Industry

An example of the unsteady behavior of such a system is the thermal spray process commonly used in coating industry.  Fine solid particles are injected into a stream of high temperature gas flow.  The particles are soften or melt before they strike onto the processed surface, providing a layer of uniform coating for protection, wear-resistance or as thermal barrier.  The process is very complicated involving high temperature, physics, fluid mechanics, material characterization, and unsteady heat transfer.  The following is a short introduction of a thermal spray process.                                                                 

        High Velocity Oxygen Fuel (HVOF) process involves the use of exhausted flame gas from a combustor to accelerate the injected coating particles to very high velocity (can be exceeding 1500 m/s well into the supersonic range).  The high energy impact of the high velocity particles will be enough to coat the material onto the surface and the completely melting (phase transition) of the particles is not required.   Therefore, it usually can provide a thicker and more uniform coating compared to other thermal process.  As ahown in the following figure, periodic structures (usually be called as shock-cells) exist inside high speed jet, as also evident from the picture following the schematic.

HVOF System from Metal Spray Supply, Inc.

Coating Process

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