Heat transfer to a metallic wire exposed to a plasma flow for great Knudsen numbers

Analytical results concerning the heat transfer to a metallic wire exposed to a plasma flow for the extreme case of the free-molecule regime and a thin plasma sheath are presented. Expressions for the heat-flux components due to all the gas species (atoms, ions and electrons) and due to various different heat-transfer mechanisms including radiation and electron emission from the wire surface are derived. It is shown that the average heat flux per unit wire surface area is independent of the wire radius and increases with increasing gas temperature, pressure and speed ratio. At low wire temperatures, the contribution to the total heat flux due to radiation and thermionic emission from the wire surface can be ignored. However, the secondary electron emission due to ion bombardment may constitute a non-negligible heat-transfer mechanism. The predicted results of the heat flux are well consistent with corresponding experimental data for a fine tungsten wire exposed to an argon plasma flow of temperature about 9000 K at 500 Pa pressure.