An experimental investigation of heat transfer from fine wires to still air at low density

Heat transfer from electrically heated wires of 0.00025 cm nominal diameter to still air at room temperature has been investigated at pressures ranging from 1 to 0.0076 atmospheres. Three wires, having ratios of length to diameter in the approximate proportion 1:5:10, were tested in the vertical position. Due to time limitations, only the shortest of these was tested in a horizontal position. A check was made to determine whether the results were influenced by the geometry of the enclosing vessel. For pressures at which the molecular mean free path is smaller than the wire diameter, the results appear to satisfy a relation of a form derived for free-molecule heat conduction. Small departures from this behavior at the higher pressures are attributed to the effects of natural convection. The Nusselt number was found not to be uniquely related to the product of the Grashof and Prandtl numbers, as has been proposed, for values of this product below 10[^-5]. It was found that, at the lowest pressures reached, solid boundaries located at distances of the order of 3 x 10[^4] diameters from the wire cannot be considered infinitely remote