Some Experimental Results of Free Convection From Horizontal Heated Wires Below Atmospheric Pressure

The cooling of horizontal wires by natural convection of air is investigated. Experiments are carried out with the use of an apparatus designed for thermal testing of electric cables for aerospace vehicles. Two wire diameters are considered: D = 0.155 mm and 0.593 mm with a length of about 2.45 m. Air pressure is varied from the atmospheric value down to 0.4 mbar, to which corresponds a Knudsen number ranging from 2·10⁻⁴ up to 2 while the Rayleigh number covers ten order of magnitude, from 10⁻¹⁰ up to 1. On the basis of the value of the Knudsen number, three regimes of heat transfer are identified: free convection in fully continuum fluid regime, the combined free molecule conduction close to the wire surface and free convection in the remaining space and the conductive regime at low pressure, when the rarefied gas is nearly quiescent. An attempt is also made to correlate the heat transfer results in all three regimes.