Decreasing Wire Diameter Research Articles

Investigations on the critical heat flux of pure liquids and mixtures under various conditions

The effects of the diameter and of the orientation of electrically heated wires on their critical heat flux, both in saturated pool boiling and in surface boiling, have been investigated. A minimum in the peak flux on horizontal wires in saturated boiling occurs at a diameter of 100 μm. The peak flux at diminishing diameter increases rapidly as a consequence of a corresponding increase in the convective heat transfer. A similar effect has been observed in surface boiling, where the slope of the peak flux vs. subcooling curves for horizontal wires increases at decreasing wire diameter. Generally, the critical heat flux on horizontal wires in pure liquids exceeds the value on vertical wires. This is caused by a premature onset of film boiling in the latter case by the formation of vapour slugs at the upper end of the heater. Contrarily, the considerably higher peak fluxes occurring in certain binary mixtures (in coincidence with a slowing down of bubble growth at low concentrations of the more volatile component) are practically independent of the orientation of the heating element, because the Marangoni-effect diminishes the possibility of bubble coalescence. A quite different behaviour of the peak flux is observed on wires and strips, which are surrounded by a narrow coaxial tube: higher values occur on vertical heating elements now, as the possibility for bubbles to escape from the inside of the tube is minimized in the horizontal position. Some preliminary investigations on the peak flux in saturated pool boiling of aqueous ternary mixtures show the existence of a direct relationship to the behaviour of the basic aqueous binary mixtures.

The resistivity of fine platinum wires

We have measured the resistivity of pure platinum wires ranging in diameter from 16 mil to 0.3 mil in the temperature range 1.2 to 4.2°K, and observed size-dependent deviations from Matthiessen’s rule. The temperature dependent portion of the resistivity is dominated in this temperature range by a term of the formAT 2, whereA increases from about 12×10−12 Ω cm/°K2 for the thickest wires to 18×10−12 Ω cm/°K2 for the thinnest ones. There is an additional resistivity contribution which appears to increase more rapidly thanT 5, and which also evidences some increase with decreasing wire diameter. The observed deviations from Matthiessen’s rule display temperature and size variations consistent with the theory ofBlatt andSatz, and the magnitude of the deviations can be accounted for by this theory taking into account only that portion of the electrical resistivity produced by electron-phonon scattering. Thus the data are consistent with arguments suggesting that interband electron-electron scattering does not lead to size-dependent deviations from Matthiessen’s rule.