Effect of absorption of radiation on thermal conductivity measurements by the transient hot-wire technique

Abstract

The paper presents an analysis of the effects of the absorption of radiation by fluids during measurements of their thermal conductivity by the transient hot-wire technique. The full integro-partial differential equation governing the simultaneous conduction and radiation in a transient hot-wire cell is simplified by means of a small number of physically reasonable assumptions and solved numerically. The numerical solution has been employed to deduce the effect of radiation absorption on the wire temperature rise in measurements of three normal alkanes. Absorption of radiation is shown to produce changes in the wire temperature rise which are comparable with the best available resolution in its measurement and are therefore not directly discernible. Nevertheless, the contribution of radiative transport to the transient heating process means that the thermal conductivity derived from such measurements is systematically in error by as much as 2.5% at 75°C. A procedure whereby thermal conductivity data may be corrected for the effects of radiation is described and the correction factor given for n-heptane, n-nonane and n-undecane in the temperature range 35–75°C and the pressures in the range 0.1–500 MPa.