Radiation Correction and Onset of Free Convection in the Transient Line-Source Technique (Experimental Observations in Fluids)

Abstract

The paper presents experimental observations on the radiation effects of lightly absorbent fluids in the transient line-source thermal conductivity measurement technique. The method of correction recommended by de Castro et al [13] was attempted. However, the present investigation shows that these corrections, which utilize a constant absorption coefficient (k), do not correct all the measurements over the total measurement period. For a given value of k, corrected measurements at large times as compared to those at short times do not agree. It is believed that a radiation correction model utilizing k which is a function of the conduction boundary layer thickness is more appropriate. An analysis of measurements in toluene is presented which shows the effect of radiation correction on temperature-time [ΔT-ln(t)] data for various values of absorption coefficient. The results are shown to be convection free by extension of the measurement period to very large times and powers and the criterion for the onset of free convection is obtained for liquid toluene.