The thermal conductivity of mixtures of nitrogen, ammonia and hydrogen

Abstract

The thermal conductivity of binary and ternary mixtures of nitrogen, ammonia and hydrogen has been measured at temperatures in the range 25 to 149°C. The apparatus used was of the hot wire type as modified by Callear & Robb, with accommodation effects almost eliminated by the use as thermometer not of the same wire as served as a source of heat but of a second wire. The accuracy of measurement was about 1% . For every binary mixture the dependence of thermal conductivity on composition (expressed in terms of mole fractions) was far from linear, and for mixtures of nitrogen and ammonia there was a composition for which the thermal conductivity was a maximum. A comparison with results obtained by earlier workers showed that the measurements of Gruss & Schmick for mixtures containing ammonia are reliable, as are those of Wassiljewa for mixtures of oxygen and hydrogen, but the results of Ibbs & Hirst for mixtures of nitrogen and hydrogen are to be rejected. The dependence of the thermal conductivity K on the mole fractions x i has been discussed in terms of the equation K = Ʃ i { K i /(1 + Ʃ j A ij x j / x i )} due to Wassiljewa. This equation, with A ij assumed independent of composition, represented the results for binary mixtures excellently. Within the limits of error attainable experimentally, there was no discernible dependence of A ij on temperature. An extrapolation of the present results to higher temperatures should therefore be fairly reliable; the thermal conductivities of the pure components are the only quantities which would have to be measured at the high temperatures concerned. A semi-empirical rule, due to Lindsay & Bromley, for predicting values of A ij , was found to give satisfactory results. The remarkably wide applicability of this rule to mixtures of polyatomic gases has been discussed. The results for N 2 + H 2 at the lowest temperature (25°C) have also been compared with the more rigorous theory of Hirschfelder. Wassiljewa’s equation, with values of A ij derived from the results for binary mixtures, gives almost as good an account of ternary mixtures, though there appear to be significant, if small, discrepancies. It follows that measurements on binary mixtures suffice to predict with reasonable accuracy the thermal conductivity of ternary mixtures. This, coupled with the approximate temperature independence of A ij , has important implications. For example, in combustion problems reliable estimates of thermal conductivities of mixtures at elevated temperatures are constantly needed.