The gaseous diffusion coefficients of methyl bromide (CH3Br ) and methyl iodide (CH3I) into dry air, nitrogen and oxygen have been measured in the temperature range 303-453 K and at atmospheric pressure by the use of the Taylor dispersion method. Both for methyl bromide and methyl iodide, the diffusion coefficients do not vary in practice on substituting pure nitrogen or oxygen for dry air. The diffusion coefficients for methyl iodide are systematically smaller than those for methyl bromide by about 11%. For the methyl iodide-oxygen system, the effect of the thermal decomposition of methyl iodide has been observed at 453 K. The present results can be reproduced well by the functional form D = ATB, where D (cm2s-1) is the diffusion coefficient at 101 325 Pa (1 atm) and T (K) is the absolute temperature. The constants A and B are as follows: methyl bromide-(air, nitrogen, oxygen), A = 5.57×10-6, B = 1.76; methyl iodide-(air, nitrogen, oxygen), A = 5.26×10-6, B = 1.75.
Numerical simulations are performed for thermal and flow fields in a transient hot-wire thermal conductivity cell designed for the study of hydrogen gas at pressures up to 100 MPa. Two-dimensional unsteady incompressible Navier-Stokes equations are solved simultaneously with the continuity and energy equations. For hydrogen at atmospheric pressure, natural convection is found to have an almost negligible effect on the predicted wire temperature. At high pressures the onset of natural convection effects is predicted to occur in less than one second in agreement with an existing empirical correlation for the critical time for natural convection in a transient hot-wire cell.