Measurements of the thermodynamic equation of state via the pressure dependence of thermophysical properties of air by a thermal-wave resonant cavity

The thermodynamic equation of state for ambient air was investigated by means of a thermal-wave resonant cavity in the pressure range between 40 and 760 Torr and at ambient, or near-ambient, temperature conditions. The pressure dependencies of the thermal diffusivity, conductivity, effusivity, and thermal-wave-source infrared emissivity were measured. The experimental results were found to be consistent with the ideal gas law for air in the foregoing pressure and temperature range. It was observed that the thermal diffusivity of air increases linearly with decreasing cavity pressure. The experimental curves obtained from the four channels (amplitude, phase, in-phase, and quadrature) of the thermal-wave signal-demodulating lock-in amplifier were fitted to thermal-wave resonant cavity theory, and the thermal conductivity and effusivity of the air in the cavity were also calculated as functions of pressure. Within the experimental error range, the thermal conductivity was found to be independent of pressure and equal to (28.9 +/- 0.2) x 10(-3) W/m K at 309-310 K. The thermal effusivity of air exhibited a linear increase with increasing pressure at approximately constant ambient temperature. In addition, the infrared emissivity of the resistively heated Cr-Ni thermal-wave thin-film strip source (cavity wall) was measured as a function of the source rms voltage at several pressures. The obtained values, ranging from 0.094 to 0.108, showed that the emissivity decreases with decreasing cavity pressure. (C) 1998 American Institute of Physics.