Experimental study of thermoconcentration convection in air–water and air–undecane mixtures

Abstract

A quantitative experimental comparison was conducted between thermal convection in dry air and thermoconcentration convection in two gas mixtures: air–water vapor and air–undecane vapor, within the temperature range of 0–80 °C at normal atmospheric pressure. Convection in these mixtures is driven by temperature and concentration gradients of water (or undecane) vapor in the air. The stationary thermoconcentration convection is accompanied by continuous phase transitions of the fluids. The quantitative results of the experiments are represented in terms of the Nusselt number Nu and the effective Rayleigh number RaE, which is the sum of the thermal RaT and concentration RaC Rayleigh numbers. Quantitative laboratory measurements were performed using the thermocouple method and were supplemented with qualitative data from visual monitoring of transparent fluid flows using holographic interferometry. The cubic and quadratic temperature dependencies of RaCRaT−1 were determined experimentally for moist air and for the air–undecane vapor mixture, respectively. The significant role of moisture phase transitions in air convection is established. Neglecting these effects at 25 °C and using the ordinary RaT instead of actual RaE would result in an error exceeding 30%. At 38 °C, this error would increase to nearly 100%. At around 80 °C, thermoconcentration convection becomes almost entirely concentration-driven, as the high molecular disordered thermal motion is suppressed by the ordered convective motion generated by evaporation and condensation of water on the opposite heat exchangers of the convective cell.