Measurement of Concentration and Temperature Gradients at Binary Mixture Boiling Bubbles

Abstract

Due to the high heat flux available, nucleate boiling is one of the most utilized processes for the transfer of large amounts of heat in chemical or power engineering applications. Nevertheless, the basic physical phenomena of this kind of heat transfer are physically not well understood, especially for multi-component mixtures in which the heat transfer coefficient is a function of the mixture composition. To apprehend the binary mixture boiling phenomena, the knowledge of the composition and temperature field surrounding a boiling bubble near the heater surface is of great impact. These quantities are measured at individual boiling bubbles by means of laser-optical methods without disturbing the system and with high spatial resolution. An optical accessible and temperature adjustable boiling chamber for the generation of single bubbles of acetone-isopropanol mixtures was constructed. As the vapor-liquid equilibriums (VLE) of these mixtures show a large gap between the saturated liquid and vapor line, significant composition alterations occur during the phase transition. Concentration and temperature gradients have been measured along a line by linear Raman spectroscopy. Due to the species specific Raman shift and the linear superposition of the inelastic scattered light intensities, qualitative and quantitative composition information can be achieved. In alcohols, e.g. isopropanol, the molecules can develop hydrogen bonds, which have an impact on the shape of the O-H bind signal in the Raman spectrum. As the ratio of molecules with and without hydrogen bonds changes with temperature, the temperature of the liquid phase can be derived from the spectra as well. The results show an enhancement of isopropanol, the less volatile component, near the phase boundary due to preferential evaporation of acetone. Furthermore, a not expected depletion of isopropanol approximately 0.75 mm away from the bubble was measured. The detected temperature increases near the boiling bubble, indicating a heat transfer from the gas phase to the surrounding liquid. The temperature distribution also has a minimum at the same position as the isopropanol distribution. A species conservation calculation with simplified assumptions was carried out and validated the measured composition distribution in the liquid surrounding a boiling bubble.