(46f) Composition and Temperature Gradient Measurements at Binary Mixture Boiling Bubbles
Nucleate Boiling is one of the most utilized processes to transfer heat to liquids in chemical engineering applications. This is due to the high heat transfer coefficient, which is the ratio of transferred heat and the temperature difference between the heated surface and the bulk fluid. In many configurations, the working fluid is often a mixture of liquids, where the heat transfer coefficient is often reduced compared to the pure components. This effect is stronger, the more the composition of liquid and vapor phase in the binary phase diagram deviate from each other. Then, composition gradients arise in the liquid surrounding the boiling bubble and the evaporation is limited by the mass transfer process1, 2. Nevertheless, the basic physical phenomena of nucleate pool boiling are not well understood for multi-component mixtures. To design plants, to validate numerical models and to apprehend the boiling process phenomena, the knowledge of the composition and temperature field surrounding a boiling bubble near the heated surface is of great impact. Laser-optical methods as Raman spectroscopy offer the possibility to measure these quantities without disturbing the system with high temporal and spatial resolution3, 4.
An optical accessible boiling chamber for the generation of only single bubbles was constructed to carry out measurements in mixtures of acetone and isopropanol. As the vapor-liquid equilibrium (VLE) of this system5 shows a large gap between the saturated liquid and vapor line, the boiling behavior differs strongly from a pure liquid. Many parameters, as mixture composition and bulk or heater temperature can be varied to study their influence on the boiling process.
A one-dimensional Raman spectroscopy setup6, 7 was applied to measure concentration and temperature gradients along a line of 3.2 mm in the liquid adjacent to the boiling bubbles. Due to the species specific Raman shift and the linear superposition of the inelastic scattered light intensities, qualitative and quantitative composition information can be achieved8. In water and alcohols, as isopropanol, the molecules can develop hydrogen bonds, which have an impact on the shape of the OH-peak in the Raman spectrum9, 10. 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. Extensive calibration measurements and a novel evaluation strategy enabled the simultaneous detection of composition and temperature, although the temperature sensitive shape of the OH-peak varied with mixture composition. The Raman spectra were recorded with an EMCCD-camera with a temporal resolution smaller than 1 µs and a spatial resolution of 160 μm. A CCD camera took a picture of the measurement volume to check whether a boiling bubble was present during acquisition of the Raman spectrum.
A species conservation calculation was carried out to validate the composition measurements. It revealed that the composition gradient is basically due to a demixing process in the spherical shell of liquid surrounding the boiling bubble. The temperature measurements indicated a heat transfer from the boiling bubble to the surrounding liquid due to condensation of vapor from the boiling bubble at its surface.
The authors gratefully acknowledge funding of parts of the project by the German National Science Foundation (DFG), which also funds the Erlangen Graduate School in Advanced Optical Technologies (SAOT) in the framework of the German excellence initiative.
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