In-situ infrared observation of molecularclustering near cooled surface

Abstract

For the saturated vapor condensation on the cooled surface, the evolution mechanism of vapor molecular in the transition zone between the bulk phase and the cooled surface is not clear yet. The molecular clustering model considers that the vapor molecules first form clusters in the gas phase before condensing on the cooled surface. However, it is difficult to observe the dynamic evolution of nanoparticles in the near-wall boundary layer, hence, the experimental verification about this model is not sufficient now. Based on the hydrogen bonded network formed inside the cluster, in this paper, the attenuated total reflection Fourier transform infrared spectroscopy is introduced to follow and detect the dynamic behavior of vapor molecules in the near-wall thin layer during the condensation process. The infrared spectra of the gas phase at different positions from the cooled surface during the condensation process are obtained. The experimental results directly verify the distribution of clusters in the near-wall region, indicating that clusters are the main units of vapor condensation and droplet growth. Moreover, the average cluster size <i>n</i> increases gradually along the direction near the cooled surface. Based on the hydrogen bond characteristics of clusters, the ethanol molecular clustering near the surface is also observed, which further verifies the rationality of this model. In addition, it’s found that the distribution region along the cooled surface of ethanol clusters during the process of condensation is smaller than that of water clusters under the same condition. This may indirectly indicate that the heat transfer boundary layer of ethanol vapor condensation is thinner than that of water vapor condensation, resulting in its weaker performance of heat transfer. This method, where we use the microstructures manufactured on the surface to regulate the distribution of clusters in the near-wall region, will provide a new insight into enhancing the process of steam condensation with non-condensable gas or efficient water capture from air.