Study Of Nucleation Site Density And Temperature Fluctuation At Nucleation Site In Nucleate Pool Boiling

A new method has been presented to predict the nucleation site density in nucleate pool boiling based on the investigation of the microstructure of boiling surface. The investigation is made by analyzing the two dimensional cross sections of boiling surface, which leads to the identification of gas trapping cavities and their dimensions as a result. A lot of data of the mouth diameters of the gas trapping cavities constitutes a data set. Many different measurement scales then are used to measure the data set. For a certain measurement scale, there is a corresponding measurement result which is division of the number of the data (in the data set) greater than the measurement scale by the area on the boiling surface the analyzed two dimensional cross sections correspond to. The measurement scale and the measurement result here can be viewed as the critical cavity diameter required for nucleation and the nucleation site density respectively because, according to classical heterogeneous nucleation theory, only those gas trapping cavities whose mouth diameters are greater than the critical cavity diameter can be activated to serve as nucleation sites. The above discrete data of measurement scales and measurement results are fitted by a mathematical formula which is used to predict the relationship between the nucleation site densities and the critical cavity radius. The prediction based on the above method agrees better with the experimental data than that based on traditional statistical method does and shows that it is the fractal relationship between the nucleation site density and the critical cavity radius.The edge detection methods in digital image processing have been employed to identify the nucleation site densities in pure component (R113) and binary mixture component (R11-R113) nucleate pool boiling in high heat fluxes through analyzing the bubbles images captured by a high-speed CCD-camera in the boiling experiments. In high heat fluxes, the images become blurring and it is very difficult for the naked eyes of people to identify accurately the number of the bubbles in the images. However, with the help of edge detection method, the blurring boundaries between the bubbles in the images captured in high heat fluxes have been detected; making the counting of the bubbles numbers more accurate. The difference between the numbers of the bubbles in the images and the numbers of the nucleation sites under the bubbles, which is mainly caused by the coalescences of the bubbles, has been considered to correct the detected bubbles numbers to get the actual numbers of the nucleation sites. The change of the nucleation site densities with the high heat fluxes has been plotted.Mathematical morphology has been used to identify the nucleation site densities in pure water and nanofluid (water plus Al₂O₃ nanoparticles) nucleatepool boiling in high heat fluxes through analyzing the infrared images reflecting the temperature distribution on the boiling surfaces. In high heat fluxes, plenty of dark spots (in the infrared images) corresponding to the nucleation sites aggregate together; it is impossible for human naked eyes to identify the spots numbers. However, after the morphological processing of the infrared images, the nucleation sites numbers are successfully identified. And the change of the nucleation site densities with the high heat fluxes has been plotted.Based on nonlinear time-series analysis technique, the fluctuations of the temperature at the nucleation sites in three different nucleate pool boiling experiments have been investigated. The entropies of the temperature-time series at the nucleation sites are calculated, which quantitatively describe the disorder character of the temperature fluctuation at the site. The two and three dimensional attractors corresponding to the time series are constructed, the correlation dimensions, quantitatively measuring the self-similar and complex character of the attractors, are calculated.