Research On Heat Transfer Characteristics Of Falling Liquid Film Based On Planar Laser-induced Fluorescence

Falling film flow,which is defined as fluid falling along the smooth vertical pipe surface and under the action of gravity alone,is widely used in various industrial processes owing to its efficient heat and mass transfer characteristics,such as evaporation and purification of fluid,nuclear reactor cooling and so on.The visualization of falling film temperature field and the study of heat transfer characteristics are helpful to understand the heat transfer mechanism,which are of great significance to improve energy efficiency and equipment performance.In this work,based on planar laser-induced fluorescence(PLIF)technique,a non-intrusive,high spatial-temporal resolution measurement of falling film temperature field in a vertical pipe is realized,and its heat transfer characteristics are quantitatively analyzed.In the process of liquid film temperature calibration,an optimal fluorescence intensity extraction method based on the maximum variance between classes is proposed,which effectively improves the calibration accuracy and achieves the visualization of liquid film temperature field.Based on the liquid film temperature field and heat transfer theory,the convective heat transfer coefficient and Nusselt number are calculated.The turbulent falling film heat transfer correlation is established and compared with a large number of liquid film heat transfer correlations.The specific work includes the following aspects:The feasibility and advantages of PLIF for the study of visualization and heat transfer characteristics of falling film temperature fields is analyzed.The high speed image acquisition system based on PLIF liquid film heat transfer research is constructed.The temperature control accuracy is improved by the drive module,embedded temperature control algorithm and upper computer interface optimization,and the high precision measurement of liquid film temperature field is achieved.The liquid film fluorescence image preprocessing is realized according to the digital image processing technology,and the optimal fluorescence intensity extraction method based on the maximum variance between classes is proposed by analyzing the gray level data of the fluorescence image.The calibration curve with good linearity is obtained by the least square fitting,and the visualization of liquid film temperature field is realized eventually.The heat transfer coefficients and Nusselt numbers of liquid film under different experimental conditions are quantitatively analyzed by the measured temperature field and the liquid film convection heat transfer theory,which is based on temperature boundary layer.A large number of liquid film heat transfer correlations and their application conditions are summarized.The turbulent falling film heat transfer correlation is established with the experimental data,and the liquid film heat transfer characteristics are analyzed by comparing with different heat transfer correlations.