Visual Observation Of Nucleate Boiling In Liquid Nitrogen And Improvement Of Its Heat Transfer Model

Cryogenic liquid with a saturation temperature much lower than room temperature and small latent heat of vaporization is apt to boil in case of heat leak or internal heat resource. Compared with the liquids with near ambient temperature, such as water and methanol, the experiments on the boiling of cryogenic liquids, especially the observation procedure, are more complex and difficult due to the obligatory insulation structures for reducing heat leakage from the surroundings. Even though the results from common liquid's boiling can be extended to the cryogenic liquids, there are many empirical parameters still requiring the verification of its applicability. The high heat flux boiling in nucleate boiling can achieve better heat transfer efficiency than other regions due to its large heat transfer coefficient with a small temperature difference. The flow and temperature field is very complex due to the production and coalescence of the large numbers of bubbles. There are still many problems to resolve. Therefore, the high heat flux boiling of the cryogenic liquids was scheduled to be studied in this work.Firstly, the evaporation process of macro-layer in the high heat flux boiling was analyzed, followed by an investigation of macro-layer evaporation model and the parameters in the model. It was found that the applicability of the physical model and empirical parameters for boiling process still needs further verification for the cryogenic liquid such as nitrogen. On this basis, according to the characteristics of the liquid nitrogen and the measured objects, the visualization and heat flux control methods were carried out to monitor the boiling phenomena on the heated surface, referring to the relevant literatures. The experimental program was then developed, and the experiment apparatus was designed and built. The parameters in the macro-layer evaporation model, such as the starting point of high heat flux boiling, the detachment frequencies of coalesced bubbles and the critical heat flux, was obtained from the measuring results. The influences of the boiling surface materials and the surface diameter on the critical heat flux and the corresponding superheat were discussed. The thickness of macro-layer and the active site density were then calculated from the equations of macro-layer evaporation model with the help of the experimental results. The influence of the boiling surface materials on the active site density was also analyzed. Finally, a boiling heat transfer model of liquid nitrogen at high heat flux was established, with which the calculation results can fit well with the results from experiments.