Visualization Of Flow Boiling Of LN₂ In Micro-tubes

For the applications in micro cryogenic surgical apparatus (MCSA), the cooling of high temperature super-conductivity (HTS) and other fields, the flow pattern of flow boiling of liquid nitrogen in micro-tubes has been investigated by the help of high-speed digital camera.The illumination and magnification of the micro-tube are two difficulties in two-phase flow experiments in micro-tubes in cryogenic temperature. Illuminating outside the dewar and macro-photography are adapted to solve the two problems and high-quality images of flow patterns are acquired. High speed digital camera is employed to acquire the typical flow patterns of LN2 flow boiling in vertical upward tubes with the inner diameters of 1.931, 1.042 and 0.531 mm. The flow patterns are bubbly flow, slug flow, churn flow and annular flow. Confined bubbles are observed in 1.042 mm and 0.531 mm tubes. The deposition of liquid droplet in churn flow is observed. Flow regime maps are depicted according to experimental results and the surface tension and diameter are considered to be the major factors affecting the flow pattern transitions. It is found that the slug/churn and churn/annular transition lines of the present experiment shift to lower superficial vapor velocity; while bubbly/slug transition lines to higher superficial vapor velocity compared to gas-water flow regime maps with the similar hydraulic diameters. The corresponding transition lines move to higher superficial velocity when reducing the inner diameter of the tube. The results indicate that no existing theoretical model can explain the flow regime maps in the present experiment.Void fraction is measured quantitatively and the results are compared to various models in which the model of Woldesemayat and Ghajar gives the best prediction. The homogeneous model over-predict the results and the probable explanation is that slip ratio in mini-tubes is much higher than that in normal-sized tubes in which homogenous model can give good predication. It is also found that the slip ratio is greater when the diameters of the tubes decrease.Periodical oscillations are observed and the flow patterns are recoreded. For the oscillations in the 1.042 mm tube, the mass flux is out of phase with the heat flux and the wall temperature is in phase with the mass flux. It appears annular flow and single liquid phase most of the time during the period. Intermittent flow patterns occupy limited time. The oscillation period prelongs when the the outlet mass quality of the micro tube increases. The oscillations may disappear when the heat flux increases or the mass flux decreases.Three kinds of two-phase flow oscillation are observed in the 1.931 mm tube and the similar characteristics of all the oscillations are: the mass flux and pressure drop are strictly out of phase with a phase angle of 180°; wall temperature and inlet liquid temperature oscillate in phase, but both of them are roughly out of phase to mass flux.Comparative experiments are carried out to explore the mechanism of the above oscillations. The results show that the oscillations are pressure-drop oscillation essentially. The corresponding theory model is proposed for the first time and agrees well with the experimental results.