Study Of Bubble Dynamics During Upward Subcooled Boiling Flow Of Liquid Nitrogen In A Vertical Tube

With the development of cryogenic technology, cryogenic liquid has been widely used. Because of its good performance, liquid nitrogen has become one of the most widely used cryogenic liquid. There are some empirical heat transfer correlations of liquid nitrogen, but most of them are confined either by applicable scope or by accuracy and cannot fulfill the development of cryogenic heat transfer. For this reason, it is necessary to adopt the wall heat partition model used by high boiling point liquids to liquid nitrogen subcooled boiling flow. When adopting wall heat partition model bubble departure diameter is an important parameter and has a great influence on the accuracy of the result. Though there are some empirical correlations about it, most of them are aimed at pool boiling heat transfer and cannot agree well in flow boiling. Based on this, the author employs the knowledge bubble dynamics to study the force balance model for liquid nitrogen in this paper.CFX was used in the calculation of this paper with two-fluid model. Wall heat partition model was employed as the boundary condition in two-fluid model. First, incorporate the empirical correlation of bubble departure diameter into wall heat partition model to calculate bubble departure diameter. Then examine the influence of outlet pressure, liquid subcooling, wall heat flux and inlet velocity on bubble departure diameter. The change of bubble departure diameter with these parameters was recorded. Based on this, implant the force balance model into the wall heat partition model, use the empirical correlation's results to calculate the magnitude of each force both parallel to the flow direction and vertical to the flow direction. By analyzing the trend of each force in different conditions, it was found that each force's change with working condition was rational and the reason was explained. Meanwhile, according to the relative magnitude of each force, it was found that the growth force was smaller than other forces obviously. Also, shear lift force is under evaluated. This proves that the correlation of growth force and shear lift force adopted in this paper cannot suit well to subcooled boiling flow of liquid nitrogen in a vertical tube. This paper gives some advice on how to revise the growth force and shear lift force.When analyzing the trend of each force with different working conditions, it was found that each force's change can be attributed to the change of bubble departure diameter or the relative change of bubble departure diameter and the velocity difference at the mass center of bubble. All of these phenomenons were caused by the change of velocity field and temperature field of two-phase flow, which can be solved by CFD easily. Besides, the use of two-fluid model guaranteed the accuracy of calculation results.