COOL-DOWN OF A VERTICAL LINE WITH LIQUID-NITROGEN

Analytical and numerical modeling is presented for predicting the ther mofluid parameters of the cool-down process of an open-to-air vertical tube carrying liquid nitrogen. A two-fluid mathematical model is empl oyed to describe the flowfield. In this model four distinct flow regio ns were analyzed: 1) fully liquid, 2) inverted annular film boiling, 3 ) dispersed flow, and 4) fully vapor. These now regimes were observed in an experimental investigation constructed for validating the mathem atical model, and also in previous experiments by other investigators. For the single-phase regions, the one-dimensional form of mass, momen tum, and energy equations were used. For the two-phase regions, the vo lume-averaged, phasic one-dimensional form of conservation equations w ere applied. The one-dimensional energy equation was formulated to det ermine the tube wall temperature history. The numerical procedure is b ased on the semi-implicit, finite-difference technique. The calculatio ns for the inverted annular film boiling were performed implicitly. Th e computations for the tube wall, fully liquid, and dispersed flow reg ions were performed explicitly. In each region, the appropriate models for heat transfer and shear stress rates are used. Results and compar isons of the predicted numerical models with the experimental data for several constant inlet flow rates of liquid nitrogen into a vertical, insulated tube are presented.