An Experimental and Analytical study on the Refilling and Rewetting of Hot Horizontal Tubes

Abstract

<p>This thesis contains the details of an experimental and analytical study on the refilling and rewetting of hot horizontal tubes. The experiments were conducted to refill and rewet two 25.4 mm 1.0., 3 m long zircaloy tubes (1 mm and 2 mm wall thicknesses) under a wide range of initial and boundary conditions. In addition to obtaining the quenching rates and the various parametric effects, the data played an important role in understanding the nature of the quench front propagation and identifying the associated flow patterns. Moreover, the data were used to obtain quench curves at different axial and circumferential locations. These quench curves helped in investigating the parametric effects on the different modes of heat transfer associated with the rewetting process as well as giving valuable insight into the rewetting mechanisms involved in the process.</p> <p>A rewetting criterion based on vapour film collapse in the film boiling region is proposed. This criterion suggests that surface rewetting is initiated when the vapour film thickness reduces down to a value equal to the sum of the amplitude of the liquid-vapour interface fluctuations and the solid surface roughness. Accordingly, a theoretical model for predicting the dynamic behaviour of the liquid-vapour interface in the film boiling region near the bottom of a horizontal tube was developed. This study showed that the hydraulic disturbances represented by the system pressure fluctuations could induce interface fluctuations capable of rewetting the surface when the equilibrium vapour film thickness was close to the experimentally estimated critical value.</p> <p>A two-fluid model incorporating the proposed rewetting criterion, as well as derived models for the partition factor and the film boiling heat transfer coefficient, was developed to predict the transients of the refilling and rewetting of hot horizontal tubes. This model is capable of predicting the experimentally observed thermal-hydraulic transients of the refilling "and rewetting processes.</p>