STUDY OF ENTRAINMENT DYNAMICS WITH OR WITHOUT PHASE CHANGE EN ROUTE TO PROCESS OF DRYOUT AROUND HEATED ROD BUNDLE

Abstract

Numerical modelling of entrainment phenomenon in annular flow is carried out with and without phase change situation to lead towards dryout prediction in rod bundle of Boiling Water Reactor (BWR) as a part of research conducted for Advanced Heavy Water Reactor (AHWR) under 3rd Generation of Indian Nuclear Power Generation Programme. First, numerical modelling of two-phase annular flow in adiabatic situation has been carried out inside a tube of 11 mm diameter using computational multi-fluid dynamics Volume of Fluid is incorporated for capturing the interface and surface tension is modelled with continuum surface force (CSF). The proposed numerical methodology has been validated with available experimental data in literature. Three different cases of varying gas-liquid relative flow rates have been considered for the simulation of the present study. The mechanism behind evolution of the disturbance wave generation has been explained with 2D and 3D contour of gas-liquid flow. Bulging of liquid at disturbance wave is illustrated which in turn results in droplet entrainment and deposition. Local reversal of velocity at liquid film above the disturbance wave is observed and wall shear stress has been calculated to substantiate the same. The liquid film thickness over various sections of the tube has been plotted over time to predict wave frequency and average film thickness. The liquid film thickness has shown decreasing trend over the axial distance. Wave velocity is found to be decreasing with increase in liquid inlet flow rate. Next, numerical modelling of two-phase annular flow boiling is carried out inside a tube using volume of fluid based computational multifluid dynamics. Entrainment from liquid-vapor interface and boiling from the solid-liquid interface are captured from basic hydrodynamics before predicting the route towards the dryout condition. The stages of bubble nucleation, growth, merging, bursting, droplet entrainment, film rewetting, and dryout has been clearly visualized for diabatic annular mist flow. The bubble evolution was quantified in terms of bubble radius and contact length over space and time. To signify the dryout condition, plots of liquid phase fraction and average heat transfer coefficient were plotted which shows decreasing trends with space. Effect of different wall degree of superheats and gas-liquid velocities at a higher working pressure of 40 bar have been studied, which can be commonly be observed in BWR conditions. Dryout is found to happen early in the axial length if the degree of superheat and flow velocities increases. xxxi The proposed numerical methodology is used to track the interfacial dynamics of dryout mechanism for flow boiling around heated cylindrical rod. The simulations are carried out for various combinations of water and water vapor velocities, wall temperatures, and heat flux conditions at a representative higher working pressure of 40 bar, which is usually observed. Various mechanisms of dryout, such as disturbance wave generation, droplet entrainment, deposition, rewetting, nucleation beneath the thin annular film, bubble growth, merging, bursting, etc. are illustrated with pictorial representations. The liquid phase fraction, wall temperature, and heat transfer coefficient are used to quantify the dryout characteristics. The dryout is observed for a range of liquid-vapor velocities and heating conditions. Finally, a significant difference in flow boiling for the concave surface inside tube and the convex surface outside tube is shown with heat transfer coefficient plots. Further, numerical investigation is carried out for understanding of interfacial dynamics during co-flow of vapor and liquid phases of water inside a typical Boiling Water Reactor (BWR), consisting of a nuclear fuel rod bundle assembly of 7 pins in a circular array. Two representative spacings between rods in a circular array are used to carry out the simulation. This work will portray near realistic vapor bubble and liquid flow dynamics in rod bundle scenario. Constant wall heat flux for fuel rod and uniform velocity of the liquid at the inlet patch is applied as a boundary condition. The saturation properties of water are taken at 30 bar pressure. The dry out phenomenon with no liquid presence is numerically observed with phase fraction contours at various axial cut-sections. The quantification of the liquid phase fraction at different axial planes is plotted over time, emphasizing the progressive dry-out mechanism. A comparison of liquid vapor distribution for inner and outer rods reveals that the inner rod's dry-out occurs sooner than that of the outer rod. The heat transfer coefficient to identify the heat dissipation capacity of each case is also reported.