ROUTE TO DRY OUT FROM ANNULAR FLOW BY APPLICATION OF EXTERNAL HEAT SUPPLY

Abstract

Numerical study of boiling has been performed around wire and also for internal annular flow inside a pipe in three-dimensional domain with the help of fixed mesh, incompressible, Volume of Fluid solver. First, to understand the basics of film boiling in presence of external heat supply in the present study pool boiling around a heated wire has been simulated at atmospheric pressure, considering the wire to be at the constant temperature heat source. Film dynamics, bubble nucleation, growth, merging and pinch off has been observed around perfectly smooth and uniform horizontal wire without any nucleation site. Effect of increase of the degree of superheat on boiling rate, measured in terms of vapour generation and bubble population has been studied, next. Similar studies have been made by inclining the wire at different angles (10°, 45°, 60° and vertical), where axial and azimuthal bubble sliding and more efficient bubble merging have been observed. Bubble size and population distribution have been investigated along the length of the inclined wire to link up with corresponding enhancement in boiling rate. For understanding the effect of active and passive neighbour, horizontal boiling around two-wire system has been studied. First horizontal wires have been stacked at the same vertical plane and then different active and passive combination have been simulated. Also boiling from two active wires placed at same horizontal plane has been studied for different wire spacings. For all these cases bubble life cycle has been investigated to understand the role of neighbouring wire. Effect of these neighbouring wires on boiling heat transfer and vapour generation rate has been reported. Ideas of bubble-bubble interaction will be quite helpful for understanding the route cause behind annular flow dryout. Taking queue from the previous effort, an annular flow boiling inside a tube has been also targeted as the next part of this study. Complex interfacial interactions like bubble nucleation, growth and merging underlying the liquid film has been studied beneath the liquid film of annular flow. Simultaneously wave generation, its subsequent deformation and entrainment in between the liquid film and vapor core has been obtained. Bubble bursting and rewetting of the pipe wall has been modelled with care. Due to boiling and bubble bursting liquid film is distorted into lamella and finally dry out has been observed. Azimuthal distribution of vi liquid film, bubbles and entrained liquid has also been presented in the cross-sectional view of the pipe at different axial location and temporal instants. Further effect of wall superheat, liquid and vapor velocities on the film evolution and dry out length has been clearly depicted. Dynamics of wavy film has been represented in terms of attractor plot of liquid phase fraction at different axial locations, which gives a better idea about the effect of parametric variations. Finally, axial variation of heat transfer coefficient for different cases has also been compared.