A two-phase, two-component bubbly flow model

The detailed simulation of fast transient two-phase flows is a yet a well-solved problem in spite of its practical importance and the progress in research for the last several decades. Indeed, such fast transients appear in many different industrial applications and processes with a variety of different initial and boundary conditions, different fluids, and different thermodynamic conditions. Particularly two-phase pipe flows have received much attention, not only because of their economic importance for the oil and nuclear industry. Although the flow in straight pipes is essentially one-dimensional and therefore relatively simple, modelling the phenomena is impeded by questions such as proper formulation of the governing equations. Over the years this has led to the development of a multitude of computer codes with different levels of accuracy and physical correctness of the underlying models. This thesis is focused on one-dimensional models for fast transient flows in a kinematic non-equilibrium. Besides the thermodynamic non-equilibrium, there is another type of non-equilibrium which appears to be very important in two-phase flow: the kinematic non-equilibrium or drift between the phases-.. Such flow models include bubbly gas/liquid flows which are characterized by strong coupling between the phases, due to the rapid interphase transfers of mass, momentum and energy. As a consequence the assumptions that the phase pressures and the phase temperatures are equal at any cross-section appear consistent with experimental observations. The set of equations includes a momentum equation which has the form of a relaxation law of the drift velocity. This equation is based on a simplified version of the so-called Voinov-Berne equation for the momentum of the gas in a bubbly flow. The ability of the model to predict steady state critical flows is tested first. This is done by means of an analysis of the sensitivity to variations of the main parameters, and also by comparing the results with two sets of original experimental data on air-water critical flows. Finally, the model is tested in transient conditions modelling the water hammer phenomena. (author)