| Two-phase flow instabilities can occur in various boiling and condensing flow systems and may cause violation of thermal limits, mechanical vibrations and failure of system components, hinder system control, affect normal operation, and influence system safety. Therefore, understanding of flow instabilities is important for the design, control, and performance prediction of any two-phase system.;In this work, both a linear frequency-domain model and a nonlinear time-domain stability model using the homogeneous and drift-flux two-phase flow models were developed and validated. The study shows that homogeneous model predictions are more conservative compared with the drift-flux model predictions. Both linear and nonlinear models were used to study the parametric effects on parallel channel and loop instabilities. The parameters studied in this thesis include channel length, diameter, inlet and exit flow restrictions, heated pipe inlet velocity, heated pipe inlet subcooling, heat flux and gravity. The parametric study shows that increasing single-phase pressure drop or decreasing two-phase pressure drop stabilizes the system. The relationship between parallel channel and loop instabilities are obtained in this work.;Complex phase change systems may experience various coupled modes of instabilities, and understanding which one is the dominant instability mode is critical for finding appropriate means to stabilize the system. Since Ledinegg, pressure-drop and density-wave instabilities are the three most important instabilities, in this thesis, the interactions among these three kinds of instabilities were investigated. If there is no compressible volume upstream of a boiling system, Ledinegg instability may occur when system operates in the negative slope region, which causes a sudden flow excursion to another operating condition at which higher order density-wave instabilities may occur. If a compressible volume exists upstream of a boiling system, in the negative slope region, when the operation point is close to the local minimum of the steady-state pressure drop - flow rate curve, there may be a pure pressure-drop oscillations region; when the operation point is close to the local maximum of the steady-state operating curve, there is a complicated transition region; between these two regions, pressure-drop oscillations superimposed with density-wave oscillations may occur. This thesis is the first work to theoretically study interacting modes of two-phase flow instabilities. In particular, pressure drop instabilities superimposed with density-wave instabilities were found, and the stability boundaries for various modes of instabilities were obtained. |
