This work consists of the studies of both hydrodynamic and thermal instabilities in boiling two-phase flow systems. In the first part, pressure-drop type oscillations and Ledinegg instability were studied both theoretically and experimentally. The second part of this work was concentrated on experimental studies of convective transition boiling and thermal oscillations.; In the experimental study of pressure-drop type oscillations, the effects of mass flow rate, heat input, inlet subcooling and the upstream compressible volume were investigated. The steady-state characteristics of system pressure-drop versus mass flow rate with stability boundaries, and typical oscillations in pressure, mass flow rate and wall temperature were obtained.; Numerical analysis based on the drift-flux two-phase flow model gave good predictions of the steady-state characteristics and oscillations of pressure and wall temperatures.; A planar model was developed for pressure-drop type oscillations and was analyzed from the perspective of dynamical system theory. A rigorous mathematical proof showed that the limit-cycle of pressure-drop type oscillations occurred after a supercritical Hopf bifurcation and the limit-cycle contracted to a stable equilibrium point after a reverse supercritical Hopf bifurcation. The stability criterion derived from the model was in very good agreement with the experimental results.; In extension to the planar model, a model including the oscillations in surge tank inlet flow rate was developed and analyzed. Bifurcation analysis produced more general results than those obtained from the planar model. This model also provides a unified bifurcation analysis for pressure-drop type oscillations and Ledinegg instability.; Experimental researches on convective transition boiling and thermal oscillations were conducted. Mechanism of thermal oscillation in the transition boiling region was studied. The effect of hydrodynamic instability on CHF was also studied.
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