Investigation On The Mechanism Of Transient Process Of Two-Phase Flow Excursion

Two-phase boiling heat transfer has been widely applied in various industrial fields because of its considerable heat dissipation potential.However,thermal-induced two-phase flow instabilities often occur in practical processes,which can cause a great restriction for the heat transfer capacity and even a fatal threat for the system safety.In this thesis,study on flow excursion instability of forced two-phase flow in a single channel is carried out,nonlinear dynamic stability analysis and experimental study are used,dynamic instability mechanism of the transient flow excursion process is revealed,and active control methods are put forward.The specific research contents and results are listed as follows:Firstly,concerning the local instability with small perturbations,the system model consisting of nonlinear second-order ordinary differential equations is established based on the continuation method.The codimension one bifurcation diagram is calculated using the bifurcation theory.It is revealed that saddle-node bifurcation points,tangent points between the internal and external curves,and the onset of flow instability are equivalent.It is not a sufficient occurring condition of flow excursion that the operating state crosses over the minimum point of the internal curve.Furthermore,two separate stable branches are obtained on the multi-valued characteristic curve by using two saddle-node points as demarcation points.Flow excursion can be represented via the step change of mass flow rate between these two stable branches.Besides,the unidirectional and irreversible nature of flow excursion determines the existence of excursive hysteresis which means that the final steady-state system solutions depend on the historical evolution process.Secondly,with respect to the global instability with large perturbations,the relationship between perturbations and state points on the phase plane is established.The attraction domain of the fixed equilibrium point is introduced to evaluate the effects of large perturbations.The system model consisting of nonlinear second-order ordinary differential equations is established using constant driving force as the boundary condition.The global stability analysis is carried out via the numerical phase portrait method.In view of the global phase portrait,the existence of the boundary of attraction domain is verified,the typical characteristics of the boundary of attraction domain are obtained and the mechanism of large perturbations on flow excursion is revealed.By combining with the instability mechanisms with small perturbations,the dynamic mechanism of flow excursion instability is produced in terms of the boundary of attraction domain.Furthermore,experimental study is conducted on transient characteristics of flow excursion during the excursive hysteresis process.It is observed that flow oscillations trigger the occurrence of flow excursion premature to the saddle-node point.By combining the experimental data with the theoretical model,the position of the boundary of the attraction domain is calculated,a general function of the boundary of the attraction domain is constructed,the mechanism of flow excursion caused by flow oscillation is revealed and the dynamic mechanism of flow excursion instability is verified.In addition,through the typical characteristics of the systematical hydrodynamic multi-valued characteristic curves obtained via experiments,an approximate prediction method of the hot flow resistance is proposed based on the cooled flow resistance,which can be used to quickly grasp the two-phase flow characteristics under heated conditions.Finally,in order to achieve the active control of flow excursion,the theoretical formula between the extreme points of the hydrodynamic multi-valued characteristic curve is deduced,combined with the empirical prediction formula of the minimum point,and a semi-empirical correlation of the parameter boundary of the flow excursion instability is obtained,and then a control method concerning the adjustment of subcooled parameters is put forward.Besides,combined with the parameter boundary,a control method concerning subcooled parameters and perturbation excitation is proposed based on the boundary of the attraction domain.