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Two Phase Flow Phase Change and Numerical Modeling Part 10

Tham khảo tài liệu 'two phase flow phase change and numerical modeling part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 260 Two Phase Flow Phase Change and Numerical Modeling constant-pressure-drop boundary condition the increase of exit pressure drop following the positive perturbation in inlet velocity that transforms into a wave of higher density will result indeed into an instantaneous drop in the inlet flow. The process is now reversed as the density wave resulting from the lower inlet velocity travels to the channel exit the pressure drop at channel exit decreases as the wave of lower density reaches the top resulting in an increase in the inlet flow rate which starts the cycle over again. With correct timing the flow oscillation can become self-sustained matched by an oscillation of pressure and by the single-phase and two-phase pressure drop terms oscillating in counter-phase. In accordance with this description as a complete oscillating cycle consists in the passage of two perturbations through the channel higher density wave and lower density wave the period of oscillations T should be of the order of twice the mixture transit time T in the heated section T 2t 2 In recent years Rizwan-Uddin 1994 proposed indeed different descriptions based on more complex relations between the system parameters. His explanation is based on the different speeds of propagation of velocity perturbations between the single-phase region speed of sound and the two-phase region so named kinematic velocity . This behaviour is dominant at high inlet subcooling such that the phenomenon seems to be more likely related to mixture velocity variations rather than to mixture density variations. In this case the period of oscillations is larger than twice the mixture transit time. 2.1 Stability maps The operating point of a boiling channel is determined by several parameters which also affect the channel stability. Once the fluid properties channel geometry and system operating pressure have been defined major role is played by the mass flow rate r the total thermal power supplied Q and the inlet .