Experimental, theoretical and computational modeling of flow boiling, flow condensation and evaporating falling films
The transition from single-phase to two-phase thermal systems in future space vehicles demands a thorough understanding of phase change methods in reduced gravity, including microgravity. In this study, phase change methods like flow boiling, flow condensation and evaporative falling-films are investigated experimentally, theoretically and computationally. The experimental part of the study consists of an investigation of the influence of inlet subcooling and two-phase inlet on flow boiling heat transfer and critical heat flux in a horizontal 2.5-mm wide by 5-mm high rectangular channel in different orientations with respect to Earth gravity using FC-72 as working fluid. High-speed video imaging is used to identify dominant interfacial characteristics for different combinations of inlet conditions and heating configurations. Gravity is shown having a dominant influence on interfacial behavior at low mass velocities, while inertia dwarfs gravity effects at high mass velocities. CHF variation between different orientations with respect to Earth gravity is large for low mass velocities and diminishes for high mass velocities. In the theoretical part of the study, a consolidated investigation of the complex trends of flow boiling CHF in a rectangular channel in both microgravity and for different orientations in Earth gravity are performed. Separate theoretical models are constructed to investigate subcooled inlet flows and saturated two-phase inlet flows. It is shown that the Interfacial Lift-off Model provides good predictions of CHF data for both gravitational environments, both single-sided and double-sided heating, and both subcooled and saturated inlet conditions. In the computational part of the study, CFD models are constructed for two separate phase change configurations. First, turbulent, free-falling liquid films subjected to evaporative heating, and second, annular flow condensation in vertical upflow configuration. Implemented in FLUENT, the models are used to predict variations of various flow and thermal parameters and compare the results with available experimental data. Energy transfer at the two-phase interface are implemented successfully with the aid of appropriate phase change models. For both phase change configurations, the CFD model was able to capture complex flow behavior observed in experiments and predict heat transfer coefficients with reasonable accuracy. Also included in this part is a comprehensive review of literature on computational modeling of various boiling and condensation applications. This part of the study is laying the groundwork for future implementation of CFD models in capturing more complicated flow boiling and CHF phenomena.
Mudawar, Purdue University.
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