An Experimental Study Of Flow Boiling Heat Transfer Enhancement In Minichannels With Porous Mesh Heating Wall

Author : Xiang Zhang
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (14 download)

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Download or read book Experimental Investigation on Multiscale Interactions in Intermittent Flow Boiling and a New Hybrid VOF-Lagrangian Modeling Approach written by Xiang Zhang and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In flow boiling, most phase change heat transfer occurs at the smallest scales: in 1 nm -- 10 [mu]m thick liquid microlayers under nucleating bubbles and in thin thermal boundary layers around such bubbles. Nucleating bubbles evolve from microscopic cavities on heated surfaces (reaching 104 - 106 sites per cm2). Theoretical closure models have been proposed for individual bubble nucleation, growth, and departure due to hydrodynamics. However, high-fidelity integration of such microscale models and theory with full channel flow processes has proven challenging due to the large range of length and time scales. Thus, contemporary boiling engineering still relies on empirical correlations. While such models are validated by large experimental databases, they are limited to specific fluids and geometries. More rigorous descriptions are needed to predict transport with emerging configurations and fluids. The first objective of this study is to experimentally characterize the coupling between micro-scale transport and large-scale hydrodynamics in flow boiling heat transfer through high-speed photography and thermal imaging. A new hybrid volume-of-fluid (VOF)-Lagrangian multi-scale simulation approach is then proposed to directly resolve and couple these processes. Experimental data from this investigation and other publications are employed to assess this modeling approach. First, a square air-oil bubble column facility is built to track the bubble trajectories at free surface and assess the hybrid VOF-Lagrangian simulation approach. High speed videos are captured from both the front and top view to measure flow parameters including: bubble size distribution (BSD), bubble rise velocity in vertical direction, as well as bubble lifetime and propagation velocity at the free surface. A hybrid VOF-Lagrangian solver is developed to characterize the coupling between micro-scale bubble transport and macro-scale hydrodynamics in such multi-scale two-phase flows. The Lagrangian model tracks the trajectory of individual injected discrete small bubbles, accounting for effects such as buoyancy, pressure, virtual mass, drag, and turbulent dispersion. Once bubbles exceed a threshold packing density or overlap with VOF structures, they are converted to the grid-scale vapor phase. An empirical bubble-lifetime model is implemented to account for the finite coalescence times of bubbles at free surfaces. Contributions of this effort include programmable closure for bubble lifetime at the free surface (before popping/coalescence), a pinning force method for bubbles at the free surface, and Lagrangian-to-VOF transition of bubbles based on packing density. Next, a two-phase flow boiling experimental facility is developed to collect simultaneous high-speed visualization and IR temperature distribution data. The test cell channel is 420 mm long with a 10 mm square-cross section. A transparent ITO coated sapphire window serves as a heater and IR interface for measuring the internal wall temperature. The facility is charged with a low boiling point fluid (HFE7000) to reduce uncertainties from heat loss. Vertical saturated flow boiling wake-nucleation interaction experiments are performed for varying volume flow rates (0.5 -- 1.5 L min-1, laminar-to-turbulent Re) and heat fluxes (0 -- 100 kW m-2). Discrete vapor slugs are injected to explore interactions with nucleate boiling processes. By measuring film-heater power, surface temperature distributions, and pressures, local instantaneous heat transfer coefficients (HTC) are obtained. A conjugate gradient method is implemented to solve the 2-D transient inverse heat conduction problem and estimate the non-uniform heat flux distribution. For nucleate flow boiling without slug injection, visualizations show that both bubble cycle and wait time are inversely proportional to the applied heat flux. The average wall temperature displays a linear relationship with heat flux. Moreover, bubble departure diameter tends to increase with increasing the bubble nucleation temperature, and stabilizes at higher temperature. With respect to the wake-nucleation interaction experiments, it is found that small nucleate bubbles only in the core region of Taylor bubble body with a certain bubble size (1.4 to 1.6 mm) would merge into the vapor slug. IR data indicate that the wake shear effect is divided into 5 stages during the whole process. Wake heat transfer enhancement is observed due to the vortex shedding and intense mixing between in the Taylor bubble wake. In addition, the nucleate boiling is firstly suppressed in the Taylor bubble body due to the falling liquid film, then recovered after the wake region. To efficiently simulate such processes, the hybrid VOF-Lagrangian model is extended to account for heat transfer, bulk- and Lagrangian-bubble-scale phase change, and nucleation site processes. The approach tracks discrete nucleation sites on walls and simulates a contact line pinning force. Bubble growth and departure is directly predicted using grid-scale VOF velocity and temperature data rather than idealized conditions. This method is first verified with literature data for pool boiling. It is then qualitatively assessed for the Taylor flow boiling processes studied experimentally.