Characterization of Two-Phase Flow Morphology Evolution during Boiling via High-Speed Visualization

Nucleate boiling is an efficient heat transfer mechanism that enables the dissipation of high heat fluxes at low temperature differences. Heat transfer phenomena during nucleate boiling are closely linked to the two-phase flow morphology that evolves in time and based on the operating conditions. In particular, the critical heat flux, which is the upper limit for the nucleate boiling regime, can be triggered by hydrodynamic mechanisms resulting from interactions between the liquid and vapor phases. The aim of this thesis is to characterize the two-phase flow morphology evolution during nucleate boiling at high heat fluxes in two configurations: pool boiling, and confined and submerged two-phase jet impingement. The characterization is performed via non-invasive, high-speed optical based diagnostic tools.

Experimental characterization of liquid-vapor interfaces during boiling is often challenging because the rapidly evolving vapor structures are sensitive to invasive probes and multiple interfaces can occlude one another along a line of sight. In this thesis, a liquid-vapor interface reconstruction technique based on high-speed stereo imaging is developed. Images are filtered for feature enhancement and template matching is used for determining the correspondence of local features of the liquid-vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. To enhance the signatures of true matches, the correlation signals for each sampled pixel are averaged over a short time ensemble correlation. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields a set of points in the physical world representing the liquid-vapor interface. The developed liquid-vapor interface reconstruction technique is a high-speed, flexible and non-invasive alternative to the various existing methods for phase-distribution mapping. This technique also has the potential to be combined with other optical-based diagnostic tools, such as tomographic particle image velocimetry, to further understand the phase interactions.

The liquid-vapor interface reconstruction technique is used to characterize liquid-vapor interfaces above the heated surface during nucleate pool boiling, where the textured interface resulting from the boiling phenomena and flow interactions near the heated surface is particularly suited for reconstruction. Application of the reconstruction technique to pool boiling at high heat fluxes produces a unique quantitative characterization of the liquid-vapor interface morphology near heated surface. Analysis of temporal signals extracted from reconstructions indicate a clear transition in the nature of the vapor flow dynamics from a plume-like vapor flow to a release mode dominated by vapor burst events. Further investigation of the vapor burst events allows identification of a characteristic morphology of the vapor structures that form above the surface that is associated to the square shape of the heat source. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of the critical heat flux during pool boiling.

As compared to pool boiling, nucleate boiling can be sustained up to significantly higher heat fluxes during two-phase jet impingement. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. The connection between two-phase flow morphology and the extension of nucleate boiling regime is investigated for a single subcooled jet of water that impinges on a circular heat source via high-speed visualization from two synchronized top and side views of the confinement gap. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, the regular formation and collapse of vapor structures that bridge the heated surface and the orifice plate is observed, which causes significant oscillations in the pressure drop across. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl-like vapor structure that recurrently shrinks, due to vapor break-off at the edge of the orifice plate, and replenishes due to vapor generation. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure, as a fluid wall jet the keeps the heated surface wetted such that discrete bubbles continue to nucleate. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn-like regime, and some liquid droplets are sheared off into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater.

The flow morphology characterizations presented in this dissertation further the understanding of flow and heat transfer phenomena during nucleate boiling. In the pool boiling configuration, the vapor release process was quantitatively described; during two-phase jet impingement, a possible mechanism for critical heat flux was identified. Opportunities for future work include the utilization of image processing techniques to extract quantitative measurements from two-phase jet impingement visualizations. Also, the developed liquid-vapor interface reconstruction technique can be applied to a boiling situation with a simpler liquid-vapor interface geometry, such as film boiling, to generate benchmark data for validation and development of numerical models.