The fundamental understandings of nucleate pool-boiling heat transfer (NPHT) is crucial to develop efficient thermal management systems of thermoelectric power generation, miniaturized electronic systems, and industrial processes. The upper heat flux limit is related to the pre-mature surface dryout, also known as the Critical Heat Flux (CHF), and this is related to numerous interlinked design/operation parameters including heater size, shape, surface wetting, working fluid properties, system pressure, hydrodynamic instability wavelength, and heater orientations. One key parameter that controls CHF is critical liquid-vapor instability wavelength, i.e., hydrodynamically-controlled liquid chocking limit. Numerous studies have shown that the critical instability wavelength can be tailored using engineered surface at the horizontal heater surface, i.e., upward facing heater surface. However, the in-depth study of the tailored CHF on the tilted heater surface is rare, although the understandings of enhanced CHF on such surface orientations, i.e., 60°-130°, are crucial to delay the catastrophic surface dryout for system design flexibility and safety. To understand the enhanced CHF on tilted heater surfaces via tailored hydrodynamic-instability wavelength, columnar-post sintered-particle wicks are employed, where the tailored hydrodynamic-instability wavelength can be achieved via efficient liquid-vapor phase separation. The columnar post wicks have array of cylindrical liquid permeable post wicks, where the liquid supply enhances via the capillary flow through the post wicks and the produced vapor efficiently escapes through pore space among the post wicks. The diameter of the columnar-post wick is dcp&nbsp;= 1.25 mm, and the center-to-center distance between the columnar post wicks, i.e., pitch distance, varies from lp = 2.5 mm to 7.5 mm. The enhanced CHF from the tailored hydrodynamic instability wavelength via the columnar post wicks are predicted using a modified interfacial lift-off CHF hydrodynamic model that incorporates classical two-dimensional interfacial instability theory, a separated flow model, an energy balance, and a criterion for separation of the wavy interface at CHF. On the tilted plain surface with the surface orientation from 60° to 130° with respect to the upward facing heater surface, the model predicts the CHF, qCHF = 126.5 to 92.5 W/cm2 at the critical hydrodynamic instability wavelength, lcr = 9.2 to 12.7 mm, respectively, using water as a working fluid. The enhanced CHF with the columnar-post wick is predicted at the surface orientations of q = 90° and 120°, showing a maximum of 185% and 250% increase, respectively, by decreasing the critical hydrodynamic instability wavelength via columnar-post wicks. The maximum enhancement occurs at lp = 2.5 mm where qCHF increases from 104 to 295 W/cm2 for q = 90°, and from 89 to 313 for q = 120°. Also, the possible experimental validations with the non-uniform columnar-post wick fabrications are discussed. The developed model will provide insights into the tailored hydrodynamic instability wavelength at tilted angle via engineered surface.

Enhanced Pool Boiling Critical Heat Flux on Tilted Heating Surfaces Using Columnar-Post Wicks

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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