a b s t r a c tLocal heat transfer distributions at high spatial resolution are obtained under two-phase transport conditions in confined and submerged impingement from arrays of miniature jets. The dielectric liquid HFE-7100 is investigated to enable direct cooling of electronic components. Three round orifice geometries with the same total orifice open area are investigated, including a single orifice of 3.75 mm diameter, a 3 Â 3 array of 1.25 mm diameter orifices, and a 5 Â 5 array of 0.75 mm diameter orifices. A thin-foil heat source backed by a magnesium-fluoride window is fabricated to allow detailed mapping of the heated surface temperature via infrared (IR) thermography. The rigorous experimental calibration procedures employed, and correction for heat spreading within the thermally conductive IR-transparent window, yield low-uncertainty local heat transfer coefficient distributions. Each of the three orifice geometries is characterized at volumetric flow rates of 450 ml/min, 900 ml/min, and 1800 ml/min, resulting in a Reynolds number range of 1920-39400. Pressure drop across the confined jets is measured for all experimental cases. The test facility and measurement techniques employed are validated against heat transfer and pressure drop correlations in the literature for single-phase jet impingement from a single round orifice. Spatially resolved temperature contour maps, along with local heat transfer coefficient and boiling curves, are presented as a function of applied heat flux. Boiling is shown to coexist with single-phase convection under the impinging liquid jets. Two-phase enhancement is exhibited at large radial distances from the single jet axis, and in regions between neighboring jets within the arrays. The arrays of jets result in higher area-averaged heat transfer than a single jet at a fixed flow rate; however, the arrays display larger relative nonuniformity in local two-phase heat transfer coefficient and surface temperature. While the 5 Â 5 array resulted in a higher (and the 3 Â 3 a lower) pressure drop than the single jet, all orifices displayed pressure drop that is independent of the applied heat flux and vapor generation.

Confined jet impingement with boiling offers unique and attractive performance characteristics for thermal management of high heat flux components. Two-phase operation of jet impingement has been shown to provide high heat transfer coefficients while maintaining a uniform temperature over a target surface. This can be achieved with minimal increases in pumping power compared to single-phase operation. To investigate further enhancements in heat transfer coefficients and increases in the maximum heat flux supported by two-phase jet impingement, an experimental study of surface enhancements is performed using the dielectric working fluid HFE-7100. The performance of a single, 3.75 mm-diameter jet orifice is compared across four distinct copper target surfaces of varying enhancement scales: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin fins), and a hybrid surface on which the pin fins are coated with the microporous layer. The heat transfer performance of each surface is compared in single-and two-phase operation at three volumetric flow rates (450 ml/min, 900 ml/min, and 1800 ml/min); area-averaged heat transfer parameters and pressure drop are reported. The mechanisms resulting in enhanced performance for the different surfaces are identified, with a special focus on the coated pin fins. This hybrid surface showed the best enhancement of all those tested, and resulted in an extension of critical heat flux (CHF) by a maximum of 2.42 times compared to the smooth flat surface at the lowest flow rate investigated; no increase in the overall pressure drop was measured.

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layered wall jet structure. Experiments were conducted using stereo particle image velocimetry at three different confinement gap heights (2, 4, and 8 jet diameters) across Reynolds numbers ranging from 1000 to 9000. The rotating flow structures within the confinement region and their interaction with the surrounding flow were dependent on the confinement gap height and Reynolds number. The recirculation core shifted downstream as the Reynolds number increased. For the smallest confinement gap height investigated, the strong recirculation caused a disruption of the wall jet development. The radial position of the recirculation core observed at this small gap height was found to coincide with the location where the maximum wall jet velocity had decayed to 15% of the impinging jet exit velocity. After this point, the self-similarity hypothesis failed to predict the evolution of the wall jet further downstream. A reduction in confinement gap height increased the growth rates of the wall jet thickness but did not affect the decay rate of the wall jet maximum velocity. For jet Reynolds numbers above 2500, the decay rate of the maximum velocity in the developing region of the wall jet was approximately −1.1, which is close to previous results reported for the fully developed region of radial wall jets. A much higher decay rate of −1.5 was found for the wall jet formed by a laminar impinging jet at Re = 1000.

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75 mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is

Understanding how turbulence impacts marine floc formation and breakup is key to predicting particulate carbon transport in the ocean. While floc formation and sinking rate has been studied in the laboratory and in-situ, the breakup response to turbulence has attracted less attention. To address this problem, the breakup response of bentonite clay particles flocculated in salt water was studied experimentally. Flocs were grown in a large aggregation tank under unmixed and mixed aggregation conditions and then subjected to turbulent pipe flow. Particle size was quantified using microscope imaging and in-situ measurements obtained from standard optical oceanographic instruments; a Sequoia Scientific LISST-100X and two WET Labs ac-9 spectrophotometers. The LISST instrument was found to capture the breakup response of flocs to turbulent energy, though the resulting particle size spectra appear to have underestimated the largest floc lengthscales in the flow while overestimating the abundance of primary particles. Floc breakup and the resulting shift towards smaller particles caused an increase in spectral slope of attenuation as measured by the ac-9 instruments. The Kolmogorov lengthscale was not found to have a limiting effect on floc size in these experiments. While the flocs were found to decrease in overall strength over the course of the two-month experimental time period, repeatable breakup responses to turbulence exposure were observed. Hydrodynamic conditions during floc formation were found to have a large influence on floc strength and breakup response. A non-constant strength exponent was observed for flocs formed with more energetic mixing. Increased turbulence from mixing during aggregation was found to increase floc fractal dimension and apparent density, resulting in a shift in the breakup relationships to higher turbulence dissipation rates. The results suggest that marine particle aggregation and vertical carbon transport concepts should include the turbulence energy responsible for aggregate formation and the resulting impact on floc strength, density, and the disruption potential.