A methodology is proposed for predictive modeling of the liquid-gas interface shape and saturated flow boiling heat transfer coefficient in two-phase microchannel flows within the annular regime. The mechanistic model accounts for the effects of surface tension and interface curvature, gravity, and shear stress in determining the liquid film shape; one-dimensional conduction is assumed to occur across the variable-thickness film to calculate wall heat transfer coefficients locally along the channel length. Model performance is benchmarked against 251 experimentally measured heat transfer coefficient values taken from the literature for annular-regime flow boiling in microchannels of a rectangular crosssection. These data are successfully predicted with a mean absolute error of 21.7%, and 72.1% of the points lie within an error band of 30%. The match to data is poorest at lower vapor qualities corresponding to the onset of the annular regime, for which the heat transfer coefficient is underpredicted; an experimental investigation is performed to better understand the disparity under these operating conditions. Liquid film shapes are measured during adiabatic annular flow through microchannels of square cross-section for a range of channel hydraulic diameters (160 mu m, 510 mu m, 1020 gm) and operating conditions, so as to control the void fraction and Weber number of the flow. Using air and water as the working gas and liquid, respectively, trends in film behavior are identified and compared against model predictions. The experimental findings reveal the non-negligible impact of capillary pumping on the interface morphology at the onset of the annular regime. (C) 2017 Elsevier Ltd. All rights reserved.

An experimental investigation is conducted to examine the effects of operating conditions and channel size on the liquid film thickness of vapor bubbles in adiabatic air-water flows within the slug flow regime. Acrylic test sections are fabricated to contain a single microchannel of square cross-section with hydraulic diameters of 510 mu m and 1020 mu m. High-speed visualizations are used to map the flow regimes in these channels in order to determine the range of liquid and gas flow rates for which slug-regime flows are sustained. Subsequently, a tomographic optical imaging technique is employed to quantitatively reconstruct the liquid gas interface of vapor bubbles at selected operating conditions. This technique relies on visualization of fluorescent particles seeded into the liquid phase in order to identify the phase boundaries within thin sections of the flow. Using the reconstructions, the thickness of the liquid film in the corner of the square channel cross-section is extracted. This film thickness is found to decrease with increasing capillary number, and a simple expression is proposed for calculation of the film thickness; predictions from this model match the measurements with a mean absolute error (MAE) of 21.6%; 85.7.% of all predicted data points fall within an error band of +/- 30%. Additionally, film thickness data from the literature are compared to model predictions and a comparable MAE of 21.8% is found with 77.8% of data points falling within an error band of +/- 30%. (C) 2017 Elsevier Ltd. All rights reserved.

Coalescence-induced jumping of condensate droplets from a superhydrophobic surface with hierarchical micro/nanoscale roughness is quantitatively characterized. Experimental observations show that the condensate droplet jumping is induced by coalescence of multiple droplets of different sizes and that the coalesced droplet trajectories typically deviate from the surface normal. A depth-from-defocus image processing technique is developed to track the out-of-plane displacement of the jumping droplets, so as to accurately measure the droplet size and velocity. The results demonstrate that the highest jumping velocity is achieved when two droplets coalesce. The jumping velocity decreases gradually with an increase in the number of coalescing droplets, despite the greater potential surface energy released upon coalescence. A general theoretical model that accounts for viscous dissipation, surface adhesion, line tension, the initial droplet wetting states and the number and sizes of the coalescing droplets is developed to explain the trends of droplet jumping velocity observed in the experiments.

A new measurement technique is developed for quantitatively mapping the liquid-gas interface profiles of air bubbles in an adiabatic microchannel slug flow environment. Water seeded with 0.5 mu m-diameter fluorescent polystyrene particles is pumped through a single acrylic microchannel of 500 mu m x 500 mu m square cross section. A periodic slug flow is achieved by the controlled injection of air into the channel. Particles are constrained to the liquid phase, and their distribution in the flow is visualized through an optical microscope in an epifluorescent configuration with pulsed laser illumination to resolve the instantaneous liquid-gas interface profile to within +/- 2.8 mu m in the focal plane. This approach is able to identify the interface profile within individual focal planes at various depths within the channel, unlike conventional backlit optical profile detection approaches that cart only resolve the interface at the midplane. A similar particle-tracking technique was previously demonstrated for interface reconstruction in annular flows; however, the additional noise within images due to the reflection and refraction of background light at the compound-curvature interfaces characteristic of slug bubbles requires texture-based image analysis to obtain interface profiles. The varying interface profile of the slug bubbles in the streamwise direction also greatly complicates the tracking procedures for achieving a three-dimensional reconstruction of slug bubbles based on the measured two-dimensional interface profiles, which requires spatial alignment of the streamwise position of liquid-vapor interfaces realized at varying depths within the channel. This is addressed during reconstruction by using the measured mid-plane slug bubble cap curvature to assign the relative streamwise positions of interface profiles obtained at other measured depths. The characterization of two different selected bubble morphologies presented here demonstrates a critical improvement in metrological capability which can provide greater insight into microchannel flow phenomena in the slug-flow regime. (C) 2015 Elsevier Ltd. All rights reserved.

The morphology of liquid-gas interfaces in adiabatic two-phase microchannel flow through a transparent acrylic microchannel of 500 μm × 500 μm square cross section is investigated. Water seeded with 0.5 μm-diameter fluorescent polystyrene particles is pumped through the channel, and the desired adiabatic two-phase flow regime is achieved through controlled air injection. The diagnostic technique relies on obtaining particle position data through epifluorescent imaging of the flow at excitation and emission wavelengths of 532 and 620 nm, respectively. The particle positions are then used to resolve interface locations to within ±2 μm in the viewing plane. This technique was previously demonstrated by the authors for a static meniscus in a capillary tube. The complete interface geometry between liquid and gas phases is obtained for operation in the annular flow regime by mapping the interface within individual focal planes at various depths within the channel. The diagnostic technique is shown to successfully locate and measure interfaces between transparent, immiscible fluids in a dynamic microchannel flow environment.

Experiments were conducted to determine the influence of local vapor quality on local heat transfer coefficient in flow boiling in an array of microchannels. Additionally, the variation of local heat transfer coefficient along the length and width of the microchannel heat sink for given operating conditions was investigated over a range of flow parameters. Each test piece includes a silicon parallel microchannel heat sink with 25 integrated heaters and 25 temperature sensors arranged in a 5×5 grid, allowing for uniform heat dissipation and local temperature measurements. Channel dimensions ranged from 100 μm to 400 μm in depth and 100 μm to 5850 μm in width; the working fluid for all cases was the perfluorinated dielectric liquid, FC-77. The heat transfer coefficient is found to increase with increasing vapor quality, reach a peak, and then decrease rapidly due to partial dryout on the channel walls. The vapor quality at which the peak is observed shows a strong dependence on mass flux, occurring at lower vapor qualities with increasing mass flux for fixed channel dimensions. Variations in local heat transfer coefficient across the test piece were examined both along the flow direction and in a direction transverse to it; observed trends included variations due to entrance region effects, two-phase transition, non-uniform flow distribution, and channel wall dryout.

Experiments were conducted to develop a microscale spatial measurement technique applicable to transparent fluid flows, with the target application being the determination of liquid geometries in multiphase flows. Air/isopropanol interfaces were used to simulate multiphase boundaries and were studied by examining menisci in capillary tubes. Fluorescent seeding particles of 0.5 μm diameter were added to the isopropanol working fluid. The use of fluorescent particles circumvented many of the noise-related issues characteristic of transparent and reflective systems. By exploiting the difference in the length scale between particle size, inter-particle spacing, and target feature size, the particle position data could be used to resolve spatial measurements within the sample, primarily the location of the interface within the field of view. Additionally, by adjusting the position of the focal plane within the sample, a three-dimensional map of the air/liquid interface was obtained.