[Show abstract][Hide abstract] ABSTRACT: Direct numerical simulation of transport in foam materials can benefit from realistic representations of the porous-medium geometry generated by employing non-destructive 3D imaging techniques. X-ray microtomography employs computer-processed X-rays to produce tomographic images or slices of specific regions of the object under investigation, and is ideally suited for imaging opaque and intricate porous media. In this work, we employ micro-CT for numerical analysis of air flow and convection through four different high-porosity copper foams. All four foam samples exhibit approximately the same relative density (6.4–6.6% solid volume fraction), but have different pore densities (5, 10, 20, and 40 pores per inch, PPI). A commercial micro-computed tomography scanner is employed for scanning the 3D microstructure of the foams at a resolution of 20 μm, yielding stacks of two-dimensional images. These images are processed in order to reconstruct and mesh the real, random structure of the foams, upon which simulations are conducted of forced convection through the pore spaces of the foam samples. The pressure drop values from this μCT based CFD analysis are compared against prior experimental results; the computational interfacial heat transfer results are compared against the values predicted by an empirical correlation previously reported, revealing excellent agreement between the numerical and experimental/empirical hydraulic and thermal results, thus highlighting the efficacy of this novel approach.
International Journal of Heat and Mass Transfer 09/2015; 88. DOI:10.1016/j.ijheatmasstransfer.2015.04.038 · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Capacitance tomography is performed on a ∼1 mm thick planar domain. A square array of electrodes is used on one side (bottom) and a single large electrode on the opposing side (top). Dielectric anomalies of interest are empty pockets (voids) in a dielectric material filling the domain. Capacitance is measured between neighboring electrodes on the bottom side, as well as between bottom electrodes and the top electrode. Because regions of sensitivity are highly localized, information regarding the 3D structure of a void must be ascertained from relatively few measurements involving the electrodes near the void. Solutions that are strictly binary are desired for the reconstructed image without incurring excessive computational expense. A new image-reconstruction algorithm is developed that is tailored to capacitance tomography of sparsely characterized domains, entitled the shape-energy evolutionary reconstruction (SEER) algorithm. The algorithm creates binary images by sequentially composing a list of occupied voxels, guided by a knowledge of the spatial sensitivity of each measurement and a voxel-to-voxel energy regularization which prefers adjoining groups of occupied voxels. Detection and reconstruction of void structures in a 1.27 mm layer of thermal gap filler material is demonstrated experimentally. Basic three-dimensional characteristics of voids are distinguished. The reconstructed images provide a means of visualizing and interpreting void characteristics, such as their volume or proximity to a substrate wall.
Sensors and Actuators A Physical 08/2015; 233:349-359. DOI:10.1016/j.sna.2015.07.019 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 μm-diameter fluorescent polystyrene particles is pumped through a single acrylic microchannel of 500 μm × 500 μm 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 μm in the focal plane.
International Journal of Heat and Mass Transfer 07/2015; 86. DOI:10.1016/j.ijheatmasstransfer.2015.02.067 · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High-fidelity simulation of flow boiling in microchannels remains a challenging problem, but the increasing interest in applications of microscale two-phase transport highlight its importance. In this paper, a volume of fluid (VOF)-based flow boiling model is proposed with computational expense-saving features that enable cost-effective simulation of two-phase flow and heat transfer in realistic geometries. The vapor and liquid phases are distinguished using a color function which represents the local volume fraction of the tracked phase. Mass conservation is satisfied by solving the transport equations for both phases with a finite-volume approach. In order to predict phase change at the liquid-vapor interface, evaporative heat and mass source terms are calculated using a novel, saturated-interface-volume phase change model that fixes the interface at the saturation temperature at each time step to achieve stability. Numerical oscillation of the evaporation source terms is thus eliminated and a non-iterative time advancement scheme can be adopted to reduce computational cost. The reference frame is set to move with the vapor slug to artificially increase the local velocity magnitude in the thin liquid film region in the relative frame, which reduces the influence of numerical errors resulting from calculation of the surface tension force, and thus suppresses the development of spurious currents. This allows use of non-uniform meshes that can efficiently resolve high-aspect-ratio geometries and flow features and significantly reduces the overall numerical expense. The proposed model is used to simulate the growth of a vapor bubble in a heated 2D axisymmetric microchannel. The bubble motion, bubble growth rate, liquid film thickness, and local heat transfer coefficient along the wall are compared against previous numerical studies.
ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems; 07/2015
[Show abstract][Hide abstract] ABSTRACT: The effect of a variety of surface enhancements on the heat transfer achieved with an array of impinging jets is experimentally investigated using the dielectric fluid HFE-7100 at different volumetric flow rates. The performance of a 5×5 array of jets, each 0.75mm in diameter, is compared to that of a single 3.75mm diameter jet with the same total open orifice area, in single-and two-phase operation. Four different target copper surfaces are evaluated: 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; area-averaged heat transfer and pressure drop measurements are reported. The array of jets enhances the single-phase heat transfer coefficients by 1.13-1.29 times and extends the critical heat flux (CHF) on all surfaces compared to the single jet at the same volumetric flow rates. Additionally, the array greatly enhances the heat flux dissipation capability of the hybrid coated pin-fin surface, extending CHF by 1.89-2.33 times compared to the single jet on this surface, with a minimal increase in pressure drop. The jet array coupled with the hybrid enhancement dissipates a maximum heat flux of 205.8W/cm2 (heat input of 1.33kW) at a flow rate of 1800ml/min (corresponding to a jet diameter-based Reynolds number of 7800) with a pressure drop incurred of only 10.9kPa. Compared to the single jet impinging on the smooth flat surface, the array of jets on the coated pin-fin enhanced surface increased CHF by a factor of over four at all flow rates.
Journal of Heat Transfer 03/2015; 137(7):071501. DOI:10.1115/1.4029969 · 1.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hierarchical micro/nanostructured superhydrophobic surfaces are developed to control the nucleation, growth, and departure of condensate droplets on copper substrates. The microscale roughness elements on the hierarchical surface yield a 40% higher droplet growth rate and a 300% increase in cumulative droplet departure volume as compared to superhydrophobic surfaces with nanostructures alone.
[Show abstract][Hide abstract] ABSTRACT: The enhancement of pool boiling heat transfer by copper-particle surface coatings is experimentally investigated, using the wetting dielectric fluid FC-72. In one technique, loose copper particles are placed on a heated copper surface to provide additional vapor nucleation sites in the cavities formed at particle-surface and particle–particle contact points, thereby enhancing boiling performance over a polished surface. This ‘free-particle’ technique is benchmarked against the more traditional technique of sintering a fixed layer of copper particles to the surface to enhance boiling heat transfer performance. The effect of particle size on the heat transfer performance is studied for particle diameters ranging from 45 μm to 1000 μm at a constant coating layer thickness-to-particle diameter ratio of approximately 4. The parametric trends in the boiling curve and the critical heat flux are compared between the two techniques, and the dominant boiling mechanisms influencing these trends are compared and contrasted. High-speed visualizations are performed to qualitatively assess the boiling patterns and bubble departure size/distribution, and thus corroborate the trends observed in the boiling curves. The measured wall superheat is significantly lower with a sintered coating compared to the free-particle layer for any given particle size and heat flux. Performance trends with respect to particle size, however, are remarkably similar for both enhancement techniques, and an optimum particle size of ∼100 μm is identified for both free particles and sintered coatings. The free-particle technique is shown to offer a straightforward method to screen the boiling enhancement trends expected from different particulate layer compositions that are intended to be subsequently fabricated by sintering.
International Journal of Heat and Mass Transfer 02/2015; 81. DOI:10.1016/j.ijheatmasstransfer.2014.09.052 · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dropwise condensation of atmospheric water vapor is important in multiple practical engineering applications. The roles of environmental factors and surface morphology/chemistry on the condensation dynamics need to be better understood to enable efficient water-harvesting, dehumidification, and other psychrometric processes. Systems and surfaces that may promote faster condensation rates and self-shedding of condensate droplets could lead to improved mass transfer rates and higher water yields in harvesting applications. In the present study, experiments are performed in a facility that allows visualization of the condensation process on a vertically oriented, hydrophobic surface at a controlled relative humidity and surface subcooling temperature. The distribution and growth of water droplets are monitored across the surface at different relative humidities (45%, 50%, 55%, and 70%) at a constant surface subcooling temperature of 15 °C below the ambient temperature (20 °C). The droplet growth dynamics exhibits a strong dependency on relative humidity in the early stages during which there is a large population of small droplets on the surface and single droplet growth dominates over coalescence effects. At later stages, the dynamics of droplet growth is insensitive to relative humidity due to the dominance of coalescence effects. The overall volumetric rate of condensation on the surface is also assessed as a function of time and ambient relative humidity. Low relative humidity conditions not only slow the absolute rate of condensation, but also prolong an initial transient regime over which the condensation rate remains significantly below the steady-state value.
International Journal of Heat and Mass Transfer 01/2015; 80:759-766. DOI:10.1016/j.ijheatmasstransfer.2014.09.080 · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A sensor concept is developed and analyzed for in situ characterization of a thin dielectric layer. An array of long, planar electrodes is flush-mounted into opposing faces of two substrates on either side of the dielectric layer. The substrates are oriented such that the lengthwise dimensions of the opposing electrodes are orthogonal. Capacitance is measured between single electrode pairs on opposite substrates while all other electrodes are grounded. The electric field between the active electrodes is sharply focused at their crossing point, resulting in high sensitivity to void content in a square detection zone of the dielectric layer. For a fixed interfacial gap size, direct proportionality of the capacitance with void fraction within the detection zone is poor for high electrode-to-electrode spacing on the substrates, but improves dramatically as this spacing is reduced. Three methods of deriving a simulation-based sensitivity response of measured capacitance to any arbitrary two-dimensional void geometry are investigated. The best method requires data from simulations of an empty air gap and a TIM-filled gap, and uses a reduced-order superposition technique to predict the normalized capacitance value obtained for any void geometry to within 10% of that predicted by a high-fidelity direct simulation. The sensing technique is demonstrated using manually introduced voids of 250 µm–2000 µm diameter in a 254 µm thick interface material layer with a dielectric constant of 4.7. The relationship of the capacitance to the void fraction is shown to fall within the predicted bounds.
[Show abstract][Hide abstract] ABSTRACT: X-ray micro computed tomography (μ-CT), originally developed for non-destructive biomedical imaging, is increasingly being employed in areas as diverse as materials characterization and reverse engineering. The technique employs computer processed X-rays to produce tomographic images or slices of specific regions of the object under investigation. This paper presents a numerical analysis of air flow through four different high-porosity ERG copper foams having different pore sizes (5, 10, 20, and 40 pores per inch, PPI), and approximately the same relative density (6.4-6.6% solid fraction). These samples were scanned with a commercial micro computed tomography scanner at a resolution of 20 μm, yielding a stack of two-imensional images. Starting with these two-dimensional images, the real, random structure of the foams was reconstructed and subsequently meshed using the commercial software Simpleware. Meshes thus produced were then exported to FLUENT for simulating the fluid flow through the pore space of the foam samples. The results of μ-CT based CFD computations are compared against experimental measurements of pressure drop that were previously obtained with the same samples. The comparison reveals excellent agreement between the numerical and experimental results, highlighting the accuracy of this novel approach.
Energy Procedia 12/2014; 45:645-652. DOI:10.1016/j.egypro.2014.01.069
[Show abstract][Hide abstract] ABSTRACT: A single molten-salt thermocline tank is a low-cost alternative to conventional multiple-tank systems for concentrating solar power thermal energy storage. Thermocline tanks are typically composed of molten salt and a filler material that provides sensible heat capacity at reduced cost; such tanks are referred to as a dual-media thermocline (DMT). However, inclusion of quartzite rock filler introduces the potential for mechanical ratcheting of the tank wall during thermal cycling. To avoid this potential thermomechanical mode of failure, the tank can be operated solely with molten salt, as a single-medium thermocline (SMT) tank. In the absence of a filler material to suppress formation of tank-scale convection eddies, the SMT tank may exhibit undesirable internal fluid flows in the tank cross-section. The performance of DMT and SMT tanks is compared under cyclic operation, assuming adiabatic external wall boundary conditions. A computational fluid dynamics model is used to solve for the spatial temperature and velocity distributions within the tank. For the DMT tank, a two-temperature model is used to account for the non-thermal equilibrium between the molten salt and the filler material, and Forchheimer's extension of Darcy's Law is added to the porous-medium formulation of the laminar momentum equation. The governing equations are solved numerically using a finite volume approach. For adiabatic external boundaries, the SMT tank yields a percentage point increase in the first and second law efficiencies relative to the DMT tank. Future work is needed to compare the thermocline tank designs with respect to capital cost and storage performance under non-adiabatic wall boundaries.
Energy Procedia 12/2014; 49:916-926. DOI:10.1016/j.egypro.2014.03.099
[Show abstract][Hide abstract] ABSTRACT: A numerical treatment is proposed to minimize the creation of unphysical, spurious currents in modeling liquid-gas slug flow using the volume of fluid-continuum surface force (VOF-CSF) method. An elongated gas slug drawn into a small circular channel initially filled with liquid is considered. To suppress spurious currents formed by numerical errors in calculation of the surface tension force at small capillary numbers (Ca < 0.01), an artificial relative reference frame is specified with motion in a direction opposite to the flow. An increase in the local relative velocity magnitude near the interface is demonstrated to be the key mechanism for spurious current suppression. A comparison of simulations performed with and without this treatment shows that spurious currents are eliminated at Ca = 0.0029; liquid film thickness, gas slug velocity, and liquid-phase circulation near the leading slug interface are preserved and the computed values agree with the literature. This demonstrates that the proposed moving reference frame method does not influence the computed physical phenomena of interest while suppressing unphysical spurious velocities.
[Show abstract][Hide abstract] ABSTRACT: We investigate hitherto-unexplored flow characteristics inside a sessile droplet evaporating on heated hydrophobic and superhydrophobic surfaces and propose the use of evaporation-induced flow as a means to promote efficient "on-the-spot" mixing in microliter-sized droplets. Evaporative cooling at the droplet interface establishes a temperature gradient that induces buoyancy-driven convection inside the droplet. An asymmetric single-roll flow pattern is observed on the superhydrophobic substrate, in stark contrast with the axisymmetric toroidal flow pattern that develops on the hydrophobic substrate. The difference in flow patterns is attributed to the larger height-to-diameter aspect ratio of the droplet (of the same volume) on the superhydrophobic substrate, which dictates a single asymmetric vortex as the stable buoyancy-induced convection mode. A scaling analysis relates the observed velocities inside the droplet to the Rayleigh number. On account of the difference in flow patterns, Rayleigh numbers, and the reduced solid-liquid contact area, the flow velocity is an order of magnitude higher in droplets evaporating on a superhydrophobic substrate as compared to hydrophobic substrates. Flow velocities in all cases are shown to increase with substrate temperature and droplet size: The characteristic time required for mixing of a dye in an evaporating sessile droplet is reduced by ∼8 times on a superhydrophobic surface when the substrate temperature is increased from 40 to 60 °C. The mixing rate is ∼15 times faster on the superhydrophobic substrate compared to the hydrophobic surface maintained at the same temperature of 60 °C.
Physical Review E 12/2014; 90(6-1):062407. DOI:10.1103/PhysRevE.90.062407 · 2.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A sensor is developed for simple, in situ characterization of dielectric thermal interface materials (TIMs) at bond line thicknesses less than 100 lm. The working principle is based on the detection of regions of contrasting electric permittivity. An array of long, parallel electrodes is flush-mounted into each opposing substrate face of a narrow gap interface, and exposed to the gap formed between the two surfaces. Electrodes are oriented such that their lengthwise dimension in one substrate runs perpendicular to those in the other. A capacitance measurement taken between opposing electrodes is used to characterize the interface region in the vicinity of their crossing point (junction). The electric field associated with each electrode junction is numerically simulated and analyzed. Criteria are developed for the design of electrode junction geometries that localize the electric fields. The capacitances between floating-ground electrodes in the electrode sensor configuration employed give rise to a nontrivial network of interacting capacitances which strongly influence the measured response at any junction. A generalized solution for analyzing the floating network response is presented. The technique is used to experimentally detect thermal grease spots of 0.2mm to 1.8mm diameter within a 25 lm interface gap. It is necessary to use the generalized solution to the capacitance network developed in this work to properly delineate regions of contrasting permittivity in the interface gap region using capacitance measurements.