Local Heat Transfer Distributions in Confined Multiple Air Jet Impingement

Heat transfer from a discrete heat source to multiple, normally impinging, confined air jets was experimentally investigated. The jets issued from short, square-edged orifices with still-developing velocity profiles on to a foil heat source which produced a constant heat flux. The orifice plate and the surface containing the heat source were mounted opposite each other in a parallel-plates arrangement to effect radial outflow of the spent fluid. The local surface temperature was measured in fine increments over the entire heat source. Experiments were conducted for different jet Reynolds numbers (5000,Re ,20,000), orifice-to-target spacing ~ 0.5,H/d,4! , and multiple-orifice arrangements. The results are compared to those previously obtained for single air jets. A reduction in orifice-to-target spacing was found to increase the heat transfer coefficient in multiple jets, with this effect being stronger at the higher Reynolds numbers. With a nine-jet arrangement, the heat transfer to the central jet was higher than for a corresponding single jet. For a four-jet arrangement, however, each jet was found to have stagnationregion heat transfer coefficients that were comparable to the single-jet values. The effectiveness of single and multiple jets in removing heat from a given heat source is compared at a fixed total flow rate. Predictive correlations are proposed for single and multiple jet impingement heat transfer

A Tomographic-PIV Investigation of Vapor-Induced Flow Structures in Confined Jet Impingement Boiling

Tomographic particle image velocimetry (PIV) is used to study the effect of confinement gap height on the liquid flow characteristics in jet impingement boiling. This first application of tomographic PIV to flow boiling is significant given the complexity of confined two-phase jet impingement. A jet of subcooled wa- ter at a Reynolds number of 5,0 0 0 impinges onto a circular heat source undergoing boiling heat transfer at a constant heat input. Confinement gap heights of 8, 4, and 2 jet diameters are investigated. A visual hull method is used to reconstruct the time-varying regions of the vapor in the flow. The vapor motion is found to govern the liquid flow pattern and turbulence generation in the confinement gap. Time-averaged velocities and regions of turbulent kinetic energy in the liquid are highest for a confinement gap height of 8 jet diameters, with lower velocity magnitude and turbulence being observed for the smaller spac- ings. Coherent vortical structures identified with the λ2 -criterion are found to occur most frequently near the moving vapor interface. The most intense regions of turbulent kinetic energy do not coincide with the location of coherent structures within the flow. Irrotational velocity fluctuations in the liquid phase caused by vapor bubble pinch-offare the primary cause of the high turbulent kinetic energy measured in these regions. At a gap height of H / d = 2 the vapor plume is constrained as it grows from the heat source, causing bulk flow oscillations in the downstream region of the confinement gap

The Petal Effect of Parahydrophobic Surfaces offers Low Receding Contact Angles that Promote Effective Boiling

Despite extensive study of boiling processes and their widespread use in industry, critical interactions between the fluid and surface during boiling remain poorly understood. Simplistic, static descriptions of the contact angle are still relied upon to describe the effects of surface wettability on dynamic interfacial processes that govern boiling. This work demonstrates the critical role of the dynamic wettability characteristics of a surface on bubble growth dynamics and boiling performance. In spite of their superior nucleation behavior, hydrophobic surfaces have received little attention for boiling applications due to their typically premature transition from efficient nucleate boiling to inefficient film boiling. Evaluation of hydrophobic surfaces with high contact angle hysteresis reveals that the heat transfer efficacy of these surfaces can be exploited in boiling, so long as the receding contact angle of the surface is sufficiently small to mitigate vapor spreading and thereby extend the nucleate boiling regime. A new paradigm of textured boiling surfaces – parahydrophobic surfaces that exhibit the “petal effect” and mimic the wetting behavior of a rose petal – are shown to have untapped potential in boiling applications resulting from highly hydrophobic behavior coupled with low receding contact angles

The Role of Dynamic Wetting Behavior during Bubble Growth and Departure from a Solid Surface

Surface wettability is known to have a major influence on the ebullition characteristics of a bubble growing from a solid surface. Yet, simplistic static characterization of the wetting behavior is still relied upon to indicate performance characteristics during boiling. In this study, a theoretical framework is developed for the wetting and dewetting processes occurring during bubble growth based upon the dynamic contact angles. This framework is incorporated into adiabatic volume-of-fluid simulations to capture the influence of the surface wettability on contact line and contact angle dynamics during bubble growth and departure. The simulations span a large range of dynamic wetting behaviors and fluid properties. The receding contact angle is shown to govern the early stages of bubble growth as the contact line recedes outward from the bubble center and is the dominant wetting characteristic that determines the maximum contact diameter and departure size. The advancing contact angle dictates the departure morphology as the contact line retracts inward and has a secondary role in determining the departure size. Following, improved reduced-order models are developed that establish fluid-property-independent correlations for the maximum contact diameter and departure diameter as a function of the dynamic contact angles. The results call for the need to redefine wettability classifications based on dynamic contact angles rather than static contact angle in the context of boiling. Hygrophilicity and hygrophobicity are redefined in this context, and an additional classification, ambiphilicity, is introduced for boiling surfaces exhibiting low receding contact angles and high advancing contact angles

An Optical Approach for Quantitative Characterization of Slug Bubble Interface Profiles in a Two-Phase Microchannel Flow

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. 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 can 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

Two-phase Flow Morphology and Local Wall Temperatures in High-Aspect-Ratio Manifold Microchannels

Manifold microchannel heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels afford a large enhancement in heat transfer area; however, the flow morphology in manifold microchannels during two-phase operation, as well as the resulting thermal performance, are not well understood. In this work, a single manifold microchan- nel representing a repeating unit in a heat sink is fabricated in silicon with a bonded glass viewing window. Samples of different channel lengths (750 μm and 1500 μm) and depths (125 μm, 250 μm, and 10 0 0 μm) are considered; channel and fin widths are both maintained at 60 μm. Subcooled fluid (HFE-7100) is delivered to the channel at a constant flow rate such that the fluid velocity at the inlet is ~1.05 m/s in all cases. A high-speed camera is used to visualize the two-phase flow in the channel through the glass sidewall; an infrared camera measures the temperature distribution on the opposite channel sidewall. The flow visualizations reveal that vapor nucleation occurs at stagnation regions below the manifold near the inlet plenum and at both corners adjacent to the channel base. For deep chan- nels (10 0 0 μm), at sufficiently high heat fluxes, vapor completely covers the base of the channels and liquid does not re-wet the surface in this region. This newly identified vapor blanketing phenomenon causes a significant decrease in performance and an increase in the measured channel wall temperatures. This study reveals the critical role of the two-phase flow morphology in manifold microchannel heat sink design

Axisymmetric Wall Jet Development in Confined Jet Impingement

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layeredwall 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

Stereo-PIV Measurements of Vapor-Induced Flow Modifications in Confined Jet Impingement Boiling

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 diam- eters from the target surface. The behavior at jet Reynolds numbers of 5,0 0 0 and 15,0 0 0 is compared for a constant jet inlet subcooling of 10 °C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbu- lence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor genera- tion from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap . Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets

Three-Dimensional Liquid-Vapor Interface Reconstruction from High-Speed Stereo Images during Pool Boiling

A technique for reconstruction of liquid-gas interfaces based on high-speed stereo-imaging is applied to the liquid-vapor interfaces formed above a heated surface during pool boiling. 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. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields the physical world representation of the liquid-vapor interface. Liquid-vapor interface reconstruction is demonstrated during pool boiling for a range of heat fluxes. Textured mushroom-like vapor bubbles that are fed by multiple nucleation sites are formed close to the heated surface. Analysis of the temporal attributes of the interface distinguishes the transition with increasing heat flux from a mode in which vapor is released from the surface as a continuous plume to one dominated by the occurrence of intermittent vapor bursts. A characteristic morphology of the vapor mushroom formed during vapor burst events is identified. This liquid-vapor interface reconstruction technique is a time-resolved, flexible and non-invasive alternative to existing methods for phase-distribution mapping, and can be combined with other opticalbased diagnostic tools, such as tomographic particle image velocimetry. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of critical heat flux during pool boiling

Design of a Non-intrusive Electrical Impedance-Based Void Fraction Sensor for Microchannel Two-Phase Flows

.A non-intrusive electrical impedance-based sensor is developed for measurement of local void fraction in air-water adiabatic flow through rectangular microchannels. Measurement of the void fraction in microchannels is essential for the formulation of two-phase flow heat transfer and pressure drop correlations, and may enable real-time flow regime control and performance prediction in the thermal regulation of high-heat-flux devices. The impedance response of the sensor to a range of flow regimes is first investigated in a crosswise (transverse) configuration with two aligned electrodes flush-mounted on opposing microchannel walls. Numerical simulations performed on a multi-phase domain constructed from three-dimensional reconstruction of experimentally observed phase boundaries along with the corresponding experimental results serve to establish the relationship between void fraction and dimensionless impedance for this geometric configuration. A reduced-order analytical model developed based on an assumption of stratified gas-liquid flow allows ready extension of these calibration results to different working fluids of interest. An alternative streamwise sensor configuration is investigated with two electrodes flush-mounted along a single wall in the flow direction in view of its potentially simpler practical implementation in arrays of microchannels. It is shown that a correlation between time-averaged impedance and void fraction can be established for this alternative configuration as well