Suresh V. Garimella

Purdue University, West Lafayette, Indiana, United States

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Publications (275)213.31 Total impact

  • Zhenhai Pan, Justin A Weibel, Suresh V Garimella
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    ABSTRACT: Prediction and manipulation of the evaporation of small droplets is a fundamental problem with importance in a variety of microfluidic, microfabrication, and biomedical applications. A vapor-diffusion-based model has been widely employed to predict the interfacial evaporation rate; however, its scope of applicability is limited due to incorporation of a number of simplifying assumptions of the physical behavior. Two key transport mechanisms besides vapor diffusion-evaporative cooling and natural convection in the surrounding gas-are investigated here as a function of the substrate wettability using an augmented droplet evaporation model. Three regimes are distinguished by the instantaneous contact angle (CA). In Regime I (CA ≲ 60°), the flat droplet shape results in a small thermal resistance between the liquid-vapor interface and substrate, which mitigates the effect of evaporative cooling; upward gas-phase natural convection enhances evaporation. In Regime II (60 ≲ CA ≲ 90°), evaporative cooling at the interface suppresses evaporation with increasing contact angle and counterbalances the gas-phase convection enhancement. Because effects of the evaporative cooling and gas-phase convection mechanisms largely neutralize each other, the vapor-diffusion-based model can predict the overall evaporation rates in this regime. In Regime III (CA ≳ 90°), evaporative cooling suppresses the evaporation rate significantly and reverses entirely the direction of natural convection induced by vapor concentration gradients in the gas phase. Delineation of these counteracting mechanisms reconciles previous debate (founded on single-surface experiments or models that consider only a subset of the governing transport mechanisms) regarding the applicability of the classic vapor-diffusion model. The vapor diffusion-based model cannot predict the local evaporation flux along the interface for high contact angle (CA ≥ 90°) when evaporative cooling is strong and the temperature gradient along the interface determines the peak local evaporation flux.
    Langmuir : the ACS journal of surfaces and colloids. 08/2014;
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    ABSTRACT: 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 investigated for a 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.
    Measurement Science and Technology 07/2014; 25(9):095301. · 1.44 Impact Factor
  • Susmita Dash, Suresh V Garimella
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    ABSTRACT: The evaporation characteristics of sessile water droplets on smooth hydrophobic and structured superhydrophobic heated surfaces are experimentally investigated. Droplets placed on the hierarchical superhydrophobic surface subtend a very high contact angle (∼160°) and demonstrate low roll-off angle (∼1°), while the hydrophobic substrate supports corresponding values of 120° and ∼10°. The substrates are heated to different constant temperatures in the range of 40-60 °C, which causes the droplet to evaporate much faster than in the case of natural evaporation without heating. The geometric parameters of the droplet, such as contact angle, contact radius, and volume evolution over time, are experimentally tracked. The droplets are observed to evaporate primarily in a constant-contact-angle mode where the contact line slides along the surface. The measurements are compared with predictions from a model based on diffusion of vapor into the ambient that assumes isothermal conditions. This vapor-diffusion-only model captures the qualitative evaporation characteristics on both test substrates, but reasonable quantitative agreement is achieved only for the hydrophobic surface. The superhydrophobic surface demonstrates significant deviation between the measured evaporation rate and that obtained using the vapor-diffusion-only model, with the difference being amplified as the substrate temperature is increased. A simple model considering thermal diffusion through the droplet is used to highlight the important role of evaporative cooling at the droplet interface in determining the droplet evaporation characteristics on superhydrophobic surfaces.
    Physical Review E 04/2014; 89(4-1):042402. · 2.31 Impact Factor
  • Susmita Dash, Suresh V. Garimella
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    ABSTRACT: The evaporation characteristics of sessile water droplets on smooth hydrophobic and structured superhydrophobic heated surfaces are experimentally investigated. Droplets placed on the hierarchical superhydrophobic surface subtend a very high contact angle (̃160°) and demonstrate low roll-off angle (̃1°), while the hydrophobic substrate supports corresponding values of 120° and ̃10°. The substrates are heated to different constant temperatures in the range of 40-60 °C, which causes the droplet to evaporate much faster than in the case of natural evaporation without heating. The geometric parameters of the droplet, such as contact angle, contact radius, and volume evolution over time, are experimentally tracked. The droplets are observed to evaporate primarily in a constant-contact-angle mode where the contact line slides along the surface. The measurements are compared with predictions from a model based on diffusion of vapor into the ambient that assumes isothermal conditions. This vapor-diffusion-only model captures the qualitative evaporation characteristics on both test substrates, but reasonable quantitative agreement is achieved only for the hydrophobic surface. The superhydrophobic surface demonstrates significant deviation between the measured evaporation rate and that obtained using the vapor-diffusion-only model, with the difference being amplified as the substrate temperature is increased. A simple model considering thermal diffusion through the droplet is used to highlight the important role of evaporative cooling at the droplet interface in determining the droplet evaporation characteristics on superhydrophobic surfaces.
    03/2014; 89(4).
  • Ravi S. Patel, Suresh V. Garimella
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    ABSTRACT: A diagnostic technique capable of characterizing interfaces between transparent, immiscible fluids is developed and demonstrated by investigating the morphology of liquid-gas interfaces in an adiabatic two-phase flow through a microchannel of 500 μm × 500 μm square cross section. 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 nm and 620 nm, respectively. The particle position data are then used to resolve interface locations to within ±1 μm in the focal plane. By mapping the interface within individual focal planes at various depths within the channel, it is possible to determine the complete liquid-gas interface geometry across the channel cross section in a dynamic flow environment. Utilizing this approach, the liquid-gas phase boundaries of annular flows within a microchannel have been successfully characterized.
    International Journal of Multiphase Flow 01/2014; · 1.72 Impact Factor
  • Susan N. Ritchey, Justin A. Weibel, Suresh V. Garimella
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    ABSTRACT: As electronics packages become increasingly thinner and more compact due to size, weight, and performance demands, the use of large intermediate heat spreaders to mitigate heat generation non-uniformities are no longer a viable option. Instead, non-uniform heat flux profiles produced from chip-scale variations or from multiple discrete devices are experienced directly by the ultimate heat sink. In order to address these thermal packaging trends, a better understanding of the impacts of non-uniform heating on two-phase flow characteristics and thermal performance limits for microchannel heat sinks is needed. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse peak-heating conditions spanning across the entire heat sink area. The investigation is conducted using a silicon microchannel heat sink with a 5 × 5 array of individually controllable heaters attached to a 12.7 mm × 12.7 mm square base. The channels are 240 μm wide, 370 μm deep, and separated by 110 μm wide fins. The working fluid is the dielectric fluorinert liquid FC-77, flowing at a mass flux of approximately 890 kg/m2 s. High-speed visualizations of the flow are recorded to observe the local flow regimes. Despite the substrate beneath the microchannels being very thin (200 μm), significant lateral conduction occurs and must be accounted for in the calculation of the local heat flux imposed. For non-uniform heat input profiles, with peak heat fluxes along the streamwise and transverse directions, it is found that the local flow regimes, heat transfer coefficients, and wall temperatures deviate significantly from a uniformly heated case. These trends are assessed as a function of an increase in the relative magnitude of the nonuniformity between the peak and background heat fluxes.
    International Journal of Heat and Mass Transfer 01/2014; 71:206–216. · 2.32 Impact Factor
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    Scott M. Flueckiger, Suresh V. Garimella
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    ABSTRACT: Molten-salt thermocline tanks are a low-cost energy storage option for concentrating solar power plants. Despite the potential economic advantage, the capacity of thermocline tanks to store sufficient amounts of high-temperature heat is limited by the low energy density of the constituent sensible-heat storage media. A promising design modification replaces conventional rock filler inside the tank with an encapsulated phase-change material (PCM), contributing a latent heat storage mechanism to increase the overall energy density. The current study presents a new finite-volume approach to simulate mass and energy transport inside a latent heat thermocline tank at low computational cost. This storage model is then integrated into a system-level model of a molten-salt power tower plant to inform tank operation with respect to realistic solar collection and power production. With this system model, PCMs with different melting temperatures and heats of fusion are evaluated for their viability in latent heat storage for solar plants. Thermocline tanks filled with a single PCM do not yield a substantial increase in annual storage or plant output over a conventional rock-filled tank of equal size. As the melting temperature and heat of fusion are increased, the ability of the PCM to support steam generation improves but the corresponding ability of the thermocline tank to utilize this available latent heat decreases. This trend results from an inherent deconstruction of the heat-exchange region inside the tank between sensible and latent heat transfer, preventing effective use of the added phase change for daily plant operations. This problem can be circumvented with a cascaded filler structure composed of multiple PCMs with their melting temperatures tuned along the tank height. However, storage benefits with these cascaded tank structures are shown to be highly sensitive to the proper selection of the PCM melting points relative to the thermocline tank operating temperatures.
    Applied Energy. 01/2014; 116:278–287.
  • Karthik K. Bodla, Suresh V. Garimella, Jayathi Y. Murthy
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    ABSTRACT: Characterization and design of fluid–thermal transport through random porous sintered beds is critical for improving the performance of two-phase heat transport devices such as heat pipes. Two-dimensional imaging techniques are quite well developed and commonly employed for microstructure and material characterization. In this study, we employ 2D image data (thin sections) for measuring critical microstructural features of commercial wicks for use in correlation-based prediction of transport properties. We employ a stochastic characterization methodology based on the two-point autocorrelation function, and compare the predicted properties such as particle and pore diameters and permeability with those from our previously published studies, in which 3D X-ray microtomography data was employed for reconstruction. Further, we implement a reconstruction technique for reconstructing a three-dimensional stochastic equivalent structure from the thin sections. These reconstructed domains are employed for predicting effective thermal conductivity, permeability and interfacial heat transfer coefficient in single-phase flow. The current computations are found to compare well with models and correlations from the literature, as well as our previous numerical studies. Finally, we propose a new parametrized model for the design of porous materials based on the nature of the two-point autocorrelation functions. Using this model, we reconstruct sample three-dimensional microstructures, and analyze the influence of various parameters on fluid–thermal properties of interest. With advances in additive manufacturing techniques, such an approach may eventually be employed to design intricate porous structures with properties tailored to specific applications.
    International Journal of Heat and Mass Transfer 01/2014; 73:250–264. · 2.32 Impact Factor
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    ABSTRACT: A three-dimensional numerical model is developed and validated to study the effect of geometric parameters such as microchannel depth and width, manifold depth, and manifold inlet and outlet lengths on the performance of a manifold microchannel (MMC) heat sink. The manifold arrangement used to distribute the flow through alternating inlet and outlet pairs greatly reduces the pressure drop incurred in conventional fluid supply arrangements due to its shorter flow paths, while simultaneously enhancing the heat transfer coefficient by limiting the growth of thermal boundary layers. The computational analysis is performed on a simple unit-cell model to obtain an optimized design for uniform thermal boundary conditions, as well as on a porous-medium model to obtain a complete system-level analysis of multiple microchannels across one manifold. The porous-medium approach can be further modified to analyze the performance under asymmetrical heating conditions. Along with conventional deterministic optimization, a probabilistic optimization study is performed to identify the optimal geometric design parameters that maximize heat transfer coefficient while minimizing pressure drop for an MMC heat sink. In the presence of uncertainties in the geometric and operating parameters of the system, this probabilistic optimization approach yields a design that is robust and reliable, in addition to being optimal. Such an optimization analysis provides a quantitative estimate of the allowable uncertainty in input parameters for acceptable uncertainties in the relevant output parameters. The approach also yields information such as the local and global sensitivities which are used to identify microchannel width and manifold inlet length as the critical input parameters to which the outputs are most sensitive. The deterministic analysis shows that the heat transfer performance of the MMC heat sink is optimal at a manifold inlet to outlet length ratio of 3. A comparison between the deterministic and probabilistic optimization approaches is presented for the unit-cell model. A probabilistic optimization study is performed for the porous-medium model and the results thus obtained are compared with those of the unit-cell model for a uniform heat flux distribution.
    International Journal of Heat and Mass Transfer 01/2014; 69:92–105. · 2.32 Impact Factor
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    ABSTRACT: Prediction and active control of the spatial distribution of particulate deposits obtained from sessile droplet evaporation are vital in printing, nanostructure assembly, biotechnology, and other applications that require localized deposits. This Letter presents surface wettability-based localization of evaporation-driven particulate deposition and the effect of superhydrophobic surface morphology on the distribution of deposits. Sessile water droplets containing suspended latex particles are evaporated on non-wetting textured surfaces with varying microstructure geometry at ambient conditions. The droplets are visualized throughout the evaporation process to track the temporal evolution of contact radius and apparent contact angle. The resulting particle deposits on the substrates are quantitatively characterized. The experimental results show that superhydrophobic surfaces suppress contact-line deposition during droplet evaporation, thereby providing an effective means of localizing the deposition of suspended particles. A correlation between deposit size and surface morphology, explained in terms of the interface pressure balance at the transition between wetting states, reveals an optimum surface morphology for minimizing the deposit coverage area.
    Applied Physics Letters 01/2014; 104(20):201604-201604-5. · 3.79 Impact Factor
  • Zhenhai Pan, Susmita Dash, Justin A Weibel, Suresh V Garimella
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    ABSTRACT: Evaporation rates are predicted and important transport mechanisms identified for evaporation of water droplets on hydrophobic (contact angle ~ 110 deg) and superhydrophobic (contact angle ~ 160 deg) substrates. Analytical models for droplet evaporation in the literature are usually simplified to include only vapor diffusion in the gas domain, and the system is assumed to be isothermal. In the comprehensive model developed in this study, evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Conjugate heat and mass transfer are solved in the solid substrate, liquid droplet, and surrounding gas. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. The influences of evaporative cooling and convection on the evaporation characteristics are determined quantitatively. The liquid-vapor interface temperature drop induced by evaporative cooling suppresses evaporation, while gas-phase natural convection acts to enhance evaporation. While the effects of these competing transport mechanisms are observed to counterbalance for evaporation on a hydrophobic surface, the stronger influence of evaporative cooling on a superhydrophobic surface accounts for an over-prediction of experimental evaporation rates by ~ 20% with vapor diffusion-based models. The local evaporation fluxes along the liquid-vapor interface for both hydrophobic and superhydrophobic substrates are investigated. The highest local evaporation flux occurs at the three-phase contact line region due to proximity to the higher temperature substrate, rather than at the relatively colder droplet top; vapor diffusion-based models predict the opposite. The numerically calculated evaporation rates agree with experimental results to within 2% for superhydrophobic substrates and 3% for hydrophobic substrates. The large deviations between past analytical models and the experimental data are therefore reconciled with the comprehensive model developed here.
    Langmuir 12/2013; · 4.38 Impact Factor
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    ABSTRACT: Label-free, rapid detection of biomolecules in microliter volumes of highly diluted solutions (sub-femtomolar) is of essential importance for numerous applications in medical diagnostics, food safety, and chem-bio sensing for homeland security. At ultra-low concentrations, regardless of the sensitivity of the detection approach, the sensor response time is limited by physical diffusion of molecules towards the sensor surface. We have developed a fast, low cost, non-faradaic impedance sensing method for detection of synthetic DNA molecules in DI water at attomolar levels by beating the diffusion limit through evaporation of a micro-liter droplet of DNA on a nanotextured superhydrophobic electrode array. Continuous monitoring of the impedance of individual droplets as a function of evaporation time is exploited to dramatically improve the sensitivity and robustness of detection. Formation of the nanostructures on the electrode surface not only increases the surface hydrophobicity, but also allows robust pinning of the droplet contact area to the sensor surface. These two features are critical for performing highly stable impedance measurements as the droplet evaporates. Using this scheme, the detection limit of conventional non-faradaic methods is improved by five orders of magnitude. The proposed platform represents a step-forward towards realization of ultra-sensitive lab-on-chip biomolecule detectors for real time point-of-care application. Further works are however needed to ultimately realize the full potential of the proposed approach to appraise biological samples in complex buffer solutions rather than in DI water.
    Lab on a Chip 09/2013; · 5.70 Impact Factor
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    Susmita Dash, Suresh V Garimella
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    ABSTRACT: We report on experiments of droplet evaporation on a structured superhydrophobic surface that displays very high contact angle (CA ∼ 160 deg), and negligible contact angle hysteresis (<1 deg). The droplet evaporation is observed to occur in a constant-contact-angle mode, with contact radius shrinking for almost the entire duration of evaporation. Experiments conducted on Teflon-coated smooth surface (CA ∼ 120 deg) as a baseline also support an evaporation process that is dominated by a constant-contact-angle mode. The experimental results are compared with an isothermal diffusion model for droplet evaporation from the literature. Good agreement is observed for the Teflon-coated smooth surface between the analytical expression and experimental results in terms of the total time for evaporation, transient volume, contact angle, and contact radius. However, for the structured superhydrophobic surface, the experiments indicate that the time taken for complete evaporation of the droplet is greater than the predicted time, across all droplet volumes. This disparity is attributed primarily to the evaporative cooling at the droplet interface due to the high aspect ratio of the droplet and also the lower effective thermal conductivity of the substrate due to the presence of air gaps. This hypothesis is verified by numerically evaluating the temperature distribution along the droplet interface. We propose a generalized relation for predicting the instantaneous volume of droplets with initial CA > 90 deg, irrespective of the mode of evaporation.
    Langmuir 08/2013; · 4.38 Impact Factor
  • Scott M. Flueckiger, Zhen Yang, Suresh V. Garimella
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    ABSTRACT: Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power systems. A review of previous experimental and numerical thermocline tank studies is performed to identify key issues associated with tank design and performance. Published models have shown that tank discharge performance improves with both larger tank height and smaller internal filler diameter due to increased thermal stratification and sustained outflow of molten salt with high thermal quality. For well-insulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heat-exchange region and increase discharge efficiency. Under nonadiabatic conditions, low flow rates become detrimental to stratification due to the development of fluid recirculation zones inside the tank. For such tanks, higher flow rates reduce molten-salt residence time inside the tank and improve discharge efficiency. Despite the economic advantages of a thermocline tank, thermal ratcheting of the tank wall remains a significant design concern. The potential for thermal ratcheting is reduced through the inclusion of an internal thermal insulation layer between the molten salt and tank wall to diminish temperature oscillations along the tank wall. Future research directions are also pointed out, including combined analyses that consider the solar receiver and power generation blocks as well as optimization between performance and economic considerations.
    Heat Transfer Engineering 08/2013; 34(10):787-800. · 0.69 Impact Factor
  • Christopher P. Migliaccio, Suresh V. Garimella
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    ABSTRACT: Evaporation of narrow water ribbons (of 5 and 7 μL volume) formed on a heated surface is investigated. Chemical and structural patterning of a silicon substrate is employed to fabricate a hydrophilic stripe that bisects hydrophobic pillar arrays of varied geometric roughness. Electrical heating of a 100 nm titanium layer on the back side of the device provides a constant heat flux. In the absence of electrical actuation, water introduced onto the substrate takes a bulging ribbon shape that is constrained to the immediate vicinity of the hydrophilic stripe. Electrowetting of the water ribbon spreads it into the hydrophobic pillar arrays on either side, leading to significant increases in maximum wetted width (up to 200%) and wettability (up to 80% reduction in contact angle). Infrared thermography is employed to characterize the cooling effect due to the spreading of the ribbon, while a goniometer monitors the ribbon shape. The heat transfer in each case is estimated through an energy balance analysis, and the results are compared with other electrowetting-based cooling techniques.
    International Journal of Heat and Mass Transfer 01/2013; 57(1):73–81. · 2.32 Impact Factor
  • Nikhil Bajaj, Ganesh Subbarayan, Suresh V. Garimella
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    ABSTRACT: Performance of thermal interface materials (TIMs) used between a microelectronic device and its associated heat spreader is largely dependent on the bulk thermal conductivity of the TIM, but the bond-line thickness (BLT) of the applied material as well as the interfacial contact resistances are also significant contributors to overall performance. Hierarchically Nested Channels (HNCs), created by modifying the surface topology of the chip or the heatsink with hierarchical arrangements of microchannels in order to improve flow, have been proposed to reduce both the required squeezing force and the final BLT at the interfaces. In the present work, a topological optimization framework that enables the design of channel arrangements is developed. The framework is based on a resistance network approximation to Newtonian squeeze flow. The approximation, validated against finite element (FE) solutions, allows efficient, design-oriented solutions for squeeze flow in complex geometries. A comprehensive design sensitivity analysis exploiting the resistance network approximation is also developed and implemented. The resistance approximation and the sensitivity analysis is used to build an automated optimal channel design framework. A Pareto optimal problem formulation for the design of channels is posed and the optimal solution is demonstrated using the framework.
    International Journal of Heat and Mass Transfer 06/2012; 55(s 13–14):3560–3575. · 2.32 Impact Factor
  • Justin A. Weibel, Suresh V. Garimella
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    ABSTRACT: The current study investigates capillary-fed boiling of water from porous sintered powder wicks used in emerging high-effective-conductivity vapor chamber heat spreaders intended for management of hot spots with heat fluxes exceeding 500 W cm−2. Characterization of 1 mm thick wicks composed of 100 μm sintered copper particles is performed in a test facility which replicates the capillary feeding conditions that occur in such devices. Boiling curves are obtained for a 5 mm × 5 mm heated input area, along with high-speed in-situ visualization of the evaporation/boiling processes. Understanding the vapor formation regimes is essential to predictive modeling of the observed characteristics. Schematic representations of such regimes along the boiling curves are presented for homogeneous and modified wick structures. In general, incipience of boiling in sintered-powder wicks reduces the effective thermal resistance and, for small heat input areas, does not cause liquid starvation due to a capillary limitation. The thermal performance enhancement provided by two different augmentation methods is quantified and explained in terms of the observed vapor formation characteristics. Patterns fabricated within the sintered powder create multi-scale wicks with regions of different pore size. These patterns reduce thermal resistance throughout the boiling regime by increasing the permeability to vapor exiting the wick, as confirmed by visualization of the preferential vapor venting from the surface. At the highest heat fluxes investigated prior to dryout, a thin liquid film is observed to form in the recessed patterned areas at the base of the wick. Integration of copper-coated carbon nanotubes on to the sintered powder reduces the required superheat for boiling incipience, thus reducing the overall thermal resistance at low heat fluxes. Evaporation and boiling regime heat transfer predictions from several available correlations are compared to the current results, and are shown to corroborate the conclusions regarding vapor permeability.
    International Journal of Heat and Mass Transfer 06/2012; 55(s 13–14):3498–3510. · 2.32 Impact Factor
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    Susmita Dash, Marie T Alt, Suresh V Garimella
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    ABSTRACT: Surfaces may be rendered superhydrophobic by engineering the surface morphology to control the extent of the liquid-air interface and by the use of low-surface-energy coatings. The droplet state on a superhydrophobic surface under static and dynamic conditions may be explained in terms of the relative magnitudes of the wetting and antiwetting pressures acting at the liquid-air interface on the substrate. In this paper, we discuss the design and fabrication of hollow hybrid superhydrophobic surfaces which incorporate both communicating and noncommunicating air gaps. The surface design is analytically shown to exhibit higher capillary (or nonwetting) pressure compared to solid pillars with only communicating air gaps. Six hybrid surfaces are fabricated with different surface parameters selected such that the Cassie state of a droplet is energetically favorable. The robustness of the surfaces is tested under dynamic impingement conditions, and droplet dynamics are explained using pressure-based transitions between Cassie and Wenzel states. During droplet impingement, the effective water hammer pressure acting due to the sudden change in the velocity of the droplet is determined experimentally and is found to be at least 2 orders of magnitude less than values reported in the literature. The experiments show that the water hammer pressure depends on the surface morphology and capillary pressure of the surface. We propose that the observed reduction in shock pressure may be attributed to the presence of air gaps in the substrate. This feature allows liquid deformation and hence avoids the sudden stoppage of the droplet motion as opposed to droplet behavior on smooth surfaces.
    Langmuir 05/2012; 28(25):9606-15. · 4.38 Impact Factor
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    S. Flueckiger, S. V. Garimella
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    ABSTRACT: The cyclic operation of a molten-salt thermocline tank is simulated to investigate the influence of internal granule diameter and external convection losses on tank performance. Practical constraints limiting thermocline tank height are taken into account. The authors two-temperature model, developed in earlier work (Solar Energy, 84, 974–985, 2010) for the analysis of heat transfer and fluid flow in the thermocline tank, is extended to monitor entropy generation and exergy transport. Storage performance is measured in terms of first- and second-law efficiency definitions, as well as a first-law efficiency used in conjunction with an outflow temperature criterion. Reducing the diameter of the fillerbed granules improves the thermocline tank performance by sustaining higher molten-salt outflow temperatures throughout the discharge phase of the cycle, which results in greater operational efficiency. External convection losses strongly influence entropy generation inside the tank fillerbed due to the development of radial temperature gradients and increased irreversible thermal diffusion. Convection losses also result in lower tank efficiencies due to the reduction of hot molten salt available inside the tank. A comparison of the different efficiency definitions employed in this work reveal that the ad hoc outflow temperature criterion used in past studies provides an overly conservative assessment of thermocline performance.
    Solar Energy. 05/2012; 86(5).
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    ABSTRACT: The thermal and hydrodynamic performance of passive two-phase cooling devices such as heat pipes and vapor chambers is limited by the capabilities of the capillary wick structures employed. The desired characteristics of wick microstructures are high permeability, high wicking capability and large extended meniscus area that sustains thin-film evaporation. Choices of scale and porosity of wick structures lead to trade-offs between the desired characteristics. In the present work, models are developed to predict the capillary pressure, permeability and thin-film evaporation rates of various micropillared geometries. Novel wicking geometries such as conical and pyramidal pillars on a surface are proposed which provide high permeability, good thermal contact with the substrate and large thin-film evaporation rates. A comparison between three different micropillared geometries – cylindrical, conical and pyramidal – is presented and compared to the performance of conventional sintered particle wicks. The employment of micropillared wick structure leads to a 10-fold enhancement in the maximum heat transport capability of the device. The present work also demonstrates a basis for reverse-engineering wick microstructures that can provide superior performance in phase-change cooling devices.
    International Journal of Heat and Mass Transfer - INT J HEAT MASS TRANSFER. 01/2012;

Publication Stats

3k Citations
213.31 Total Impact Points

Institutions

  • 2001–2014
    • Purdue University
      • • School of Mechanical Engineering
      • • Department of Mechanical Engineering Technology (MET)
      West Lafayette, Indiana, United States
  • 2011
    • Peking University
      • Department of Energy & Resources Engineering
      Beijing, Beijing Shi, China
    • Gonzaga University
      Spokane, Washington, United States
  • 2010
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
    • Sony Corporation
      Edo, Tōkyō, Japan
    • Tsinghua University
      Peping, Beijing, China
  • 2008
    • Korea Institute of Machinery and Materials
      Sŏul, Seoul, South Korea
    • University of Houston
      • Department of Mechanical Engineering
      Houston, TX, United States
  • 2003
    • Technische Universität Dresden
      Dresden, Saxony, Germany
  • 1992–2002
    • University of Wisconsin - Milwaukee
      • Department of Mechanical Engineering
      Milwaukee, WI, United States
  • 1990–1991
    • University of California, Berkeley
      • Department of Mechanical Engineering
      Berkeley, CA, United States