Ram Ranjan

Analysis of the Wicking and Thin-Film Evaporation Characteristics of Microstructures

The topology and geometry of microstructures play a crucial role in determining their heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures are modeled using SURFACE EVOLVER for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), are considered. Nondimensional capillary pressure, average distance of the liquid free-surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area, and thin-film area are computed. These performance parameters are presented as functions of the nondimensional geometrical parameters characterizing the microstructures, the volume of the liquid filling the structure, and the contact angle between the liquid and solid. Based on these performance parameters, hexagonally-packed spheres on a surface are identified to be the most efficient microstructure geometry for wicking and thin-film evaporation. The solid-liquid contact angle and the nondimensional liquid volume that yield the best performance are also identified. The optimum liquid level in the wick pore that yields the highest capillary pressure and heat transfer is obtained by analyzing the variation in capillary pressure and heat transfer with liquid level and using an effective thermal resistance model for the wick. DOI: 10.1115/1.3160538

Assessment of Nanostructured Capillary Wicks for Passive, Two-Phase Heat Transport

The major factors limiting the thermal performance of passive two-phase heat-spreading devices are the ability of the wick structures to transport liquid by means of capillary forces and the thermal resistance posed by the wicks. Nanoscale geometric enhancements to the wick structure, through the use of carbon nanotubes and metallic nanowires, promise to enhance the capillary transport while at the same time decreasing the thermal resistance due to their high intrinsic thermal conductivity. We analyze the performance of nanostructured wicks in heat-spreading applications. We report that the flow resistance of nanostructures constitutes a major barrier to their use as passive flow-conveying media and identify geometrical parameters that yield high rates of thin-film evaporation while minimizing the flow resistance. The analysis shows that the use of nanostructures as the sole wicking element in a two-phase thermal spreader restricts its footprint area to a size of 4 cm2 for heat flux inputs as low as 1 W/cm2 due to the large flow resistance in the nanowick. To overcome nanowick flow resistance, we propose a nanostructure-enhanced sintered particle wick microstructure that leads to a decrease in the wick thermal resistance by 14% relative to the corresponding wick with the same flow resistance and without nanostructures.

A Numerical Study of Fluid Flow and Heat Transfer around a Square Cylinder at Incidence using Unstructured Grids

A numerical investigation of flow and heat transfer around a square cylinder at incidence (α = 0° − 45°) is presented for a range of Reynolds numbers ( Re = 60 − 150). A finite-volume code suitable for unstructured grids has been developed to simulate the flow. The unstructured grid has been generated using the Delaunay triangulation algorithm. A modified pressure-velocity correction scheme with semi-explicit time-stepping is implemented to solve the Navier-Stokes equations. Collocated grid arrangement has been used for the dependent variables. Convective terms have been discretized using a second order upwind least squares scheme. The formation of Karman vortex street has been captured and the Strouhal number associated with the wake has been determined. The dependence of Strouhal number, force coefficients (drag and lift), moment coefficient and average Nusselt number on Reynolds number, and angle of incidence for a fixed blockage ratio has been reported and analyzed.

Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications

Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K-333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm-0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2 , while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs 50 (>;W/cm2).

A numerical model for transport in heat pipes considering wick microstructure effects

A three-dimensional flow and heat transfer model for heat pipes and vapor chambers is presented. The Navier-Stokes equations along with the energy equation are solved numerically for the liquid and vapor flows in the heat pipe. A porous medium formulation is used for the wick region. Evaporation and condensation at the liquid-vapor interface are modeled using kinetic theory. The effect of the microstructured wick on evaporation and condensation mass fluxes at the liquid-vapor interface is accounted for by integrating a microstructure-level evaporation model (micromodel) with the device-level model (macromodel). Meniscus curvature in the wick is calculated at every location as a result of this coupling. The model accounts for the change in interfacial area in the wick pore, thin-film evaporation, and Marangoni convection effects during phase change at the liquid-vapor interface. The coupled model is used to predict the performance of a screen-mesh wick in a heat pipe and the implications of the coupling are discussed.

Bubble dynamics during capillary-fed nucleate boiling in porous media

Boiling from structured surfaces offers an effective heat transfer enhancement strategy. While there have been attempts to develop models to predict the boiling heat transfer coefficient from sintered screen meshes or sintered particles, less is known about the detailed mechanisms of bubble growth and departure in a porous medium. In this work, we study the growth of a vapor bubble in a micro-scale porous medium in which bubble growth is impeded by the drag offered by the porous medium. We use a volume-of-fluid model to track the liquid-vapor interface during bubble growth. Isotropic and anisotropic arrangements of sintered particles are modeled as uniform particles aligned in hexagonal and square-packed arrangements. A porosity range of 25%-75% is considered, while the particle diameter and wick thickness are held at 200 μm and 1 mm, respectively. The growth of a water vapor bubble nucleating at the interface between the porous medium and the substrate at a wall superheat of 5 K is investigated. Uniform bubble departure, present during the pool boiling of water, is not observed; instead, vapor columns are formed in the porous medium. A moderate wick porosity (50-70%) is found to assist the formation of vertical vapor columns in the wick pores. In contrast, laterally distributed vapor structures form at small porosities (<;50%). Based on the observed vapor column structures in the wick pores, an approximate mathematical model is proposed to optimize the wick thickness for maximum boiling performance. A wick thickness-to-particle diameter ratio in the range of 4 to 5 is found to optimize the heat transfer performance during boiling.

An Experimentally Validated Model for Transport in Thin, High Thermal Conductivity, Low CTE Heat Spreaders

Passive phase-change thermal spreaders such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In the present work, a numerical model for ultra-thin vapor chambers has been developed which is suitable for reliable predictions of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled and the model predictions are compared with experiments on custom-fabricated devices. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive.Copyright

Numerical Study of Evaporation Heat Transfer From the Liquid-Vapor Interface in Wick Microstructures

A numerical model of the evaporating liquid meniscus under saturated vapor conditions in wick microstructures has been developed. Four different wick geometries representing the common wicks used in heat pipes, viz., wire mesh, rectangular grooves, sintered wicks and vertical microwires, are modeled and compared for evaporative performance. The solid-liquid combination considered is copper-water. Steady evaporation is modeled and the liquid-vapor interface shape is assumed to be static during evaporation. Liquid-vapor interface shapes in different geometries are obtained by solving the Young-Laplace equation using Surface Evolver. Mass, momentum and energy equations are solved numerically in the liquid domain, with the vapor assumed to be saturated. Evaporation at the interface is modeled by using appropriate heat and mass transfer rates obtained from kinetic theory. Thermo-capillary convection due to non-isothermal conditions at the interface is modeled for all geometries and its role in heat transfer enhancement from the interface is quantified for both low and high superheats. More than 80% of the evaporation heat transfer is noted to occur from the thin-film region of the liquid meniscus. Very small Capillary and Weber numbers arising due to small fluid velocities near the interface for low superheats validate the assumption of static liquid meniscus shape during evaporation. Solid-liquid contact angle, wick porosity, solid-vapor superheat and liquid level in the wick pore are varied to study their effects on evaporation from the liquid meniscus.© 2009 ASME

Characterization of Microstructures for Heat Transfer Performance in Passive Cooling Devices

The topology and geometry of microstructures play a crucial role in determining heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures have been modeled using the program, Surface Evolver, for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), have been modeled. Non-dimensional capillary pressure, average distance of the free liquid surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area and thin-film percentage of surface area of the liquid meniscus have been computed. These parameters are presented as functions of the non-dimensional geometrical parameters of the microstructures, volume of the liquid filling the structure, as well as the contact angle between the liquid and solid. Based on these non-dimensional performance parameters, the microstructure, contact angle and non-dimensional liquid volume for the best performance are identified.Copyright

Thermoelectric package design for high ambient temperature electronics cooling

With the emergence of wide bandgap power devices, there is now ability to reduce the size and weight of power converters through the use of high frequency switching and high temperature (high-T) operation. Due to the lack of maturity of high temperature signal electronics, it can be cost effective to utilize low-temperature rated components in high-T conditions. Active heat pumping can potentially maintain a safe, low temperature of the electronic components. In this paper, a thermally-insulated electronics package has been designed and tested for the thermal management of low temperature rated electronics in high-T environments. Solid-state thermoelectric heat pumping is exploited to maintain a low electronics device temperature while operating in ambient temperatures higher than 20°C over the electronics device. A physics-based model has been developed for the package optimization which couples the heat sink and thermoelectric performance. The package has been fabricated and tested at various ambient temperature conditions where an excellent agreement with the model predictions is noted. The advantages and drawbacks of employing thermoelectric heat pumping for such applications are discussed.