Van P. Carey

Pool boiling enhancement techniques for water at low pressure

Saturated pool boiling of water at low pressures from a heated, 12.7*12.7 mm horizontal surface was examined. Rectangular fins, fluidized particulate beds, and surface finishes were used to enhance heat transfer. Water at low pressure significantly decreases the boiling performance below that of water at atmospheric pressure. However, surface temperatures are reduced to values acceptable for cooling electronic components. All rectangular fin geometries were found to enhance heat transfer, although certain geometries were more effective. The finned surfaces extended the base area critical heat flux beyond the critical heat flux for the flat plate. Rougher surface finishes reduced wall superheat temperatures. The addition of non-wetting, TFE particles also decreased wall superheat temperatures for the isolated bubble regime.<<ETX>>

Thermal Bubble Formation on Polysilicon Micro Resistors

Thermal bubble formation in the microscale is of importance for both scientific research and practical applications. A bubble generation system that creates individual, spherical vapor bubbles from 2 to 500 μm in diameter is presented. Line shape, polysilicon resistors with a typical size of 50 x 2 x 0.53 μm 3 are fabricated by means of micromachining. They function as resistive heaters and generate thermal microbubbles in working liquids such as Fluorinert fluids (inert, dielectric fluids available from the 3M company), water, and methanol. Important experimental phenomena are reported, including Marangoni effects in the microscale; controllability of the size of microbubbles; and bubble nucleation hysteresis. A one-dimensional electrothermal model has been developed and simulated in order to investigate the bubble nucleation phenomena. It is concluded that homogeneous nucleation occurs on the microresistors according to the electrothermal model and experimental measurements.

A Review of Heat Transfer Physics

With rising science contents of the engineering research and education, we give examples of the quest for fundamental understanding of heat transfer at the atomic level. These include transport as well as interactions (energy conversion) involving phonon, electron, fluid particle, and photon (or electromagnetic wave). Examples are 1. development of MD and DSMC fluid simulations as tools in nanoscale and microscale thermophysical engineering. 2. nanoscale thermal radiation, where the characteristic structural size becomes comparable to or smaller than the radiation (electromagnetic) wavelength. 3. laser-based nanoprocessing, where the surface topography, texture, etc., are modified with nanometer lateral feature definition using pulsed laser beams and confining optical energy by coupling to near-field scanning optical microscopes. 4. photon-electron-phonon couplings in laser cooling of solids, where the thermal vibrational energy (phonon) is removed by the anti-Stokes fluorescence; i.e., the photons emitted by an optical material have a mean energy higher than that of the absorbed photons. 5. exploring the limits of thermal transport in nanostructured materials using spectrally dependent phonon scattering and vibrational spectra mismatching, to impede a particular phonon bands. These examples suggest that the atomic-level heat transfer builds on and expands electromagnetism (EM), atomic-molecular-optical physics, and condensed-matter physics. The theoretical treatments include ab initio calculations, molecular dynamics simulations, Boltzmann transport theory, and near-field EM thermal emission prediction. Experimental methods include near-field microscopy. Heat transfer physics describes the kinetics of storage, transport, and transformation of microscale energy carriers (phonon, electron, fluid particle, and photon). Sensible heat is stored in the thermal motion of atoms in various phases of matter. The atomic energy states and their populations are described by the classical and the quantum statistical mechanics (partition function and combinatoric energy distribution probabilities). Transport of thermal energy by the microscale carriers is based on their particle, quasi-particle, and wave descriptions; their diffusion, flow, and propagation; and their scattering and transformation encountered as they travel. The mechanisms of energy transitions among these energy carriers, and their rates (kinetics), are governed by the match of their energies, their interaction probabilities, and the various hindering-mechanism rate (kinetics) limits. Conservation of energy describes the interplay among energy storage, transport, and conversion, from the atomic to the continuum scales. With advances in micro- and nanotechnology, heat transfer engineering of micro- and nanostructured systems has offered new opportunities for research and education. New journals, including Nanoscale and Microscale Thermophysical Engineering, have allowed communication of new specific/general as well directly useful/educational ideas on heat transfer physics. In an effort to give a more collective perspective of such contribution, here we put together a collection on small-scale heat transfer involving phonon, electron, fluid particle, and photon. These are 1. Development of MD and DSMC fluid simulations as tools in nanoscale and microscale thermophysical engineering (Carey) 2. Nanoscale thermal radiation (Chen) 3. Laser-based nanoprocessing (Grigoropoulos) 4. Photon-electron-phonon couplings in laser cooling of solids (Kaviany) 5. Exploring the limits of thermal transport in nanostructured materials (Majumdar).

Annular film-flow boiling of liquids in a partially heated, vertical channel with offset strip fins

Abstract Flow visualization photographs and measured local heat transfer data are presented for annular film-flow boiling of saturated liquids in a vertical channel with offset strip fins. A special test section was used in this study which permitted direct visual observation of the boiling process while simultaneously measuring local heat transfer coefficients at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for convective boiling of water, methanol and n -butanol at atmospheric pressure over wide ranges of mass flux and quality. Photographs of the flow indicate that virtually no nucleate boiling is present when the flow is in the film-flow regime. For the fin matrix studied here, complete dryout of the film on the heated surface was not observed to occur at a single downstream location. Instead, dry patches are observed to form at specific locations in the matrix, with the patches increasing in size with downstream distance until the entire film is gone. An approximate analytical model of transport in the liquid film is also presented. A closed-form correlation for the boiling heat transfer coefficient is derived from this model which is in good agreement with our measured data.

Exploration of a Potential-Flow-Based Compact Model of Air-Flow Transport in Data Centers

This paper summarizes an exploration of a compact model of air flow and transport in data centers developed from potential flow theory. Boundaries for the airflow in the data center are often complex due to the numerous rows of servers and other equipment in the facility, and there are generally multiple air inlets and outlets, which produce a fairly complex three-dimensional flow field in the air space in the data center. The general problem of airflow and convective transport in a data center requires accurate treatment of a turbulent flow in a complex flow passage with some buoyancy effects. As a result, full CFD thermofluidic models tend to be time-consuming and tedious to set up for such complex flow circumstances. In this initial study, we formulated an approximate model that retains only the most basic physical mechanisms of the flow. The resulting model of air flow in the data center is based on potential flow theory, which is exact for irrotational inviscid flow. The temperature field resulting from server heat input is determined by solving the convective energy transport equation along potential flow streamlines. This innovative approach, which takes advantage of the irrotational character of the modeled flow, provides a fast computational method for determining the temperature field and convective transport of thermal energy in the data center. Computations to predict the three-dimensional flow and temperature fields with the model typically require less than 60 seconds to complete on a laptop computer. Flow and temperature field results predicted by the model for typical data center flow circumstances are presented and limitations of the model are assessed. Features of an intuitive graphical user interface for the model that simplifies input of the data center design parameters are also described. Results for case studies indicate low sensitivity to mesh size and convergence criteria. Although the flow and temperature field models developed here are more approximate than full CFD methods, they are good first approximations that provide the means to rapidly explore the parameter space for the data center design. This model can be used to quickly identify the optimal region of the design space, whereupon a more detailed CFD modeling can be used to fine-tune an optimal design. The results of this investigation demonstrate that this type of fast compact model can be a very useful tool when used as a precursor to full CFD modeling in data center design optimization.© 2009 ASME

Transport near a vertical ice surface melting in saline water: experiments at low salinities

Time-exposure photographs of the buoyancy-driven flow adjacent to a submerged vertical ice surface melting in 10‰ saline water are presented for ambient water temperatures between 1 and 15°C. For ambient temperatures greater than 1·9°C, the thermal and saline components of the buoyancy force are at least partially opposed to each other. Since most past studies of ice melting in saline water have concentrated on oceanic salinities, little is known about the complicated flow behaviour which results from these opposed buoyancy effects at ambient water salinities between fresh water and oceanic salinities (35‰). The results presented here provide new insight concerning the subtle mechanisms which arise in such flows. Photographs of the entire flow field document the many different and complicated flow configurations that arise. At 10‰, as the ambient temperature is increased from 1 to 15°C, regimes of upward, bi-directional and split flow are observed. In the latter circumstance, the flow is laminar and bi-directional over part of the ice surface and turbulent over the rest of the surface. Bi-directional flow results from reversal of part of the upward wake above the top of the ice surface. Split flow appears to be a consequence of the transition to turbulence of either the inner or outer portion of the laminar bi-directional flow. Measured velocity profiles, surface heat-transfer rates and interface temperatures agree well with the analytical results reported in a previous study for conditions that result in conventional boundary-layer flow. These experimental results, together with those of previous studies, indicate the approximate extent of the different flow regimes that arise for a vertical ice surface melting in cold, low-salinity water.

Disjoining Pressure Effects in Ultra-Thin Liquid Films in Micropassages—Comparison of Thermodynamic Theory With Predictions of Molecular Dynamics Simulations

The concept of disjoining pressure, developed from thermodynamic and hydrodynamic analysis, has been widely used as a means of modeling the liquid-solid molecular force interactions in an ultra-thin liquid film on a solid surface. In particular, this approach has been extensively used in models of thin film transport in passages in micro evaporators and micro heat pipes. In this investigation, hybrid μPT molecular dynamics (MD) simulations were used to predict the pressure field and film thermophysics for an argon film on a metal surface. The results of the simulations are compared with predictions of the classic thermodynamic disjoining pressure model and the Born-Green-Yvon (BGY) equation. The thermodynamic model provides only a prediction of the relation between vapor pressure and film thickness for a specified temperature. The MD simulations provide a detailed prediction of the density and pressure variation in the liquid film, as well as a prediction of the variation of the equilibrium vapor pressure variation with temperature and film thickness. Comparisons indicate that the predicted variations of vapor pressure with thickness for the three models are in close agreement. In addition, the density profile layering predicted by the MD simulations is in qualitative agreement with BGY results, however the exact density profile is dependent upon simulation parameters. Furthermore, the disjoining pressure effect predicted by MD simulations is strongly influenced by the allowable propagation time of injected molecules through the vapor region in the simulation domain. A modified thermodynamic model is developed that suggests that presence of a wall-affected layer tends to enhance the reduction of the equilibrium vapor pressure. However, the MD simulation results imply that presence of a wall layer has little effect on the vapor pressure. Implications of the MD simulation predictions for thin film transport in micro evaporators and heat pipes are also discussed.

Boiling binary mixtures at subatmospheric pressures

The authors present a study of boiling binary mixtures of water with methanol or 2-propanol at subatmospheric pressures. Liquid-phase equilibrium vapor pressures, binary phase equilibrium thermodynamic properties, heat transfer characteristics, and the critical heat flux (CHF) condition are determined for saturated pool boiling from a localized heat source while varying the concentrations of methanol and 2-propanol in water. The heat source is an upward-facing copper surface submerged in a laterally confined, finite pool. Low-pressure boiling of aqueous mixtures provides a means of removing high heat fluxes while maintaining low surface temperatures. Small additions of alcohol to water increase the CHF condition above that of pure water. Higher concentrations of alcohol begin decreasing the CHF condition to that of the pure alcohol. While single-component correlations using mole weighted binary liquid thermodynamic properties have been shown to predict ideal binary mixture boiling behavior, they are unsuccessful in predicting the characteristics of aqueous mixtures. The significance of the results obtained to the use of binary coolants for electronics cooling applications is discussed.<<ETX>>

Transport at large downstream distances in mixed convection flow adjacent to a vertical uniform-heat-flux surface

Abstract A perturbation analysis of mixed convection flow over a vertical semi-infinite surface with uniform heat flux is presented. A matched asymptotic expansion technique is used to construct inner and outer expansions including, for the first time, both mixed convection and higher-order boundary layer effects. It is shown that these effects must be simultaneously included to obtain consistent higher-order approximations to mixed convection boundary layer flow at large downstream distances. Numerical calculations are presented for Pr = 0.733 and 6.7 which indicate the relative magnitudes of mixed convection and non-boundary layer effects. In addition, new experimental measurements of surface heat transfer rates and velocity and temperature profiles are presented for mixed convection flow adjacent to a vertical uniformheat-flux surface in air. The measured profiles are found to be in excellent agreement with those predicted by the analysis. The predicted variation of the Nusselt number is also seen to agree well with the values inferred from the measurements.

An Exergy-Based Figure-of-Merit for Electronic Packages

Chip power consumption and heat dissipation have become important design issues because of increased energy costs and thermal management limitations. As a global compute utility evolves, seamless connectivity from the chip to the data center will become increasingly important. The optimization of such an infrastructure will require performance metrics that can adequately capture the thermodynamic and compute behavior at multiple physical length scales. In this paper, an exergy-based figure-of-merit (FoM), defined as the ratio of computing performance (in MIPS) to the thermodynamic performance (in exergy loss), is proposed for the evaluation of computational performance. The paper presents the framework to apply this metric at the chip level. Formulations for the exergy loss in simple air-cooled heat sink packages are developed, and application of the proposed approach is illustrated through two examples. The first comparatively assesses the loss in performance resulting from different cooling solutions, while the second examines the impact of non-uniformity in junction power in terms of the FoM. Modeling results on a 16 mm×24 mm chip indicate that uniform power and temperature profiles lead to minimal package irreversibility (and therefore the best thermodynamic performance). As the nonuniformity of power is increased, the performance rapidly degrades, particularly at higher power levels. Additionally, the competing needs of minimization of junction temperature and minimization of cooling power were highlighted using the exergy-based approach. It was shown that for a given power dissipation and a specific cooling architecture (such as an air-cooled heat sink solution), an optimal thermal resistance value exists beyond which the costs of increased cooling may outweigh any potential benefits in performance. Thus, the proposed FoM provides insight into thermofluidic inefficiencies that would be difficult to gain from a traditional first-law analysis. At a minimum, the framework presented in this paper enables quantitative evaluation of package performance for different nonuniform power inputs and different choices of cooling parameters. At best, since the FoM is scalable, the proposed metric has the potential to enable a chip-to-data-center strategy for optimal resource allocation.