Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

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Heat and Mass Transfer

Entropy Generation Minimization

Condensation is a phase change phenomenon often encountered in nature, as well as used in industry for applications including power generation, thermal management, desalination, and environmental control. For the past eight decades, researchers have focused on creating surfaces allowing condensed droplets to be easily removed by gravity for enhanced heat transfer performance. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces with even higher droplet mobility and, in some cases, coalescence-induced droplet jumping. Here, we provide a review of new insights gained to tailor superhydrophobic surfaces for enhanced condensation heat transfer considering the role of surface structure, nucleation density, droplet morphology, and droplet dynamics. Furthermore, we identify challenges and new opportunities to advance these surfaces for broad implementation in thermofluidic systems.

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Condensation Heat Transfer on Superhydrophobic Surfaces

United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center)

Condensation heat transfer on superhydrophobic surfaces

Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in the removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively, sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ∼78°) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less-wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in the condensate collection rate, up to 19% (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of noncondensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.

Enhancing dropwise condensation through bioinspired wettability patterning.

The continually increasing heat generation rates in high performance electronics, radar systems and data centers require development of efficient heat exchangers that can transfer large heat loads. In this paper, we present the design of a new high-performance heat exchanger capable of transferring 1000 W while consuming less than 33 W of input electrical power and having an overall thermal resistance of 0.05 K/W. The low thermal resistance is achieved by using a loop heat pipe with a single evaporator and multiple condenser plates that constitute the array of fins. Impellers between the fins are driven by a custom permanent magnet synchronous motor in a compact volume of 0.1 × 0.1 × 0.1 m to maximize the heat transfer area and reduce the required airflow rate and electrical power. The design of the heat exchanger is developed using analytical and numerical methods to determine the important parameters of each component. The results form the basis for the fabrication and experimental characterization that is currently under development.

Design of an Integrated Loop Heat Pipe Air-Cooled Heat Exchanger for High Performance Electronics

Objective: To test a method for performing electrical impedance myography (EIM) in the mouse hind limb for the assessment of disease status in neuromuscular disease models. Methods: An impedance measuring device consisting of a frame with electrodes embedded within an acrylic head was developed. The head was rotatable such that data longitudinal and transverse to the major muscle fiber direction could be obtained. EIM measurements were made with this device on 16 healthy mice and 14 amyotrophic lateral sclerosis (ALS) animals. Repeatability was assessed in both groups. Results: The technique was easy to perform and provided good repeatability in both healthy and ALS animals, with intrasession repeatability (mean 6 SEM) of 5% 61% and 12% 62%, respectively. Significant differences between healthy and ALS animals were also identified (e.g., longitudinal mean 50 kHz phase was 1860.6u for the healthy animals and 1461.0u for the ALS animals, p=0.0025). Conclusions: With this simple device, the EIM data obtained is highly repeatable and can differentiate healthy from ALS animals. Significance: EIM can now be applied to mouse models of neuromuscular disease to assess disease status and the effects of therapy.

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A technique for performing electrical impedance myography in the mouse hind limb: data in normal and ALS SOD1 G93A animals.

Shear-induced softening of nanocrystalline metal interfaces at cryogenic temperatures

Modern computer processors require significant
cooling to achieve their full performance. The "efficiency" of heat
sinks is also becoming more important: cooling of electronics
consumes 1% of worldwide electricity use by some estimates.
Unfortunately, current cooling technologies often focus on
improving heat transfer at the expense of efficiency. The present
work focuses on a unique, compact, and efficient air cooled heat
sink which addresses these shortcomings. While conventional air
cooled heat sinks typically use a separate fan to force air flow
over heated fins, the new design incorporates centrifugal fans
directly into the body of a loop heat pipe with multiple planar
condensers. These "integrated fans" rotate between the planar
condensers, in close proximity to the hot surfaces, establishing a
radially outward flow of cooling air. The proximity of the rotating
impellers to the condenser surfaces results in a marked enhancement
in the convective heat transfer coefficient without a large
increase in input power. To develop an understanding of the heat
transfer in integrated fan heat sinks, a series of experiments was
performed to simultaneously characterize the fan performance and
average heat transfer coefficients. These characterizations were
performed for 15 different impeller profiles with various
impeller-to-gap thickness ratios. The local heat transfer
coefficient was also measured using a new heated-thin-film infrared
thermography technique capable of applying various thermal boundary
conditions. The heat transfer was found to be a function of the
flow and rotational Reynolds numbers, and the results suggest that
turbulent flow structures introduced by the fans govern the
transport of thermal energy in the air. The insensitivity of the
heat transfer to the impeller profile decouples the fan design from
the convection enhancement problem, greatly simplifying the heat
sink design process. Based on the experimental results, heat
transfer and fan performance correlations were developed (most
notably, a two-parameter correlation that predicts the
dimensionless heat transfer coefficients across 98% of the
experimental work to within 20% relative RMS error). Finally,
models were developed to describe the scaling of the heat transfer
and mechanical power consumption in multi-fan heat sinks. These
models were assessed against experimental results from two
prototypes, and suggest that future integrated fan heat sink
designs can achieve a 4x reduction in thermal resistance and 3x
increase in coefficient of performance compared to current
state-of-the-art air cooled heat sinks.

Active heat transfer enhancement in integrated fan heat sinks

A supercritical hydrogen liquefaction cycle is proposed and analyzed numerically. If hydrogen is to be used as an energy carrier, the efficiency of liquefaction will become increasingly important. By examining some difficulties of commonly used industrial liquefaction cycles, several changes are suggested and a readily scalable, supercritical, helium-cooled hydrogen liquefaction cycle is proposed. An overlap in flow paths of the two coldest stages allowed the heat exchanger losses to be minimized and the use of a single-phase liquid expander eliminates the pressure reduction losses associated with a Joule-Thomson valve. A computational model of the cycle was developed to investigate the effects of altering component efficiencies and various system parameters on the cycle efficiency. Furthermore, a heat exchanger simulation was developed to verify the feasibility and to estimate the approximate size of the heat exchangers in the cycle simulation. For a large, 50-ton-per-day plant with reasonable estimates of achievable component efficiencies, the proposed cycle offers a modest improvement in efficiency over the current state of the art. In comparison to the 30-40% Second Law efficiencies of today’s most advanced industrial plants, efficiencies of 39-44% are predicted for the proposed cycle, depending on the heat exchange area employed.

Wayne L. Staats

Papers

Design of an Integrated Loop Heat Pipe Air-Cooled Heat Exchanger for High Performance Electronics

A technique for performing electrical impedance myography in the mouse hind limb: data in normal and ALS SOD1 G93A animals.

Shear-induced softening of nanocrystalline metal interfaces at cryogenic temperatures

Active heat transfer enhancement in integrated fan heat sinks

Analysis of a supercritical hydrogen liquefaction cycle