Timothy A. Shedd

Next Generation Spray Cooling: High Heat Flux Management in Compact Spaces

For many years, spray cooling has been known as a promising technology for the removal of high heat fluxes. However, that promise has yet to be fully realized. This work presents a current understanding of the mechanisms of spray cooling and its limitations. To address these limitations, a novel spray nozzle array is described that allows for scalable, high-performance cooling. However, fluid management remains a challenging concern that may limit the application of this technology.

Syndecan-1 is required to maintain intradermal fat and prevent cold stress.

Homeostatic temperature regulation is fundamental to mammalian physiology and is controlled by acute and chronic responses of local, endocrine and nervous regulators. Here, we report that loss of the heparan sulfate proteoglycan, syndecan-1, causes a profoundly depleted intradermal fat layer, which provides crucial thermogenic insulation for mammals. Mice without syndecan-1 enter torpor upon fasting and show multiple indicators of cold stress, including activation of the stress checkpoint p38α in brown adipose tissue, liver and lung. The metabolic phenotype in mutant mice, including reduced liver glycogen, is rescued by housing at thermoneutrality, suggesting that reduced insulation in cool temperatures underlies the observed phenotypes. We find that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for adipocyte differentiation in vitro. Intradermal fat shows highly dynamic differentiation, continuously expanding and involuting in response to hair cycle and ambient temperature. This physiology probably confers a unique role for Sdc1 in this adipocyte sub-type. The PPARγ agonist rosiglitazone rescues Sdc1−/− intradermal adipose tissue, placing PPARγ downstream of Sdc1 in triggering adipocyte differentiation. Our study indicates that disruption of intradermal adipose tissue development results in cold stress and complex metabolic pathology.

Reprint of: A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Abstract Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.

Measurements of the dynamic contact angle for conditions relevant to immersion lithography

The semiconductor industry has used optical lithography to create impressively small features. However, the resolution of optical lithography is approaching limits based on light wavelength and numerical aperture. Immersion lithography is a means to extend the resolution by inserting a liquid with a high index of refraction between the lens and wafer. This enables the use of higher numerical aperture optics. Several engineering obstacles must be overcome before immersion lithography can be used on an industry-wide scale. One such challenge is the deposition of the immersion liquid onto the wafer during the scanning process; any residual liquid left on the wafer is a potential defect mechanism. The residual liquid deposition is controlled by the details of the fluid management system, and is strongly dependent on the three-phase contact line. Therefore, this work concentrates on understanding the behavior of this contact line, specifically by measuring the dynamic contact angle and the critical velocity for liquid deposition. A contact angle measurement technique is developed and verified; the technique is subsequently applied to measure the dynamic advancing and receding contact angle for a series of resist-covered surfaces under conditions that are relevant to immersion lithography.

Characteristics of the Liquid Film and Pressure Drop in Horizontal, Annular, Two-phase Flow Through Round, Square and Triangular Tubes

A unique set of liquid film thickness and pressure drop data has been obtained for horizontal, annular flow of air and water through round, square and triangular tube using a noninvasive, optical liquid film thickness measurement system. In the square and triangular tubes, the liquid film was thinned in the corners, indicating an effect of turbulent secondary flows in these geometries

Visualization of two-phase flow through microgrooved tubes for understanding enhanced heat transfer

Abstract Though widely used, questions remain as to the mechanisms by which micro-fin or microgrooved tubes enhance heat transfer performance. In this work, new experimental liquid film thickness profiles in adiabatic, air–water flow through clear tubes with 20 microgrooves at three different helix angles are presented. These results correlate well with the asymmetrical temperature measurements and heat transfer coefficients seen by previous investigators. Important observations include increased wall wetting for a given flow condition, decreasing influence of the grooves with increasing gas velocity, and a rotational redistribution of the liquid film by helical grooves, though without indication of a swirling behavior.

Cross-Sectional Imaging of the Liquid Film in Horizontal Two-Phase Annular Flow

Planar laser induced fluorescence (PLIF) was applied to horizontal air/water two-phase annular flow in order to clearly image the liquid film and interfacial wave behavior at the top, side and bottom of the tube. The visualization section was fabricated from FEP, which has nearly the same refractive index as water at room temperature. This index-matched test section was used to allow imaging of the water to within approximately 10 microns of the 15.1 mm I.D. tube wall. A small amount of dye was added to the water with a peak excitation wavelength near that of a pulsed Nd:YAG laser (532 nm). The laser system generated an approximately 5 ns pulsed light sheet at 30 Hz. Images of the liquid film were captured using a digital video camera with a macro lens for a resolution of about 8.2 microns/pixel. Cross-sectional data at 68 annular flow conditions were obtained. The observations of the liquid film between waves indicated that the film thickness was relatively insensitive to both gas and liquid flow in the annular regime, confirming film thickness measurements reported elsewhere. In addition, the structure of waves changes significantly from wavy-annular, where peaked or cresting waves dominate, to fully annular, where the waves are much more turbulent and unstructured. The wave height decreases with increased gas flow and is relatively insensitive to increased liquid flow in the annular regime. The entrainment of gas in the liquid by the waves is very apparent from these images. Although the precise entrainment mechanisms are not entirely clear, a viable folding action mechanism is proposed. The visualization results will be discussed in relation to both conceptual and computational annular flow modeling.© 2004 ASME

Correlation of Heat Transfer Data to Film Thickness Data of the Thin Film Found in Spray Cooling

As electronic circuit design and packaging technology progresses, the density and power levels of electronic components is increasing at a nearly exponential rate. The higher heat loads dissipated by these devices are nearing the limits of traditional cooling techniques. One method capable of removing heat fluxes as high as 100 W/cm2 is spray cooling. This process involves the impingement of liquid droplets onto a heated surface, forming a thin two-phase film. In order to create reliable models of the heat transfer during spray cooling, the behavior of the film must be understood. This paper presents an investigation into the behavior of the thin film found in spray cooling. A study was performed to relate experimental measurements of the heat transfer coefficients to experimental measurements of film thickness as they vary spatially over a die surface. Both a single nozzle and a multi-nozzle array were investigated. Measured heat transfer coefficients ranged from 0.2 to 1.2 W/m2 K and film thicknesses ranged from 90 to 300 μm.Copyright

Static and Dynamic Wetting Characteristics of Nano-patterned Surfaces

In efforts to increase the scanning speeds of the current state-of-the-art IC chip manufacturing process, immersion lithography, the present work investigates the liquid wetting effects of oriented surface nanopatterns with varying depth. In this set of experiments, both static and forced dynamic contact line behaviors of two liquids were studied on two substrate materials imprinted with line-space nanopatterns of varying depth and hybridization (second tier of nano-posts). These experiments found a variety hydrophobic and hydrophilic behaviors, some of which deviate from conventional wetting theories. Specifically, the static and dynamic liquid contact angles are found to consistently depend on the depth of patterns with the same interfacial areas, and a surface with contact angle less than 90° was turned highly hydrophobic with the addition of patterns.

Static and dynamic contact angles of water on photoresist

Fluid management issues in an immersion lithography system include the retention of the liquid (i.e., prevention of residual liquid on the wafer) and the possible entrainment of gas bubbles within the immersion fluid. Three key parameters strongly influence the control of fluid within the lens gap: the static liquid/resist contact angle, the contact angle hysteresis, and the dynamic contact angle characteristics. This article presents a comprehensive set of data for these parameters on silicon wafers coated with six different photoresists and describes the experimental apparatus and data reduction techniques used to collect the data. Measurements for six candidate photoresists, one with a top-coat, indicate that air entrainment due to contact line motion is highly unlikely for typical immersion lithography systems. However, significant contact angle hysteresis does exist that may lead to meniscus failure and to the deposition of droplets at low to moderate wafer velocities. In addition, the receding dynam...