Ann M. Anderson

Functional significance of variation in bryophyte canopy structure

In most bryophytes, the thickness of boundary layers (i.e., unstirred layers) that surrounds plant surfaces governs rates of water loss. Architectural features of canopies that influence boundary layer thickness affect the water balance of bryophytes. Using field samples (9.3 cm diameter cushions) from 12 populations (11 species) of mosses and liverworts, we evaluated the relationship between canopy structure and boundary layer properties. Canopy structure was characterized using a contact surface probe to measure canopy depth along perpendicular transects at spatial scales ranging from 0.8 to 30 mm on 186 points per sample. Semivariance in depth measurements at different spatial scales was used to estimate three architectural properties: surface roughness (L(r)), the scale of roughness elements (S(r)), and fine-scale surface texture, the latter characterized by the fractal dimension (D) of the canopy profile. Boundary layer properties were assessed by evaporation of ethanol from samples in a wind-tunnel at wind speeds from 0.6 to 4.2 m/s and applied to characterize mass transfer using principles of dynamic similarity (i.e., using dimensionless representations of conductance and flow). In addition, particle image velocimetry (PIV) was used to visualize and quantify flow over two species. All cushions exhibited the characteristics of turbulent as opposed to laminar boundary layers, and conductance increased with surface roughness. Bryophyte canopies with higher L(r) had greater conductances at all wind speeds. Particle image velocimetry analysis verified that roughness elements interacted with flow and caused turbulent eddies to enter canopies, enhancing evaporation. All three morphological features were significantly associated with evaporation. When L(r), S(r), and D were incorporated with a flow parameter into a conductance model using multiple linear regression, the model accounted for 91% of the variation in mass transfer.

Applying heat transfer coefficient data to electronics cooling

This paper reviews the options for defining the heat transfer coefficient, shows how the problem arises, and then describes the steps necessary to properly use the existing heat transfer coefficient data base. There are two options for applying the existing data base to a fully powered array: One can either calculate the adiabatic temperatures of the components from the known heat release distribution in the array or calculate the values of h m that can be used with T m , again using the known heat release distribution

Hydrophobic Silica Aerogels: Review of Synthesis, Properties and Applications

There are many applications for which a material must be water-resistant. Silica aerogels can have unusual properties, including high surface area, low density, low thermal conductivity, and good optical translucency. This combination of properties makes hydrophobic silica aerogels attractive materials for use in applications ranging from transparent insulation systems to drug delivery platforms. These aerogel materials have been prepared using a wide variety of techniques, including incorporation of silica precursors with non-polar substituents into the sol–gel matrix and surface modification of the matrix following gelation. In this chapter we describe the different aerogel synthesis methods, present a discussion of techniques for measuring hydrophobicity and review the extensive literature on hydrophobic silica aerogels, including information on their physical properties and applications.

A Transient Technique for Calibrating Thermochromic Liquid Crystals: The Effects of Surface Preparation, Lighting and Overheat

This paper describes a transient technique for thermochromic liquid crystal hue-temperature calibration and presents results on the effects of surface preparation, lighting and overheat on that calibration. An image acquisition system and calibration test plate were designed for transient measurement and are described. The transient calibration results compare well to steady state calibration results for cooling and heating rates of 0.03 °C/s. Hysteresis occurred when the surface was heated above the play range of the crystals but was not permanent. We found that a poorly prepared liquid crystal surface yielded a “steep” calibration curve, was subject to light ‘warm up’ effects and the hysteresis effect was severe upon overheating.Copyright

Preparing silica aerogel monoliths via a rapid supercritical extraction method.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10(-3) molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

The Effects of Film Thickness, Light Polarization, and Light Intensity on the Light Transmission Characteristics of Thermochromic Liquid Crystals

Thermochromic liquid crystal materials change their crystalline structure and optical properties with temperature, making them useful in temperature measurement applications. This paper presents the results of a study to develop a temperature measurement system that uses light transmission through thermochromic liquid crystals instead of light reflection. We painted Hallcrest R25C10W sprayable liquid crystals on a clear surface and placed it in a spectrophotometer. The amount of light transmitted at monochromatic wavelengths from 400 nm to 700 nm was measured for temperatures from 25°C to 55°C under conditions of nonpolarized, linearly polarized, and cross-polarized light, for three light intensity levels, and three liquid crystal layer thicknesses. As the temperature was increased the amount of light transmitted through the liquid crystal layer increased. When the liquid crystals are in their active range the transmission spectra exhibit an s-curve shape and the percent of light transmitted through the liquid crystals at a fixed temperature increases with increasing wavelength. We detected significant changes in the transmission spectra for temperatures from 27°C to 48°C, whereas when used with reflected light the thermochromic liquid crystals are useful over a significantly smaller range. As the thickness of the thermochromic liquid crystal layer increases or as the incoming light intensity decreases, the amount of light transmitted through the liquid crystals decreases. We also investigated the effects of temperature overheat on the transmission spectra and found that heating the thermochromic liquid crystals above their active range increases the amount of light transmission. However, when the liquid crystals are cooled below their active range they return to their original state. We have analyzed the spectrophotometer data in a number of ways including: (a) total amount of light transmitted, (b) amount of red, green, and blue light transmitted; and (c) spectral curve shape characteristics (peak transmission, inflection wavelength and wavelength for peak transmission) all as a function of temperature. A linear relationship exists between temperature and all of these variables which we believe can be exploited for the development of a charge coupled light camera based light transmission system for temperature measurement.

Aerogels as Platforms for Chemical Sensors

Sensing of chemical species in air, in water and in other solvents is important for a wide variety of applications, including but not limited to monitoring chemical species that might have environmental, health, forensic, manufacturing, or security implications. The unusual properties of aerogels – very high surface area, high porosity, low density – render them particularly appealing for sensing applications. In this chapter, we survey the published reports of the application of aerogels to chemical sensing. These include sensors based on silica, silica composite, titania, carbon and clay aerogels, with spectroscopic and conductimetric detection methods.

Using Objective-Driven Heat Transfer Lab Experiences to Simultaneously Teach Critical Thinking Skills and Technical Content

The heat transfer course at Union College has been redesigned to improve critical thinking and problem solving skills, provide an understanding of the origin of the data and parameters used in heat transfer analysis, and familiarize students with modern experimental and computer analysis tools. The course is taught in the junior year and combines a 4-hr/wk lecture with a 3-hr/wk integrated lab component during a 10 week term. The lecture covers the traditional conduction, convection and radiation heat transfer material whereas in the lab students combine hands on experimental measurements with the use of sophisticated design tools. Our method for achieving the course goals is to use objective-driven lab exercises. Students are asked directed questions about a concrete problem involving heat transfer and are required to develop an experimental/numerical plan to answer the questions. Initial labs are more directed and are designed to introduce experimental / numerical analysis techniques which students may then use in the end of term design project which is more open ended. We have focused on the use of small scale inexpensive equipment which allows us to use multiple set-ups and have students work in small (2-3 person) groups. In this paper we present the weekly lab exercises and discuss how they contribute to the pedagogical goals for the lab, course, and curriculum.Copyright

Elements of a general correlation for turbulent heat transfer

Typically, heat transfer researchers present results in the form of an empirically based relationship between a length-based Nusselt number, a length-based Reynolds number, and a fluid Prandtl number. This approach has resulted in a multitude of heat transfer correlations, each tied to a specific geometry type. Two recent studies have contributed key ideas that support the development ofa more general correlation for turbulent heat transfer that is based on local parameters. Maciejewski and Moffat found that wall heat transfer rates scale with streamwise turbulent velocity fluctuations and Anderson and Moffat found that the adiabatic temperature rise is the driving potential for heat transfer. Using these two concepts and a novel approach to dimensional analysis, the present authors have formulated a general correlation for turbulent heat transfer. This correlation predicts wall heat flux as a function of the turbulent velocity fluctuations, the adiabatic temperature rise, and the fluid properties (density, specific heat, thermal conductivity, and viscosity). The correlation applies to both internal and external flows and is tested in air, water, and FC77.

The Alula and Its Aerodynamic Effect on Avian Flight

The alula, a small thumb-like appendage on a bird wing, is often credited with increasing lift and decreasing the risk of stall during bird flight. Using field based studies; researchers have observed that the alula lifts away from the wing at critical moments in flight, such as take-off and landing. However, to date, there has been no conclusive experimental evidence to support the idea that use of the alula affects lift. To determine the effect of the alula on avian flight, we used a wind tunnel to study the wings of four ducks: the Wood Duck (Aix sponsa), the Redhead Duck (Aythya americana), the Black Scoter (Melanitta americana), and the Lesser Scaup (Aythya affinis). We used a combination of lift/drag measurements and Particle Image Velocimetry (PIV) to test the wings at velocities from 10–16 m/s and angles of attack from −20 to 25 degrees. The alula was observed to naturally lift as the stall angle was approached. Of the four wings, the Black Scoter demonstrated the largest maximum lift coefficient (1.4), followed by the Wood Duck (1.3), the Lesser Scaup (1.2) and lastly, the Redhead Duck (0.9). All four wings had minimum drag coefficients near 0.1. The Lesser Scaup was the only wing which had a measurable change in lift (10%) attributable to alula deployment. PIV results for the flow field around the Lesser Scaup wing showed higher velocities on the top side of the wing when the alula was deflected.Copyright