Nancy R. Hall

Multi-scale kinetics of a field-directed colloidal phase transition

Polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We anneal such gels formed from magneto-rheological fluids by toggling the field strength at varied frequencies. This processing allows the arrested structure to relax periodically to equilibrium—colloid-rich, cylindrical columns. Two distinct growth regimes are observed: one in which particle domains ripen through diffusive relaxation of the gel, and the other where the system-spanning structure collapses and columnar domains coalesce apparently through field-driven interactions. There is a stark boundary as a function of magnetic field strength and toggle frequency distinguishing the two regimes. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. Such directed assembly may be harnessed to create unique materials from dispersions of colloids.

Experimental Investigation of Flow Condensation in Microgravity

Future manned space missions are expected to greatly increase the space vehicles size, weight, and heat dissipation requirements. An effective means to reducing both size and weight is to replace single-phase thermal management systems with two-phase counterparts that capitalize upon both latent and sensible heat of the coolant rather than sensible heat alone. This shift is expected to yield orders of magnitude enhancements in flow boiling and condensation heat transfer coefficients. A major challenge to this shift is a lack of reliable tools for accurate prediction of two-phase pressure drop and heat transfer coefficient in reduced gravity. Developing such tools will require a sophisticated experimental facility to enable investigators to perform both flow boiling and condensation experiments in microgravity in pursuit of reliable databases. This study will discuss the development of the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS), which was initiated in 2012 in collaboration between Purdue University and NASA Glenn Research Center. This facility was recently tested in parabolic flight to acquire condensation data for FC-72 in microgravity, aided by high-speed video analysis of interfacial structure of the condensation film. The condensation is achieved by rejecting heat to a counter flow of water, and experiments were performed at different mass velocities of FC-72 and water and different FC-72 inlet qualities. It is shown that the film flow varies from smooth-laminar to wavy-laminar and ultimately turbulent with increasing FC-72 mass velocity. The heat transfer coefficient is highest near the inlet of the condensation tube, where the film is thinnest, and decreases monotonically along the tube, except for high FC-72 mass velocities, where the heat transfer coefficient is enhanced downstream. This enhancement is attributed to both turbulence and increased interfacial waviness. One-ge correlations are shown to predict the average condensation heat transfer coefficient with varying degrees of success, and a recent correlation is identified for its superior predictive capability, evidenced by a mean absolute error of 21.7%.

Experimental investigation into the impact of density wave oscillations on flow boiling system dynamic behavior and stability

In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.

Multiphase Flow Separators in Reduced Gravity

Gas phase and liquid phase separation is necessary for one of two reasons. First, system-critical components are designed to specifically operate in a single phase mode only. Pumps, especially centrifugal pumps, lose their prime when gas bubbles accumulate in the impellor housing. Turbines and compressors suffer from erosion problems when exposed to vapor laden with liquid droplets. The second reason is that system performance can be significantly enhanced by operating in a single phase mode. The condensation heat transfer coefficient can be enhanced when the liquid of an entering two-phase stream is stripped thus permitting initial direct contact of the vapor with the cold walls of the condenser. High efficiency and low mass Environmental Control and Life Support Systems invariably require multiphase processes. These systems consist of water filtration and purification via bioreactors that encounter two phase flow at the inlets from drainage streams associated with the humidity condensate, urine, food processing, and with ullage bubble effluent from storage tanks. Entrained gases in the liquid feed, could have deleterious effects on the performance of many of these systems by cavitating pumps and poisoning catalytic packed bed bioreactors. Phase separation is required in thermal management and power systems whereby it is necessary to have all vapor entering the turbine and all liquid exiting the condenser and entering the pump in order to obtain the highest reliability and performance of these systems. Power systems which utilize Proton Exchange Membrane Fuel Cells generate a humidified oxygen exit stream whereby the water vapor needs to be condensed and removed to insure reliable and efficient system operation. Gas-liquid separation can be achieved by a variety of means in low gravity. Several active and passive techniques are examined and evaluated. Ideally, a system that functions well in all gravity environments that the system experiences is a requirementCopyright

Realization of hydrodynamic experiments on quasi-2D liquid crystal films in microgravity

Abstract Freely suspended films of smectic liquid crystals are unique examples of quasi two-dimensional fluids. Mechanically stable and with quantized thickness of the order of only a few molecular layers, smectic films are ideal systems for studying fundamental fluid physics, such as collective molecular ordering, defect and fluctuation phenomena, hydrodynamics, and nonequilibrium behavior in two dimensions (2D), including serving as models of complex biological membranes. Smectic films can be drawn across openings in planar supports resulting in thin, meniscus-bounded membranes, and can also be prepared as bubbles, either supported on an inflation tube or floating freely. The quantized layering renders smectic films uniquely useful in 2D fluid physics. The OASIS team has pursued a variety of ground-based and microgravity applications of thin liquid crystal films to fluid structure and hydrodynamic problems in 2D and quasi-2D systems. Parabolic flights and sounding rocket experiments were carried out in order to explore the shape evolution of free floating smectic bubbles, and to probe Marangoni effects in flat films. The dynamics of emulsions of smectic islands (thicker regions on thin background films) and of microdroplet inclusions in spherical films, as well as thermocapillary effects, were studied over extended periods within the OASIS (Observation and Analysis of Smectic Islands in Space) project on the International Space Station. We summarize the technical details of the OASIS hardware and give preliminary examples of key observations.

OBSERVATION AND ANALYSIS OF SMECTIC ISLANDS IN SPACE (OASIS)

1 Principal Investigator, Professor, Department of Physics, University of Colorado, Boulder, CO. 2 Co. Investigator, Research Professor, Department of Physics, University of Colorado, Boulder, CO. 3 Co. Investigator, Research Professor, Department of Physics, University of Colorado, Boulder, CO. 4 Research Scientist, Department of Physics, University of Colorado, Boulder, CO. 5 Co. Principal Investigator, Professor, Department of Physics, University of Magdeburg, Magdeburg, Germany. 6 Project Scientist, National Cneter for Space Exploration, NASA Glenn Research Center 7 Project Manager, NASA Glenn Research Center 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 4 7 January 2011, Orlando, Florida AIAA 2011-1199

Optical Thin Film Measurements using a Vapor Phase Catalytic Ammonia Removal System at Elevated Temperatures

A Vapor Phase Catalytic Ammonia Removal (VPCAR) test system was developed for the purpose of purifying water from wastewater products in low gravity environments. The wiped film rotating disk inside of the engineering test apparatus has been outfitted with a heater element on its back side to produce temperatures adequate to increase evaporation rates and even boil water. One of the main characteristic indicators of the VPCAR efficiency is the water film thickness present on the wiped film rotating disk (WFRD). This paper examines two different VPCAR optical film measurement systems and their performance in the VPCAR system during operations near boiling point. The film measurement method is based on trace amounts of an organic tracer dye in solution. The fluorescence emission energy and thus the interpretation of film thickness become more complicated at elevated temperatures. With elevated temperature variance, emission is no longer strictly a function of the excitation energy, dye concentration and optical path length (i.e. film thickness). The temperature of the dye/water solution has a profound effect on the emission amplitude making fluorescence-based film thickness calibration essential for anticipated elevated temperatures to compensate for this temperature dependence.

Extensional Properties of a Dilute Polymer Solution Following Preshear in Microgravity

The Shear History Extensional Rheology Experiment (SHERE) is an International Space Station (ISS) experiment designed to study the effects of a preshear history on the transient extensional viscosity of a dilute polymer solution in a uniaxial stretching flow. The absence of gravitational body forces allows us to measure the capillary thinning of the fluid filament after cessation of the extensional deformation without sagging of the liquid bridge. Understanding the deformation and thinning of polymeric solutions in complex flows containing both shearing and extensional kinematics is particularly relevant in a wide variety of industries, including fiber-spinning, injection molding, food and consumer product processing, as well as future “containerless processing” operations. The SHERE experiment offers the ability to preshear the test samples before imposing a uniaxial stretching flow in order to explore the impact of this preshearing on the sample extensional viscosity and elastocapillary thinning. After the SHERE main hardware was launched to the ISS on-board Shuttle Mission STS-120, two batches of 20 and 25 fluid samples were successively launched on-board Shuttle Missions STS123 and STS-126. The SHERE experiments were performed by astronauts Greg Chamitoff and Mike Fincke between July 2008 and January 2009. In this talk, we will focus on the main results obtained for a well-characterized dilute polymer solution and compare them to ground-based experiments. In addition, we will show potential applications of the current microgravity experimental findings.