Brent C. Houchens

Modeling the Floating Zone: Instabilities in the Half Zone and Full Zone

Linear stability analyses are performed on steady, axisymmetric base flows in a laterally heated floating zone (full zone) in microgravity. We treat fully three-dimensional disturbances for a range of Prandtl numbers less than 0.2 and aspect ratios (total zone height to diameter) varying from 1/2 to 2. In all cases, the critical mode is a transition to a steady three-dimensional flow. Axial symmetry of the disturbance, and the critical wave number, vary with both Prandtl number and aspect ratio

Hands-On Water Purification Experiments Using the Adaptive WaTER Laboratory for Undergraduate Education and K-12 Outreach

A bench-top educational system, the Adaptive Water Treatment for Education and Research (WaTER) Laboratory, has been developed as part of a year-long capstone design project. The Adaptive WaTER Lab teaches students about the effectiveness of various water purification techniques. Stackable housings employ six different filtration and purification methods including: sediment filtration, carbon filtration, chemical disinfection, reverse osmosis, forward osmosis, and ultraviolet light disinfection. Filtration pressure is supplied by a hand or foot pump, and two rechargeable batteries are required for the UV sterilization unit. The advantages and limitations of each technique are investigated, with learning performance criteria measured by knowledge of: material costs, contaminant removal or neutralization capabilities (from large sediment to bacteria and viruses to chemicals), robustness and longevity, and power requirements and efficiencies. Finally, suitable combinations of treatment techniques are studied for specific contamination issues, with the ultimate goal of producing potable water. The importance of sustainable water use is also discussed. Background information and suggested experiments are introduced through accompanying educational packets. This system has had a successful impact on undergraduate education. The metrics of success include a published journal article, an awarded EPA P3 educational grant and a pending patent for the undergraduates involved in the development of the Lab. Other undergraduates are currently involved in a design for manufacturability study. Finally, the Lab has served as a demonstration tool in a new interdisciplinary engineering course “Integrated Approaches to Sustainable Development.” The Adaptive WaTER Lab has also been used in hands-on outreach to over 300 underrepresented K-12 students in the Houston area. Two high school students borrowed the original prototype of the Lab to use in an Earth Day demonstration, and one student recently worked on an individual project using the Lab. Because the Lab is portable and requires only human and solar power (to recharge the batteries via a solar backpack), it is also ideal for educational efforts in developing nations. Labs are currently being produced for outreach and donation via three international projects to install water purification systems and/or educational Labs in schools and clinics in Mexico, Lesotho and Swaziland, in collaboration with the Beyond Traditional Borders and Rice 360 health initiatives.Copyright

The Fluid Mechanics of Membrane Filtration

Reverse osmosis and nanofiltration membranes remove colloids, macromolecules, salts, bacteria and even some viruses from water. In crossflow filtration, contaminated water is driven parallel to the membrane, and clean permeate passes through. A large pressure gradient exists across the membrane, with permeate flow rates two to three orders of magnitude smaller than that of the crossflow. Membrane filtration is hindered by two mechanisms, concentration polarization and caking. During filtration, the concentration of rejected particles increases near the membrane surface, forming a concentration polarization layer. Both diffusive and convective transport drive particles back into the bulk flow. However, the increase of the apparent viscosity in the concentration polarization layer hinders diffusion of particles back into the bulk and results in a small reduction in permeate flux. Depending on the number and type of particles present in the contaminated water, the concentration polarization will either reach a quasi-steady state or particles will begin to deposit onto the membrane. In the later case, a cake layer eventually forms on the membrane, significantly reducing the permeate flux. Contradictive theories suggest that the cake layer is either a porous solid or a very viscous (yield stress) fluid. New and refined models that shed light on these theories are presented.Copyright

Volunteerism in engineering outreach: Motivations and surprising outcomes for undergraduate mentors

The motivations for volunteering of former DREAM Head Mentors is investigated. DREAM is an engineering outreach program at Rice University in collaboration with underserved Houston, Texas high schools. The existing Volunteer Functions Inventory and Volunteer Motivation Inventory have been adapted for this study. The internally developed Mentors Self-Assessment Survey has been improved with new qualitative items, and scale items from another outreach program (TEAMS). Results indicate that volunteer undergraduate Head Mentors were overwhelmingly motivated by the Values function or category, related to humanitarian concern for others. There is no volunteer requirement at Rice University and no mentors have ever requested certification of volunteer hours to maintain scholarships or fellowships, supporting the findings of highly altruistic motivations. Among former Head Mentors, 47% have pursed STEM graduate degrees - approximately twice the rate of Rice alumni. Half of these are women and half are Hispanic. Head Mentors are leaders at the schools and often participate in research and publication of findings. This study suggests that leadership development and introduction to research through engineering outreach may be an untapped pathway to diversify the undergraduate engineering pipeline and simultaneously encourage the pursuit of graduate degrees in engineering, particularly among women and underrepresented students.

Simulation of colloidal fouling by coupling a dynamically updating velocity profile and electric field interactions with Force Bias Monte Carlo methods for membrane filtration.

Pressure-driven flow through a channel with membrane walls is modeled for high particulate volume fractions of 10%. Particle transport is influenced by Brownian diffusion, shear-induced diffusion, and convection due to the axial crossflow. The particles are also subject to electrostatic double layer repulsion and van der Waals attraction, from both particle-particle and particle-membrane interactions. Force Bias Monte Carlo (FBMC) simulations predict the deposition of the particles onto the membranes, where both hydrodynamics and the change in particle potentials determine the probability that a proposed move is accepted. The particle volume fraction is used to determine an apparent local viscosity observed by the continuum flow. As particles migrate, the crossflow velocity field evolves in quasi-steady fashion with each time instance appearing fully developed in the downstream direction. Particles subject to combined hydrodynamic and electric effects (electrostatic double layer repulsion and van der Waals attraction) reach a more stable steady-state as compared to systems with only hydrodynamic effects considered. As expected, at higher crossflow Reynolds numbers more particles remain in the crossflow free stream.

Learning Outcomes From Design of Wind Turbines Carried Out by Underserved High School Mentees Participating in DREAM

The fall 2011 DREAM design project required teams to design and build a working wind turbine. The turbines were tested at three wind speeds, with the most points awarded for power production at the lowest speed, forcing mentees to optimize their blade designs. The Pre-engineering Concept Inventory (PCI) and Intuition Inventory (II), which focus on wind energy, were used to measure pre- and post-content knowledge of high school mentees. The use of mini-lectures to convey content simultaneously with design is discussed.The Inventories show that participation in DREAM generally increases mentees’ understanding of wind energy concepts. However, insufficient mathematics foundations hamper their ability to understand algebraic representations and evaluate numerical predictions. Results provide feedback on current practices and help pinpoint specific areas for improvement to increase the efficacy of DREAM in future years.Copyright

Mechanical Wheelchair Propulsion System for Patients With Arthrogryposis

Arthrogryposis is a congenital disorder characterized by extreme joint stiffness that inhibits strength and flexibility in upper and lower extremities. Cases vary in severity, but this research focuses on those in which patients require a wheelchair for mobility. Currently, two conventional designs exist: mechanical and electric wheelchairs. For most arthrogryposis patients, existing mechanical wheelchairs are insufficient for independent propulsion as their joints are severely impaired, prohibiting them from reaching the outer handrails on the wheels and expending enough force to propel and steer. Existing devices that improve the mechanical advantage of wheelchairs are insufficient for the needs of these patients who have very limited and specific ranges of motions, which are not compatible with the required force inputs. Though electric wheelchairs allow independent mobility, they are expensive to maintain and not easily portable, limiting their use by socioeconomically disadvantaged patients.Arthrogryposis patients require a lightweight, portable and durable mechanical wheelchair that takes advantage of the user’s specific strengths, and is easily maneuvered in all directions without assistance. A design is presented for a socioeconomically disadvantaged teenage client with arthrogryposis. After taking data regarding the ranges of motion and strength of the client, a new propulsion system was designed and retrofitted to a conventional wheelchair. Prototye I has been tested and a second-generation design which fits the needs of a wider audience suffering from arthrogryposis is presented.Copyright

Comparison of the 4th Order Stream Function and 2nd Order Vorticity Transport Representations for Modeling Float-Zone Crystal Growth With a Constant Magnetic Field

During optically heated float-zone crystal growth processing, a cylindrical melting zone forms from a polycrystalline rod by lateral heating, then re-solidifies into single crystal rod. Thermocapillary forces drive a flow in the melt region. This base flow is susceptible to instabilities, which lead to nonuniformities and defects in the crystal. In order to minimize these imperfections, a magnetic field is employed to damp the base flow within the melt region. Modeling this base flow has proven challenging, and two representations are discussed here. The float-zone crystal growth base flow under the influence of a constant magnetic field and neglecting buoyancy is investigated with a full-zone model. The flow field and temperature distributions are calculated by a steady state, axisymmetric spectral collocation method using Chebyshev polynomials as basis functions. A 2nd order vorticity transport representation is compared with a 4th order stream function formulation. The results show high consistency between the models.Copyright

Numerical Issues Associated With the Full-Zone Model of the Optically Heated Floating-Zone Used in Semiconductor Processing

Recent models of the thermocapillary driven liquid bridge or full-zone (FZ) have highlighted numerical difficulties in the system, associated with the large velocity gradients near the free surface and the geometric singularity at r = 0. High resolution spectral solutions have been developed to account for these issues. These result in complex representations and highly specialized numerical procedures. After a brief review of these methods, a simplified formulation for the FZ model with strong form boundary conditions is proposed and discussed. Comparisons are made using base flows and stability analyses. Existing solutions have overcome the geometric singularity either by moving the grid away from the r = 0 axis, or by maintaining the correct Taylor series expansion in the representation of each dependent variable. The former has the weakness that an important constraint is not applied. The later formulation is rigorous, but results in complex expressions for the governing equations. To decrease the load associated with the mathematical manipulation and numerical implementation of this method, here a more general Chebyshev polynomial representation of the stream function is applied to the axisymmetric base flow. This removes the need to maintain the proper expansion and instead offers a set of equations in strong form by treating the axis as a boundary known from the spatial symmetry of the model. However, this does not guarantee that momentum is conserved at the internal symmetry boundaries. Various applications of the other boundary conditions are also studied. In the most accurate representation, all boundary conditions except the thermocapillary condition are cast in the strong form via orthogonality. These strong equations must be chosen carefully to avoid introducing redundant conditions. However, the result is a mathematically simpler representation that mimics the accuracy of previous methods.Copyright

Stability of Thermoelectromagnetic Convection

The role of thermoelectromagnetic convection (TEMC) on the stability of a range of flows is investigated. Here we discuss the general features of TEMC, and describe experiments in which this effect is thought to have significance. The general formulation for TEMC at a solid-liquid interface is presented. Initial results are benchmarked with existing analytical and numerical solutions.Copyright