Browsing by Author "Tarek Echekki, Committee Member"

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Batch Sedimentation in an Impulsively Heated System.
(2010-07-22) Joshi, Ameya; Thomas Ward, Committee Chair; Alexei Saveliev, Committee Member; Zhilin Li, Committee Member; Tarek Echekki, Committee Member
An Experimental Evaluation of the Performance of the Amorphous Silicon PV Array on the NCSU AFV Garage
(2007-04-24) Christy, Daniel William; James Leach, Committee Member; Tarek Echekki, Committee Member; Herbert M. Eckerlin, Committee Chair
A comprehensive performance test has been conducted on the 3 kW amorphous silicon photovoltaic (PV) system on the roof of the Alternative Fuel Vehicle Garage of the North Carolina Solar Center. The purpose of this testing program was to measure the performance of the PV system, to determine if any deterioration has occurred over the past three years since installation, and to evaluate the performance of the individual circuits that makeup the PV system. Test conducted on the individual circuits of the PV system showed significant differences. This is particularly true for the two different solar panel models, which were installed using different techniques. Numerous tests were conducted on these circuits to isolate the problem. The current-voltage curves of the factory-laminated panels were much worse than the self-laminated panels. No cause of the poor performance could be definitively established. Discussions with the PV panel manufacturer are continuing to identify the cause of the variation in PV circuit performance. Comparisons made to performance data recorded in 2003-2004 show a similar kWh production over 3-month periods, this is encouraging. However, comparisons between global irradiance and AC power production show a 9% reduction in power production. Continued research is recommended to further evaluate the circuit issues and to study how PV panel temperatures can be reduced so as to improve over PV system efficiencies.
Experimental Quantification of Transient Stretch Effects from Vortices Interacting with Premixed Flames
(2008-12-01) Danby, Sean James; William Roberts, Committee Chair; Tarek Echekki, Committee Member; Richard Gould, Committee Member; Jack Edwards, Committee Member
The understanding of complex premixed combustion reactions is paramount to the development of new concepts and devices used to increase the overall usefulness and capabilities of current technology. The evolution from laminar spherically propagating flames to turbulent chemistry is a logical and necessary process to study the complex interactions which occur within any modern practical combustion device. Methane-air flames were chosen to observe the mild affects of thermo-diffusive stability. Five primary propane equivalence ratios were utilized for investigation: 0.69, 0.87, 1.08, 1.32, and 1.49. The choice of equivalence ratio was strategically made so that the 0.69/1.49 and 0.87/1.32 mixtures have the same undiluted flame propagation rate, dr/dt. Therefore, in the undiluted case, there are two flame speeds represented by these mixtures. Three vortices were selected to be used in this investigation. The vortex rotational velocities were measured to be 77 cm/s, 266 cm/s and 398 cm/s for the â€œweakâ€ , â€œmediumâ€ and â€œstrongâ€ vortices, respectively. Ignition of the flame occurred in two ways: (1) spark-ignition or (2) laser ignition using an Nd:YAG laser at its second harmonic in order to quantify the effect of electrode interference. Accompanying high-speed chemiluminescence imaging measurements, instantaneous pressure measurements were obtained to give a more detailed understanding of the effect of vortex strength on reactant consumption rate over an extended time scale and to explore the use of a simple measurement to describe turbulent mixing. Further local flame-vortex interface analysis was conducted using non-invasive laser diagnostics, such as particle image velocimetry and planer laser induced fluorescence of the OH radical. The dependence of heat release rate on temperature provides an estimation of the strain rate dependence of the reaction rate.
Mathematical Modeling of Single Phase Flow and Particulate Flow Subjected to Microwave Heating
(2006-12-19) Zhu, Jianxi; Andrey V. Kuznetsov, Committee Chair; K. P. Sandeep, Committee Member; William L. Roberts, Committee Member; Tarek Echekki, Committee Member
The purpose of this research is to numerically investigate heat transfer in liquids and liquids with carried solid particles as they flow continuously in a duct that is subjected to microwave irradiation. During this process, liquid flows in an applicator tube. When flow passes through the microwave cavity, the liquid absorbs microwave power and its temperature quickly increases. The spatial variation of the electromagnetic energy and temperature fields in the liquid was obtained by solving coupled momentum, energy and Maxwell's equations. A finite difference time domain method (FDTD) is used to solve Maxwell's equations simulating the electromagnetic field. The effects of dielectric properties of the liquid, the applicator diameter and its location, as well as the geometry of the microwave cavity on the heating process are analyzed. For modeling particulate flow subjected to microwave heating, the hydrodynamic interaction between the solid particle and the carrier fluid is simulated by the force-coupling method (FCM). The Lagrangian approach is utilized for tracking particles. The electromagnetic power absorption, temperature distribution inside both the liquid and the particles are taken into account. The effects of dielectric properties and the inlet position of the particle on electromagnetic energy and temperature distributions inside the particle are studied. The effect of the particle on power absorption in the carrier liquid is analyzed. The effect of the time interval between consecutive injections of two groups of particles on power absorption in particles is analyzed as well.
A Methodology for Translating Detonation Wave Effects between One and Two Dimensions
(2008-05-12) Susi, Bryan; Kevin Lyons, Committee Chair; Tarek Echekki, Committee Member; Tiegang Fang, Committee Member
This research focuses on evaluating empirical methods and implementing a prototype Transitional Airblast Model (TRAM) for facilitating communication between one-dimensional and two-dimensional airblast models. An overview of detonation phenomena is presented, especially concerning detonation waves and accompanying airblast effects. Two existing airblast models are discussed that were designed to predict the effects of a detonation in two separate types of geometries, one-dimensional and two-dimensional. The functionality and behavior of each airblast model will be scrutinized giving particular insight into their performance in applications with both one-dimensional and two-dimensional components. The strengths and deficiencies of the different airblast models will offer motivation for the development of the TRAM prototype. The TRAM prototype consists of two separate methodologies, one for translating one-dimensional airblast propagation to two dimensions, and another for translating two-dimensional airblast propagation to one dimension. The selection of those two methodologies will be presented, along with results of detonation scenarios using both existing airblast models as well as the TRAM prototype. The TRAM prototype performed well for both types of detonation scenarios and is recommended for further development.
Numerical Modeling and Experimental Investigation of the Hydroentanglement Process
(2007-11-19) Xiang, Ping; Abdel-Fattah M. Seyam, Committee Co-Chair; Andrey V. Kuznetsov, Committee Chair; Kevin M. Lyons, Committee Member; Tarek Echekki, Committee Member
Numerical Simulation of Airflow, Particle Deposition and Drug Delivery in a Representative Human Nasal Airway Model
(2007-08-07) Shi, Huawei; Tarek Echekki, Committee Member; Chong Kim, Committee Member; Kevin Lyons, Committee Member; Clement Kleinstreuer, Committee Chair; Zhe Zhang, Committee Co-Chair
The human nasal cavities, each with an effective length of only 10cm, feature a wide array of basic flow phenomena due to their complex geometries. Dependent on such airflow fields are the transport and deposition of micro- and nano- particles in the human nasal cavities, of interest to engineers, scientists, air-pollution regulators, and healthcare officials. By utilizing advanced CAD and reverse engineering skills, a realistic model of the human nasal cavity was constructed from MRI image data for 3-D computer simulations. Assuming laminar quasi-steady airflow, dilute micro- and nano-particle suspension flows and local deposition efficiencies were analyzed for 7.5<=Q<=20L⁄min and 1nm<=dp<=50um . Simulation results are in good agreement with experimental measurements, assuring that computational fluid-particle dynamics (CFPD) is an effective and efficient tool to predict both toxic and therapeutic aerosol dynamics in the nasal cavities. Both nano- and micro- particle deposition efficiencies are influenced by particle size and airflow rate. Specifically, deposition of nanoparticles is governed by particle diffusion or Brownian motion, and decreases with increasing particle size and airflow rate in the nasal cavity. For microparticle deposition, the major mechanism is particle inertia. As a result, microparticle deposition increases for larger particles and higher airflow rates. Computational efforts were extended to nasal drug delivery, i.e., a droplet-spray model was developed which can be used to simulate nasal sprays. However, it turned out, after varying several droplet-spray parameters and trying different inlet conditions, that direct nasal sprays cannot achieve efficient drug delivery to the desirable surface area, e.g., the human olfactory region. However, a new nasal drug delivery method, called bi-directional nasal drug delivery, was successfully tested. The simulation results indicate that bi-directional nasal drug delivery overcomes major shortcomings of nasal sprays and may be a good candidate for the next generation of nasal drug delivery systems.
Semiconductor Crystal Growth by Vertical Bridgman and Gradient Freezing Processes with Applied Fields
(2007-05-08) Wang, Xianghong; Kara Peters, Committee Member; Kevin Lyons, Committee Member; Tarek Echekki, Committee Member; Nancy Ma, Committee Chair
Integrated circuits and optoelectronic devices are produced on surfaces of thin wafers sliced from a semiconductor crystal. The performance of the semiconductor is directly related to the uniformity of its composition. The crystal's composition generally changes due to a changing melt composition with segregation coefficient not equal to unity. Therefore, a major objective during the growth of any semiconductor crystal is to minimize the variations of the crystal's dopant or alloy composition. Externally-applied fields such as magnetic and electric fields can be used to provide electromagnetic damping or stirring of the melt motion in order to minimize the dopant or alloy segregation in the melt and thus in the crystal. This research focuses on investigations of various semiconductor crystal growth processes from the melt in the presence of externally-applied fields. These processes are (1) the Bridgman-Stockbarger process in steady magnetic fields, (2) the vertical gradient freezing process using submerged heater growth in steady magnetic and electric fields, (3) the Bridgman process using submerged heater growth in a rotating magnetic field, and (4) the Bridgman process using submerged heater growth in a combination of steady and rotating magnetic fields. Numerical models are developed using a Chebyshev spectral method with Gauss-Lobatto collocation points. These models provide predictions of the temperature, velocity and concentration fields in the melt as well as the dopant or alloy concentration in the entire crystal.
Soot Formation in Laminar Jet Diffusion Flames at Elevated Pressures
(2008-08-28) Yelverton, Tiffany Leigh Berry; Tiegang Fang, Committee Member; Kevin M. Lyons, Committee Member; Tarek Echekki, Committee Member; William L. Roberts, Committee Chair; Adriana de Souza e'Silva, Committee Member