Browsing by Author "Andrey V. Kuznetsov, Committee Member"

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Investigation of a spark ignition flame kernel interacting with a laminar vortex toroid
(2002-08-19) Xiong, Yin; Andrey V. Kuznetsov, Committee Member; Kevin M. Lyons, Committee Member; Richard Gould, Committee Member; William L. Roberts, Committee Chair
Chemistry-turbulence interactions play a critical role in most practical combustion environments. Understanding the interaction between a flame kernel and a vortex is an important fundamental step. This dissertation presents high-speed movies of combustion luminosity during the interaction of a laminar vortex toroid with a spark -generated premixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts. This dissertation also describes experiments that were conducted to study the Lewis number effect on the flame kernel-vortex interaction. The influence of a time varying strain rate on kernel growth was investigated by studying both lean methane-air (thermo-diffusively unstable) and lean propane-air (thermo-diffusively stable) flame kernels, using both natural CH/OH emission image sequences acquired by a high-speed intensified camera to show details of the disturbed flame kernel growth, and OH-PLIF images to determine the true two-dimensional nature of the interaction. Significant differences are observed in the highly curved regions on the backside of the invading vortex in the two different mixtures. Lewis number effects on local burning rate variations, flame front wrinkling, and pocket formation are reported, and in general, the results are in agreement with predictions from asymptotic theory assuming low stretch rates. Local mixture enrichment by direct injection in the vicinity of the spark plug at the time of ignition can affect flame kernel development and extend the lean limit of flammability of a fuel/air mixture. In the third set of the experiments, flame kernels were ignited in a lean premixed CH4/air mixture with an equivalence ratio of 0.6, while CH4/air mixtures at six different equivalence ratios ranging from 0 to infinity were used to generate the vortex. Chemiluminescence images of kernel -vortex interactions have been captured using both anICCD camera and a high-speed intensified camera. Details about flame kernel-vortex interactions of the six test cases are presented and discussed by comparing image sequences for different cases.
Modeling and Design of a Novel Cooling Device for Microelectronics using Piezoelectric Resonating Beams
(2003-12-29) Wu, Tao; Paul I. Ro, Committee Chair; Andrey V. Kuznetsov, Committee Member; Fuh-Gwo Yuan, Committee Member; Paul D. Franzon, Committee Member
As thermal management in microelectronics becomes more and more important in insuring the reliable operation, a novel and effective cooling device by smart materials such as piezoelectric bimorph needs to be developed. Investigation of modeling and design of piezoelectric resonating structures was conducted. A dynamic performance prediction method was proposed to calculate tip deflections at resonances and investigate the effect of finite stiffness bonding layer in piezoelectric bimorph. Considering the product of resonance frequency and dynamic tip deflection as a performance merit, the effects of length and location of the actuators on passive piezoelectric structures as well as the boundary conditions were analyzed for generating acoustic streaming which may be used for cooling microelectronic components. The cooling effects generated by vibrating non-slot and slotted piezoelectric bimorphs were experimentally investigated. A prototype, which is comprised of a piezoelectric bimorph actuator, an aluminum block with commercial cartridge heater served as heat source, four micrometer heads to adjust the gap size between bimorph and heat source, was constructed. Validated finite element analyses were employed to simulate the vibration characteristics including the natural frequencies and mode shapes of the proposed prototype. Setting the operation frequency at the fundamental resonance frequency, the cooling effects were measured by the temperature drops of the heat source above the vibrating bimorph. Electric field applied on the bimorph and the gap between heat source and actuator were adjusted to find out the best cooling result. Heat transfer coefficients between the heat source and vibrating bimorphs were calculated by ANSYS steady state thermal analysis and the lumped energy balance method. Air flow patterns around the bimorph actuator were visualized using particle tracking velocimetry (PTV) as well. The experiments showed that there exists an optimal gap between the heat source and the vibrating bimorph which brings the maximum temperature drop and the cooling effect increases with the electric field strength. The enhancement of heat transfer between the heat source and the non-slot bimorph can be up to 210% with the acoustic streaming generated by the bimorph vibration. The presence of slots in the bimorphs may enhance the mixing of streams outside and inside the channel resulting in an amplified heat transfer performance. However, the number, location and size of slots may influence the vibration characteristics and the formation of swirling streaming in the channel between the heat source and the bimorph. Finally, the heat transfer coefficient of the prototyped cooling device in terms of mean Nusselt number was correlated as a function of streaming Reynolds number. This study may provide useful information on modeling the vibration characteristics of piezoelectric actuators and designing the miniature cooling device utilizing bimorph vibrations.
Modeling and Optimization of a Rotary Dryer in a Manufacturing Plant
(2003-12-27) Ahluwalia, Harshdeep Singh; Andrey V. Kuznetsov, Committee Member; Dr. Herbert M. Eckerlin, Committee Member; Dr. James W. Leach, Committee Chair
The aim of this thesis is to predict the effects of reducing excess air levels and fuel burned in a rotary dryer being operated in a mining facility in North Carolina. The dryer is now being operated at very high excess combustion air levels, resulting in high stack losses and low thermal efficiency. Reducing the excess air entering the burner and the amount of fuel burned would result in energy and cost savings. However, experimental data to confirm that drying rates can be maintained at the required level by simultaneously increasing the combustion air temperature and reducing the air flow are unavailable. This work attempts to predict the dryer performance at variable air flows from a computer model. The modeling involves an analysis of the heat and mass transfer that occurs in the drying process. The model is validated by comparing predicted performance to actual performance at present operating condition. The excess air levels and the fuel burned are then adjusted to obtain the best efficiency. It is expected that the model prediction will encourage the mining facility to experiment with combustion air flow.
Overall Heat Transfer Coefficients and Axial Temperature Distribution of Fluids in a Triple Tube Heat Exchanger
(2003-11-06) Batmaz, Ediz; Andrey V. Kuznetsov, Committee Member; Brian E. Farkas, Committee Member; K.P. Sandeep, Committee Chair
Computation of overall heat transfer coefficients in a triple tube heat exchanger (TTHE) is complicated when compared to a double tube heat exchanger (DTHE) since the two overall heat transfer coefficients are not independent of each other and must be solved for simultaneously. Previous methods established towards calculation of these parameters either include assumptions that are not valid for all flow conditions and fluid flow rates or use empirical correlations which may cause significant deviations from actual values of these parameters. A more generic technique was developed for calculation of overall heat transfer coefficients and axial temperature distribution of fluids in a triple tube heat exchanger. The developed procedure has been used for calculation of these parameters at various fluid flow rates and product inlet temperatures. Theoretical double tube heat exchanger results were also tabulated for comparison purposes. The advantages of using a TTHE over a DTHE has been both conceptually explained and demonstrated using the results obtained. However, it was also shown that design of TTHE experiments is critical, especially in the co-current flow arrangement, since the relative flow rates of the fluids may result in a decrease in the effectiveness. The effect of fluid flow rates, product inlet temperature, and flow arrangement on values of overall heat transfer coefficients, total amount of heat transferred, and effectiveness were also investigated. These results were analyzed using SAS and interpreted for the consistency of the obtained results with the literature.