Mechanical Engineering
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Item The Structure of the Blue Whirl: A Soot-Free Reacting Vortex Phenomenon(2017) Hariharan, Sriram Bharath; Gollner, Michael J; Oran, Elaine S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent experiments have led to the discovery of the blue whirl, a small, stable flame that evolves from a fire whirl, and burns typically sooty hydrocarbons without producing soot. The distinct physical structure of the flame is investigated through digital imaging techniques, which suggest that the transition and shape of the flame may be influenced by vortex breakdown. The thermal structure of the blue whirl reveals a peak temperature around 2000 K, and that most of the combustion occurs in a relatively small, visibly bright vortex ring. The formation of the flame is shown to occur over a variety of surfaces, including water and flat metal, all of which indicate that the formation of the blue whirl is strongly influenced by the flow structure over the incoming boundary layer. Finally, a schematic structure of the blue whirl is proposed, based on the measurements presented here and previous literature on fire whirls and vortex breakdown.Item Gasification and Combustion of Large Char Particles and Tar(2015) Molintas, Henry; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although diffusion is known to play an important role for gasification and combustion of large char particles, their effects on conversion rates, kinetic parameters and other relevant factors have not been thoroughly analyzed. Similarly, tar reduction is not yet well understood. Central to these challenges is the shortage of experimental data for reduction of tar and large char particles. Likewise, analytical models for reduction processes have not been systematically examined. In this study, large char particles between 1.5 to 7 mm are gasified and combusted non-isothermally with initial temperatures up to 1000 degree celcius using various oxidants. Tar is also reduced with steam and vitiated air continuously and non-isothermally. In the absence of mathematical tools for large particle reduction analysis, models are proposed and derived in this study. Carbon and large near-spherically or irregularly shaped particles are modeled as large disk-shaped and spherically-shaped particles, respectively. One-film ash segregated core and random pore models are explored to analyze char reduction data and these are found to provide consistent and inconsistent results, respectively. Thiele analysis is also used and it indicates that less porous particles are consumed more externally at the surface than internally. For C + O2⇒ CO2 reductions, disk-shaped particles ignite when reactor temperature reaches 584 degree and these processes are purely kinetic controlled for 1.5 mm thick samples. Reduction of spherically-shaped particles shows that O2 enrichment as compared to a 50 degree celcius rise in reactor temperature substantially improves conversion. Oxygen enrichment with steam also significantly increases conversion of 5.5 mm thick disk-shaped particle up to 600 % under identical reactor conditions. For C + CO2⇒2CO reductions, conversion rates increased five-fold when reactor temperature is increased from 850 to 1000 degree Celsius. Increasing initial reactor temperatures and O2 enrichment provide an increase in char reactivity, diffusional rate, conversion, reduction rate and surface temperature. Most of the large particle reductions investigated here operate near kinetic-diffusion controlled regime. Calculated total energy released during combustion is within the range of Dulong’s empirical formula. At higher tar concentrations, CO and H2 production moderately increase between 814 to 875 degree celsius.Item Numerical Simulation of Low-Pressure Explosive Combustion in Compartment Fires(2008-11-19) Hu, Zhixin (Victor); Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A filtered progress variable approach is adopted for large eddy simulations (LES) of turbulent deflagrations. The deflagration model is coupled with a non-premixed combustion model, either an equilibrium-chemistry, mixture-fraction based model, or an eddy dissipation model. The coupling interface uses a LES-resolved flame index formulation and provides partially-premixed combustion (PPC) modeling capability. The PPC sub-model is implemented into the Fire Dynamic Simulator (FDS) developed by the National Institute of Standards and Technology, which is then applied to the study of explosive combustion in confined fuel vapor clouds. Current limitations of the PPC model are identified first in two separate series of simulations: 1) a series of simulation corresponding to laminar flame propagation across homogeneous mixtures in open or closed tunnel-like configurations; and 2) a grid refinement study corresponding to laminar flame propagation across a vertically-stratified layer. An experimental database previously developed by FM Global Research, featuring controlled ignition followed by explosive combustion in an enclosure filled with vertically-stratified mixtures of propane in air, is used as a test configuration for model validation. Sealed and vented configurations are both considered, with and without obstacles in the chamber. These pressurized combustion cases present a particular challenge to the bulk pressure algorithm in FDS, which has robustness, accuracy and stability issues, in particular in vented configurations. Two modified bulk pressure models are proposed and evaluated by comparison between measured and simulated pressure data in the Factory Mutual Global (FMG) test configuration. The first model is based on a modified bulk pressure algorithm and uses a simplified expression for pressure valid in a vented compartment under quasi-steady conditions. The second model is based on solving an ordinary differential equation for bulk pressure (including a relaxation term proposed to stabilize possible Helmholtz oscillations) and modified vent flow velocity boundary conditions that are made bulk-pressure-sensitive. Comparisons with experiments are encouraging and demonstrate the potential of the new modeling capability for simulations of low pressure explosions in stratified fuel vapor clouds.Item EFFECTS OF PREHEATED COMBUSTION AIR ON LAMINAR COFLOW DIFFUSION FLAMES UNDER NORMAL AND MICROGRAVITY CONDITIONS(2005-08-30) Ghaderi Yeganeh, Mohammad; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Global energy consumption has been increasing around the world, owing to the rapid growth of industrialization and improvements in the standard of living. As a result, more carbon dioxide and nitrogen oxide are being released into the environment. Therefore, techniques for achieving combustion at reduced carbon dioxide and nitric oxide emission levels have drawn increased attention. Combustion with a highly preheated air and low-oxygen concentration has been shown to provide significant energy savings, reduce pollution and equipment size, and uniform thermal characteristics within the combustion chamber. However, the fundamental understanding of this technique is limited. The motivation of the present study is to identify the effects of preheated combustion air on laminar coflow diffusion flames. Combustion characteristics of laminar coflow diffusion flames are evaluated for the effects of preheated combustion air temperature under normal and low-gravity conditions. Experimental measurements are conducted using direct flame photography, particle image velocimetry (PIV) and optical emission spectroscopy diagnostics. Laminar coflow diffusion flames are examined under four experimental conditions: normal-temperature/normal-gravity (case I), preheated-temperature/normal gravity (case II), normal-temperature/low-gravity (case III), and preheated-temperature/low-gravity (case IV). Comparisons between these four cases yield significant insights. In our studies, increasing the combustion air temperature by 400 K (from 300 K to 700 K), causes a 37.1% reduction in the flame length and about a 25% increase in peak flame temperature. The results also show that a 400 K increase in the preheated air temperature increases CH concentration of the flame by about 83.3% (CH is a marker for the rate of chemical reaction), and also increases the C2 concentration by about 60% (C2 is a marker for the soot precursor). It can therefore be concluded that preheating the combustion air increases the energy release intensity, flame temperature, C2 concentration, and, presumably, NOx production. Our work is the first to consider preheated temperature/low-gravity combustion. The results of our experiments reveal new insights. Where as increasing the temperature of the combustion air reduces the laminar flame width under normal-gravity, we find that, in a low-gravity environment, increasing the combustion air temperature causes a significant increase in the flame width.Item Direct Numerical Simulation of Non-premixed Combustion with Soot and Thermal Radiation(2005-07-14) Wang, Yi; Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct numerical simulation (DNS) is a productive research tool in combustion science used to provide high-fidelity computer-based observations of the micro-physics in turbulent reacting flows. It is also a unique tool for the development and validation of reduced model descriptions used in macro-scale simulations of engineering-level systems. Because of its high demand of computational power, current state-of-the-art DNS remains limited to small computational domains, small Reynolds numbers, and simplified problems corresponding to adiabatic, non-sooting, gaseous flames in simple geometries. This Ph.D. study is part of a multi-institution collaborative research project aimed at using terascale technology to overcome many of the current DNS limitations. Two different tracks are followed in the present work: a DNS development track, and a DNS production track corresponding to a study of flame-wall interactions. Due to project management issues, the two tracks remain separate in this work. In the first track, we develop numerical and physical models to enhance the capability of our fully compressible DNS solver for turbulent combustion. The Acoustic Speed Reduction (ASR) method is a new perturbation method designed to reduce the stiffness associated with acoustic waves found in slow flow simulations and to thereby enhance computational efficiency. The Navier-Stokes Characteristic Boundary Conditions (NSCBC) are modified to allow for successful simulations of turbulent counterflow flames. In addition, a semi-empirical soot model and a parallel thermal radiation model based on a ray-tracing method are developed and implemented into our DNS code. All the models are validated, showing that the capability of our DNS tool is greatly enhanced. In the second track, we perform a DNS study of non-premixed flame-wall interactions. The structure of the simulated wall flames is studied in terms of a classical fuel-air-based mixture fraction and a new variable, called the excess enthalpy variable, which characterizes deviations from adiabatic behavior. Using the excess enthalpy variable, a modified flame extinction criterion is proposed and tested against DNS data. While beyond the scope of this Ph.D. thesis, it is expected that follow-up studies of flame-wall interactions will take advantage of the new DNS software features developed in the first track of the present work.Item IN SITU INFRARED DIAGNOSTICS FOR A MICRO-SCALE COMBUSTION REACTOR(2004-08-19) Heatwole, Scott; Buckley, Steve G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of centimeter to millimeter scale engines and power supplies have created a need for micro-scale combustion diagnostics. Fuel concentrations, product concentrations, and temperature are useful measurements in determining combustion behavior, chemical efficiency, and flame structures. However, to the present there have been few efforts to develop non-intrusive diagnostic techniques appropriate for application in such small engines. Non-intrusive measurements in these engines are complicated by short path length and lack of optical access. In this thesis in situ FTIR spectroscopy is used to measure temperature and concentrations of fuel, and carbon dioxide in a micro-combustor. The measurements are made through silicon walls spaced a few millimeters apart. This is possible because silicon is transmissive in the infrared. Experimental issues, including the optical setup, limitations associated with etaloning, calibration, and interpretation of the resulting spectra using wide-band models are discussed in detail.