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Title: METHANE AND DIMETHYL ETHER OXIDATION AT ELEVATED TEMPERATURES AND PRESSURE.
Creator: Zinner, Christopher, Basu, Saptarshi, University of Central Florida
Abstract / Description: Autoignition and oxidation of two Methane (CH4) and Dimethyl Ether (CH3OCH3 or DME) mixtures in air were studied in shock tubes over a wide range of equivalence ratios at elevated temperatures and pressures. These experiments were conducted in the reflected shock region with pressures ranging from 0.8 to 35.7 atmospheres, temperatures ranging from 913 to 1650 K, and equivalence ratios of 2.0, 1.0, 0.5, and 0.3. Ignition delay times were obtained from shock-tube endwall pressure traces for...
Show moreAutoignition and oxidation of two Methane (CH4) and Dimethyl Ether (CH3OCH3 or DME) mixtures in air were studied in shock tubes over a wide range of equivalence ratios at elevated temperatures and pressures. These experiments were conducted in the reflected shock region with pressures ranging from 0.8 to 35.7 atmospheres, temperatures ranging from 913 to 1650 K, and equivalence ratios of 2.0, 1.0, 0.5, and 0.3. Ignition delay times were obtained from shock-tube endwall pressure traces for fuel mixtures of CH4/CH3OCH3 in ratios of 80/20 percent volume and 60/40 percent volume, respectively. Close examination of the data revealed that energy release from the mixture is occurring in the time between the arrival of the incident shock wave and the ignition event. An adjustment scheme for temperature and pressure was devised to account for this energy release and its effect on the ignition of the mixture. Two separate ignition delay correlations were developed for these pressure- and temperature-adjusted data. These correlations estimate ignition delay from known temperature, pressure, and species mole fractions of methane, dimethyl ether, and air (0.21 O2 + 0.79 N2). The first correlation was developed for ignition delay occurring at temperatures greater than or equal to 1175 K and pressures ranging from 0.8 to 35.3 atm. The second correlation was developed for ignition delay occurring at temperatures less than or equal to 1175 K and pressures ranging from 18.5 to 40.0 atm. Overall good agreement was found to exist between the two correlations and the data of these experiments. Findings of these experiments also include that with pressures at or below ten atm, increased concentrations of dimethyl ether will consistently produce faster ignition times. At pressures greater than ten atmospheres it is possible for fuel rich mixtures with lower concentrations of dimethyl ether to give the fastest ignition times. This work represents the most thorough shock tube investigation for oxidation of methane with high concentration levels of dimethyl ether at gas turbine engine relevant temperatures and pressures. The findings of this study should serve as a validation for detailed chemical kinetics mechanisms.
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Date Issued: 2008
Identifier: CFE0002096, ucf:47539
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0002096

Title: COMBUSTION DYNAMICS AND FLUID MECHANICS IN ACOUSTICALLY PERTURBED NON-PREMIXED SWIRL-STABILIZED FLAMES.
Creator: Idahosa, Uyi, Basu, Saptarshi, University of Central Florida
Abstract / Description: The prevalence of gas turbines operating in primarily lean premixed modes is predicated on the need for lower emissions and increased efficiency. An enhancement in the mixing process through the introduction of swirl in the combustion reactants is also necessary for flame stabilization. The resulting lean swirling flames are often characterized by a susceptibility to feedback between velocity, pressure and heat release perturbations with a potential for unstable self-amplifying dynamics. The...
Show moreThe prevalence of gas turbines operating in primarily lean premixed modes is predicated on the need for lower emissions and increased efficiency. An enhancement in the mixing process through the introduction of swirl in the combustion reactants is also necessary for flame stabilization. The resulting lean swirling flames are often characterized by a susceptibility to feedback between velocity, pressure and heat release perturbations with a potential for unstable self-amplifying dynamics. The existing literature on combustion dynamics is predominantly dedicated to premixed flame configurations motivated by power generation and propulsive gas turbine applications. In the present research effort, an investigation into the response of atmospheric, non-premixed swirling flames to acoustic perturbations at various frequencies (fp = 0-315Hz) and swirl intensities (S=0.09 and S=0.34) is carried out. The primary objective of the research effort is to broaden the scope of fundamental understanding in flame dynamics in the literature to include non-premixed swirling flames. Applications of the research effort include control strategies to mitigate the occurrence of thermoacoustic instabilities in future power generation gas turbines. Flame heat release is quantitatively measured using a photomultiplier with a 430nm bandpass filter for observing CH* chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. Acoustic perturbations are generated with a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes, u'/Uavg in the 0.03-0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. The effect of varying fuel flow rates on the flame response is also examined using with dynamic time-dependent fuel supply rates over the data acquisition period. The Particle Image Velocimetry (PIV) method is used to study the isothermal flow field associated with acoustic pulsing. The acoustic impedance, wavelet analysis, Rayleigh criteria and phase conditioning methods are used to identify fundamental mechanisms common to highly responsive flame configurations.
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Date Issued: 2010
Identifier: CFE0003186, ucf:48607
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0003186

Title: WATER VAPOR AND CARBON DIOXIDE SPECIES MEASUREMENT IN NARROW CHANNELS.
Creator: Lambe, Derek, Basu, Saptarshi, University of Central Florida
Abstract / Description: A novel method has been implemented for measuring the concentration of gas species, water vapor and carbon dioxide, within a narrow channel flow field non-invasively using tunable diode laser absorption spectroscopy (TDLAS) in conjunction with a laser modulated at a high frequency [Wavelength Modulation Spectroscopy (WMS)] tuned to the ro-vibrational transition of the species. This technique measures the absorption profile which is a strong function of the species concentration across short...
Show moreA novel method has been implemented for measuring the concentration of gas species, water vapor and carbon dioxide, within a narrow channel flow field non-invasively using tunable diode laser absorption spectroscopy (TDLAS) in conjunction with a laser modulated at a high frequency [Wavelength Modulation Spectroscopy (WMS)] tuned to the ro-vibrational transition of the species. This technique measures the absorption profile which is a strong function of the species concentration across short path lengths and small time spans, as in PEM fuel cells during high load cycles. This method has been verified in a transparent circular flow 12 cm path length and a 12 mm rectangular flow channel. Distinct absorption peaks for water vapor and carbon dioxide have been identified, and concentrations of water vapor and carbon dioxide within the test cells have been measured in situ with high temporal resolutions. A comparison of the full width at half maximum (FWHM) of the absorption lineshapes to the partial pressure of water vapor and carbon dioxide showed a predominantly linear relationship, except in the lower partial pressure regions. Test section temperature was observed to have very minimal impact on these curves at low partial pressure values. A porous media like a membrane electrode assembly (MEA) similar to those used in PEM fuel cells sandwiched between two rectangular flow channels was also tested. Some of the scattered radiation off the MEA was observed using a photodiode at high gain, allowing for more localized species detection. The technique was used to monitor the humidity on either side of the MEA during both temperature controlled and super-saturated conditions. The measurements were observed to be repeatable to within 10 %.
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Date Issued: 2009
Identifier: CFE0002805, ucf:48096
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0002805

Title: Numerical Simulation of Conventional Fuels and Biofuels Dispersion and Vaporization Process in Co-flow and Cross-flow Premixers.
Creator: Gu, Xin, Kumar, Ranganathan, Basu, Saptarshi, Kapat, Jayanta, Chow, Louis, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: In order to follow increasingly strict regulation of pollutant emissions, a new concept of Lean Premixed pre-vaporized (LPP) combustion has been proposed for turbines. In LPP combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the pre-mixer are the key factors required to stabilize the combustion process and improve the efficiency. A numerical study is conducted for the fundamental understanding of the liquid fuel dispersion and vaporization...
Show moreIn order to follow increasingly strict regulation of pollutant emissions, a new concept of Lean Premixed pre-vaporized (LPP) combustion has been proposed for turbines. In LPP combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the pre-mixer are the key factors required to stabilize the combustion process and improve the efficiency. A numerical study is conducted for the fundamental understanding of the liquid fuel dispersion and vaporization process in pre-mixers using both cross-flow and co-flow injection methods. First, the vaporization model is validated by comparing the numerical data to existing experiments of single droplet vaporization under both low and high convective air temperatures. Next, the dispersion and vaporization process for biofuels and conventional fuels injected transversely into a typical simplified version of rectangular pre-mixer are simulated and results are analyzed with respect to vaporization performance, degree of mixedness and homogeneity. Finally, collision model has been incorporated to predict more realistic vaporization performance. Four fuels, Ethanol, Rapeseed Methyl Esters (RME), gasoline and jet-A have been investigated. For mono-disperse spray with no collision model, the droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. The diameter histogram near the pre-mixer exit showed a wide droplet diameter distribution for all the fuels. In general, pre-heating of the fuels before injection improved the vaporization performance. An improvement in the drag model with Stefan flow correction showed that a low speed injection and high cone angle improved performance. All fuels achieved complete vaporization under a spray cone angle of 140(&)deg;. In general, it was found that cross-flow injection achieved better vaporization performance than co-flow injection. A correlation is derived for jet-A's total vaporization performance as a function of non-dimensional inlet air temperature and fuel/air momentum flux ratio. This is achieved by curve-fitting the simulated results for a broad range of inlet air temperatures and fuel/air momentum flux ratios. The collision model, based on no-time-counter method (NTC) proposed by Schmidt and Rutland, was implemented to replace O'Rourke's collision algorithm to improve the results such that the unphysical numerical artifact in a Cartesian grid was removed and the results were found to be grid-independent. The dispersion and vaporization processes for liquid fuel sprays were simulated in a cylindrical pre-mixer using co-flow injection method. Results for jet-A and Rapeseed Methyl Esters (RME) showed acceptable grid independence. At relatively low spray cone angle and injection velocity, it was found that the collision effect on the average droplet size and the vaporization performance were very high due to relatively high coalescence rate induced by droplet collisions. It was also found that the vaporization performance and the level of homogeneity of fuel-air mixture could be significantly improved when the dispersion level is high, which can be achieved by increasing the spray cone angle and injection velocity. In order to compare the performance between co-flow and cross-flow injection methods, the fuels were injected at an angle of 40(&)deg; with respect to the stream wise direction to avoid impacting on the wall. The cross-flow injection achieved similar vaporization performance as co-flow because a higher coalescence rate induced by droplet collisions cancelled off its higher heat transfer efficiency between two phases for cross-flow injections.
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Date Issued: 2012
Identifier: CFE0004192, ucf:49004
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004192

Title: Evaporation, Precipitation Dynamics and Instability of Acoustically Levitated Functional Droplets.
Creator: Saha, Abhishek, Kumar, Ranganathan, Basu, Saptarshi, Kapat, Jayanta, Deng, Weiwei, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Evaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates...
Show moreEvaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates chemical and thermal contamination with surfaces. This work uses an ultrasonic levitation technique to investigate the vaporization dynamics under radiative heating, with focus on evaporation characteristics, precipitation kinetics, particle agglomeration, structure formation and droplet stability. Timescale and temperature scales are developed to compare convective heating in actual sprays and radiative heating in the current experiments. These relationships show that simple experiments can be conducted in a levitator to extrapolate information in realistic convective environments in spray drying. The effect of acoustic streaming, droplet size and liquid properties on internal flow is important to understand as the heat and mass transfer and particle motion within the droplet is significantly controlled by internal motion. Therefore, the droplet internal flow is characterized by Particle Image Velocimetry for different dropsize and viscosity. Nanosuspension droplets suspended under levitation show preferential accumulation and agglomeration kinetics. Under certain conditions, they form bowl shaped structures upon complete evaporation. At higher concentrations, this initial bowl shaped structure morphs into a ring structure. Nanoparticle migration due to internal recirculation forms a density stratification, the location of which depends on initial particle concentration. The time scale of density stratification is similar to that of perikinetic-driven agglomeration of particle flocculation. The density stratification ultimately leads to force imbalance leading to a unique bowl-shaped structure. Chemically active precursor droplet under acoustic levitation shows events such as vaporization, precipitation and chemical reaction leading to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process followed by formation of precipitate. Ex-situ analyses using TEM and SEM reveal highly porous morphology with trapped gas pockets and nanoceria crystalline structures at 70 degree C. Inhomogeneity in acoustic pressure around the heated droplet can induce thermal instability. Short wavelength (Kelvin-Helmholtz) instability for diesel and bio-diesel droplets triggers this secondary atomization, which occurs due to relative velocity between liquid and gas phase at the droplet equator. On the other hand, liquids such as Kerosene and FC43 show uncontrollable stretching followed by a catastrophic break-up due to reduction in surface tension and viscosity coupled with inhomogeneity of pressure around the droplet. Finally, a scaling analysis has been established between vaporizing droplets in a convective and radiative environment. The transient temperature normalized by the respective scales exhibits a unified profile for both modes of heating. The analysis allows for the prediction of required laser flux in the levitator experiments to show its equivalence in a corresponding heated gas stream. The theoretical equivalence shows good agreement with experiments for a range of droplet sizes.
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Date Issued: 2012
Identifier: CFE0004436, ucf:49346
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004436

Title: Experimental Investigation of Breakup and Coalescence Characteristics of a Hollow Cone Swirling Spray.
Creator: Lee, Joshua, Kumar, Ranganathan, Deng, Weiwei, Kapat, Jayanta, Basu, Saptarshi, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Atomization can be achieved by discharging liquid at relative high velocities into a slow moving environment (hydraulic nozzles) or by discharging liquid at low velocities into a fast moving gas flow (air-blast nozzles). These two types of injector nozzles are featured in majority of the industry applications such as power generation, food or pharmaceutical powder formation, spray painting, petroleum refining, and thermal sprays. The most common atomizer used in combustion engines is the...
Show moreAtomization can be achieved by discharging liquid at relative high velocities into a slow moving environment (hydraulic nozzles) or by discharging liquid at low velocities into a fast moving gas flow (air-blast nozzles). These two types of injector nozzles are featured in majority of the industry applications such as power generation, food or pharmaceutical powder formation, spray painting, petroleum refining, and thermal sprays. The most common atomizer used in combustion engines is the pressure-swirl nozzle (Simplex nozzle) to obtain a homogenous mixture at different equivalence ratios. The experimental studies performed with pressure-swirl nozzles have reported contradictory results over the last few years. Thus, the fundamentals of spray dynamics, such as spray formation, liquid breakup length, droplet breakup regimes, and coalescence still need to be understood for a pressure-swirl nozzle.An experimental study of the breakup characteristics of various liquids and fuels with different thermal physical properties emanating from hollow cone hydraulic injector nozzles induced by pressure-swirling was investigated. The experiments were conducted using two nozzles with different orifice diameters 0.3mm and 0.5mm and injection pressures (0.3-4MPa) which correspond to Rep = 7,000-31,000 depending on the liquids being tested. Three laser-based techniques, i.e., Shadowgraph, Particle Image Velocimetry (PIV) and Phase Doppler Particle Anemometry (PDPA) were utilized in this study. Although each technique had its limitation in different flow regimes, the results were cross-validated, and generally showed correct trends in axial and radial measurements of velocity and diameter for different nozzles, Weber and Reynolds numbers.The spatial variation of diameter and velocity arises principally due to primary breakup of liquid films and subsequent secondary breakup of large droplets due to aerodynamic shear. Downstream of the nozzle, coalescence of droplets due to collision is also found to be significant. Different types of liquid film break up was considered and found to match well with the theory. The spray is subdivided into three zones: near the nozzle, a zone consisting of film and ligament regime, where primary breakup and some secondary breakup take place; a second zone where the secondary breakup process continues, but weakens, and the centrifugal dispersion becomes dominant, and a third zone away from the spray where coalescence is dominant. Each regime has been analyzed in detail to understand the effect of surface tension and viscosity. Surface tension and viscosity were engineered to mimic fuels, which were then compared with real fuels such as Ethanol, Jet-A and Kerosene. Results show similarity in the diameter in the beginning stages of breakup but in the coalescence regime, the values deviate from each other, indicating that the vapor pressure also plays a major role in this regime.
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Date Issued: 2013
Identifier: CFE0005021, ucf:50014
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005021

Title: Agglomeration, Evaporation and Morphological Changes in Droplets with Nanosilica and Nanoalumina Suspensions in an Acoustic Field.
Creator: Tijerino Campollo, Erick, Kumar, Ranganathan, Deng, Weiwei, Chow, Louis, Basu, Saptarshi, University of Central Florida
Abstract / Description: Acoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore...
Show moreAcoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore their respective timescales are important to control the final shape. The balance of forces acting on the droplet, such as the acoustic pressure and surface tension, determine the geometry of the levitated droplet. Thus, the morphology of the resultant structure can be controlled by manipulating the amplitude of the levitator and the fluid properties of the precursor nanosuspensions. The interface area in colloidal nanosuspensions is very large even at low particle concentrations. The effects of the presence of this interface have large influence in the properties of the solution even at low concentrations.This thesis focuses on the dynamics of particle agglomeration in acoustically levitated evaporating nanofluid droplets leading to shell structure formation. These experiments were performed by suspending 500(&)#181;m droplets in a pressure node of a standing acoustic wave in a levitator and heating them using a carbon dioxide laser. These radiatively heated functional droplets exhibit three distinct stages, namely, pure evaporation, agglomeration and structure formation. The temporal history of the droplet surface temperature shows two inflection points. Morphology and final precipitation structures of levitated droplets are due to competing mechanisms of particle agglomeration, evaporation and shape deformation. This thesis provides a detailed analysis for each process and proposes two important timescales for evaporation and agglomeration that determine the final diameter of the structure formed. It is seen that both agglomeration and evaporation timescales are similar functions of acoustic amplitude (sound pressure level), droplet size, viscosity and density. However it is shown that while the agglomeration timescale decreases with initial particle concentration, the evaporation timescale shows the opposite trend. The final normalized diameter hence can be shown to be dependent solely on the ratio of agglomeration to evaporation timescales for all concentrations and acoustic amplitudes. The experiments were conducted with 10nm silica, 20nm silica, 20nm alumina and 50nm alumina solutions. The structures exhibit various aspect ratios (bowls, rings, spheroids) which depend on the ratio of the deformation timescale (tdef) and the agglomeration timescale (tg).
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Date Issued: 2012
Identifier: CFE0004610, ucf:49914
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004610

Acoustic Levitation (2)	+ -
Acoustic Impedance (1)	+ -
Acoustic Streaming (1)	+ -
Acoustically Levitated Droplet (1)	+ -
Agglomeration (1)	+ -

Alumina (1)	+ -
Biofuels (1)	+ -
Breakup (1)	+ -
Coalesecence (1)	+ -
Collision Modeling (1)	+ -
Combustion Dynamics (1)	+ -
Crossflow (1)	+ -
Droplet (1)	+ -
Droplet Evaporation (1)	+ -
Evaporation (1)	+ -
Forced Flames (1)	+ -
Fuels (1)	+ -
Infrared (1)	+ -
LPP (1)	+ -
Laser (1)	+ -
Nanofluid (1)	+ -
Nanoparticle (1)	+ -
Nanosuspension (1)	+ -
Non-Premixed Flames (1)	+ -
Numerical Simulation (1)	+ -
PEMFC (1)	+ -
Precursor Solution (1)	+ -
Premixer (1)	+ -
Silica (1)	+ -
Simplex Nozzle (1)	+ -

Basu, Saptarshi (7)	+ -
University of Central Florida (7)	+ -
Kumar, Ranganathan (4)	+ -
Deng, Weiwei (3)	+ -
Kapat, Jayanta (3)	+ -

Shivamoggi, Bhimsen (3)	+ -
Chow, Louis (2)	+ -
Gu, Xin (1)	+ -
Idahosa, Uyi (1)	+ -
Lambe, Derek (1)	+ -
Lee, Joshua (1)	+ -
Saha, Abhishek (1)	+ -
Tijerino Campollo, Erick (1)	+ -
Zinner, Christopher (1)	+ -

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