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- Title
- FLAME-TURBULENCE INTERACTION FOR DEFLAGRATION TO DETONATION.
- Creator
-
Chambers, Jessica, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
Detonation is a high energetic mode of pressure gain combustion that exploits total pressure rise to augment high flow momentum and thermodynamic cycle efficiencies. Detonation is initiated through the Deflagration-to-Detonation Transition (DDT). This process occurs when a deflagrated flame is accelerated through turbulence induction, producing shock-flame interactions that generate violent explosions and a supersonic detonation wave. There is a broad desire to unravel the physical mechanisms...
Show moreDetonation is a high energetic mode of pressure gain combustion that exploits total pressure rise to augment high flow momentum and thermodynamic cycle efficiencies. Detonation is initiated through the Deflagration-to-Detonation Transition (DDT). This process occurs when a deflagrated flame is accelerated through turbulence induction, producing shock-flame interactions that generate violent explosions and a supersonic detonation wave. There is a broad desire to unravel the physical mechanisms of turbulence induced DDT. For the implementation of efficient detonation methods in propulsion and energy applications, it is crucial to understand optimum turbulence conditions for detonation initiation. The study examines the role of turbulence-flame interactions on flame acceleration using a fluidic jet to generate turbulence within the reactant flow field. The investigation aims to classify the turbulent flame dynamics and temporal evolution of the flame stages throughout the turbulent flame regimes. The flame-flow interactions are experimentally studied using a detonation facility and high-speed imaging techniques, including Particle Image Velocimetry (PIV) and Schlieren flow visualization. Flow field measurements enable local turbulence characterization and analysis of flame acceleration mechanisms that result from the jet�s high level of turbulent transport. The influence of initial flame turbulence on the turbulent interaction is revealed, resulting in higher turbulence generation and overall flame acceleration. Turbulent intensities are classified, revealing a dynamic fluctuation of flame structure between the thin reaction zone and the broken reaction regime throughout the interaction.
Show less - Date Issued
- 2016
- Identifier
- CFH2000024, ucf:45578
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH2000024
- Title
- INITIATION OF SUSTAINED REACTION IN PREMIXED, COMBUSTIBLE SUPERSONIC FLOW VIA A PREDETONATOR.
- Creator
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Rosato, Daniel A, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
The propagation of a shock and flame from a detonation wave injected orthogonally into a combustible, supersonic flow was observed. The detonation wave was generated through the use of a miniaturized detonation tube, henceforth referred to as a predetonator. Conditions within the test section, including stagnation pressure and equivalence ratio, were varied between cases. Through the use of high-speed schlieren, shadowgraph, and broadband OH chemiluminescence imaging, the leading shock and...
Show moreThe propagation of a shock and flame from a detonation wave injected orthogonally into a combustible, supersonic flow was observed. The detonation wave was generated through the use of a miniaturized detonation tube, henceforth referred to as a predetonator. Conditions within the test section, including stagnation pressure and equivalence ratio, were varied between cases. Through the use of high-speed schlieren, shadowgraph, and broadband OH chemiluminescence imaging, the leading shock and reaction were recorded as they moved through the test section. Variation of stagnation pressure affected the propagation of the leading shock. Higher stagnation pressures caused greater deflection of the shock wave and jet issued by the predetonator. It was seen that at sufficiently high equivalence ratios, the shock and reaction were able to travel upstream from the test section into the diverging section of the converging-diverging nozzle. Shortly after the shock entered the nozzle, evidence of the initiation of shock induced combustion was observed. Stagnation pressure variation in the range tested had little effect on the ability to initiate a reaction. Multiple behaviors of the shock-induced-combustion were observed, dependent upon the equivalence ratio of the flow through the test section. Behaviors include sustained reaction on the edges of the flow, sustained reaction in the core of the flow, and periodic, non-sustained reaction.
Show less - Date Issued
- 2018
- Identifier
- CFH2000549, ucf:45673
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH2000549
- Title
- MECHANISMS OF LEAN FLAME EXTINCTION.
- Creator
-
Lasky, Ian M, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to...
Show moreLean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to distinguish the physical mechanisms of blowout. Flame-vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of flame-generated vorticity coupled with an increase of downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. Frequency analysis is performed to characterize the dynamical changes of the hydrodynamic instability modes during flame extinction. Additionally, various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for different inflow velocity regimes.
Show less - Date Issued
- 2018
- Identifier
- CFH2000369, ucf:45710
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH2000369
- Title
- DESIGN AND INVESTIGATION OF VITIATED-AIR HEATER FOR OBLIQUE DETONATION-WAVE ENGINE.
- Creator
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Hoban, Matthew M, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
A facility was designed to provide high-enthalpy, hypersonic flow to a detonation chamber. Preliminary investigation identified 1300 K and Mach 5 as the total temperature and Mach number require to stabilize an oblique detonation wave inside the detonation chamber. Vitiated-air heating was the preheating method chosen to meet these capabilities. The vitiator facility heats compressed air while still retaining about 50% of the original oxygen content. Schlieren flow visualization and...
Show moreA facility was designed to provide high-enthalpy, hypersonic flow to a detonation chamber. Preliminary investigation identified 1300 K and Mach 5 as the total temperature and Mach number require to stabilize an oblique detonation wave inside the detonation chamber. Vitiated-air heating was the preheating method chosen to meet these capabilities. The vitiator facility heats compressed air while still retaining about 50% of the original oxygen content. Schlieren flow visualization and conventional photography was performed at the exit plane of a choke plate, which simulated the throat of a converging-diverging nozzle. A shock diamond formation was observed within the jet exhausting out of the choke hole. This is a clear indication that the facility is capable of producing hypersonic flow. A stoichiometric propane-air mixture was burned inside the combustion chamber. A thermocouple survey measured an average temperature of 1099 K at the exit plane of the mixing chamber; however, the actual temperature is likely higher than this, because cool, ambient air could be seen mixing with the hot, vitiated air near the exit plane. Because the adiabatic flame temperature of propane-air is lower than that of hydrogen-air, if hydrogen is used to vitiate the air, the facility is capable of meeting the 1300-K objective.
Show less - Date Issued
- 2016
- Identifier
- CFH0000236, ucf:44676
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH0000236
- Title
- Detailed Understanding of Flow, Heat Transfer, and Pressure Drop Behavior in a Square Channel With 45 Deg Ribs.
- Creator
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Ahmed, Lumaya, Kapat, Jayanta, Gordon, Ali, Ahmed, Kareem, Shivamoggi, Bhimsen, University of Central Florida
- Abstract / Description
-
Internal Duct Cooling (IDC) with rib turbulators is one of the common cooling techniques applied inside the turbine airfoils. It is very important for the gas turbine industry to design and develop an optimized cooling channel that maximizes the amount of heat removed, while simultaneously minimizing the pressure drop for a target overall cooling effectiveness. Angled ribs perform superior to the transverse ribs due to additional secondary flow associated with them. However, they result in a...
Show moreInternal Duct Cooling (IDC) with rib turbulators is one of the common cooling techniques applied inside the turbine airfoils. It is very important for the gas turbine industry to design and develop an optimized cooling channel that maximizes the amount of heat removed, while simultaneously minimizing the pressure drop for a target overall cooling effectiveness. Angled ribs perform superior to the transverse ribs due to additional secondary flow associated with them. However, they result in a highly non-homogenous heat transfer distribution, which is a manifestation of the complex, turbulent flow field inside the channel. It is very important to comprehend the secondary flow physics to characterize the heat transfer distribution in such angled ribbed channels. Additionally, due to the manufacturing constraint, the gas turbine industry encounters a challenge to make ribs edge sharp and results in ribs with rounded edges. The one of the main objectives of the present study is to provide a fundamental understanding of the flow physics on the heat transfer and pressure drop behavior in 45(&)deg; ribbed channels both with sharp and rounded-edge ribs. It is found that the secondary flow has a significant effect on the heat transfer behavior for both types of ribs. There is a great need of high-fidelity PIV flow field data in the inter-rib space for an angled ribbed channel which can be used for CFD validation, especially for LES. The current study provides benchmarking flow field data in the inter-rib space in a square channel with 45(&)deg; ribs using stereoscopic PIV technique. Besides the experiments, numerical studies were also conducted by using LES and different RANS models. The LES results show an excellent prediction capability for aerothermal behavior in such channels. However, the prediction capability of RANS models is found to be inconsistent for different rib configurations and flow conditions.
Show less - Date Issued
- 2018
- Identifier
- CFE0007302, ucf:52171
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007302
- Title
- THE EXPLORATION OF ROTATING DETONATION DYNAMICS INCORPORATING A COAL-BASED FUEL MIXTURE.
- Creator
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Rogan, John P., Ahmed, Kareem, Bhattacharya, Samik, University of Central Florida
- Abstract / Description
-
This investigation explores the detonation dynamics of a rotating detonation engine (RDE). Beginning with the general understanding and characteristics of hydrogen and compressed air as a detonation fuel source, this study further develops the experimental approach to incorporating a coal-based fuel mixture in an RDE. There is insufficient prior research investigating the use of coal as part of a fuel mixture and insignificant progress being made to improve thermal efficiency with...
Show moreThis investigation explores the detonation dynamics of a rotating detonation engine (RDE). Beginning with the general understanding and characteristics of hydrogen and compressed air as a detonation fuel source, this study further develops the experimental approach to incorporating a coal-based fuel mixture in an RDE. There is insufficient prior research investigating the use of coal as part of a fuel mixture and insignificant progress being made to improve thermal efficiency with deflagration. The U.S. Department of Energy's Office of Fossil Energy awarded the Propulsion and Energy Research Laboratory at the University of Central Florida a grant to lead the investigation on the feasibility of using a coal-based fuel mixture to power rotating detonation engines. Through this study, the developmental and experimental understanding of RDEs has been documented, operability maps have been plotted, and the use of a coal-based fuel mixture in an RDE has been explored. The operability of hydrogen and compressed air has been found, a normalization of all operable space has been developed, and there is evidence indicating coal can be used as part of a fuel mixture to detonate an RDE. The study will continue to investigate coal's use in an RDE. As the most abundant fossil fuel on earth, coal is a popular fuel source in deflagrative combustion for electrical power generation. This study investigates how the combustion of coal can become significantly more efficient.
Show less - Date Issued
- 2018
- Identifier
- CFH2000437, ucf:45741
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH2000437
- Title
- Structured Light-Field Focusing 3D Density Measurements of A Supersonic Cone.
- Creator
-
Shigematsu, Ryonosuke, Ahmed, Kareem, Bhattacharya, Samik, Das, Tuhin, University of Central Florida
- Abstract / Description
-
This study describes three-dimensional (3D) quantitative visualization of density field in a supersonic flow around a cone spike. A measurement of the density gradient is conducted within a supersonic wind tunnel facility at the Propulsion and Energy Research Laboratory at the University of Central Florida utilizing Structured Light-Field Focusing Schlieren (SLLF). In conventional schlieren and Shadowgraph techniques, it is widely known that a complicated optical system is needed and yet...
Show moreThis study describes three-dimensional (3D) quantitative visualization of density field in a supersonic flow around a cone spike. A measurement of the density gradient is conducted within a supersonic wind tunnel facility at the Propulsion and Energy Research Laboratory at the University of Central Florida utilizing Structured Light-Field Focusing Schlieren (SLLF). In conventional schlieren and Shadowgraph techniques, it is widely known that a complicated optical system is needed and yet visualizable area depends on an effective diameter of lenses and mirrors. Unlike these techniques, SLLF is yet one of the same family as schlieren photography, it is capable of non-intrusive turbulent flow measurement with relatively low cost and easy-to-setup instruments. In this technique, cross-sectional area in the flow field that is parallel to flows can be observed while other schlieren methods measure density gradients in line-of-sight, meaning that it measures integrated density distribution caused by discontinuous flow parameters. To reconstruct a 3D model of shock structure, two-dimensional (2D) images are pictured to process in MATLAB. The ultimate goal of this study is to introduce a novel technique of SLLF and quantitative 3D shock structures generated around a cone spike to reveal the interaction between free-stream flow and the high-pressure region.
Show less - Date Issued
- 2018
- Identifier
- CFE0007096, ucf:51965
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007096
- Title
- A Multi-Species Single-LED Hazardous Gas Sensor for Commercial Space Applications.
- Creator
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Parupalli, Akshita, Vasu Sumathi, Subith, Ahmed, Kareem, Chow, Louis, University of Central Florida
- Abstract / Description
-
In the interest of furthering both commercial and government-funded opportunities for deep space exploration, the safety of life and equipment onboard must be absolutely certain. In this regard, the presence of any hazardous gases or combustion events onboard space vehicles must be quickly characterized and detected. Several hazardous gases of interest have absorption features in the mid-infrared range and can be detected with an infrared light source, via the principles of absorption...
Show moreIn the interest of furthering both commercial and government-funded opportunities for deep space exploration, the safety of life and equipment onboard must be absolutely certain. In this regard, the presence of any hazardous gases or combustion events onboard space vehicles must be quickly characterized and detected. Several hazardous gases of interest have absorption features in the mid-infrared range and can be detected with an infrared light source, via the principles of absorption spectroscopy. A non-dispersive infrared (NDIR) sensor that follows these principles has been developed to utilize light-emitting diodes (LEDs) for gas detection and quantification. LEDs contain a particular advantage in this situation because they have low power requirements, are robust and easily adaptable, and they are cheaper than existing laser-based systems. The design has successfully performed several laboratory, environmental chamber, and high-altitude balloon flight tests. The main purpose of these various tests was to place the sensor in challenging environments, examine the effects on sensor performance, and adjust accordingly.The current sensor design utilizes a single 4.2?m LED and a rotating diffraction grating to detect both carbon dioxide (CO2) and nitrous oxide (N2O) within a single scan. These measurements were further validated using two distributed feedback quantum cascade lasers (QCL) centered at 4.25?m and 4.58?m. The sensor collected data on a wavelength range of 4117nm to 4592nm. Mixtures containing the concentrations of the two species of interest varying from 0.2% to 0.8% were analyzed. The integrated absorbance data was calculated for each species and compared with theoretical predictions. The results show that the data follows the expected behavior and correlates better at lower concentrations. Subsequent work on this sensor will focus on increasing the quantity of identifiable gases and on further testing in hazardous environments.
Show less - Date Issued
- 2019
- Identifier
- CFE0007898, ucf:52752
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007898
- Title
- Characterization of Acoustic Modes in Aeroengines.
- Creator
-
Otero, Michelle, Ahmed, Kareem, Kapat, Jayanta, Bhattacharya, Samik, University of Central Florida
- Abstract / Description
-
Acoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community....
Show moreAcoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community. The scope of this study was to design, optimize and characterize acoustic behaviors of a scaled rocket combustion chamber simulating acoustic pressure waves. Tangential and longitudinal acoustic waves of the system were extracted and validated through analytical and computational fluids dynamics models. The results of this study will assist with the process of extracting dominant oscillation frequencies of a system essential in the design of acoustic suppression devices for attenuation of critical frequencies.
Show less - Date Issued
- 2018
- Identifier
- CFE0007354, ucf:52081
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007354
- Title
- Compressible Turbulent Flame Speed of Highly Turbulent Standing Flames.
- Creator
-
Sosa, Jonathan, Ahmed, Kareem, Kassab, Alain, Kapat, Jayanta, University of Central Florida
- Abstract / Description
-
This work presents the first measurement of turbulent burning velocities of a highly-turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame-turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind...
Show moreThis work presents the first measurement of turbulent burning velocities of a highly-turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame-turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.
Show less - Date Issued
- 2018
- Identifier
- CFE0007102, ucf:51955
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007102
- Title
- Investigation of Heat Transfer Enhancement Within a Concentric Annulus.
- Creator
-
Hanhold, Alexander, Kapat, Jayanta, Ahmed, Kareem, Vasu Sumathi, Subith, University of Central Florida
- Abstract / Description
-
Effective heat exchange is key for many energy applications including heat exchangers, heat extraction from heat source, and heat rejection to ambient thermal sink. This study focuses on the investigation for a specific heat exchange configuration, namely heat removal within a concentric annular passage using helical turbulators and jet impingement. Numerical testing was used to see how the different geometric parameters affect the heat transfer and pressure drop within the annulus by using...
Show moreEffective heat exchange is key for many energy applications including heat exchangers, heat extraction from heat source, and heat rejection to ambient thermal sink. This study focuses on the investigation for a specific heat exchange configuration, namely heat removal within a concentric annular passage using helical turbulators and jet impingement. Numerical testing was used to see how the different geometric parameters affect the heat transfer and pressure drop within the annulus by using helicoil turbulators. A vast range of designs were studied by changing the turbulator shape, pitch, and blockage ratio while maintaining a constant Reynolds number of 25,000. CFD was performed in STARCCM+ using the realizable ?-? turbulence model. Results show that turbulence and heat transfer increase with a higher blockage ratio and smaller pitch but the pressure drop is subsequently increased as well. The square turbulator promoted higher heat transfer compared to the circle turbulator but the pressure drop was significantly increased when the helix angle was greater than 20(&)deg; and blockage ratio greater than 0.48.Experimental and numerical efforts were used to find the heat transfer due to impingement jets on the target surface. Multiple flows as a function of jet Reynolds number ranging from 16,000-33,000 were tested for two geometries. Temperature Sensitive Paint (TSP) was utilized to observe local heat transfer. It was observed that jet degradation occurs after the 6th row of stream-wise impingement jets for both cases experimentally and it was difficult to numerically capture the effect of the cross flow from previous jets but managed to follow the same trend. The numerical results showed that they can be used with good agreement to predict the surface averaged Nusselt number to be within the 12% uncertainty found from experimental efforts. Geometry B was determined to perform better in terms of heat transfer as opposed to Geometry A with the same pressure loss.
Show less - Date Issued
- 2017
- Identifier
- CFE0007286, ucf:52155
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007286
- Title
- Analysis of Heat Transfer on Turbulence Generating Ribs using Dynamic Mode Decomposition.
- Creator
-
Elmore, Michael, Kapat, Jayanta, Ahmed, Kareem, Bhattacharya, Samik, University of Central Florida
- Abstract / Description
-
Ducts with turbulence-promoting ribs are common in heat transfer applications. This study usesa recent modal extraction technique called Dynamic Mode Decomposition (DMD) to determinemode shapes of the spatially and temporally complex flowfield inside a ribbed duct. One subjectmissing from current literature is a method of directly linking a mode to a certain engineeringquantity of interest. Presented is a generalized methodology for producing such a link utilizing thedata from the DMD...
Show moreDucts with turbulence-promoting ribs are common in heat transfer applications. This study usesa recent modal extraction technique called Dynamic Mode Decomposition (DMD) to determinemode shapes of the spatially and temporally complex flowfield inside a ribbed duct. One subjectmissing from current literature is a method of directly linking a mode to a certain engineeringquantity of interest. Presented is a generalized methodology for producing such a link utilizing thedata from the DMD analysis. Theory suggests exciting the modes which are identified may causethe flow to change in such a way to promote the quantity of interest, in this case, heat transfer. Thistheory is tested by contouring the walls of the duct by the extracted mode shapes.The test procedure is taken from an industrial perspective. An initial, unmodified geometry pro-vides a baseline for comparison to later contoured models. The initial case is run as a steady-stateReynolds-Averaged Navier-Stokes model. Large-Eddy Simulation generates the necessary datafor the DMD analysis. Several mode shapes extracted from the flow are applied to the duct wallsand run again in the RANS model, then compared to the baseline, and their relative performanceexamined.
Show less - Date Issued
- 2018
- Identifier
- CFE0007328, ucf:52123
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007328
- Title
- Theoretical Paschen's Law Model for Aerospace Vehicles: Validation Experiment.
- Creator
-
Mulligan Aroche, Jaysen, Ahmed, Kareem, Kapat, Jayanta, Bhattacharya, Samik, University of Central Florida
- Abstract / Description
-
Aerospace vehicles often experience triboelectric charging while traversing the atmosphere. Triboelectric charging occurs when a material come into frictional contact with a different material. Aerospace vehicles triboelectrically charge due to frictional contact with dust and ice crystals suspended in the atmosphere. Launch vehicles traversing ice clouds in low-pressure atmosphere are especially prone to electrostatic discharge events (i.e. sparks). These conditions are hazardous and affect...
Show moreAerospace vehicles often experience triboelectric charging while traversing the atmosphere. Triboelectric charging occurs when a material come into frictional contact with a different material. Aerospace vehicles triboelectrically charge due to frictional contact with dust and ice crystals suspended in the atmosphere. Launch vehicles traversing ice clouds in low-pressure atmosphere are especially prone to electrostatic discharge events (i.e. sparks). These conditions are hazardous and affect the vehicle's launch commit criteria. In 2010, engineers from an ARES-I rocket launch reported concerns with triboelectric charging over their self-destruct system antenna. This concern was addressed by putting the antenna through harsh conditions in a laboratory environment. The need for laboratory testing could have been avoided if there was a mathematical model to predict these events. These discharge events can typically be predicted by the Classical Paschen's Law, which relates discharge voltage to pressure, material and distance between the charged and ground surfaces (i.e. electrodes). However, the Classical Paschen's Law does not capture any aerodynamic considerations such as large bulk flow and compressibility effects. It became apparent that a new model would be needed to predict a discharge voltage with aerodynamic considerations. This research focused on defining a theoretical model and providing experimental data to validate the model. The hypothesis of this work is that charged ions are removed too quickly for enough charge to build up and result in an electrostatic discharge at the voltage that is predicted by the Classical Paschen's Law. The wind tunnel testing for this experiment was conducted at the Center for Advanced Turbomachinery (&) Energy Research (CATER) facility. A charged electrode was exposed to flows at Mach numbers 1.5 to 3.5. It was found that the supersonic flow suppressed the electrostatic discharge events. The voltage required for an electrostatic discharge at supersonic conditions increased by a factor of three. The modified Paschen's Law can help in defining the launch commit criteria of aerospace vehicles.
Show less - Date Issued
- 2018
- Identifier
- CFE0007059, ucf:51994
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007059
- Title
- Influence of Transverse Slot Jet on Premixed Flame Acceleration.
- Creator
-
Tarrant, Dylan, Ahmed, Kareem, Bhattacharya, Samik, Vasu Sumathi, Subith, University of Central Florida
- Abstract / Description
-
This work aims to identify the key flow parameters that influence flame acceleration in a semi-confined square channel. A transverse fluidic jet was used as an active flow blockage mechanism and to introduce turbulence into the propagating flame. Three experimental parameters were used to examine the relative influence of (1) mixture reactivity defined here as system equivalence ratio (SER), (2) jet mixture composition (JMC), and the momentum ratio (MR) on the acceleration of laminar premixed...
Show moreThis work aims to identify the key flow parameters that influence flame acceleration in a semi-confined square channel. A transverse fluidic jet was used as an active flow blockage mechanism and to introduce turbulence into the propagating flame. Three experimental parameters were used to examine the relative influence of (1) mixture reactivity defined here as system equivalence ratio (SER), (2) jet mixture composition (JMC), and the momentum ratio (MR) on the acceleration of laminar premixed methane flame. High-speed PIV and schlieren photography were utilized to characterize the instantaneous flow-field conditions throughout the flame-jet interaction. Using these diagnostic techniques, flame front positions and local velocity vector fields have been spatially and temporally resolved. Changes in flame properties including flame structure, velocity, and vorticity were tracked as a function of time. Stoichiometric equivalence ratios were more effective in the production of vorticity and the promotion of flame acceleration. The stoichiometric condition accelerated the flame to the highest final flame velocity of the three parameters examined. Different compositions of the jet mixture demonstrated that the flame acceleration is primarily affected by the jet turbulence and not on the reactivity of the jet compositions. Out of the three parameters examined, the momentum ratio parameter had the least amount of influence on the flow field and flame acceleration. The increase of 33 % in the momentum ratio had negligible effect in the final flame front velocity and implies that the jet turbulence is the main driving mechanism for flame acceleration.
Show less - Date Issued
- 2018
- Identifier
- CFE0007255, ucf:52186
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007255
- Title
- Ignition Studies of Oxy-Syngas/CO2 Mixtures Using Shock Tube for Cleaner Combustion Engines.
- Creator
-
Barak, Samuel, Vasu Sumathi, Subith, Kapat, Jayanta, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
In this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% - 85% by volume...
Show moreIn this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% - 85% by volume. A 10% fuel concentration was consistently used throughout the experiments. This study looked at the effects of changing the equivalence ratios (?), between 0.33, 0.5, and 1.0 as well as changing the fuel ratio (?), hydrogen to carbon monoxide, from 0.25, 1.0 and 4.0. The study was performed at 1.61-1.77 atm and a temperature range of 1006-1162K. The high-speed imaging was performed through a quartz end wall with a Phantom V710 camera operated at 67,065 frames per second. From the experiments, when increasing the equivalence ratio, it resulted in a longer ignition delay time. In addition, when increasing the fuel ratio, a lower ignition delay time was observed. These trends are generally expected with this combustion reaction system. The high-speed imaging showed non-homogeneous combustion in the system, however, most of the light emissions were outside the visible light range where the camera is designed for. The results were compared to predictions of two combustion chemical kinetic mechanisms: GRI v3.0 and AramcoMech v2.0 mechanisms. In general, both mechanisms did not accurately predict the experimental data. The results showed that current models are inaccurate in predicting CO2 diluted environments for syngas combustion.
Show less - Date Issued
- 2018
- Identifier
- CFE0006974, ucf:52909
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006974
- Title
- Shock Tube Investigations of Novel Combustion Environments Towards a Carbon-Neutral Future.
- Creator
-
Barak, Samuel, Vasu Sumathi, Subith, Kapat, Jayanta, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
Supercritical carbon dioxide (sCO2) cycles are being investigated for the future of power generation. These cycles will contribute to a carbon-neutral future to combat the effects of climate change. These direct-fired closed cycles will produce power without adding significant pollutants to the atmosphere. For these cycles to be efficient, they will need to operate at significantly higher pressures (e.g., 300 atm for Allam Cycle) than existing systems (typically less than 40 atm). There is...
Show moreSupercritical carbon dioxide (sCO2) cycles are being investigated for the future of power generation. These cycles will contribute to a carbon-neutral future to combat the effects of climate change. These direct-fired closed cycles will produce power without adding significant pollutants to the atmosphere. For these cycles to be efficient, they will need to operate at significantly higher pressures (e.g., 300 atm for Allam Cycle) than existing systems (typically less than 40 atm). There is limited knowledge on combustion at these pressures or at the high dilution of carbon dioxide. Nominal fuel choices for gas turbines include natural gas and syngas (mixture of CO and H2). Shock tubes study these problems in order to understand the fundamentals and solve various challenges. Shock tube experiments have been studied by the author in the sCO2 regime for various fuels including natural gas, methane and syngas. Using the shock tube to take measurements, pressure and light emissions time-histories measurements were taken at a 2-cm axial location away from the end wall. Experiments for syngas at lower pressure utilized high-speed imaging through the end wall to investigate the effects of bifurcation. It was found that carbon dioxide created unique interactions with the shock tube compared to tradition bath gasses such as argon. The experimental results were compared to predictions from leading chemical kinetic mechanisms. In general, mechanisms can predict the experimental data for methane and other hydrocarbon fuels; however, the models overpredict for syngas mixtures. Reaction pathway analysis was evaluated to determine where the models need improvements. A new shock tube has been designed and built to operate up to 1000 atm pressures for future high-pressure experiments. Details of this new facility are included in this work. The experiments in this work are necessary for mechanism development to design an efficient combustor operate these cycles.
Show less - Date Issued
- 2019
- Identifier
- CFE0007781, ucf:52359
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007781
- Title
- Heat Transfer, Friction, and Turbulent Analysis on Single Ribbed-Wall Square Channel.
- Creator
-
Vergos, Christopher, Kapat, Jayanta, Vasu Sumathi, Subith, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
An experimental investigation of heat transfer and friction behavior for a fully developed flow in a non-rotating square channel was conducted under a wide range of Reynolds numbers from 6,000 to 180,000. The rig used in this study was a single ribbed wall variant of Ahmed et al.'s [ 1 ] rig from which results of this rig were compared. Ahmed et al.'s rig was a replica of Han et al.'s square channel [ 2 ] used to validate their work, and expand the Reynolds number range for both heat transfer...
Show moreAn experimental investigation of heat transfer and friction behavior for a fully developed flow in a non-rotating square channel was conducted under a wide range of Reynolds numbers from 6,000 to 180,000. The rig used in this study was a single ribbed wall variant of Ahmed et al.'s [ 1 ] rig from which results of this rig were compared. Ahmed et al.'s rig was a replica of Han et al.'s square channel [ 2 ] used to validate their work, and expand the Reynolds number range for both heat transfer and friction data. The test section was 22 hydraulic diameters (Dh) long, and made of four aluminum plates. One rib roughened bottom wall, and three smooth walls bounded the flow. Glued brass ribs oriented at 45(&)deg; to the flow direction, with a ratio of rib height to channel hydraulic diameter (e/Dh) and a ratio of pitch to rib height (p/e) of 0.063 and 10, respectively, lined the bottom wall. A 20Dh long acrylic channel with a continuation of the test section's interior was attached at the inlet of the test section to confirm the fully developed flow. Heat transfer tests were conducted in a Reynolds number range of 20,000 to 150,000. During these tests, the four walls were held under isothermal conditions. Wall-averaged, and module-averaged Nusselt values were calculated from the log-mean temperature differences between the plate surface temperature and calculated, by energy balance, fluid bulk temperature. Streamwise Nusselt values become constant at an x/Dh of 8 within the tested Reynolds number range. Wall averaged Nusselt values were determined after x/Dh=8, and scaled by the Dittus-Boelter correlation, Nuo, for smooth ducts to yield a Nusselt augmentation value (Nu/Nuo). Non-heated friction tests were conducted from a Reynolds number range of 6,000 to 180,000. Pressure drop along the channel was recorded, and channel-averaged Darcy-Weisbach friction factor was calculated within the range of Reynolds number tested. Scaling the friction factor by the smooth-wall Blasius correlation, fo, gave the friction augmentation (f/fo). The thermal performance, a modified ratio of the Nusselt and friction augmentation used by Han et al. [ 2 ], was then calculated to evaluate the bottom-line performance of the rig. It was found that the Nusselt augmentation approached a constant value of 1.4 after a Reynolds number of 60,000 while friction augmentation continued to increase in a linear fashion past that point. This caused the overall thermal performance to decline as Reynolds number increased up to a certain point. Further studies were conducted in an all acrylic, non-heated variant of the rig to study the fluid flow in the streamwise direction on, and between two ribs in the fully developed region of the channel. Single-wire hot-wire anemometry characterized velocity magnitude profiles with great detail, as well as turbulence intensity for Reynolds numbers ranging from 5,000 to 50,000. As the Reynolds number increased the reattachment point between two ribs remained about stationary while the turbulence intensity receded to the trailing surface of the upstream rib, and dissipated as it traveled. At low Reynolds numbers, between 5,000 and 10,000, the velocity and turbulence intensity streamwise profiles seemed to form two distinct flow regions, indicating that the flow over the upstream rib never completely attached between the two ribs. Integral length-scales were also derived from the autocorrelation function using the most turbulent signal acquired at each Reynolds number. It was found that there is a linear trend between Reynolds number and the integral length-scale at the most turbulent points in the flow. For example, at Re=50,000 the most the length scale found just past the first rib was on the order of two times the height of the rib. Rivir et al. [ 30 ] found in a similar case that at Re = 45,000, it was 1.5 times the rib height. Several factors could influence the value of this integral length-scale, but the fact that their scale is on the order of what was obtained in this case gives some level of confidence in the value.
Show less - Date Issued
- 2017
- Identifier
- CFE0007138, ucf:52318
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007138
- Title
- Ellipsometric Measurements of Alternative Fuels.
- Creator
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Nash, Leigh, Vasu Sumathi, Subith, Kapat, Jayanta, Ahmed, Kareem, University of Central Florida
- Abstract / Description
-
Alternative jet fuels will be important in the future to ensure cleaner burning, reliable, and reasonably priced air transportation. One important property that must meet certification standards is the fuel's thermal stability, or its ability to withstand heating before breaking down. Jet fuels are used as engine coolants, and thermally unstable fuels can form deposits in the fuel delivery systems, leading to a loss of fuel flow. In the past, the thermal stability of a fuel was rated using a...
Show moreAlternative jet fuels will be important in the future to ensure cleaner burning, reliable, and reasonably priced air transportation. One important property that must meet certification standards is the fuel's thermal stability, or its ability to withstand heating before breaking down. Jet fuels are used as engine coolants, and thermally unstable fuels can form deposits in the fuel delivery systems, leading to a loss of fuel flow. In the past, the thermal stability of a fuel was rated using a color standard method. The color of the deposit left on a metal tube that had been heated and exposed to a test fuel were matched with a color standard to rate the level of deposition, and thus the fuel's thermal stability. Ellipsometry, which is an optical technique that uses changes in a beam of light's polarization after it reflects off a sample to determine the thickness of any film on that sample, has recently been implemented to improve the thermal stability characterization standard.Various aspects of the ellipsometric method have been investigated in this work. In addition, several thermal stability studies were carried out. The effect of increasing temperature on the thermal stability of Sasol Iso-Paraffinic Kerosene, Jet A, JP-8, and Gevo jet fuel have been analyzed, and the effect of varying levels of the additive naphthalene in Sasol IPK has also been investigated. Various theoretical optical models have been evaluated for their ability to predict deposit thickness. Finally, attempts to validate these measurements were made using scanning electron microscopy, ellipsometric tube rating, interferometric tube rating, and reference tubes. The analysis carried out in this work was used to make recommendations for improving the thermal stability test standard.
Show less - Date Issued
- 2017
- Identifier
- CFE0007130, ucf:52323
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007130
- Title
- Compressibility Effect on Turbulent Flames and Detonation Initiation and Propagation.
- Creator
-
Sosa, Jonathan, Ahmed, Kareem, Kassab, Alain, Kapat, Jayanta, University of Central Florida
- Abstract / Description
-
This work presents the first measurement of turbulent burning velocities of a highly-turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame-turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind...
Show moreThis work presents the first measurement of turbulent burning velocities of a highly-turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame-turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.Additionally, this study presents the exploration of detonation wave propagation dynamics in premixed supersonic flows using a novel rotating detonation engine (RDE) configuration. An RDE with a coupled linear extension, referred to as ?DE, is used to divide detonations traveling radially in the RDE into linearly propagating waves. A tangential propagating wave is directed down a modular tangential linearized extension to the engine for ease of optical diagnostics and hardware configuration investigations. A premixed Mach 2 supersonic linear extension is coupled to the ?DE to investigate the effects of varying crossflow configurations for detonation propagation, particularly the interaction between detonations and supersonic reactive mixtures. Detonation waves are generated at the steady operating frequency of the RDE and visualized using high speed schlieren and broadband OH* chemiluminescence imaging. The stagnation pressure was varied from over- to ideally-expanded supersonic regimes. Experimental analysis of detonation interaction with the supersonic regimes show that the detonation propagates freely in the ideally-expanded regime. Deflagration-to-detonation transition (DDT) occurs in the over-expanded regime. Based on the data collected, the DDT process favors supersonic flow with higher source pressures. Lastly, this work presents the experimental evidence of controlled detonation wave initiation and propagation in hydrogen-air premixed hypersonic Mach 5 flows. A Mach 5 high-enthalpy facility is used to provide the premixed hydrogen-air stream targeted to match the boundary conditions (Chapman-Jouguet, CJ) for stable detonations. The work shows for the first-time flame deflagration-to-detonation transition through coupled mechanism of turbulent flame acceleration and shock-focusing in a premixed Mach 5 flow. The paper defines three new distinct regimes in a Mach 5 premixed flow: Deflagration-to-Detonation Transition (DDT), Unsteady Compressible Turbulent Flames, and Shock-Induced Combustion. With rising national interest in hypersonics and reduced combustion emissions, the discovery and classification of these new combustion regimes allows for a possible pathway to develop and integrate detonation technology enabling hypersonic propulsion technology and advanced power systems.
Show less - Date Issued
- 2019
- Identifier
- CFE0007534, ucf:52607
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007534
- Title
- Development of a Single Sensor Approach for Capturing Three-Dimensional, Time Resolved Flame and Velocity Information.
- Creator
-
Reyes, Jonathan, Ahmed, Kareem, Kassab, Alain, Kapat, Jayanta, University of Central Florida
- Abstract / Description
-
Performing non-intrusive measurements is the key to acquiring accurate information representative of what is being observed. The act of measuring often changes the environment being observed altering the information that is being obtained. Due to this, the community of fluid scientists have gravitated towards using laser-based measurements to observe the phenomena occurring in their experiments. The study of fluids has advanced since this point, utilizing techniques such as planar laser...
Show morePerforming non-intrusive measurements is the key to acquiring accurate information representative of what is being observed. The act of measuring often changes the environment being observed altering the information that is being obtained. Due to this, the community of fluid scientists have gravitated towards using laser-based measurements to observe the phenomena occurring in their experiments. The study of fluids has advanced since this point, utilizing techniques such as planar laser induced florescence (PLIF), particle image velocimetry (PIV), laser doppler velocimetry (LDV), particle doppler anemometry (PDA), etc. to acquire chemical species information and velocity information. These techniques, though, are inherently two-dimensional and cannot fully describe a flow field. In the area of reacting flow fields (combustion) acquiring the local fuel to air ratio information is increasingly important. Without it, scientist must rely on global one-dimensional metering techniques to correlate the fuel to air ratio of their flow field of interest. By knowing the fuel to air ratio locally and spatially across a flame, the location of products and reactants can be deduced, giving insight into any inefficiencies associated with a burner. Knowing the spatial fuel air field also gives insights into the density gradient associated with the flow field. Discussed in this work will be the development of a non-intrusive local fuel-air measurement technique and an expansion of the PIV technique into the third dimension, tomographic PIV, utilizing only one camera to do so for each measurement. The local fuel-air measurement is performed by recording two species (C2* and CH*) simultaneously and calibrating their ratio to the known fuel-air field. Tomographic PIV is performed by utilizing fiber coupling to acquire multiple viewpoints utilizing a single camera.
Show less - Date Issued
- 2019
- Identifier
- CFE0007523, ucf:52602
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007523
