Browsing by Subject "Combustion"

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Characteristics of radially propagating smoldering combustion in a sawdust bed
(2015-12) Bush, Robert H., III; Ellzey, Janet L.
In this thesis, experimental work on smoldering of sawdust beds is presented and discussed. Extensive study has been done on wood burning cook stoves with an emphasis on performance characterization and optimization. Few studies, however, have focused on the smoldering process with the goal of understanding the propagation of the front and the production of emissions. In this study, photographs, temperature and emission measurements on smoldering sawdust clearly showed the evolution of the combustion process: an initial conversion of the raw sawdust to char followed by the conversion of char to ash. In general, the char front propagated symmetrically in the radial direction while the ash front was not symmetric, and typically followed paths where oxygen was most readily available. Further analysis was accomplished by observing the characteristics of the sawdust bed before transition to flaming occurred. Contrary to expected results, flaming did not occur as the air flow was increased, but rather once it was decreased, suggesting that flaming is determined by a balance between generation of volatiles and dilution by incoming air. Experiments with vitiated air, in which the oxygen content of air is diluted by adding nitrogen, were conducted to determine a limit at which combustion was no longer self-sustaining. Experiments showed that vitiated air with 7% oxygen in the supply air did not support self-sustaining combustion. Finally, a comparison between poplar and walnut was conducted to show the effect of wood species. Comparison of temperature, hydrocarbon, and carbon monoxide outputs identified characteristic differences between the poplar and walnut species.
A computational fluid dynamics simulation model for flare analysis and control
(2006) Castiñeira Areas, David; Edgar, Thomas F.
Industrial flares are units designed to safely dispose of waste hydrocarbon gases from chemical and petrochemical plants by burning gases to carbon dioxide and steam, which are then released to the atmosphere. There is still great uncertainty about flare efficiency and the resultant gas emissions under different operating conditions. For this reason, environmental agencies have encouraged the development of predictive models for flare gas combustion systems, so effective control and mitigation strategies can be implemented. The principal focus of this dissertation is to develop mathematical models of industrial flares that predict the efficiency of these industrial combustion systems. For this purpose, a computational fluid dynamics (CFD) simulation model is implemented to analyze the effects of variables such as ambient wind velocity, gas heating value, and steam injection on flare combustion efficiency. Some advanced chemistry and turbulence submodels are also implemented to describe the complex flare flow phenomena. Simulation results show that flares may represent an important source of gas emissions due to inefficient operation at high crosswinds and large steam/fuel ratios. The predictive models presented in this work will allow for better estimation of the resulting gas emissions from industrial plants. Use of these simulation models will also yield economic savings for environmental studies compared to setting up expensive flare experiments. In addition, these predictive models allow for a detailed analysis of species concentration profiles and turbulent flow patterns within the flames, data which is not available experimentally. Furthermore, several instrumentation and control strategies for industrial flares are analyzed in this dissertation. A new approach for flare monitoring based on multivariate image analysis is proposed so that flare combustion efficiency can be measured in real-time.
Computational modeling of high pressure plasmas for plasma assisted combustion, liquid reforming and thermal breakdown applications
(2019-01-22) Sharma, Ashish, 1990-; Raja, Laxminarayan L.; Varghese, Philip L; Goldstein, David B; Bisetti, Fabrizio; Hallock, Gary A
The goal of the present work is to study high pressure non-equilibrium plasma discharges in chemically reactive systems. In this work, we present coupled computational studies of high pressure nanosecond pulsed plasmas for multiphysics applications ranging from plasma assisted combustion ignition, large gap thermal breakdown, to electric discharge in liquids for fuel reforming and biomedical applications. In the first part of the work, we report the results of a computational study which explores argon surface streamers as a low-voltage mechanism for thermal breakdown of large interelectrode gaps and investigate the effect of impurities (molecular oxygen) on the development of continuous surface streamer channels under atmospheric-pressure conditions. In pure argon, a continuous conductive streamer successfully bridges the gap between two electrodes indicating high probability of transition to arc. Presence of oxygen impurities in small concentrations (less than 5%) is found to be conducive to streamer induced thermal breakdown as it reduces the threshold voltage of streamer formation and minimizes unwanted streamer branching effects while maintaining a high probability of streamer to arc transition. Higher oxygen impurity levels > 5% are found to significantly deteriorate the continuous conductivity of streamer channel and lead to a much lower probability for transition to thermal arcs. In the second part of the work, we present a computational study of nanosecond streamer discharges in helium gas (He) bubbles suspended in distilled water (H₂O) for liquid reforming applications. The model takes into account the presence of water vapor in the gas bubble for an accurate description of the discharge kinetics. The objective is to study the kinetics and dynamics of streamer evolution and maximize active species production within the gas bubbles which is the quantity of interest for plasma processing of liquids. We investigate two parameters, namely a) trigger voltage polarity and b) the presence of multiple bubbles, which are found to significantly influence the characteristics of the discharge in gas bubbles. A substantial difference is observed in initiation, transition and evolution stages of streamer discharge for positive and negative trigger voltages. The volumetric distribution of species in the streamer channel is more uniform for negative trigger voltages on account of the formation of multiple streamers. In case of the presence of more than one gas bubble, we see the phenomenon of streamer hopping between bubbles where the high electric field in the sheath of the first bubble triggers the streamer discharge in the adjacent bubble. The presence of multiple immersed bubbles reduces the breakdown voltage of the plasma discharge and results in more uniform generation of active species. It is concluded that a negative pin trigger with multiple immersed gas bubbles maximizes the active species generation which is conducive to plasma assisted liquid reforming applications. In the final part of the work, a coupled two-dimensional computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H₂ – air mixture in a high pressure environment is described. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effect of three important plasma properties that influence combustion ignition and flame propagation, namely a) plasma gas temperature, b) plasma-produced primary combustion radicals O, OH, and H densities, and c) plasma-generated charged and electronically excited radical densities. Preliminary zero-dimensional studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduces the ignition delay of the H₂ – air mixture and becomes especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H₂ – air mixture (φ=0.3) at 1 and 3.3 atm and a range of initial tem- perature from 1000 - 5000 K. The plasma is accompanied by fast gas heating due to N₂ metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (∼ 0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H₂ – air mixture. The generation of other plasma specific species results only in a slight ∼ 10 % improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H₂⁺, O₂⁻, O⁻ ) and oxygen metastable species (O₂ [superscript a1], O₂ [superscript b1], O₂ [superscript *]) primarily contributing to com- bustion enhancement. These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.
Controlling parameters of excess enthalpy combustion
(2014-05) Belmont, Erica Lynn; Ellzey, Janet L.
Excess enthalpy combustion utilizes heat recirculation, in which heat is transferred from hot products to cold reactants to effectively preheat the reactants, in order to achieve improved combustion performance through the extension of flammability limits and increased burning rate. This research examines the effect of key parameters in excess enthalpy combustion on combustion stability, fuel conversion, and product species production through experimental and numerical investigation. Operating condition parameters that are studied include inlet reactant equivalence ratio and inlet velocity, and reactor geometry parameters that are studied include reactor channel height and length. Premixed reactants, including gaseous and liquid fuels, are investigated at rich and lean conditions. The examination of liquid fuels and the ability of a reactor to support rich and lean combustion of both gaseous and liquid fuels is a significant demonstration of a reactor’s flexibility for practical applications. This research experimentally and numerically examines excess enthalpy combustion in a counter-flow reactor. First, the counter-flow reactor, previously used for thermal partial oxidation of gaseous hydrocarbon fuels, is used in experiments to reform a liquid hydrocarbon fuel, heptane, to syngas. The effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and product composition is explored. Second, lean combustion is demonstrated through experiments in the same counter-flow reactor previously used in reforming studies. The effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and pollutant concentrations in combustion products is studied. An analytical model, previously developed for rich combustion, is adapted to qualitatively predict the behavior of the counter-flow reactor in response to changes in lean operating conditions. Third, lean combustion in the counter-flow reactor is further studied by examining the combustion of increasingly complex gaseous and liquid fuels. Again, the effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and pollutant concentrations in combustion products is studied. Fourth and finally, a computational scaling study examines the impact of counter-flow reactor channel geometry on combustion stability, temperature increase above adiabatic values, heat recirculation, and fuel and product species conversion efficiency.
Development of a multiple-pass Raman spectrometer for flame diagnostics
(2013-05) KC, Utsav; Varghese, Philip L.
A multiple-pass cell is developed and applied to enhance the Raman signal from methane-air flames for temperature measurements. Stable operation of the cell was demonstrated and studied in two alignment modes. In the ring mode, the beams are focused into a ring of ~ 3 mm diameter at the center of the cell, and spectra were recorded at low dispersion (0.26 nm/pixel). Temperature is calculated from the ratio of the intensity of Stokes to anti-Stokes signal from nitrogen. Temperature is also inferred from the shapes of the Stokes and anti-Stokes peaks in the spectrum. The uncertainty in the value of flame temperature in these measurements was ±50 K. The signal gain from 100 passes is a factor of 83. Signal to noise ratio (SNR) improved by a factor of 9.3 in room temperature air with an even higher factor in flames. The improvement in SNR depends on the acquisition time and is best for short acquisition times. In the two point mode, multi passing is achieved simultaneously with high spatial resolution as the laser is focused at two small regions separated by ~ 2 mm at the center of the cell. The probe regions are 300 [mu]m × 200 [mu]m. The vast improvement in the spatial resolution is achieved at the cost of a reduced number of passes and signal gain. The two point mode is operated with 25 passes at each point with a signal gain factor of ~20; the SNR gain depends on the data acquisition time. Spectra were recorded at high dispersion (~0.03 nm/pixel). Temperature is inferred from curve fitting to the high resolution Stokes spectrum of nitrogen in methane-air flames. The curve fit is based on very detailed simulation of Raman spectrum of nitrogen. The final model includes the angular dependence of Raman scattering, electrical and mechanical anharmonicity in the polarizability matrix elements, and the presence of a rare isotope of nitrogen in air. The uncertainty in the value of temperature in the least noisy data is ±9 K. The sources of uncertainty in temperature and their contribution to the total uncertainty are also identified.
Direct numerical simulation and reaction path analysis of titania formation in flame synthesis
(2012-08) Singh, Ravi Ishwar; Ezekoye, Ofodike A.; Raman, Venkat
Flame-based synthesis is an attractive industrial process for the large scale generation of nanoparticles. In this aerosol process, a gasifi ed precursor is injected into a high-temperature turbulent flame, where oxidation followed by particle nucleation and other solid phase dynamics create nanoparticles. Precursor oxidation, which ultimately leads to nucleation, is strongly influenced by the turbulent flame dynamics. Here, direct numerical simulation (DNS) of a canonical homogeneous flow is used to understand the interaction between a methane/air flame and titanium tetrachloride oxidation to titania. Detailed chemical kinetics is used to describe the combustion and precursor oxidation processes. Results show that the initial precursor decomposition is heavily influenced by the gas phase temperature field. However, temperature insensitivity of subsequent reactions in the precursor oxidation pathway slow down conversion to the titania. Consequently, titania formation occurs at much longer time scales compared to that of hydrocarbon oxidation. Further, only a fraction of the precursor is converted to titania, and a signi cant amount of partially-oxidized precursor species are formed. Introducing the precursor in the oxidizer stream as opposed to the fuel stream has only a minimal impact on the oxidation dynamics. In order to understand modeling issues, the DNS results are compared with the laminar flamelet model. It is shown that the flamelet assumption qualitatively reproduces the oxidation structure. Further, reduced oxygen concentration in the near-flame location critically a ffects titania formation. The DNS results also show that titania forms on the lean and rich sides of the flame. A reaction path analysis (RPA) is conducted. The results illustrate the di ffering reaction pathways of the detailed chemical mechanism depending on the composition of the mixture. The RPA results corroborate with the DNS results that titania formation is maximized at two mixture fraction values, one on the lean side of the flame, and one on the rich side.
Experimental investigation of film cooling and thermal barrier coatings on a gas turbine vane with conjugate heat transfer effects
(2013-05) Kistenmacher, David Alan; Bogard, David G.
In the United States, natural gas turbine generators account for approximately 7% of the total primary energy consumed. A one percent increase in gas turbine efficiency could result in savings of approximately 30 million dollars for operators and, subsequently, electricity end-users. The efficiency of a gas turbine engine is tied directly to the temperature at which the products of combustion enter the first stage, high-pressure turbine. The maximum operating temperature of the turbine components’ materials is the major limiting factor in increasing the turbine inlet temperature. In fact, current turbine inlet temperatures regularly exceed the melting temperature of the turbine vanes through advanced vane cooling techniques. These cooling techniques include vane surface film cooling, internal vane cooling, and the addition of a thermal barrier coating (TBC) to the exterior of the turbine vane. Typically, the performance of vane cooling techniques is evaluated using the adiabatic film effectiveness. However, the adiabatic film effectiveness, by definition, does not consider conjugate heat transfer effects. In order to evaluate the performance of internal vane cooling and a TBC it is necessary to consider conjugate heat transfer effects. The goal of this study was to provide insight into the conjugate heat transfer behavior of actual turbine vanes and various vane cooling techniques through experimental and analytical modeling in the pursuit of higher turbine inlet temperatures resulting in higher overall turbine efficiencies. The primary focus of this study was to experimentally characterize the combined effects of a TBC and film cooling. Vane model experiments were performed using a 10x scaled first stage inlet guide vane model that was designed using the Matched Biot Method to properly scale both the geometrical and thermal properties of an actual turbine vane. Two different TBC thicknesses were evaluated in this study. Along with the TBCs, six different film cooling configurations were evaluated which included pressure side round holes with a showerhead, round holes only, craters, a novel trench design called the modified trench, an ideal trench, and a realistic trench that takes manufacturing abilities into account. These film cooling geometries were created within the TBC layer. Each of the vane configurations was evaluated by monitoring a variety of temperatures, including the temperature of the exterior vane wall and the exterior surface of the TBC. This study found that the presence of a TBC decreased the sensitivity of the thermal barrier coating and vane wall interface temperature to changes in film coolant flow rates and changes in film cooling geometry. Therefore, research into improved film cooling geometries may not be valuable when a TBC is incorporated. This study also developed an analytical model which was used to predict the performance of the TBCs as a design tool. The analytical prediction model provided reasonable agreement with experimental data when using baseline data from an experiment with another TBC. However, the analytical prediction model performed poorly when predicting a TBC’s performance using baseline data collected from an experiment without a TBC.
Fuel economy predictions for heavy‐duty vehicles and quasi‐dimensional DI diesel engine numerical modeling
(2016-05) Ates, Murat, 1982-; Matthews, Ronald D.; Hall, Matthew John; Ellzey, Janet L.; Ezekoye, Ofodike A.; Biros, George; Roberts, Charles E.
A research team developed the University of Texas Fuel Economy Model to estimate the fuel consumption of both light-duty and heavy-duty vehicles operated on Texas roads. One of the objectives of the model was to be as flexible as possible in order to be capable of simulating a variety of vehicles, payloads, and traffic conditions. For heavy-duty vehicles, there are no prescribed driving cycles, there are no coastdown coefficients available from the EPA, and we relied on experimental brake specific fuel consumption maps for a few heavy-duty diesel engines. Heavy-duty vehicle drive cycles highly depend upon the vehicle load, the grade of the road, the engine size, and the traffic conditions. In order to capture real driving conditions 54 drive cycles with three different Class 8 trucks, three weight configurations, three traffic congestion levels, and two drivers are collected. Drive cycles obtained in this research include road grade and vehicle speed data with time. Due to the lack of data from EPA for calculating the road load force for heavy-duty vehicles, coastdown tests were performed. To generate generic fuel maps for the fuel economy model, a direct injection quasi-dimensional diesel engine model was developed based on in-cylinder images available in the literature. Sandia National Laboratory researchers obtained various images describing diesel spray evolution, spray mixing, premixed combustion, mixing controlled combustion, soot formation, and NOx formation via imaging technologies. Dec combined all of the available images to develop a conceptual diesel combustion model to describe diesel combustion from the start of injection up to the quasi-steady form of the jet. The end of injection behavior was left undescribed in this conceptual model because no clear image was available due to the chaotic behavior of diesel combustion. A conceptual end-of-injection diesel combustion behavior model was proposed to capture diesel combustion in its life span. A full-cycle quasi-dimensional direct injection diesel engine model was developed that represents the physical models, utilizing the conceptual model developed from imaging experiments and available experiment-based spray models, of the in-cylinder processes. The compression, expansion, and gas exchange stages are modeled via zero-dimensional single zone calculations. A full cycle simulation is necessary in order to capture the initial conditions of the closed section of the cycle and predict the brake specific fuel consumption accurately.
Large-Eddy simulation of gas turbine combustors using Flamelet Manifold methods
(2015-12) Lietz, Christopher Fernandez; Clemens, Noel T.; Raman, Venkat; Ezekoye, Ofodike A; Goldstein, David B; Varghese, Philip L
The main objective of this work was to develop a large-eddy simulation (LES) based computational tool for application to both premixed and non- premixed combustion of low-Mach number flows in gas turbines. In the recent past, LES methodology has emerged as a viable tool for modeling turbulent combustion. LES is particularly well-suited for the compu- tation of large scale mixing, which provides a firm starting point for the small scale models which describe the reaction processes. Even models developed in the context of Reynolds averaged Navier-Stokes (RANS) exhibit superior results in the LES framework. Although LES is a widespread topic of research, in industrial applications it is often seen as a less attractive option than RANS, which is computationally inexpensive and often returns sufficiently accurate results. However, there are many commonly encountered problems for which RANS is unsuitable. This work is geared towards such instances, with a solver developed for use in unsteady reacting flows on unstructured grids. The work is divided into two sections. First, a robust CFD solver for a generalized incompressible, reacting flow configuration is developed. The computational algorithm, which com- bines elements of the low-Mach number approximation and pressure projection methods with other techniques, is described. Coupled to the flow solver is a combustion model based on the flamelet progress variable approach (FPVA), adapted to current applications. Modifications which promote stability and accuracy in the context of unstructured meshes are also implemented. Second, the LES methodology is used to study three specific problems. The first is a channel geometry with a lean premixed hydrogen mixture, in which the unsteady flashback phenomenon is induced. DNS run in tandem is used for establishing the validity of the LES. The second problem is a swirling gas turbine combustor, which extends the channel flashback study to a more practical application with stratified premixed methane and hydrogen/methane mixtures. Experimental results are used for comparison. Finally, the third problem tests the solver’s abilities further, using a more complex fuel JP-8, Lagrangian fuel droplets, and a complicated geometry. In this last configu- ration, experimental results validate early simulations while later simulations examine the physics of reacting sprays under high centripetal loading.
Particulate matter formation from volatile chemical products including combustion and non-combustion sources
(2018-10-09) Dhulipala, Surya Venkatesh; Hildebrandt Ruiz, Lea; Corsi, Richard; Allen, David; Rochelle, Gary; Xu, Ying
New evidence on the contribution of volatile chemical products from non-combustion sources to ambient particulate matter formation has renewed interest in policy-makers and atmospheric scientists to quantify these emissions which have historically been under-reported. In this dissertation, several representative compounds are chosen to provide a holistic comparison of particulate matter formation from both combustion and non-combustion sources. For this purpose, an environmental chamber is employed with state-of-art-instruments to monitor the formation and decay of air pollutants in the gas-phase and particle-phase. The Aerosol Chemical Speciation Monitor (ACSM) is used to measure the total concentration and bulk composition of particulate matter less than 1 µm in size (PM₁). The High Resolution Time-of-Flight-Mass-Spectrometer (HR-Tof-CIMS) along with a Filter Inlet for Aerosols and Gases (FIGAERO) is used to measure the gas-phase composition in real-time and the particle-phase composition in a semi-continuous manner. Toluene, an aromatic compound, is commonly used in solvents. Its reaction with OH radicals, the most abundant radical in the atmosphere, is well understood but its reaction with chlorine radicals is not. Chlorine is an important oxidant in both in-land and coastal areas. Here, high oxidative states of organic aerosol (component of PM) and gas-phase products formed during toluene-Cl photo-oxidation are reported. Secondary OH radical production is also observed. Long-chain alkanes are associated with vehicular exhaust and their reactions with chlorine also remain poorly understood. Here, the Cl-initiated photo-oxidation of alkanes with 10 carbons – n-decane, 2-methyl nonane and butyl cyclohexane are reported under low NO [subscript x] environments and variable relative humidity (0% and 35-55%). Presence of gas-phase and particle-products associated with OH radical chemistry are reported in the presence of chlorine. A class of compounds defined as Low Vapor Pressure Volatile Organic Compounds (LVP-VOCs) by policy-makers and referred to as Intermediate-Volatility Organic Compounds (IVOCs) by atmospheric scientists includes commercial grade mineral spirits, diethylene mono butyl glycol ether (DEGBE) and Texanol®, which are commonly used in solvents and coatings. Here, the Cl- and OH-radical initiated particulate matter formation from these IVOCs are discussed and compared to alkanes of similar volatility but originating from a combustion source (n-pentadecane and 2,6,10 trimethyl dodecane).
Studies of rich and ultra-rich combustion for syngas production
(2012-12) Smith, Colin Healey; Ellzey, Janet L.; Ezekoye, Ofodike A; Hidrovo, Carlos H; Berberoglu, Halil; Raja, Laxminarayan L
Syngas is a mixture of hydrogen (H2), carbon monoxide (CO) and other species including nitrogen (N2), water (H2O), methane (CH4) and higher hydrocarbons. Syngas is a highly desired product because it is very versatile. It can be used for combustion in turbines or engines, converted to H2 for use in fuel cells, turned into diesel or other high-molecular weight fuels by the Fischer-Tropsch process and used as a chemical feedstock. Syngas can be derived from hydrocarbons in the presence of oxidizer or water as in steam reforming. There are many demonstrated methods to produce syngas with or without water addition including catalytic methods, plasma reforming and combustion. The goal of this study is to add to the understanding of non-catalytic conversion of hydrocarbon fuels to syngas, and this was accomplished through two investigations: the first on fuel conversion potential and the second on the effect of preheat temperature. A primarily experimental investigation of the conversion of jet fuel and butanol to syngas was undertaken to understand the potential of these fuels for conversion. With these new data and previously-published experimental data, a comparison amongst a larger set of fuels for conversion was also conducted. Significant soot formation was observed in experiments with both fuels, but soot formation was so significant in the jet fuel experiments that it limited the range of experimental operating conditions. The comparison amongst fuels indicated that higher conversion rates are observed with smaller molecular weight fuels, generally. However, equilibrium calculations, which are often used to determine trends in fuel conversion, showed the opposite trend. In order to investigate preheat temperature, which is one important aspect of non-catalytic conversion, experiments were undertaken with burner-stabilized flames that are effectively 1-D and steady-state. An extensive set of model calculations were compared to the obtained experimental data and was used to investigate the effect of preheat temperatures that were beyond what was achievable experimentally. Throughout the range of operating conditions that were tested experimentally, the computational model was excellent in its predictions. Experiments where the reactants were preheated showed a significant expansion of the stable operating range of the burner (increasing the equivalence ratio at which the flame blew off). However, increasing preheat temperature beyond what is required for stabilization did not improve syngas yields.