Premixed Air Research Articles

The Structure and Stability of Premixed CH4, H2, and NH3/H2 Flames in an Axially Staged Can Combustor

Abstract An experimental investigation of flame structure, stability, and emissions performance was conducted in a two-stage lab-scale generic combustor design operated with CH4, H2, and NH3/H2 fuel blends. The main flame zone features a premixed bluff body stabilized flame, with a secondary flame zone initiated downstream by injecting premixed air and fuel using two opposing radial jets. The total power and air flowrate are kept constant between the different fueling cases, while the air split between stages and equivalence ratios are varied to explore conditions relevant to gas turbine operation. Given the relative novelty of the configuration, special emphasis is given to analyzing the structure of the opposing jet flames in the secondary stage. In contrast to previous literature on reacting jets in cross flow, these interact significantly due to their proximity, leading to a merged flame zone at the impingement location in the center of the combustion chamber, and some flame propagation upstream of the jet location. As the jet-to-crossflow momentum ratio increases, the merged flame zone changes shape, reaching close to the walls for the methane cases but remaining very compact when operating with almost pure hydrogen. For the hydrogen flames, diverting more air to the second stage allows higher total thermal power conditions to be reached, while avoiding flashback, and eliminates thermoacoustic instabilities. For ammonia-hydrogen flames, air is diverted to the second stage, while a constant fuel flow is sent to the primary stage, resulting in some locally rich conditions in the primary flame. A local minima in terms of NOX occurs when the primary flame is operated at an equivalence ratio of 1.15. Analysis of the flame structure suggests that this state corresponds to almost complete combustion or pyrolysis of NH3 in the main flame, with the remaining hydrogen burned in an inverse diffusion flame in the secondary zone.

Comparison of the Flame Dynamics of a Liquid-Fueled Swirl-Stabilized Combustor for Different Degrees of Fuel-Air Premixing

Abstract This study investigates the flame dynamics of lean premixed kerosene combustion for two different degrees of fuel–air premixing using a swirl stabilized burner with an axially movable twin fluid fuel injection nozzle. Thermal power, equivalence ratio, and atomizing air mass flow are varied systematically for both nozzle positions investigated. Measurements of the droplet size distribution at the nozzle exit are provided for all operation points. NOx emission measurements and OH*-chemiluminescence flame images show that stationary combustion characteristics significantly change with the nozzle position. Flame Transfer Functions (FTFs) are presented and interpreted for all operation points. The FTFs for the two configurations differ most in the low frequency range where the influence of the droplet dynamics is expected to be highest. For both configurations, a change in thermal power does not affect droplet size, flame shape, NOx emissions, and FTF. The observed trends in response to changes in equivalence ratio and atomizing air mass flow are opposite for both configurations. NOx emissions and flame shape are independent of the atomization air mass flow in the highly premixed configuration but not in the partially premixed configuration. In contrast to this, the FTF is affected by changes of the atomization air mass flow in both configurations, but again the trends are opposite. The observed trends for the highly premixed configuration are modeled and reproduced by a change in the phase relation between the equivalence ratio fluctuations and other instability driving mechanisms.

Maximal utilization of purge gas in methanol plants with improved primary furnace burners: A CFD study

The focus of this study is to achieve a better design of burners installed in petrochemical steam methane reformeing (SMR) furnaces. The initial design for the burners of the primary furnace in the Marjan petrochemical plant was problematic; hot spots occured on the side walls of the radiant chamber when the purge gas of the plant was added to the fuel stream. Three distinctive improvements have been recommended to alleviate this problem and provide higher combustion efficiency. These three suggestions include introducing pre-mixing of air and fuel streams, lowering the fuel nozzle velocity, and additional staggered nozzle holes to the base design to achieve more uniform flames. The non-premixed combustion computational fluid dynamics (CFD) simulations are conducted with ANSYS Fluent software on only one array of the burners, given the periodic boundary conditions at the two sides of each array. The resulting temperature and velocity distributions are obtained to compare the designs. The results indicate that a more uniform temperature distribution will be achievable by adding the staggered nozzle holes, which provide a heat flux of 7604 W/m2 and reduce the radiant wall’s average temperature by 0.598%. However, the combustion phenomenon occurs at its highest efficiency—99.998%—through the second design, where the fuel velocity is lowered at the nozzles. Although the first design with the pre-mixing of air and fuel performs more effectively in pushing flame towards the reformer tubes and reducing temperature variations along the radiant walls, the possibility of flash-back occurrence is high; so further safety actions must be taken into account. Eventually, the third design is chosen for implementation as economic analysis indicates that $10 million/year will be saved by lowering the amount of natural gas injected into the burners.

Effects of Fuel Concentration Gradient on Stabilization of Oblique Detonation Waves in Kerosene–Air Mixtures

The transition structures and wave stabilization features of the wedge-induced oblique detonation wave (ODW) affect the combustion of the hypersonic air-breathing detonation engine, which is associated with the inhomogeneity of the inflow mixture. This study numerically investigates the influence of inhomogeneous kerosene–air mixtures on the stabilization of ODW for the first time, considering the inter-phase heat and mass transfers and focusing on the flow structure. The multiphase reacting flows are solved by the two-way coupling Eulerian–Lagrangian method. The inhomogeneous degree of fuel–air premixing is represented by the gradient of the liquid fuel equivalence ratio. A new pattern of transition wave structure from the shock-induced deflagration to oblique detonation is found. Under the fuel-rich condition before the shock-induced deflagration wave, a diamond-shaped wave structure is generated due to the large fuel concentration gradients. This flow structure is formed on the wedge without oscillations and is expected for a well-stabilized ODW. The initiation length of ODW is used to value the combustion performance. Its dependence on the inhomogeneous premixing degree displays a W-shaped curve. The chemical heat release influences the initiation length more obviously than the evaporative cooling in the fuel-lean conditions before the shock-induced deflagration. The ODW stabilization is enhanced by the heat released from the fuel-rich chemical reaction. Generally, the two-phase oblique detonation is determined by the competitiveness between the evaporative heat loss and chemical heat release. A uniform fuel–air mixture may not be optimal for detonation initiation based on the results of the present study.

Experimental study on effects of premixed air distribution on preheating combustion characteristics and NOx emission of pulverized coal

Air distribution is the key factor affecting combustion characteristics and NOx emission of pulverized coal with the use of the self-preheating burning technology. Nevertheless, previous research is mainly about the effects of air distribution in the down-fired combustor, but not on the effects of premixed air distribution. To make up for such shortage, we focused on the effects of premixed air distribution on combustion characteristics and NOx emission. Relevant experimental variables included the ratio of premixed air flow to total secondary air flow (CP), premixed position, and the ratio of first-staged premixed air flow to second-staged premixed air flow (C12). During preheating, N2 was the predominant conversion form of released coal-N. During combustion, the influence of the premixed air on temperature distribution differed among regions, and it may be regulated by the premixed position. At above 800 mm, the temperature was lower with the premixed air, and decreased and increased with the rising CP when premixed air entered from points A and B respectively. At below 1200 mm, the temperature with the premixed air was higher and lower when premixed air entered from points A and B respectively, but it always decreased as CP rose in these two conditions. As C12 increased, the temperature dropped at above 1200 mm, while climbed first and then declined at below 1200 mm. Introducing the premixed air improved the combustion efficiency (η) and reduced the NOx emission, but the effect of CP on η was regulated by the premixed position. Specifically, η rose first and then decreased as CP increased when the premixed air entered from point A, while its regularity was just the opposite when the premixed air entered from point B. On the contrast, NOx emission always decreased in these two conditions. Additionally, increasing C12 properly can reduce NOx emission while improving η.

Experimental investigation of the flame structure of dilute sprays issuing into a hot and low-oxygen coflow

The combustion of liquid fuels in a hot and low-oxygen environment is commonly encountered in a range of practical situations. To enable investigation of the fundamental combustion processes relating to such applications, liquid fuels were injected into the reaction zone as dilute sprays in this study. Droplets of ethanol, n-heptane, and n-heptane/toluene blends were produced via an ultrasonic nebuliser, and were carried by air through a central jet to a hot coflow of combustion products. The resulting flames were then analysed using four simultaneous laser diagnostic techniques. Planar laser-induced fluorescence (PLIF) was implemented to perform imaging of key intermediate species, including hydroxyl (OH) and formaldehyde (CH2O), while the Mie scattering technique was used to detect the location of droplets. The sooting behaviour of these flames was also investigated, via the laser-induced incandescence (LII) technique. The existence of distinct inner and outer reaction zones is a key feature of all of the flames studied, and this “double flame structure” was found to be related to partial premixing of air and fuel, as well as penetration of droplets into the inner reaction zone. A change in the stabilisation of the inner flame front was observed with variations in fuel type, with a greater likelihood of ignition kernels in the case of the n-heptane and n-heptane/toluene flames, whereas the equivalent ethanol flame displays a bifurcation structure. Variations in the jet Reynolds number and liquid fuel loading were also found to have a notable impact on the distribution and evaporation of droplets, which was in turn found to affect the formation of the double flame structure. Due to the complex coupling between turbulence, chemistry and droplet evaporation in these flames, the accurate prediction of such results a priori is not within the limits of current modelling capabilities. These findings provide a valuable insight to enable future advancements in spray combustion modelling and the design of practical combustion devices.

Methane-air partially premixed flame behaviors in a micro plate slit

The size of the combustion space has a significant effect on the flame behavior in the micro combustor. In this paper, the structures and dynamics of laminar partially premixed methane/air were experimentally investigated as the combustion space gradually decreases. In the experiment, five flame regimes were observed in sequence: the stable flame, transient oscillating flame, extinction, flame with repetitive extinction and ignition (FREI), and quenching. The effects of the mixture flow rate, primary equivalence ratio, and separating distance of plates on the different flame behaviors were analyzed. It was revealed that increasing the mixture flow rate could improve the flame’s stability while increasing the primary premixed air, and the flame tended to become unstable. Moreover, in the FREI regime, the larger the separating distance, the lower the frequency of the FREI flame; meanwhile, the FREI frequency increased as the primary equivalent ratio decreased. The FREI frequency exhibited a non-monotonic change with the mixture flow rate and peaked at 1.06 m/s. Furthermore, it was demonstrated that the smaller the primary equivalent ratio, the smaller the quenching distance. The mixture flow rate had no significant effect on the quenching distance. The research is helpful to the design of the micro combustion device.

Numerical study of characteristics of liquefied petroleum gas flames in coflow air

Liquefied Petroleum Gas (LPG) is a fossil fuel mixture, widely used in domestic and industrial burners for several applications. Non-premixed flames are commonly preferred in these burners owing to their stability and controllability. However, based on operating conditions, non-premixed flames emit emissions such as CO and soot. Several techniques, including supply of coflow air and partial premixing of air with fuel, are used to abate these emissions. Due to multicomponent nature of LPG and its complex oxidation pathway, numerical studies on LPG flames are scarce in literature. However, such studies are helpful in the analysis of flames in several burners. With this motivation, systematic numerical simulations of canonical non-premixed coflow flames of LPG and air, without and with partial premixing of air in the fuel stream, have been presented. A two-dimensional axisymmetric domain is used to represent coflow burner. The numerical model includes sub-models to account for soot formation and its oxidation, and radiation energy loss due to participating species and soot. A short kinetic mechanism with 43 species and 392 elementary reactions is used. Results from the numerical model have been validated for flames of propane, n-butane and LPG. Coflow air is varied as proportions of stoichiometric air. Partial premixing of air to the fuel stream results in a reactant mixture that is not flammable. Characteristics of flames in all these cases are presented systematically using the fields of temperature, flow and species mass fractions. A relative comparison of soot production between all these cases is made. Results reveal that in coflow flames, the net soot emissions increase initially, reach a local maximum and then decrease. In partially premixed flames, soot emissions continuously decrease with an increase in the primary air and become almost negligible after a specified air addition to the fuel stream.

Linear and nonlinear entropy-wave response of technically-premixed jet-flames-array and swirled flame to acoustic forcing

Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It comprises two components: direct noise originating from the unsteady heat release of the flame, and indirect noise resulting from the acceleration of entropy fluctuations in the turbine stages. Not only do these entropy waves contribute to the noise pollution of aeroengines, they can also have a crucial role in the feedback loop leading to thermoacoustic instabilities, which induce vibrations and thermal loads that are highly detrimental for combustors and turbine components. Thus, there is a critical need to understand and model the complex mechanisms associated with the generation and the advection of these waves. This study presents quantitative measurements of the production of entropy waves in a technically premixed turbulent combustor, subject to acoustic forcing. Entropy transfer function (ETF) relating acoustic input, obtained with microphones, to entropy wave output, obtained with OH-LIF thermometry at a distance of four flame heights from the burner outlet, were measured between 40 Hz and 90 Hz. These ETF were obtained using two burners of same length with technical premixing of air and natural gas, operated at the same thermal power: a matrix burner producing an array of turbulent jet flames, and a burner producing a single swirled turbulent flame. It is found that the ETF of the matrix burner exhibits a low-pass behavior, with a gain ranging from approximately 0.7 at 40 Hz down to 0.25 at 90 Hz, while the gain of the swirled flame ETF was not exceeding 0.2. It is also demonstrated that entropy wave production with the matrix burner is highly nonlinear, with a dramatic drop of the ETF gain occurring beyond a certain level of acoustic forcing amplitude. These measurements can be used to derive predictive nonlinear models of thermoacoustic instabilities involving entropy wave feedback.

Effect of hydrogen addition on the ionization of partially premixed methane flame

The flame ionization characteristic is a key point to realize the combustion diagnosis and control of hydrocarbon flame based on electric fields. In this paper the effects of hydrogen addition on the ionization of partially premixed methane flame were investigated experimentally and computationally. The flame current was measured by applying a small electric field across the partially premixed methane flame with different hydrogen addition and the effects of hydrogen addition on the ionization intensity and the flame ionization structure were analyzed. Furthermore, the intrinsic reaction mechanism of hydrogen addition affecting the ionization of partially premixed methane flame was studied based on a comprehensive reaction mechanism, which combines GRI 3.0 for neutral species and an 85-steps flame ionization reaction for 13 kinds of charged species. The results show that hydrogen addition can promote the ionization of partially premixed flame by affecting the chemical ionization process. However, when the hydrogen content reaches 50% of the total fuel amount, the promotion effect will be significantly weakened. Moreover, for partially premixed methane flame, the effect of hydrogen addition on the flame ionization was affected by the amount of primary premixed air. The larger the amount of primary premixed air, the more significant the promotion of hydrogen addition. What’s more, the hydrogen addition mainly affects the concentration of e− in the flame through the reactions H + CH3 (+M) ⇔ CH4 (+M) and H + O2 ⇔ O + OH while affects the concentration of H3O + through the reactions OH + CH3 ⇔ CH2 (S) + H2O and H3O+ + e− ⇔ OH + 2H.

A reduced reaction mechanism of biodiesel surrogates with low temperature chemistry for multidimensional engine simulation

A reduced biodiesel mechanism composed of 156 species and 589 reactions is reduced from an original complex mechanism (3299 species and 10806 reactions) based on MD, MD9D, and n-heptane as the surrogates. The mechanism reduction is conducted using the path flux analysis method, which considers multiple reaction path generations in the analysis of species interactions, and isomer lumping. Calculations of homogeneous auto-ignition and perfectly stirred reactor (PSR) combustion on a variety of reaction states, including pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2, are the basis of the reduction. The initial temperatures are from 700 to 1800 K for the auto-ignition, and the inlet temperature is 300 K for the PSR. These reaction states cover the high-pressure and low-temperature operating conditions of future engines using advanced combustion technologies characterized by fuel–air premixing and auto-ignition. The fidelity of the resulting reduced mechanism with low-temperature chemistry is examined using a variety of applications. Close agreements between the reduced and original mechanisms are obtained in the predictions of ignition delay, history of mixture temperature, and species mole fraction during homogeneous auto-ignition and the temperature profile in PSR. The reduced mechanism, further integrated with a nitrogen oxides chemistry and a two-step soot model, is implemented into the KIVA/CHEMKIN program for the 3D simulation of biodiesel spray combustion. The predicted liquid and vapor penetrations agree with the experimental data in a non-reactive biodiesel spray simulation, indicating an accurate estimation of biodiesel physical properties. In the simulation of biodiesel spray combustion, predicted spatial distributions of hydroxyl radical and soot also agree with the corresponding experimental data.

Use of Flame Color and Chemiluminescence for Early Detection of Lean Blowout in Gas Turbine Combustors at Different Levels of Fuel–Air Premixing

ABSTRACTLean combustion is adopted in the practical gas turbine combustors as it offers a solution to reduce the emission of NOx. But lean blowout (LBO) is considered as the main threat in taking the ultra-lean premixed combustion. LBO can also occur in aircraft engines during rapid decrease in speed or power. Although gas turbine flames can be of both premixed and partially premixed in nature, early detection of LBO of partially premixed flames has received relatively less attention compared to premixed flames. In this paper, attention has been focused on the early detection of LBO in combustors with different levels of premixing. Apart from that, a detail investigation on the electromagnetic emission of the flame is performed in order to have the insight of the dominating chemiluminescence. The primary chemiluminescence species, OH*, CH*,and C2*, are considered in this study. The flame dynamics is captured using spectral signal obtained from a spectrometer. Also, the spectrums of different colors, emitted by the flame at different conditions, are studied using both spectrometer and color Charge Coupled Device (CCD) camera. At lean condition of the premixed flame, three narrowband emissions in the visible (VIS) range, namely, blue, violet, and indigo colors, are observed. Three primary parameters, , , and , are introduced based on the color ratios when numerator of the ratio is the intensity of red band. Near LBO, each of these parameters becomes significantly low. The observation range for this analysis is considered from stoichiometric condition to the extreme lean region, where LBO occurs. At the end, the behavior of the red, green, and blue color ratios toward the LBO using both camera and spectrometer is compared. The similarity between the results of camera and spectrometer indicates that flame imaging through commercial color CCD camera can provide a low cost, simple tool for LBO detection.

Evolution of In-Cylinder Diesel Engine Soot and Emission Characteristics Investigated with Online Aerosol Mass Spectrometry

To design diesel engines with low environmental impact, it is important to link health and climate-relevant soot (black carbon) emission characteristics to specific combustion conditions. The in-cylinder evolution of soot properties over the combustion cycle and as a function of exhaust gas recirculation (EGR) was investigated in a modern heavy-duty diesel engine. A novel combination of a fast gas-sampling valve and a soot particle aerosol mass spectrometer (SP-AMS) enabled online measurements of the in-cylinder soot chemistry. The results show that EGR reduced the soot formation rate. However, the late cycle soot oxidation rate (soot removal) was reduced even more, and the net effect was increased soot emissions. EGR resulted in an accumulation of polycyclic aromatic hydrocarbons (PAHs) during combustion, and led to increased PAH emissions. We show that mass spectral and optical signatures of the in-cylinder soot and associated low volatility organics change dramatically from the soot formation dominated phase to the soot oxidation dominated phase. These signatures include a class of fullerene carbon clusters that we hypothesize represent less graphitized, C5-containing fullerenic (high tortuosity or curved) soot nanostructures arising from decreased combustion temperatures and increased premixing of air and fuel with EGR. Altered soot properties are of key importance when designing emission control strategies such as diesel particulate filters and when introducing novel biofuels.

A comparative experimental study on engine operating on premixed charge compression ignition and compression ignition mode

New combustion concepts have been recently developed with the purpose to tackle the problem of high emissions level of traditional direct injection Diesel engines. A good example is the premixed charge compression ignition combustion. A strategy in which early injection is used causing a burning process in which the fuel burns in the premixed condition. In compression ignition engines, soot (particulate matter) and NOx emissions are an extremely unsolved issue. Premixed charge compression ignition is one of the most promising solutions that combine the advantages of both spark ignition and compression ignition combustion modes. It gives thermal efficiency close to the compression ignition engines and resolves the associated issues of high NOx and particulate matter, simultaneously. Premixing of air and fuel preparation is the challenging part to achieve premixed charge compression ignition combustion. In the present experimental study a diesel vaporizer is used to achieve premixed charge compression ignition combustion. A vaporized diesel fuel was mixed with the air to form premixed charge and inducted into the cylinder during the intake stroke. Low diesel volatility remains the main obstacle in preparing premixed air-fuel mixture. Exhaust gas re-circulation can be used to control the rate of heat release. The objective of this study is to reduce exhaust emission levels with maintaining thermal efficiency close to compression ignition engine.

A new-generation 3D ozone FACE (Free Air Controlled Exposure)

To artificially simulate the impacts of ground-level ozone (O3) on vegetation, ozone FACE (Free Air Controlled Exposure) systems are increasingly recommended. We describe here a new-generation, three-dimensional ozone FACE, with O3 diffusion through laser-generated micro-holes, pre-mixing of air and O3, O3 generator with integral oxygen generator, continuous (day/night) exposure and full replication. Based on three O3 levels and assumptions on the pre-industrial O3 levels, we describe principles to calculate relative yield/biomass and estimate impacts even at lower-than-ambient O3 levels. The case study is called FO3X, and is at present the only ozone FACE in Mediterranean climate and one of the very few ozone FACEs investigating more than one stressor at a time. The results presented here will give further impulse to the research on O3 impacts on vegetation all over the world.

Evaluation of Two Measurement Techniques to Quantify Fuel–Air Mixing of a Gas Turbine Premixer at Atmospheric Conditions

In the present study, two measurement techniques are adopted to evaluate the fuel–air mixing under atmospheric conditions using an industrial fuel–air premixer. These techniques are CO2 mixing and planar laser induced fluorescence (PLIF) in water. In these techniques, CO2 and fluorescent dye are injected as fuel simulants. CO2 measurements are used to validate PLIF in water. In the CO2 technique, CO2 concentrations are converted to fuel mass fractions, whereas in the PLIF technique, a modified post processing method is used to convert the LIF signal into fuel mass fraction. The experiments are conducted at the same Reynolds number and momentum flux ratio for two injection strategies. To study the effect of the flow aerodynamics on the mixing results, high-speed particle image velocimetry (PIV) measurements are conducted in water at the same Reynolds number. A comparison of fuel concentrations measured with the CO2 and PLIF techniques shows good quantitative agreement at all momentum flux ratios. However, deviations between the two techniques are observed at locations of high fuel concentration gradients. The unsteady mixing is evaluated using the PLIF technique with high temporal resolution. Analysis of PIV and PLIF data shows that unsteady mixing is lower at regions of high fluctuations in velocity. Moreover, it is found that there is high unsteady mixing at locations of high concentration gradient.

A comparison of the characteristics of disk stabilized lean propane flames operated under premixed or stratified inlet mixture conditions

The work investigates some of the differences and similarities between fully-premixed and stratified lean propane flames stabilized in an axisymmetric bluff-body swirl burner configuration. In this arrangement a double-cavity propane–air premixer, formed along three concentric disks, supplies the afterbody disk recirculation region with a regulated radial equivalence ratio gradient. Flame stabilization can be established at a range of co-flow swirl intensities under, either uniformly premixed or stratified inlet mixture conditions. Measurements of turbulent velocities, temperatures, chemiluminescence imaging of OH* and CH* and gas analysis provided information for lean and close to blow-off flame configurations. Supporting Large Eddy Simulations were undertaken with the Thickened Flame Model and a nine-step mechanism for propane combustion to complement the experiments. The combined methodology helped to elucidate some of the parametric characteristics of the diverse flame topologies obtained in the above burner. The performance of the adopted turbulent chemistry closure and the reduced chemical scheme in addressing the effect of mixture heterogeneities on the flame anchoring and structure of the studied configurations is also evaluated. The interaction of the toroidal flame fronts with the recirculation stabilization region, the impact of heat release on the development of the axisymmetric wakes and the flame front disposition at ultra-lean and close to blow-off operation are presented and discussed for the different fuel–air inlet mixture profiles.

Combustion Characteristics for Turbulent Prevaporized Premixed Flame Using Commercial Light Diesel and Kerosene Fuels

Experimental study has been carried out for investigating fuel type, fuel blends, equivalence ratio, Reynolds number, inlet mixture temperature, and holes diameter of perforated plate affecting combustion process for turbulent prevaporized premixed air flames for different operating conditions. CO2, CO, H2, N2, C3H8, C2H6, C2H4, flame temperature, and gas flow velocity are measured along flame axis for different operating conditions. Gas chromatographic (GC) and CO/CO2infrared gas analyzer are used for measuring different species. Temperature is measured using thermocouple technique. Gas flow velocity is measured using pitot tube technique. The effect of kerosene percentage on concentration, flame temperature, and gas flow velocity is not linearly dependent. Correlations for adiabatic flame temperature for diesel and kerosene-air flames are obtained as function of mixture strength, fuel type, and inlet mixture temperature. Effect of equivalence ratio on combustion process for light diesel-air flame is greater than for kerosene-air flame. Flame temperature increases with increased Reynolds number for different operating conditions. Effect of Reynolds number on combustion process for light diesel flame is greater than for kerosene flame and also for rich flame is greater than for lean flame. The present work contributes to design and development of lean prevaporized premixed (LPP) gas turbine combustors.

Influence of Carbonate Ester Molecular Structure on Compression Ignition Combustion and Emissions

A primary motivation for the development of future fuels is to halt the increasing concentration of atmospheric carbon dioxide. As such, a fuel that can potentially be produced from a feedstock of alcohols and atmospheric carbon dioxide is an attractive proposition. Symmetrical carbonate esters may thus offer a sustainable means of converting alcohols to larger molecules that are potentially better suited to use in compression ignition engines. This paper presents experimental investigations carried out on a compression ignition engine fuelled with a range of binary fuel mixtures to determine the effect of carbonate molecular structure on combustion and exhaust emissions. Six symmetrical carbonate esters, dimethyl carbonate, diethyl carbonate, di-n-butyl carbonate, di-1-methyl-propyl carbonate, di-2-methyl-propyl carbonate, and ditert-butyl carbonate, one ester, butyl valerate, and one ketone, 5-nonanone, were investigated so as to ascertain the way in which the alkyl chain length, branching in the alkyl chain, and molar oxygen content of similar molecules affects compression ignition combustion and emissions. The engine tests of the fuels were carried out as binary mixtures with 30% (w/w) n-decane, as they were found not to combust in a steady manner as single component fuels; dimethyl and diethyl carbonate were also tested as binary mixtures with n-decane at a range of proportions. All fuels were initially tested at constant injection and constant ignition timings and repeated at two constant ignition delay timings, which were achieved through the addition of small quantities of ignition improver (2-ethylhexyl nitrate). Relative to di-n-butyl carbonate, decreasing the alkyl chain length and also introducing methyl branches to the alkyl moiety increased the duration of ignition delay, while decreasing the molar oxygen content to form butyl valerate and 5-nonone decreased the ignition delay. The respective changes in ignition delay with molecular structure were attributed to the importance of the alkyl moieties of the fuels tested in determining low temperature reactivity. When the dimethyl carbonate or diethyl carbonate content of blends with n-decane was increased, levels of peak heat released increased linearly, suggesting an influence of ignition delay and the carbonate physical properties on the extent of fuel and air premixing. At constant ignition delay timings, an effect of decreasing particulate matter and CO emissions with an increasing level of fuel bound oxygen was observed, while at all conditions NOx emissions were most significantly influenced by in-cylinder thermal conditions.

Effects of intake pressure on particle size and number emissions from premixed diesel low-temperature combustion

In this study, 10 premixed diesel low-temperature combustion engine operating conditions were chosen based on engine intake pressure (1.2–1.6 bar), intake oxygen concentration (10%, 11%, and 12%), and injection timing (−24° after top dead centre in all test conditions). At each intake oxygen concentration, the effects of intake pressure on combustion parameters and emission measurements (carbon monoxide, hydrocarbons, nitrogen oxides, particulate matter mass concentration, and particle size distributions) were analyzed. Although increased intake pressure resulted in higher in-cylinder charge air density that improved fuel/air premixing and late-cycle oxidation quality, higher intake pressure also advanced the start of combustion and thereby decreased the time available for fuel and air premixing. But even with the decrease in premixing time available before start of combustion, increased intake pressure caused significant decreases in carbon monoxide, hydrocarbon, particulate matter mass, and particle num...