Lean Regions Research Articles

In cylinder visualization of stratified combustion of E85 and main sources of soot formation

The combustion process and soot formation in spark ignited spray guided stratified combustion of E85 was investigated in a single cylinder optical engine with direct injection of fuel using an outward opening piezo actuated injector. The effect of engine rotation frequency, fuel quantity, injection sequence and ignition timing was studied. Combustion, soot formation and soot oxidation was analyzed using cylinder pressure measurements, images recorded using high speed video cameras, the flame emission spectrum and OH∗ chemiluminescence and soot incandescence imaging. A maximum injection duration was found to exist for direct ignition of the fuel spray. Engine rotation frequency had little effect on the initial and maximum rate of combustion. The maximum rate of combustion decreased with increasing cycle fuel mass when a single injection was used. The rate of combustion and indicated mean effective pressure increased and the combustion variability decreased when the single injection was split into multiple injections in close succession to deliver the same total fuel mass and the last fuel spray was ignited. Ignition of the first fuel spray resulted in a more pronounced change. The absence of soot incandescence during the initial flame propagation suggested flame propagation in a partially mixed fuel and air mixture with stoichiometric to fuel lean regions. A single fuel injection resulted in piston pool fires due to fuel spray impingement on the piston and was the primary source of soot formation. The pool fires persisted until after conditions favorable to oxidation of the soot had ended. Soot formation in the gas phase occurred while favorable soot oxidation conditions existed and was efficiently oxidized. The magnitude of the piston pool fires was reduced using multiple injections. The reduction is attributed to a reduction of the fuel spray penetration length and a smaller effective injection orifice area, resulting in a shorter total duration of fuel spray impingement on the piston crown. Soot formation occurred primarily in the gas phase when the first of two fuel sprays was ignited and persisted due to the second fuel spray entering an existing flame leading to fuel rich combustion.

Direct numerical simulation of PRF70/air partially premixed combustion under IC engine conditions

A three-dimensional direct numerical simulation (DNS) is performed to study the ignition and the combustion process of a primary reference fuel (PRF) under conditions relevant to partially premixed combustion (PPC) engines. Detailed transport properties together with a skeletal PRF chemical kinetic mechanism are employed in the simulations. The initial partially premixed charge is prescribed according to a two-injection of fuel strategy with a 50% exhaust gas recirculation (EGR). The motion of the piston, hence, the effect of compression of the charge due to the piston motion is considered. The simulations are performed with a fine mesh with a spatial resolution of 1.2 micrometers in a cubic domain of 614 micrometers on each side. The results show that there are three distinctive combustion regions in a domain under PPC conditions, a lean homogeneous mixture region, a fuel-rich region, and in between a region with stoichiometric mixture. Auto-ignition occurs first in the lean and stoichiometric mixture regions, followed by a partially premixed flame in the fuel-rich region and at last a CO oxidation phase in the fuel-rich region. In the lean mixture region, the fuel and the combustion intermediates such as CO are consumed almost completely due to the relatively fast combustion and the abundant oxygen. The CO formed in the regions with the stoichiometric and the fuel-rich mixture is consumed in a mixing controlled mode, which requires a high-level turbulence mixing or a sufficiently long residence time. NO is formed mainly due to the premixed combustion in the region with the stoichiometric mixture.

Improving the Understanding of Intake and Charge Effects for Increasing RCCI Engine Efficiency

The present experimental engine efficiency study explores the effects of intake pressure and temperature, and premixed and global equivalence ratios on gross thermal efficiency (GTE) using the reactivity controlled compression ignition (RCCI) combustion strategy. Experiments were conducted in a heavy-duty single-cylinder engine at constant net load (IMEPn) of 8.45 bar, 1300 rev/min engine speed, with 0% EGR, and a 50% mass fraction burned combustion phasing (CA50) of 0.5 CA ATDC. The engine was port fueled with E85 for the low reactivity fuel and direct injected with 3.5% 2-ethylhexyl nitrate (EHN) doped into 91 anti-knock index (AKI) gasoline for the high-reactivity fuel. The resulting reactivity of the enhanced fuel corresponds to an AKI of approximately 56 and a cetane number of approximately 28. The engine was operated with a wide range of intake pressures and temperatures, and the ratio of low- to high-reactivity fuel was adjusted to maintain a fixed speed-phasing-load condition. This allowed for the investigation of several combinations of intake temperature, intake pressure, and charge stratification at otherwise constant thermodynamic conditions. The results show that sources of engine inefficiency compete as functions of premixed and global equivalence ratios. Losses are minimized through proper balancing of intake pressure and temperature,more » such that the global equivalence ratio ( global) is as lean as possible without overly lean regions of the stratified charge causing an increase in incomplete combustion. The explored speed-load-phasing combination shows that losses are minimized at conditions where approximately 2/3 of the fuel is fully premixed. The results exhibit a pathway for achieving simultaneous increases in combustion and fuel efficiency through proper fuel reactivity and initial condition management.« less

Analysis of Flame Characteristics in a Laboratory-Scale Turbulent Lifted Jet Flame via DNS

A fully compressible 3D solver for reacting flows has been developed and applied to investigate a turbulent lifted jet flame in a vitiated coflow by means of direct numerical simulation (DNS) to validate the solver and analyze the flame characteristics. An eighth-order central differencing scheme is used for spatial discretization and a fourth-order Runge-Kutta method is employed for time integration. The DNS results agree well with the experimental measurements for the conditional means of reactive scalars. However, the lift-off height is under predicted. The mean axial velocity develops into a self-similar profile after x/D = 6. The normalized flame index is employed to characterize the combustion regime. It is found that at the flame base the gradients of the reactants are opposed and diffusion combustion is dominant. Further downstream, the contribution of premixed combustion increases and peaks at x/D = 8. Finally, the stabilization process is examined. The turbulent lifted flame is proved to stabilize in the lean mixtures and low scalar dissipation rate regions.

Model predictive control of HCCI using variable valve actuation and fuel injection

Control of work output and combustion phasing on a Homogeneous Charge Compression Ignition (HCCI) engine is essential to realize the benefits of superior efficiency and emissions. This paper presents a model predictive control approach for cycle-by-cycle control of HCCI while respecting constraints on actuators that might exist on a production implementation. The strategy is based on a physical model developed in previous work and uses valve actuation and split fuel injection to achieve the control objectives. In addition, it considers constraints on air–fuel ratio, ensuring that the system stays away from very lean or rich regions. Simulation and experimental results show that the controller works well over a range of conditions, and demonstrate the potential of this approach as a practical cycle-by-cycle control strategy for HCCI.

Investigations of emission characteristics and thermal efficiency in a spark-ignition engine fuelled with natural gas-hydrogen blends

This paper presents the experimental results of a single cylinder Enfield engine using an electronically controlled fuel injection system which was developed to carry out exhaustive tests using neat compressed natural gas (CNG), and mixtures of hydrogen in CNG (HCNG) as 5, 10, 15 and 20% by energy. Experiments were performed at 2000, 2400 and 2800 rpm with wide open throttle and varying the equivalence ratio. Hydrogen, which has a fast burning rate, when added to CNG, enhances its flame propagation rate. The emissions of HC, CO, decreased with increasing percentage of hydrogen but NOx was found to increase. The results indicated a marked improvement in the brake thermal efficiency with the increase in percentage of hydrogen added. The improved thermal efficiency was clearly observed to be more in lean regions when compared with rich regions.

The internal structure of igniting turbulent sprays as revealed by complex chemistry DNS

A parametric study of spark ignition in a uniform monodisperse turbulent spray is performed with complex chemistry three-dimensional Direct Numerical Simulations in order to improve the understanding of the structure of the ignition kernel. The heat produced by the kernel increases with the amount of fuel evaporated inside the spark volume. Moreover, the heat sink by evaporation is initially higher than the heat release and can have a negative effect on ignition. With the sprays investigated, heat release occurs over a large range of mixture fractions, being high within the nominal flammability limits and finite but low below the lean flammability limit. The burning of very lean regions is attributed to the diffusion of heat and species from regions of high heat release, and from the spark, to lean regions. Two modes of spray ignition are reported. With a relatively dilute spray, nominally flammable material exists only near the droplets. Reaction zones are created locally near the droplets and have a non-premixed character. They spread from droplet to droplet through a very lean interdroplet spacing. With a dense spray, the hot spark region is rich due to substantial evaporation but the cold region remains lean. In between, a large surface of flammable material is generated by evaporation. Ignition occurs there and a large reaction zone propagates from the rich burned region to the cold lean region. This flame is wrinkled due to the stratified mixture fraction field and evaporative cooling. In the dilute spray, the reaction front curvature pdf contains high values associated with single droplet combustion, while in the dense spray, the curvature is lower and closer to the curvature associated with gaseous fuel ignition kernels.

Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray and vaporization model and a model for fully developed turbulent combustion. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, large variations in turbulence intensity, equivalence ratio, and enthalpy along the stretched and wrinkled spark plasma channel favor localized ignition spot formations in rich-mixture regions. In combination with strong local flow velocity, multiple successful ignition events along the re-striking spark lead to early non-spherical turbulent flame fronts. In this paper, SparkCIMM is enhanced by: (1) criteria to capture localized flame extinction phenomena, (2) a formulation of early flame kernel propagation based on the G-equation theory that includes effects of non-unity Lewis numbers, and (3) an extended equation to compute turbulent burning velocities of stretched flames in stratified mixtures. Localized rich ignition along the spark leads to early flames, whose propagation is, due to initially small turbulent Damköhler numbers, significantly influenced by molecular fuel properties. The analysis reveals that non-unity Lewis number curvature effects, intensified by heavy dilution by exhaust gas recirculation, strongly affect the early flame-kernel development in spray-guided gasoline engines. In particular, these effects significantly bias the flammability limit of flame kernels towards rich-mixtures while inhibiting their propagation in lean regions. Favorable initial conditions for combustion are found in rich-mixture regions, albeit in the presence of substantial equivalence ratio fluctuations and scalar dissipation rates. This paper demonstrates that the full complexity of the model equations developed here is required to reproduce the characteristic experimental features (spark channel stretching, multiple re-strikes, localized flame kernel formation, and early turbulent flame front corrugation) of spray-guided ignition phenomena.

Improvement of room temperature ductility for Mo and Fe modified Ti 2AlNb alloy

The effect of hot bar rolling and a subsequent annealing in the (B2 + α 2) two-phase region on the mechanical properties was investigated for the Ti–25Al–14Nb–2Mo–1Fe (mol%) orthorhombic phase base alloy. After this thermomechanical treatment in the (B2 + α 2) two-phase region, a ‘Van Gogh's Sky (VGS)’ structure, with wavy and curled bands of spherical α 2 precipitates, was obtained. Electron probe micro-analysis (EPMA) examinations indicated that the VGS bands correspond to the Nb and Mo lean regions. The width and spacing of VGS bands varied with thermomechanical treatment conditions (forging temperature before hot bar rolling as well as annealing temperature after the rolling). The samples with the VGS structure exhibited the higher tensile elongation-to-failure at room temperature. The creep resistance at 923 K decreased with the existence of the VGS structure. These mechanical properties were affected by the width and spacing of the VGS bands.

Skeletal mechanism generation with CSP and validation for premixed n-heptane flames

An automated procedure has been previously developed to generate simplified skeletal reaction mechanisms for the combustion of n-heptane/air mixtures at equivalence ratios between 0.5 and 2.0 and different pressures. The algorithm is based on a Computational Singular Perturbation (CSP)-generated database of importance indices computed from homogeneous n-heptane/air ignition solutions. In this paper, we examine the accuracy of these simplified mechanisms when they are used for modeling laminar n-heptane/air premixed flames. The objective is to evaluate the accuracy of the simplified models when transport processes lead to local mixture compositions that are not necessarily part of the comprehensive homogeneous ignition databases. The detailed mechanism was developed by Curran et al. and involves 560 species and 2538 reactions. The smallest skeletal mechanism considered consists of 66 species and 326 reactions. We show that these skeletal mechanisms yield good agreement with the detailed model for premixed n-heptane flames, over a wide range of equivalence ratios and pressures, for global flame properties. They also exhibit good accuracy in predicting certain elements of internal flame structure, especially the profiles of temperature and major chemical species. On the other hand, we find larger errors in the concentrations of many minor/radical species, particularly in the region where low-temperature chemistry plays a significant role. We also observe that the low-temperature chemistry of n-heptane can play an important role at very lean or very rich mixtures, reaching these limits first at high pressure. This has implications to numerical simulations of non-premixed flames where these lean and rich regions occur naturally.

Theoretical modelling of digital radiographs of composite rocket nozzles for the analysis of defects

X-ray radiography techniques are widely used for the non-destructive evaluation of composite nozzle liners of rocket motors. In conventional radiography it is difficult to distinguish the images corresponding to excess resin and resin lean regions in the composite, since the variation in the optical density is very small. Also, porous layers and minor delaminations will cast similar images in the radiographic projection. In this work, theoretical modelling is carried out for various defect conditions to study their effects on the X-ray images. The results are useful for characterisation of the defects in the nozzle liners by radiographic non-destructive evaluation. The observations provide very good input for the interpretation of radiographs in silicaphenolic and carbon-phenolic rocket nozzles. The details of the software developed and its application to solve specific problems are discussed. The changes in optical density with respect to the width of the defect, location of the defect and circumferential extension are investigated.

Temperature-independent ceria- and Pt-doped nano-size TiO2 oxygen lambda sensor using Pt/SiO2 catalytic filter

We report for the first time the elimination of overlap of TiO2-based oxygen sensors responses in the rich and lean regions at various temperatures, by using a 1.0 wt% Pt/SiO2 catalytic filter located in front of the sensors. Nano-size TiO2 was prepared by a microemulsion method and then doped with 1.0 wt% Pt and 10.0 wt% CeO2 by an impregnation method. The sensor was exposed to synthetic exhaust gases with λ values in the range of 0.8–1.4. All of the sensors show low-high transitions at about λ = 1.0. However, without the use of the catalytic filter, the responses of the sensors in the rich and lean regions overlap at different temperatures. By using the catalytic filter only CO or O2 reaches the sensors. Thus, the low-high transition gap strongly enhances, and the overlap of the sensors responses between the rich and lean regions at various temperatures is eliminated. The transition gaps between λ = 0.9 and λ = 1.1 regions at 700 °C increase from 6.1, 17.5, and 14.5 times changes for TiO2, Pt/TiO2, and Ce/TiO2 without the filter to 431, 1881, and 16,149 times changes, respectively, when the catalytic filter is placed prior to the sensors.

Output Evaluation of Resistive Oxygen Sensor having Ce0.9Zr0.1O2 Sensing Material and Zr0.8Y0.2O2-.DELTA. Temperature Compensating Material in Model Exhaust Gas

We fabricated resistive-type oxygen sensors having a Ce0.9Zr0.1O2 thick film as a sensing material and a Zr0.8Y0.2O2-δ thick film as a temperature compensating material for the first time, and measured the resistance of the two thick films and the output of the sensors in model exhaust gas which was a combustion gas of propane. Then, we considered whether the sensor could be used as a “lambda sensor,” which can identify whether the air-to-fuel ratio of exhaust gas is rich or lean region. As a result, we confirmed that the resistance of the Ce0.9Zr0.1O2 thick film was low and high in the case of rich and lean regions, respectively. The activation energy of the resistance of the Ce0.9Zr0.1O2 film was 0.41 and 1.20 eV in rich and lean regions, respectively, and the mean value of them was 0.8 eV. The resistance of the Zr0.8Y0.2O2-δ thick film in rich was almost the same as that in lean. The activation energy was 0.92 and 0.98 eV in rich and lean, respectively, and similar to the mean value of the activation energies of the Ce0.9Zr0.1O2 film in rich and lean. This result shows that the combination of the Ce0.9Zr0.1O2 and Zr0.8Y0.2O2-δ films is better for the “lambda sensor.” The electrode structures of the sensor were optimized so that the mean value of the resistances of the Ce0.9Zr0.1O2 film in rich and lean was the same as the resistance of the Zr0.8Y0.2O2-δ film. The difference in output of the resistive oxygen sensor having the optimized electrode between rich and lean regions was more than 0.7 V, when the source voltage was 1.0 V. The output dramatically changed on the stoichiometric air-fuel ratio. From above-mentioned results, we conclude that the resistive oxygen sensors fabricated in this study are able to be used as “lambda sensors.”

Measurement of flow parameters in solid–liquid slugging fluidised beds

Impedance measurements have been applied to solid–liquid suspensions in order to infer the liquid and solid phase distributions in a fluidised slugging bed. The bed is constituted by soda spheres with a particle to column diameter ratio equal to 0.125. The instantaneous liquid fraction has been measured and used to obtain the average solid concentrations inside the different regions along the slug. The signal provided by the impedance probes is processed in terms of probability density functions. The local values of the liquid hold-up can thus be inferred. This study demonstrates the reliability of the presented approach and provides information on solid concentrations in the dense and lean regions of square-nose slugging suspensions.

NO reburning in ultrarich filtration combustion of methane

Reburning of nitrogen oxide in combustion waves formed under filtration of methane/air mixtures in a packed bed of randomly arranged inert alumina pellets is studied experimentally and numerically. Due to effective heat regeneration, the studied range of equivalence ratios spans from 0.15 to 3 covering the ultralean, lean, rich, and ultrarich regions. The data suggest strong influence of equivalence ratio on NO x emissions for pure methane/air waves due to the essential variation of the combustion chemistry. Nitrogen oxide reburning studies were performed with input concentrations of NO from 10 to 1000 ppm. Destruction and removal efficiencies above 60% were observed in ultrarich combustion waves.

Horizontal pneumatic conveying: a 3d distinct element model

Pneumatic conveying is widely used for transporting bulk solids in chemical, process and agricultural industries. It is environmentally friendly, flexible and can be fully automated. But it can also involve high power consumption, wear, abrasion, blockage and particle degradation. Hence understanding the physics can help to optimise design and operation. Conveying in a horizontal pipe involves complex multiphase flows, potentially with lean and dense phase regions, stationary particles and blockage.

Quantitative planar laser-induced fluorescence of naphthalenes as fuel tracers

In planar laser-induced fluorescence (PLIF), naphthalenes are commonly used as tracers for diesel-like fuels and fuel surrogates. Previously, such a technique was used to visualize the air–fuel mixing in a mesoscale (∼10 cm 3 device volume) burner prototype running on dodecane as a single-component JP-8 surrogate. For quantification of this technique, the influence of temperature, oxygen quenching, and the presence of the fuel surrogate was investigated through spectrally resolved LIF of naphthalenes in a heated jet as well as through broadband imaging. For excitation at 266 nm, naphthalene and 1-methylnaphthalene show very similar fluorescence behavior. Fluorescence is not influenced by the presence of dodecane but is strongly quenched by oxygen. Due to this strong oxygen quenching, fluorescence intensity was found to be linear with equivalence ratio at all temperatures. Both increased temperature and increased oxygen concentrations lead to a spectral broadening and red-shift. This effect is minor for oxygen addition and saturates at about 10% oxygen, but is significant for temperature increase and continuous throughout the temperature range investigated. This red-shift was exploited to simultaneously image temperature and equivalence ratio. A dichroic beamsplitter was used to obtain a long-wavelength and a short-wavelength image whose ratio represents temperature. Calibrations were performed on the heated jet, and subsequently this two-color PLIF method was applied to the mesoscale burner. After averaging the images from 20 processed single-shot measurements, the equivalence ratio and temperature images obtained had signal/noise ratios of 25 and 11, respectively. Temperature correction of the equivalence ratio resulted in excessive noise. Likely inaccuracies in temperature were identified in very lean regions where the signal is low and sensitive to the background corrections employed.

Hydrogen as burner fuel: modelling of hydrogen-hydrocarbon composite fuel combustion and NOx formation in a small burner

The objective of this work is to investigate numerically the turbulent non-premixed hydrogen (H2) and hydrogen–hydrocarbon flames in a small burner. Numerical studies using Fluent code were carried out for air-staged and non-staged cases. The effects of fuel composition from pure hydrogen to natural gas (100%H2, 70%H2+30%CH4, 10%H2+90%CH4, and 100%CH4) were also investigated. The predictions are validated and compared against the experimental results previously obtained and results from the literature. Turbulent diffusion flames are investigated numerically using a finite volume method for the solution of the conservation equations and reaction equations governing the problem. Although, three different turbulence models were tested, the standard k–e model was used for the modelling of the turbulence phenomena in the burner. The temperature and major pollutant concentrations (CO and NOx) distributions are in good agreement with the existing experimental results. Air staging causes rich and lean combustion regions thus lower NOx emissions through the combustor exit. Blending hydrogen with methane causes considerable reduction in temperature levels and thus NO emissions. Increasing the mixture ratio from stoichiometric to leaner mixtures also decreases the temperature and thus NO emissions. Hydrogen may be considered a good alternative fuel for burners, as its use reduces the emission of pollutants, and as it is a renewable synthetic fuel. Copyright © 2005 John Wiley & Sons, Ltd.

High-speed imaging of OH* and soot temperature and concentration in a stratified-charge direct-injection gasoline engine

In-cylinder soot formation and oxidation in a stratified-charge spark-ignited direct-injection (SIDI) engine have been studied with a high-speed (9000 frames/s) camera system with three intensified detectors. Continuous records of soot temperature and relative soot concentration within individual engine cycles are evaluated from images of soot radiation at two wavelengths (650 and 750 nm). Combustion is followed simultaneously by imaging OH* chemiluminescence (306 nm). The spatially and temporally resolved data allow the two most important soot sources to be quantified separately for a typical part-load operating condition. (1) Soot first forms in partially premixed flame propagation through rich zones, but the high soot temperatures (∼2000–2400 K) and rapid mixing with surrounding hot lean regions lead to rapid soot oxidation and burnout. (2) Soot formation in pool fires (diffusion flames fed by thin films of liquid fuel on the piston) begins later and continues until late in the cycle, when further significant soot oxidation is unlikely (soot temperatures have dropped to ∼1500 K, and OH* chemiluminescence is no longer detectable). For this SIDI engine design and operating condition, pool fires are the dominant source of engine-out soot.

Investigations of hydrogen and hydrogen–hydrocarbon composite fuel combustion and [formula omitted] emission characteristics in a model combustor

In this paper, the numerical simulation of a turbulent non-premixed hydrogen (H 2 ) diffusion flame has been performed in a model combustor. CFD studies using Fluent code were carried out changing fuel composition from pure hydrogen to natural gas (100% H 2 , 70% H 2 + 30% CH 4 , 10% H 2 + 90% CH 4 , and 100% CH 4 ). The model prediction studies have been extended to combustion air staging. Air 25% was staged and introduced through the two tangential inlets. The predictions are validated and compared against the experimental results obtained in this study and results from the literature. Turbulent diffusion flames are investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The standard k – ε model is used for modelling of turbulent flow as the model was far enough for the turbulence phenomena in the combustor. The chemical combustion reactions are described by seven species and three steps. A NO x post-processor has been used for predicting NO x emissions from the combustor. The temperature and major pollutant concentration (CO and NO x ) distributions are in good agreement with the experimental measurements. The overall flame temperature increases as hydrogen is added or decreases as methane is added to the fuel mixture. The addition of methane to hydrogen decreases the flame temperature and thus NO x emissions considerably. Air staging causes rich and lean combustion regions and thus lower NO x emissions through the combustor exit.