Air Equivalence Ratio Research Articles

Measurements of laminar premixed methane—air flame thickness at ambient conditions

A new experimental data set for the flame thickness of laminar premixed methane—air flames at ambient conditions is presented. Prior to combustion the temperature and pressure of the premixed mixture were 298 K>5 K and 100 kPa>5 kPa, respectively. The fuel—air equivalence ratio was varied from 0.8 to 1.4 in 0.05 steps. The data were obtained using simultaneous imaging of the visible and Schlieren cones on the central axis of the symmetrical flame front of a non-cooled burner configuration. The data were analysed and the plane obtained from the Schlieren image corrected using a one-dimensional energy balance to obtain the position of the cold surface of the flame and hence allow determination of the flame thickness. The data presented agree within experimental errors with the limited existing published data. For the range of equivalence ratios studied, a second-order equation was fitted and presented for methane—air flames at atmospheric pressure and ambient temperatures.

Study on premixed combustion in cylindrical micro combustors: Transient flame behavior and wall heat flux

The micro combustor is a key component of the micro thermophotovoltaic (TPV) system. Improving the wall temperature of the micro combustor is an effective way to elevate the system efficiency. An experimental study on the wall temperature and radiation heat flux of a series of cylindrical micro combustors (with a backward-facing step) was carried out. For the micro combustors with d = 2 mm, the regime of successful ignition (under the cold wall condition) was identified for different combustor lengths. Acoustic emission was detected for some cases and the emitted sound was recorded and analyzed. Under the steady-state condition, the effects of the combustor diameter ( d), combustor length ( L), flow velocity ( u 0) and fuel–air equivalence ratio ( Ф) on the wall temperature distribution were investigated by measuring the detailed wall temperature profiles. In the case that the micro combustor is working as an emitter, the optimum efficiency was found at Ф ≈ 0.8, independent of the combustor dimensions ( d and L) and the flow velocity. Under the experimental conditions employed in the present study, the positions of the peak wall temperature were found to be about 8–11 mm and 4–6 mm from the step for the d = 3 mm and d = 2 mm micro combustors, respectively, which are 8–11 and 8–12 times of their respective step heights. This result suggests that the backward-facing step employed in the combustor design is effective in stabilizing the flame position.

Experimental and numerical study of the wall temperature of cylindrical micro combustors

The micro combustor is a key component of the micro thermophotovoltaic (TPV) system. Premixed combustion of a hydrogen–air mixture in micro combustors with an inner diameter of 2 mm is studied experimentally and numerically. A backward-facing step (s = 0.5 mm) is employed in the cylindrical tube for flame stabilization. The effects of flow velocity, fuel–air equivalence ratio and combustor length on the wall temperature are investigated. Based on the measured wall temperature, the radiation heat flux through the combustor wall is quantified. The experimental results (measurements and direct photos) indicate that the position of the peak wall temperature is nearly localized within a narrow zone (4–6 mm from the step), independent of flow velocity, fuel–air equivalence ratio and combustor length. The combustion flows are numerically modeled by solving the 2D governing equations in both the gas phase and solid phase. The numerical results indicate that at Re > 500 (u0 = 8 m s−1) modeling the turbulence (k–ε model) is necessary to ensure good agreement with the experimental values. The numerical model is shown to be applicable over the full span of both flow velocity (8 ⩽ u0 ⩽ 16 m s−1) and fuel–air equivalence ratio (0.6 ⩽ Φ ⩽ 1.0).

Effect of altitude and palm oil biodiesel fuelling on the performance and combustion characteristics of a HSDI diesel engine

Altitude above sea level and fuel’s chemical and physical nature affect engine performance and combustion characteristics. In this work, a combustion diagnosis model including exergy analysis was applied to a turbocharged (TC) automotive diesel engine fuelled with neat palm oil biodiesel (B100) and No. 2 diesel fuel (B0). Tests were performed under steady state operating conditions, at two altitudes above sea level: 500 and 2400 m. Biodiesel fuelling and altitude had an additive effect on the advance in injection and combustion timings. The duration of the premixed combustion stage increased with altitude and decreased with biodiesel. When B100 was used, the transition between this stage and the diffusion stage was practically suppressed. As altitude increased, biodiesel fuelling led to shorter combustion duration, and higher in-cylinder pressures and fuel–air equivalence ratios. Brake thermal efficiency decreased with altitude for both fuels, but in a greater extent for B0. For all fuels and altitudes, exergy destruction rose sharply when combustion started, indicating that this process was the main source of irreversibilities. At both altitudes, the cumulative exergy destruction was higher for B100 due to its earlier and faster combustion process. Some of the results obtained in this work indicate that palm oil biodiesel fuelling can lead to a better engine performance at high altitudes.

Premixed turbulent flame front structure investigation by Rayleigh scattering in the thin reaction zone regime

Premixed turbulent flames of methane–air and propane–air stabilized on a bunsen type burner were studied using planar Rayleigh scattering and particle image velocimetry. The fuel–air equivalence ratio range was from lean 0.6 to stoichiometric for methane flames, and from 0.7 to stoichiometric for propane flames. The non-dimensional turbulence rms velocity, u′/ S L, covered a range from 3 to 24, corresponding to conditions of corrugated flamelets and thin reaction zones regimes. Flame front thickness increased slightly with increasing non-dimensional turbulence rms velocity in both methane and propane flames, although the flame thickening was more prominent in propane flames. The probability density function of curvature showed a Gaussian-like distribution at all turbulence intensities in both methane and propane flames, at all sections of the flame. The value of the term Dκ, the product of molecular diffusivity evaluated at reaction zone conditions and the flame front curvature, has been shown to be smaller than the magnitude of the laminar burning velocity. This finding questions the validity of extending the level set formulation, developed for corrugated flames region, into the thin reaction zone regime by increasing the local flame propagation by adding the term Dκ to laminar burning velocity.

Development of 1D model for the analysis of heat transport in cylindrical micro combustors

A 1D flame model was developed to analyze the heat transport occurring in the cylindrical micro combustors. The one-step global reaction mechanisms were employed for three fuel–air mixtures (H 2–air, CH 4–air and C 3H 8–air) to account for the difference of fuel property in terms of the kinetics. The effects of various parameters such as the combustor size, fuel property, fuel–air equivalence ratio and unburned mixture temperature on the heat loss ratio (defined as Q l/ Q in) and the heat recirculation ratio (defined as Q recir/ Q l) were investigated. The results indicated that these parameters have significant effects on the two ratios, and therefore should be carefully managed in order to achieve efficient and stable combustion. After comparing the results of different fuel–air mixtures, it is concluded that hydrogen is superior to methane and propane as the fuel for micro combustion engines owing to its higher flame temperature and thinner flame thickness, which favors the reduction of heat loss from the flame zone.

Experimental Investigation of Biomass Gasification in a Fluidized Bed Reactor

This paper aims to catch the influence of various operating conditions and catalyst addition on the property of gas product and tar evolution. The gasification of three local biomass samples (sawdust, peanut shell, and wheat straw) was performed using a fluidized bed gasification reactor, and the gas product and liquid tar were analyzed with gas chromatography (GC). First, the influence of biomass property, gasification temperature, and air equivalence ratio was investigated. The biomass feeding rate was set at ∼2.37 kg/h; the furnace temperature variant was between 750 and 850 °C; and the equivalence ratio (ER) was 0.15−0.35. It can be observed that a lower heating value (LHV) of gas product from sawdust is higher than peanut shell and straw, while the tar content is also much higher than the other two samples, which might be attributed to the high volatile content. At 800 °C, with the increase of ER, the gas yield increased rapidly from 1.14 to 1.93 m3/kg, while the LHV decreased from 7.09 to 3.26 MJ/m3. Meanwhile, the variation of ER also showed a great effect on tar species. With the increase in temperature, combustible gas content, gas yield, and LHV all increased significantly, while the tar content decreased sharply from 13.24 to 6.53 g/m3, which indicated that high temperature was favorable for biomass gasification. Then, three additives (dolomite, magnesite, and olivine) were introduced into the gasification process as catalyst for tar cracking. It is great for the upgrading of gas product quality, and tar removal efficiencies are all above 50%. It is significant for the development of biomass gasification technology.

Mathematical analysis of spark ignition engine operation via the combination of the first and second laws of thermodynamics

This study aims at the theoretical exergetic evaluation of spark ignition engine operation. For this purpose, a two-zone quasi-dimensional cycle model was installed, not considering the complex calculation of fluid motions. The cycle simulation consists of compression, combustion and expansion processes. The combustion phase is simulated as a turbulent flame propagation process. Intake and exhaust processes are also computed by a simple approximation method. The results of the model were compared with experimental data to demonstrate the validation of the model. Principles of the second law are applied to the model to perform the exergy (or availability) analysis. In the exergy analysis, the effects of various operational parameters, i.e. fuel–air equivalence ratio, engine speed and spark timing on exergetic terms have been investigated. The results of exergy analysis show that variations of operational parameters examined have considerably affected the exergy transfers, irreversibilities and efficiencies. For instance, an increase in equivalence ratio causes an increase in irreversibilities, while it decreases the first and also the second law efficiencies. The irreversibilities have minimum values for the specified engine speed and optimum spark timing, while the first and second law efficiencies reach a maximum at the same engine speed and optimum spark timing.

Syngas production from olive tree cuttings and olive kernels in a downdraft fixed-bed gasifier

This study presents a laboratory fixed-bed gasification of olive kernels and olive tree cuttings. Gasification took place with air, in a temperature range of 750–950°C, for various air equivalence ratios (0.14–0.42) and under atmospheric pressure. In each run, the main components of the gas phase were CO, CO2, H2 and CH4. Experimental results showed that gasification with air at high temperatures (950°C) favoured gas yields. Syngas production increased with reactor temperature, while CO2, CH4, light hydrocarbons and tar followed the opposite trend. An increase of the air equivalence ratio decreased syngas production and lowered the product gas heating value, while favouring tar destruction. It was found that gas from olive tree cuttings at 950°C and with an air equivalence ratio of 0.42 had a higher LHV (9.41MJ/Nm3) in comparison to olive kernels (8.60MJ/Nm3). Olive kernels produced more char with a higher content of fixed carbon (16.39w/w%) than olive tree cuttings; thus, they might be considered an attractive source for carbonaceous material production.

Improving Signal-To-Interference Ratio in Rich Hydrocarbon—Air Flames Using Picosecond Coherent Anti-Stokes Raman Scattering

There is growing interest in the use of short-pulse lasers for coherent anti-Stokes Raman scattering (CARS) to minimize non-resonant background (NRB) contributions in a variety of applications. Using time-coincident picosecond (ps) pump and Stokes beams and a time-delayed ps probe beam, we show that a three orders of magnitude reduction in NRB interference can be achieved in rich hydrocarbon-air flames while preserving 60% to 80% of the CARS signal. This represents a significant improvement in signal-to-interference ratio compared with previous measurements in room temperature air and is attributable to reduced rates of collisional dephasing and relaxation at flame temperatures. Measurements within the flame zone of a laminar flat-flame burner are used to investigate the characteristics of time-coincident and probe-delayed broadband ps N(2)-CARS spectra for C(2)H(4)-air equivalence ratios of 0.5 to 1.2. Up to three ro-vibrational bands of N(2) are excited with each laser shot using 135 ps pump and 106 ps Stokes beams, and the CARS signal is generated using a 135 ps probe beam delayed by 165 ps. The enhanced signal-to-interference ratio achieved in the current work is one to two orders of magnitude higher than that previously achieved using polarization-selection techniques without sensitivity to the effects of birefringence caused by density gradients or test cell windows. Moreover, the use of a 135 ps laser source in this study enables frequency domain "broadband" CARS with sufficient resolution to extract ro-vibrational spectral features under various flame conditions. The effect of probe delay and NRB suppression on characteristics of these broadband CARS spectra are investigated, and evidence of preferential collisional dephasing and relaxation of different ro-vibrational transitions is not detected. This is a promising but preliminary result to be investigated further in future work.

Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fuelled with primary reference fuels

The effects of exhaust gas recirculation (EGR) of primary reference fuel on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine are investigated. The results show that EGR delays the ignition timing, slows down the combustion reaction rate, reduces the pressure in cylinder, and extends the HCCI operation region. The maximum indicated thermal efficiency of fuels with different octane numbers appears in the region of high EGR rate and large fuel: air equivalence ratio. In the region of high EGR rate, hydrocarbon emissions rise up sharply as the EGR rate increases. Carbon monoxide emissions increase with the increase in EGR rate. Nitrogen oxides emissions increase abruptly as knocking combustion occurs.

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.

PREDICTION OF STABILITY LIMITS FOR LP AND LPP GAS TURBINE COMBUSTORS

ABSTRACT The prediction and the systematic suppression of self-sustained combustion instabilities in combustors for gas turbine applications still suffer from the lack of physical models for the dynamic flame characteristics of Lean-Premixed natural gas and Lean-Prevaporized-Premixed kerosene flames. Hence, the experimental determination of the flame transfer functions of LPP swirl flames was achieved using a prevaporization unit for kerosene. A time-independent and spatial homogeneous mixture of kerosene and combustion air was generated at the burner exit. By exchangeable fuel nozzles the flame dynamics were as well detected for natural gas LP swirl flames with identical operation conditions. The flame dynamics are strongly affected by the formation and in-phase reaction of coherent vortex structures, well known as drivers of combustion instabilities. These structures have been visualized with a phase-correlated imaging technique. The results discussed in this paper lead to a basic understanding and quantitative prediction of the frequency-dependent dynamics of LP and LPP swirl flames. Especially, the influence of preheating temperature, air equivalence ratio and the type of fuel on the amplitude responses and phase angle functions were investigated in detail. Based on theoretical considerations concerning the burning velocity of steady-state premixed flames a physical model and – derived from that it–scaling laws for the prediction of unstable operation modes in dependence on main operation parameters of the flame were formulated and validated by the measurements. Thus, it is now possible to scale and quantitatively predict the stability limits for the formation of periodic combustion instabilities of lean-premixed combustors with swirl-stabilized flames.

Burning velocity measurement of fluorinated compounds by the spherical-vessel method

Burning velocity has been measured using the spherical-vessel (SV) method for four hydrofluorocarbons (HFCs), i.e., difluoromethane (HFC-32), 1,1,2-trifluoroethane (HFC-143), 1,1,1-trifluoroethane (HFC-143a), and 1,1-difluoroethane (HFC-152a). Experiments were conducted for initial pressures in the range 78–108 kPa and initial temperatures in the range 280–330 K, over wide ranges of HFC–air equivalence ratios. The burning velocities were determined from the rate of pressure increase by applying a spherical-flame propagation model. Flame propagation behavior was observed in a cylindrical vessel equipped with optical windows by employing schlieren photography. It was found that the values of burning velocity derived from the spherical-vessel method are in good agreement with the ones obtained with schlieren method. It is found that the SV method is adequate for determining the burning velocity for weakly flammable HFCs as well as for moderately flammable compounds. The burning velocity of each HFC was obtained as a function of temperature, pressure, and equivalence ratio. The maximum burning velocities of HFC-32, HFC-143, HFC-143a, and HFC-152a were determined to be 6.7, 13.1, 7.1, and 23.6 cm s −1, respectively. The maximum burning velocity occurred for slightly fuel-rich concentrations. The magnitude of the burning velocity is strongly dependent on the ratio of H atoms to F atoms in the HFC molecules.

NO x formation from ammonia, hydrogen cyanide, pyrrole, and caprolactam under incinerator conditions

The oxidation of NH 3, HCN, pyrrole, and caprolactam has been investigated under incinerator conditions. The residence time resolved measurements have been conducted within a ceramic plug flow reactor built up in semi-technical scale. The experiments cover a temperature range from 1100 to 1350 K, and the air equivalence ratio varies between 0.93 and 1.5. A very important aspect is the presence of H 2O and CH 4 under these conditions. In the temperature range investigated, water and methane have an essential influence on the levels of the important radicals H, O, OH, and HO 2 and, thereby, affect the reaction rates of the reaction system. The measurements for the oxidation of ammonia in the presence of methane show a characteristic conversion of ammonia to hydrogen cyanide of about 10% of the initial NH 3 content, occurring at fuel rich and fuel lean conditions. The only reaction path for the generation of HCN in such an extent proceeds via methylamine arising from the reaction of CH 3 with NH 2. Furthermore, with growing temperature a distinct increase in the NO emissions has been observed, with reaction NO + NH 2 ⇔ N 2 + H 2O playing a key role in the NO removal. The experimental concentration profiles have been simulated using a detailed mechanism from the literature with substantial modifications. The kinetic model is able to predict all characteristics of the experiments very well. The experiments for the decomposition of pyrrole and caprolactam reveal that HCN is formed as intermediate species in very high concentrations. The hydrogen cyanide is then partially oxidized to NO. The destruction of pyrrole has been simulated with the modified mechanism for NH 3 and HCN oxidation, extended by a modified model for pyrrole decomposition from the literature. The agreement of experiments and simulations is very good. For the modelling of the destruction of caprolactam, a new mechanism has been derived that reproduces the experimental results very well.

Investigation of light load HCCI combustion using formaldehyde planar laser-induced fluorescence

Planar laser-induced fluorescence of formaldehyde was used to investigate quenching of the homogeneous-charge compression-ignition (HCCI) combustion process at light load in an n-heptane-fueled internal combustion engine. Detailed chemical kinetic calculations indicated that the persistence of formaldehyde after the end of heat release coincided with the existence of a significant mole fraction of carbon monoxide at the end of the cycle. At fuel–air equivalence ratios greater than 0.16, the data indicate that formaldehyde is uniformly formed within the combustion chamber during the first-stage ignition and then completely consumed during the second-stage ignition event. At leaner equivalence ratios, the formaldehyde fluorescence was found to persist after the second-stage ignition process; formaldehyde fluorescence was observed as late at 60 crankangle degrees after top dead center at an equivalence ratio of 0.09. The laser-induced fluorescence images were acquired in the central portion of the combustion chamber away from the cool combustion chamber surfaces. Therefore, the mechanism by which formaldehyde persists is considered to be bulk gas quenching as opposed to quenching due to thermal stratification within the combustion chamber. Data acquired from a uniform formaldehyde mixture over a range of crankangle timings in a motored engine were used to investigate the fluorescence signal as a function of temperature and pressure, and to provide a first order estimation of the absolute concentration. The fluorescence per formaldehyde molecule was found to be constant for a window about top dead center, but was found to increase for advanced or retarded timings where the pressure and temperature were lower.

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.

Investigating the effects of LPG on spark ignition engine combustion and performance

Abstract A quasi-dimensional spark ignition (SI) engine cycle model is used to predict the cycle, performance and exhaust emissions of an automotive engine for the cases of using gasoline and LPG. Governing equations of the mathematical model mainly consist of first order ordinary differential equations derived for cylinder pressure and temperature. Combustion is simulated as a turbulent flame propagation process and during this process, two different thermodynamic regions consisting of unburned gases and burned gases that are separated by the flame front are considered. A computer code for the cycle model has been prepared to perform numerical calculations over a range of engine speeds and fuel–air equivalence ratios. In the computations performed at different engine speeds, the same fuel–air equivalence ratios are selected for each fuel to make realistic comparisons from the fuel economy and fuel consumption points of view. Comparisons show that if LPG fueled SI engines are operated at the same conditions with those of gasoline fueled SI engines, significant improvements in exhaust emissions can be achieved. However, variations in various engine performance parameters and the effects on the engine structural elements are not promising.

Performance maps of a diesel engine

This paper suggests a mechanism for determining the constant specific-fuel consumption curves of a diesel engine using artificial neural-networks (ANNs). In addition, fuel–air equivalence ratio and exhaust temperature values have been predicted with the ANN. To train the ANN, experimental results have been used, performed for three cooling-water temperatures 70, 80, 90, and 100 °C for the engine powers ranging from 1000 to 2300 – for six different powers of 75–450 kW with incremental steps of 75 kW. In the network, the back-propagation learning algorithm with two different variants, single hidden-layer, and logistic sigmoid transfer function have been used. Cooling water-temperature, engine speed and engine power have been used as the input layer, while the exhaust temperature, break specific-fuel consumption (BSFC, g/kWh) and fuel–air equivalence ratio (FAR) have also been used separately as the output layer. It is shown that R 2 values are about 0.99 for the training and test data; RMS values are smaller than 0.03; and mean errors are smaller than 5.5% for the test data.

In situ investigation of laser-induced ignition and the early stages of methane–air combustion at high pressures using a rapidly tuned diode laser at 2.55 μm

The laser-induced ignition of methane/air-mixtures at elevated pressures was investigated by an absorption spectroscopic technique. A room temperature continuous wave InGaAsSb/AlGaAsSb quantum well ridge diode laser was wavelength tuned around 2.55 μm by periodically modulating the injection current from 0 to 174 mA at a 5 kHz repetition rate. The laser heat sink temperature was fixed at 291 K. The infrared laser beam was sent through the pressurized combustion vessel perpendicularly to the igniting laser beam (Nd:YAG laser, 10 ns pulse duration, 20 mJ) at the position of the ignition spark. Fuel-rich to fuel-lean mixtures of methane/air (air equivalence ratio 0.89, 1.06, 1.42, 2.50) were investigated at initial pressures of up to 3 MPa. The initial temperature was 473 K, the volume of the combustion vessel 0.9×10 −3 m 3. The formation of water vapor in the vicinity of the laser spark was tracked by the diode laser. The time resolution of the measurements was 0.2 ms for a total continuous measurement time of up to 1 s. In this way, the laser-induced ignition and its accompanying effects could be investigated on a time scale spanning four orders of magnitude. Apart from the absorbance of water vapor which could be determined semi-quantitatively (due to the effects of severe pressure broadening at high pressures and the ignorance of the exact temperature distribution after ignition), the emissions from the flame (broadband, 1–10 μm) and a gas inhomogeneity index were recorded. The gas inhomogeneity index was obtained by extracting a frequency variable from the time-dependent fluctuations of the transmitted laser intensities and calculating its derivation. The absorbance of water vapor, the emissions from the flame and the gas inhomogeneity index were found to be a powerful tool to characterize laser-induced ignition. Major implications of in situ species concentration measurements at high pressures for the design and development of high-load combustors are presented.