Initial Mixture Temperature Research Articles

Effect of natural gas composition on the laminar burning velocities at elevated temperatures

Laminar burning velocities of fuel mixtures of methane/ethane/propane with the compositions 100/0/0, 80/20/0, 80/0/20 and 80/10/10 vol% burning with air were determined experimentally using the heat flux method at 1 atm and initial gas temperatures 298, 318 and 338 K. The mixtures were selected as surrogates for natural gas, with the aim to investigate the effect of heavier hydrocarbons on the laminar burning velocity of the main component, methane. It was found, in agreement with the literature data, that the heavier hydrocarbons increase laminar burning velocity compared to that of methane + air flames. A common correlation for the temperature dependence of the burning velocity SL = SL0(T/T0)α, where T0 is a reference temperature and SL0 is the laminar burning velocity at this temperature, was used to interpret new measurements. The power exponents, α, were derived from the experimental data for methane and three surrogates for natural gas. It was found that the temperature dependence of the burning velocities is practically identical for all mixtures studied. The measurements have been compared with the modelling using two kinetic schemes: recent version of the Aramco mech 2.0 and an updated version of a model developed by the authors. Both kinetic mechanisms show systematic trends in slight over- and under-prediction of the burning velocities, respectively, for all fuel blends. However, the temperature dependence of the burning velocities is accurately reproduced by these two models. Further analysis indicates that even though rate constants of the reactions determining flame propagation are somewhat different for the two mechanisms, the power exponents α are not sensitive to the differences. This indicates that, detailed kinetic schemes capable in predicting burning velocities at a specific initial mixture temperature are able to operate at higher temperatures as well, at least from lean to moderately rich natural gas mixtures.

Laminar Burning Velocities of High-Performance Fuels Relevant to the Co-Optima Initiative

<div class="section abstract"><div class="htmlview paragraph">Laminar burning velocity (LBV) measurements are reported for promising high-performance fuels selected as drop-in transportation fuels to automotive grade gasoline as part of the United States Department of Energy’s Co-Optimization of Fuels and Engines Initiative (Co-Optima). LBV measurements were conducted for ethanol, methyl acetate, and 2-methylfuran with synthetic air (79.0 % N<sub>2</sub> and 21.0 % O<sub>2</sub> by volume) within a constant-volume spherical combustion rig. Mixture initial temperature was fixed at 428±4 K, with the corresponding initial pressure of 1.00±0.02 atm. Current LBV of ethanol is in good agreement with literature data. LBV of ethanol and 2-methylfuran showed similar values over the range of equivalence ratios, while methyl acetate exhibited an LBV significantly lower over the range of tested equivalence ratios. The maximum laminar burning velocity occurred at slightly richer equivalence ratio from the stoichiometric value for all fuels tested. LBV data were compared to simulations by chemical kinetic mechanisms. The predicted LBVs of ethanol and methyl acetate were in reasonable agreement with data, however, those for 2-methlyfuran were slightly under-predicted. Current data will serve as valuable validation targets for future chemical kinetic mechanisms for Co-Optima fuels.</div></div>

Thermodynamic simulation of porous-medium combustion chamber under diesel engine-like conditions

Currently, one of the most important issue of the diesel engines is the non-homogeneity of the in-cylinder fuel-air mixture. Non-homogeneous mixture causes the non-uniform heat release and high-temperature gradient in the combustion chamber that followed by the production of unburnt hydrocarbons, carbon monoxide, nitrogen oxides and soot. Homogenous combustion decreases the production of the emissions. One novel method for achieving homogenous combustion is the use of porous medium (PM) within the combustion chamber of engine so as to separate the mixture formation process from combustion process. The present article thermodynamically investigates the effect of utilizing PM in a constant-volume combustion chamber via solving the energy equations for both the solid and fluid phases considering the chemical reaction of the fuel combustion. At first, numerical result is validated according to the experimental result of diesel fuel injection in a constant-volume combustion chamber with PM. Then, effects of the initial mixture and PM temperature, the porosity of the PM, the mean pores diameter as well as the use of fuels such as decane, isooctane, methanol, ethanol, propane and hydrogen on combustion initiation, and also pressure and temperature of the combustion chamber studied.

An experimental investigation on laminar burning velocity of castor bio-diesel blended with gas oil

In this article we describe how the laminar burning velocity values were determined experimentally for different blends of castor bio-diesel with gas oil. The experiments were performed with a constant volume combustion bomb using pressure-time history. The castor bio-diesel obtained from pyrolysis was blended with 5, 10, 25 and 50% of castor bio diesel with gas oil by volume. The experiments were performed for a range of equivalence ratios (0.8–1.2), initial mixture temperatures (573–773K) and initial mixture absolute pressures (0.5–1 bar). It was found that blends of castor bio-diesel with gas oil exhibit lower burning velocities than pure gas oil fuel which was attributed to the oxygen content and lower heat of combustion of the blends. The experimental data showed that the burning velocity of bio fuels was lower by about 25–30% than that of gas oil. Also, the maximum value of burning velocity was obtained at an equivalence ratio of approximately 1.05, and the laminar burning velocity increased considerably as the initial temperature increased but reduced as the initial pressure increased.HighlightsCastor bio-diesel is synthesized by using pyrolysis process with the presence of a catalyst.The synthesized castor bio-diesel proves to conform to ASTM D-6751 for bio-diesel.The laminar burning velocity is decreased by increasing the ratio of castor bio-diesel.The higher initial temperature leads to a higher laminar burning velocity.Higher initial pressure leads to a lower laminar burning velocity.

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.

Sensitivity of Emissions to Uncertainties in Residual Gas Fraction Measurements in Automotive Engines: A Numerical Study

Initial conditions of the working fluid (air-fuel mixture) within an engine cylinder, namely, mixture composition and temperature, greatly affect the combustion characteristics and emissions of an engine. In particular, the percentage of residual gas fraction (RGF) in the engine cylinder can significantly alter the temperature and composition of the working fluid as compared with the air-fuel mixture inducted into the engine, thus affecting engine-out emissions. Accurate measurement of the RGF is cumbersome and expensive, thus making it hard to accurately characterize the initial mixture composition and temperature in any given engine cycle. This uncertainty can lead to challenges in accurately interpreting experimental emissions data and in implementing real-time control strategies. Quantifying the effects of the RGF can have important implications for the diagnostics and control of internal combustion engines. This paper reports on the use of a well-validated, two-zone quasi-dimensional model to compute the engine-out NO and CO emission in a gasoline engine. The effect of varying the RGF on the emissions under lean, near-stoichiometric, and rich engine conditions was investigated. Numerical results show that small uncertainties (~2–4%) in the measured/computed values of the RGF can significantly affect the engine-out NO/CO emissions.

Effects of composition and temperature of initial mixture on the formation and properties of polyurethane foam

AbstractThe purpose of the study is to optimize the initial conditions of the molding process of a multicomponent mixture into a refrigerator cabinet. Polyurethane foaming has been studied with different starting compositions and temperatures of the initial mixture. The kinetic mechanism has involved four reactions and species species. Using a standard recipe, the chemical kinetics and the numerical implementation have been first validated against the experimental values of the filling time and the density of the final foam. Thereafter, 11,763 different initial conditions are calculated. The results put in evidence that the filling time can be reduced without any significant change in the precursors of the rigidity of the foam. The final temperature of the foam has always remained lower than the limiting temperature so that thermal material deformations in the walls of the cabinet can be avoided. Increasing the physical blowing agent mass fraction beyond the standard recipe does not appear to be a useful option.

Artificial intelligence for the removal of benzene, toluene, ethyl benzene and xylene (BTEX) from aqueous solutions using iron nanoparticles

Abstract Magnetic nanosorbents proved to be highly effective in inorganic and organic contaminants removal from aqueous solutions, especially nano zero valent iron (nZVI). The main purpose of this study is to investigate the effect of using nZVI in removing benzene, toluene, ethyl benzene and xylene (BTEX) contaminants from aqueous solutions. The nZVI and the standard BTEX solution were prepared in the laboratory. X-ray diffraction (XRD), UV spectrophotometry, and scanning electron microscopy (SEM) analysis were used for nZVI characterization. The effects of contact time, initial BTEX mixture concentration, adsorbent dose, temperature, and pH on the amount of BTEX absorbed were investigated. The highest removal efficiency of 97% for the BTEX mixture was achieved at a stirring rate of 100 rpm, temperature of 60°C, and pH 7. The minimum effective time for efficient removal was 30 min, while the effective dose for BTEX compounds removal was 0.22 g/L. The Freundlich model was the best fit of experimental data. An artificial neural network (ANN) was used to predict the BTEX removal efficiency. Modeling results showed that ANN with average absolute error of 0.6272% is reliable in describing the adsorption of BTEX onto the iron nanoparticles. It is estimated that the cost of BTEX removal by nZVI under the optimal conditions will be about 3.5 USD per cubic meter.

EFFECT OF HYDROGEN ADDITION ON FLAME PROPAGATION CHARACTERISTICS THROUGH TUBE OF METHANE-AIR MIXTURES USING OPTICAL TECHNIQUE

The effect of Hydrogen addition on laminar flame speed (Uf) of Methane – Air premixed mixtures using optical technique has been experimentally investigated inside a tube. the flame front location had been positioned by a photocell. The (Uf) measured at a laboratory conditions for an extensive range of equivalence ratios (F). In order to use density ratio method for the calculation of laminar burning velocity (UL), all experimental work was carried out at constant pressure (Pre-pressure period). The flame temperature (Tb) has been calculated theoretically. Mixture strength (F) and hydrogen content (RH) dependence of (UL) is represented by empirical correlation over the ranges F = (0.6-1.4), RH = (0, 0.1, 0.2, 0.3, 0.4), all at initial unburned mixture temperature Tu = 298 K and a pressure of (1 atm). In overlapping ranges, the results were found in satisfactory agreement with those previously published.

Effects of CO2/N2 dilution on laminar burning velocity of stoichiometric DME-air mixture at elevated temperatures

The laminar burning velocity of CO2/N2 diluted stoichiometric dimethyl ether (DME) air mixtures is determined experimentally at atmospheric pressure and elevated mixture temperatures using a mesoscale high aspect-ratio diverging channel with inlet dimensions of 25mm×2mm. In this method, planar flames at different initial temperatures (Tu) were stabilized inside the channel using an external electric heater. The magnitude of burning velocities was acquired by measuring the flame position and initial temperature. The mass conservation of the mixture entering the inlet and the stationary planar flame front is applied to obtain the laminar burning velocity. Laminar burning velocity at different initial mixture temperatures is plotted with temperature ratio (Tu/Tu,o), where a reference temperature (Tu,o) of 300K is used. Enhancement in the laminar burning velocity is observed with mixture temperature for DME-air mixtures with CO2 and N2 dilutions. A significant decrease in the burning velocity and slight increase in temperature exponent of the stoichiometric DME-air mixture was observed with dilution at same temperatures. The addition of CO2 has profound influence when compared to N2 addition on both burning velocity and temperature exponent.

Effects of oil and water contamination on natural gas engine combustion processes

Abundant availability and potential for lower emissions are drivers for increased utilization of natural gas in automotive engines for transportation applications. A novel bimodal engine has been developed that allows on-board refueling of natural gas by utilizing the engine as a compressor. Engine compression however, results in altering the initial state of the natural gas. Increase in temperature and addition of oil are two key effects attributed to the onboard refueling process. A secondary effect is the presence of water in the natural gas supply line. This study investigates the effect of upstream conditions of natural gas on three parameters - autoignition temperature, ignition delay, and laminar flame speed. These parameters play key roles in the engine combustion process. Parametric studies are conducted by varying the initial mixture temperature, water, and oil content in the fuel. The studies utilize numerical simulations conducted with detailed chemistry for natural gas with n-heptane used as a surrogate for oil. Water addition to natural gas at 1–5% by volume did not result in any major changes in the combustion processes, other than a slight reduction in laminar flame speeds. Oil addition of 1–5% by volume reduced autoignition temperature by 5–10% and ignition delay by 27–95% depending on the initial temperature. Sensitivity analysis showed that this was likely due to decrease in the sensitivity of two recombination reactions with oil addition. Evolution profiles of key radical species also showed increasing mole fraction of the hydroperoxy radical at lower temperature that likely aids in reducing the ignition delay. Oil addition resulted in a relatively small increase in the laminar flame speed of 1.7% along with an increase in the adiabatic flame temperature. These results help inform the combustion process and performance to be expected from the bimodal engine.

Thermal and Ludwig–Soret diffusion effects on near-boundary ignition behavior of reacting mixtures

The autoignition behavior is key towards providing insight into a fuel's behavior and performance and equally important towards developing kinetic models. However, there has been evidence in past literature that under certain conditions legacy reactor experiments may exhibit inhomogeneities that could potentially reduce the scientific value of the reported data. Additionally, the initiation of knock in spark ignition engines has been observed to occur close to the cylinder wall. In this study, detailed one-dimensional simulations were carried out in order to provide additional insight into the aforementioned observations. The main focus of the present investigation is on the effects of a colder wall relatively to the core of the reacting mixture, and the attendant development of thermal boundary layers. It is determined that the thermal stratification could alter the ignition behavior for fuels that exhibit distinct negative temperature coefficient behavior and could cause also light and heavy species concentration stratification due to Ludwig–Soret diffusion. More specifically, simulations performed for n-heptane/air mixtures revealed that localized exothermic centers could develop for a range of initial mixture temperatures. Furthermore, simulations for hydrogen/oxygen/argon mixtures showed that species stratification caused by Ludwig–Soret diffusion could lead to increased local heat release rates.

The Effect of Thermophoresis on Flame Propagation in Nano-Aluminum and Water Mixtures

This study investigates the important role of thermophoresis in flame propagation speed of nano-sized aluminum particles and water mixtures. To this aim, a one-dimensional steady state model was developed based on mass and energy conservation equations considering the effect of thermophoresis. In order to simulate the flame propagation speed of nano-sized aluminum particles and liquid water mixtures, the flame structure (the computational domain) was divided into three zones: liquid water zone, preheat zone and reaction zone. The governing conservation equations were solved numerically using a finite-difference method. The predicted results were in reasonable agreement with literature data for the flame propagation speed. The modeling results demonstrated that although the effect of thermophoresis is negligible at micro-scales, it cannot be ignored for nano-sized particles. Furthermore, as the particle size decreases and the equivalence ratio and initial mixture temperature increases; the effect of thermophoresis on the flame propagation speed becomes more significant.

The Effect of Temperature and Carbon to Hematite Ratio on the Formation of Cementite During the Couple of STMA and Partial Melting Processes

In this research, the role of the couple of simultaneous thermal-mechanical activation (STMA) and partial melting (PM) processes in the carbothermic reduction of hematite and cementite formation was studied. For this purpose, the STMA process was performed for 6 hours at 1073 K (800 °C) on the mixture of hematite and graphite with stoichiometry ratio (22 wt pct C) in argon atmosphere in the first stage, and then this process was coupled to the PM process at 1453 K (1180 °C) for 25 minutes. The results obtained showed that the percentage of cementite phase in the product of the STMA process was only about 24 wt pct and after carrying out the PM process, this value reached 77 wt pct in the final product. In the second stage, the effect of the ratio of the parameters of carbon to hematite on the initial mixture (1:1, 1:25, and 1:5 times the stoichiometry ratio) and STMA process temperature [1073 K, 1123 K, and 1173 K (800 °C, 850 °C, and 900 °C)] was studied. The results were indicative of the fact that by an increase in the ratio of carbon to hematite and at STMA temperature, the percentage of cementite in the final product obtained from the PM process significantly increased. In fact, the specimens obtained from the couple of the STMA and PM processes with a carbon to hematite ratio of 1.5:1 at 1073 K (800 °C) and that of 1.25:1 at 1173 K (900 °C) showed the greatest percentage of cementite. The mechanism regarding the processes showed that by controlling the amount and manner of free carbon distribution in the STMA product based on the parameters of the ratio of carbon to hematite and temperature, it was possible to obtain pure cementite. Therefore, the couple of STMA and PM processes can also be brought up as a new method in the production of pure cementite.

Synthesis and physicochemical properties of macroporous cryogels

The current review reveals the analysis of recent research in the field of synthesis and characterization of cryogels. The theoretical aspects of the process of cryotropic gelation are considered. The influence of freezing temperature and concentrations of initial monomer mixture on the pore size of the material is discussed. The concept of process of cryotropic gelation is revealed. The potential need for further research in the field of cryotropic gelation is highlighted.

Kinetics of premixed acetaldehyde + air flames

Non-stretched laminar burning velocities, SL, of acetaldehyde+air mixtures at initial gas mixture temperatures, T, of 298, 318, 338, 348 and 358K are reported for the first time. The flames were stabilized on a perforated plate burner at 1atm using the heat flux method at conditions where the net heat loss from the flame to the burner is zero. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be typically better than ±1cm/s. Experimental results were compared with predictions of several kinetic models from the literature. Recent model of Leplat et al. (2011) [30] developed for acetaldehyde and ethanol oxidation showed the closest agreement with the measurements as compared to the Konnov and San Diego models. The effects of initial temperature on the adiabatic laminar burning velocities of acetaldehyde were interpreted using the correlation SL=SL0 (T/T0)α. Particular attention was paid to the variation of the power exponent α with equivalence ratio. The existence of a minimum in α in the slightly rich mixtures is demonstrated experimentally and confirmed computationally. The model of Leplat et al. was further analyzed using sensitivity analysis and it was concluded that the deviation of the modelled results when comparing with experiments is not a result of the fuel specific reactions but rather the sub-mechanisms of C1 and H2/O2.

Laminar Burning Velocities of Dimethyl Carbonate with Air

Laminar burning velocities of dimethyl carbonate (DMC) + air flames at initial gas mixture temperatures of 298, 318, 338, and 358 K are reported. Nonstretched flames were stabilized on a perforated plate burner at atmospheric pressure, and the laminar burning velocities were determined using the heat flux method. The overall accuracy of the burning velocities was evaluated to be typically better than ±1 cm/s. The effects of unburned mixture temperature on the laminar burning velocity of DMC were analyzed using the correlation SL = SL0 (Tu/Tu0)α. The present experimental results indicated that the power exponent α reaches a minimum in slightly rich mixtures corresponding to the maximum burning velocity. Modeling of these results has been attempted using the mechanism developed by Glaude et al. It was found that this model significantly overpredicts laminar burning velocities of methanol, ethanol, and DMC; however, it accurately reproduces the temperature power exponent α for dimethyl carbonate flames.

Analysis of the Current Signature in a Constant-Volume Combustion Chamber

Identifying charged species and their relative concentrations as a function of initial mixture composition, temperature, and pressure can play an important role in the diagnostics and control of combustion devices. This article aims to identify important charged species and their temporal variation in a spark-ignited methane/air mixture using the equilibrium constant method. The equilibrium composition of 20 neutral and seven charged species [(e−), CHO+, H3O+, NO+, OH−, , O−] was obtained using a novel adaptation of the Newton–Raphson method for highly stiff nonlinear system of equations. A two-zone combustion model was used to study the effect of air–fuel ratio (AFR) on the temporal variation of charged species concentration. Temporal variation of the current computed from the equilibrium concentration of charged species was found to compare well with experimental data for several AFRs.

Laminar burning velocities of acetone in air at room and elevated temperatures

Laminar burning velocities of acetone+air mixtures at initial gas mixture temperatures of 298, 318, 338 and 358K are reported. Non-stretched flames were stabilized on a perforated plate burner at 1atm, and laminar burning velocities were determined using the heat flux method, at conditions where the net heat loss from the flame to the burner is zero. The overall accuracy of the burning velocities was estimated to be better than ±1.0cms−1. Very good reproducibility of the results and excellent agreement with modeling using a recently updated chemical kinetic model brings confidence in the validity of the reported results. Previous determinations of laminar burning velocities for acetone have provided inconsistent results. In the present work it is suggested that this can in part be attributed to the chemically aggressive nature of acetone.

The Temperature and Pressure Dependencies of Propagation Characteris-tics for Premixed Laminar Ethanol-Air Flames

Laminar burning velocity is strongly dependent on mixture characteristics, e.g. initial temperature, pressure and equivalence ratio. In this work, spherically expanding laminar premixed flames, freely propagating from a spark ignition source in initially quiescent ethanol-air mixtures, have been imaged and then the laminar burning velocities were obtained at initial temperatures of 358 K to 500K, pressure of 0.1 to 0.2 MPa and equivalence ratio of 0.7 to 1.4. The measured re-sults and literature data on ethanol laminar burning velocities were accumulated, to analyze the effects of initial tempera-ture and pressure on the propagation characteristics of laminar ethanol-air flames. A correlation in the form of ul=ulo(Tu/Tu0)αT (Pu/Pu0)βP , and validated over much wide temperature, pressure and equivalence ratio ranges. The global activation temperatures were determined in terms of the laminar burning mass flux for ethanol-air flames. And the Zel’dovich numbers were estimated as well. The dependencies of global activation temperature and Zel’dovich number on initial mixture pressure, temperature and equivalence ratio were explored. Additionally, an alterna-tive correlation of laminar burning velocities, from the view of theoretical arguments, was proposed on the basis of the de-termined ethanol-air laminar mass burning flux. Good agreements were obtained in its comparison with the literature data.