Constant Volume Bomb Research Articles

Chemical Kinetic Mechanism Reduction Scheme for Diesel Fuel Surrogate

In this study, performance of the DRG-based chemical kinetic mechanism reduction techniques was evaluated using a diesel fuel surrogate model, which is the n-hexadecane mechanism. Following that, a new mechanism reduction scheme was developed to generate a reduced mechanism which is suitable to be applied in diesel engine applications.As a result, areduced mechanism with 49 species and 97 elementary reactions was successfully derived from the detailed mechanismwithan overall 97% reduction in species number and computational runtime in zero-dimensional closed homogeneous batch reactor simulations. After that, the reduced n-hexadecane mechanism was applied to simulate spray combustion in a constant volume bomb using OpenFOAM software. Results show that n-hexadecane alone is inappropriate to be employed as a single-component diesel surrogate as its high cetane number has resulted in advanced ignition timing. This agrees with recent study andthus fuel blending is suggested in order to match the diesel fuel kinetics and compositions.

Flame-Front Instabilities of Outwardly Expanding Isooctane/n-Butanol Blend–Air Flames at Elevated Pressures

Experiments were conducted in a constant volume bomb filled with isooctane/n-butanol blend–air mixtures over a wide range of n-butanol blending ratios, equivalence ratios, and initial pressures. High-speed schlieren photography was used to measure the critical radius for the onset of flame-front instability. Results show that, for a given n-butanol blending ratio, cellular instabilities appear earlier and the critical Peclet number, which represents the onset of instability, is decreased with the increase of the initial pressure and equivalence ratio. Under fuel-rich conditions, the critical Peclet number increases with the increase of the n-butanol blending ratio. Theoretical calculation of the critical Peclet number was also conducted to interpret the effects of n-butanol on the flame-front instabilities of the blends. Results show that, for rich mixture conditions, with the increase of the n-butanol blending ratio, the overall activation energy of the mixtures is decreased and, thus, the thermal–diffusional instabilities are suppressed. While under fuel-lean conditions, the thermal–diffusional instabilities are favored. As a consequence, with an increasing n-butanol blending ratio, the critical Peclet number is expected to be decreased under fuel-lean conditions and increased under fuel-rich conditions.

Laminar burning velocities of rich near-limiting flames of hydrogen

In the present work, near-limiting hydrogen flames were investigated both experimentally and numerically. Very rich hydrogen + air flames were studied in a constant volume bomb equipped with a pressure sensor and a Schlieren system for optical registration of the flame front movement. The mixtures contained 70% and 75% of hydrogen, the rest being air. The measurements were conducted at pressures from 1 to 4 atm for 70% H2 + air mixture and from 0.7 to 1.4 atm for 75% H2 + air mixture. Two methods for determination of the laminar burning velocity were used: from the temporal evolution of the flame front movement and from the pressure records at nearly constant pressure. These methods were compared and discussed in terms of accuracy and implicit assumptions behind them. Markstein lengths were also extracted and compared with the literature by using different extrapolation models. An important role of the critical radius for extraction of the burning velocity and Markstein length is demonstrated. New experimental data are compared with three models for hydrogen combustion to elucidate the need for their further development.

Burning velocity of methane-hydrogen mixtures at elevated pressures and temperatures

The effect of the initial pressure, temperature, and equivalence ratio on a number of combustion characteristics of methane-air mixtures with hydrogen additives in a closed vessel is experimentally studied. Experiments are conducted at 1, 5, and 10 atm and temperatures from 22 to 300°C. The hydrogen content in the fuel is 0, 10, and 20 vol %. The fuel equivalence ratio varies from 0.6 to 1.0. The limitations imposed by buoyancy on measurements of the laminar burning velocity by the constant-volume bomb method with recording of pressure-time histories are analyzed. It is shown that the laminar burning velocity can be appreciably increased by adding no less than 20 vol % of hydrogen to the fuel.

Effect of CF3Br on C1–C3 ignition and laminar flame speed: Numerical and experimental evaluation

Halon 1301 (CF3Br) is a well-known flame inhibitor, the understanding of which can guide the search of novel compounds with similar capabilities. Despite its importance as a benchmark for new fire suppressants, there are comparatively few detailed data for CF3Br that can be used to fine tune its chemical kinetics or to use as a baseline for candidate suppressants, particularly when interacting with hydrocarbon fuels. This paper presents new experimental data on ignition delay times and laminar flame speeds for mixtures containing CH4, C2H6, or C3H8 over a range of equivalence ratios with various amounts of CF3Br added to the mixture. Reflected-shock experiments were performed with fuel–O2 mixtures highly diluted in Argon (98%) for temperatures between 1250 and 2250K at an average pressure of 1.4atm and equivalence ratios of 0.5, 1.0, and 2.0; Halon 1301 was added at levels up to about 10% of the fuel for each hydrocarbon studied. Data obtained from the shock-tube experiments included ignition delay times and OH* time histories. Laminar flame speeds were measured using a windowed, constant-volume bomb at initial pressures of 1atm for fuel–air mixtures at equivalence ratios between 0.7 and 1.3; CF3Br was added at levels of 0.5% and 1.0% to each mixture. A chemical kinetics mechanism was used herein to compare with the data, assembled from a recent, well-studied C0–C5 hydrocarbon mechanism from NUI Galway and a recent CF3Br mechanism. The model is very accurate with regard to the methane ignition behavior (with CF3Br actually accelerating ignition). However, for ethane and propane, although the model correctly predicted that the halon should slow down ignition, the level of inhibition was drastically under-predicted for the entire range of conditions studied. For the OH* time histories, the correct trends with respect to the addition of CF3Br were predicted, but major improvements can be made for the relative effect of the suppressant. With regard to the laminar flame speed results, the model also correctly predicted the basic trends of lowering flame speed with increasing levels of CF3Br addition, but it rather poorly predicted the magnitude of the impact for all three fuels. Using the assembled mechanism, the relative thermal and chemical flame inhibition contributions of CF3Br were estimated for each fuel, with the experimental chemical effect ranging from 72% to 87%, being much larger than the thermal effect (from 13% to 28%). Finally, ignition delay time and flame speed sensitivity analyses were conducted to identify the rate-controlling reactions for the systems analyzed. Given the importance of CF3Br as a model fire suppressant, and based upon the results of this study, further improvements in its kinetics mechanism are warranted.

Burning Behaviour of High-Pressure CH4-H2-Air Mixtures

Experimental characterization of the burning behavior of gaseous mixtures has been carried out, analyzing spherical expanding flames. Tests were performed in the Device for Hydrogen-Air Reaction Mode Analysis (DHARMA) laboratory of Istituto Motori—CNR. Based on a high-pressure, constant-volume bomb, the activity is aimed at populating a systematic database on the burning properties of CH4, H2 and other species of interest, in conditions typical of internal combustion (i.c.) engines and gas turbines. High-speed shadowgraph is used to record the flame growth, allowing to infer the laminar burning parameters and the flame stability properties. Mixtures of CH4, H2 and air have been analyzed at initial temperature 293÷305 K, initial pressure 3÷18 bar and equivalence ratio = 1.0. The amount of H2 in the mixture was 0%, 20% and 30% (vol.). The effect of the initial pressure and of the Hydrogen content on the laminar burning velocity and the Markstein length has been evaluated: the relative weight and mutual interaction has been assessed of the two controlling parameters. Analysis has been carried out of the flame instability, expressed in terms of the critical radius for the onset of cellularity, as a function of the operating conditions.

Experimental and Computational Studies on Ignition Characteristics of Diesel in a Premixed Ethanol–Air Mixture Atmosphere

It was shown in former experimental studies that the ignition of diesel or n-heptane can be delayed by adding ethanol fuel. However, detailed analysis of the interaction between two fuels in a flame has been lacking. To investigate the underlying mechanism, a constant-volume bomb was used to study the ignition characteristics of diesel in a pure air atmosphere (AA) and an ethanol–air mixed atmosphere (EAA). Experimental results show that both the ignition delay and lift-off length of the diesel fuel were prolonged by ethanol fuel. A zero-dimensional model was developed to analyze the effects of ethanol on ignition properties of n-heptane. Simulation results further reveal that ethanol suppresses the low-temperature reaction of n-hepthane, whereas ethanol promotes the high-temperature reaction of n-hepthane. When two fuels are blended, OH is converted into H2O2 in the presence of ethanol, while H2O2 is thus frozen to ignition because of its high activation energy for decomposition. Therefore, the reaction activity before ignition is decreased. Both experiments and simulation show that ethanol plays a dominant role in delaying the ignition of diesel fuel.

Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H2/CO/N2/CO2/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4e1.0). This is mainly due to the high reactivity of H2 leading to high production rate of H and OH radicals. Reactions R3 and R2 play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and

A method to determine ignition delay times for Diesel surrogate fuels from combustion in a constant volume bomb: Inverse Livengood–Wu method

In this work a method to determine the ignition delay of Diesel surrogate fuels using data from a constant volume combustion bomb is presented. The studied fuels are n-heptane (a common PRF) and a mixture of 50% of n-heptane and 50% of toluene (in mass) because both fuels have been proposed as Diesel fuel surrogates under HCCI. There are different concepts of spontaneous ignition times in engines or other devices. Auto-ignition times are experimentally obtained in a constant volume combustion bomb or in compression ignition engines, for variable conditions of pressure and temperature. In this work they are obtained in a combustion bomb using four different criteria (pressure, mechanical vibrations, OH* and CH* chemiluminescence emissions). The paper describes an original approach based on an inverse Livengood–Wu method to obtain the ignition delays (as a function of pressure and temperature) from the auto-ignition times. The obtained expressions for the fuels and their mixtures are compared with those of other authors with a reasonable agreement.

Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds

Experiments and computational simulations were used to study effects of diluents on laminar flame speeds of stoichiometric, laminar, premixed dimethyl ether (DME)/air flames. The experiments were conducted in a constant volume bomb under initial temperature of 298K and initial pressure of 0.1MPa. Outwardly propagating spherical flames were used to measure the laminar flame speeds of mixtures with varying concentration of Ar, N2 or CO2 addition. Laminar flame speeds were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics. Predictions showed a good agreement with experiments. For all the three diluting agents invested in this work, laminar flame speed decreases with the increase of dilution ratio, while for a given dilution ratio, the significance of suppression effect on laminar flame speed is in the order of CO2, N2 and Ar. The suppression effects of N2 and Ar on flame propagation are mainly caused by reduced reactant concentration and modified heat capacity, both of which result in a decreased flame temperature, while besides that, CO2 further influences the chemical kinetics because it is a major product of combustion. Further experimental study on flame speed and simulation study on flame structure at fixed adiabatic flame temperature were conducted to evidence this speculation and the possible reaction pathways which contribute to the chemical retarding effect by CO2 addition were recognized through sensitivity analysis.

Effect of inert and reactive admixtures on the initiation and propagation of laminar spherical flames

The initial stage of the propagation of discharge-initiated laminar spherical flame in stoichiometric natural gas- and isobutylene-oxygen mixtures containing krypton or carbon dioxide and in the hydrogen-air mixture at atmospheric pressure in a constant-volume bomb has been investigated by high-speed color cinematography. Dilution of the combustible mixtures with the admixtures increases the steady-state flame front formation time by a factor larger than 10. The phlegmatizing effect of carbon dioxide on hydrocarbon combustion is stronger than that of krypton. For given combustible mixtures, the ratio of steady-state flame front formation times is inversely proportional to the ratio of the temperature rise values for the flames. The introduction of a minor amount of a reactive admixture (1.2% isobutylene) into the stoichiometric hydrogen-air mixture greatly increases the steady-state flame front formation time.

Initial stage of the explosion of ammonium nitrate and its powder mixtures

There is an obvious contradiction between the statistics of the devastating explosions that take place with the participation of ammonium nitrate and explosive properties of this material determined in standard tests. Pure ammonium nitrate does not burn under normal conditions and has a very low sensitivity to conventional mechanical and thermal stimuli. So far, ammonium nitrate has been detonated only by using high explosives. Causes of accidental explosions involving large masses of ammonium nitrate are likely to be found in a nonconventional behavior of ammonium nitrate. These changes may arise due to different chemical or physical factors, such as those associated with the presence of active additives, crushing of particles, etc., and lead to acceleration of the process at the initial stage of explosion. This work is devoted studying the convective burning and the initial stage of deflagration-to-detonation transition in dry and wet mixtures of ammonium nitrate with various, largely combustible additives. Experiments were conducted on loose-packed charges in a constant-volume bomb and by using the method of the critical bed height with recording pressure-time diagrams by a piezoelectric sensor. Ammonium nitrate of two different types was used: granular and powdered. The fuel additives were charcoal and aluminum powder, whereas the additives inhibiting the combustion of ammonium nitrate were water and monosodium salt of phosphoric acid. In addition, finely dispersed mixture of four components (ammonium nitrate, aluminum, powdered sugar, and TNT in a proportion of 76: 8: 12: 4) was used. The experiments in the constant-volume bomb were supplemented by numerical simulations, which made it possible to obtain a better understanding of the convective burning of the test mixtures and to evaluate the possibility of using a constant-volume bomb to collect quantitative information on the intensity of the combustion of the mixture at the initial stage of the explosion.

Spray characteristics of emulsified castor biodiesel on engine emissions and deposit formation

Castor plants are a common wild poisonous plant in Asia, and especially a weed seen across Taiwan. Due to its high viscosity and water content, straight castor oil cannot be used as a fuel for DI engine. Transesterification and emulsion technologies have been utilized to improve the spray characteristics of castor oil. Without heat, the castor biodiesel (CBD) completes the transesterification reaction under ambient temperature. Gas chromatography indicates that the CBD yield rate is 97% or above. After long-term biodiesel generator test, the emulsified castor biodiesel (EBD) leads to the problem of engine deposition. Thus, this study aimed to investigate the EBD spray characteristics on DI engine emission and deposit formation. A constant-volume bomb was established to analyze the biodiesel spray characteristics under elevated temperature. A biofuel deposit simulator was developed to solve the EBD deposit problem. Thermal gravimetric analysis, Fourier transformation infrared spectroscopy, and scanning electron microscope were utilized to analyze the EBD deposit formation mechanisms. The experimental results indicated that biodiesel generator operated on EBD can improve the fossil diesel emissions. The high NO X emission of CBD was solved by water-biodiesel emulsion technology. The biofuel deposit simulator provided some potential deposit control additives for EBD during the laboratory research stage. Without changing the engine structure, when the injection pressure was increased by 5–10%, the optimum combination was 82.8% of castor biodiesel, 15% of water, 2% of bioethanol, and 0.2% of composite surfactant Span-Tween.

Multidimensional Modeling of Natural Gas Jet and Mixture Formation in Direct Injection Spark Ignition Engines—Development and Validation of a Virtual Injector Model

During the last few years, the integration of CFD tools in the internal combustion (IC) engine design process has continually increased, allowing time and cost savings as the need for experimental prototypes has diminished. Numerical analyses of IC engine flows are rather complex from both the conceptual and operational sides. In fact, these flows involve a variety of unsteady phenomena and the right balance between numerical solution accuracy and computational cost should always be reached. The present paper is focused on computational modeling of natural gas (NG) direct injection (DI) processes from a poppet-valve injector into a bowl-shaped combustion chamber. At high injection pressures, the gas efflux from the injector and the mixture formation processes include turbulent and compressible flow features, such as rarefaction waves and shock formation, which are difficult to accurately capture with numerical simulations, particularly when the combustion chamber geometry is complex and the piston and intake/exhaust valve grids are moving. In this paper, a three-dimensional moving grid model of the combustion engine chamber, originally developed by the authors to include simulation of the actual needle lift, has been enhanced by increasing the accuracy in the proximity of the sonic section of the critical valve-seat nozzle, in order to precisely capture the expansion dynamics the methane undergoes inside the injector and immediately downstream from it. The enhanced numerical model was then validated by comparing the numerical results to Schlieren experimental images for gas injection into a constant-volume bomb. Numerical studies were carried out in order to characterize the fuel-jet properties and the evolution of mixture formation for a centrally mounted injector configuration in the case of a pancake-shaped test chamber and the real engine chamber. Finally, the fluid properties calculated by the model in the throat section of the critical nozzle were taken as reference data for developing a new effective virtual injector model, which allows the designer to remove the whole computational domain upstream from the sonic section of the nozzle, keeping the flow properties virtually unchanged there. The virtual injector model outcomes were shown to be in very good agreement with the results of the enhanced complete injector model, substantiating the reliability of the proposed novel approach.

Influence of inert and active additives on the initiation and propagation of laminar spherical flames in stoichiometric mixtures of methane, pentane, and hydrogen with air

The propagation of a laminar spherical flame in stoichiometric mixtures of methane, and pentane with air in the presence of argon and carbon dioxide and in hydrogen-air-propylene mixtures at atmospheric pressure in a constant-volume bomb is investigated using high-speed color cinematography. It is shown that, under the experimental conditions employed (at T0 = 298 K and a spark energy of E0 = 0.91 J), dilution of the combustible mixtures with these additives can cause a more than 10-fold increase in the time of formation of a steady flame front, with the inhibiting effect of carbon dioxide being stronger than that of argon. Small additives of propylene, a chemically active inhibitor, are demonstrated to substantially increase the time it takes to form a steady flame front and reduce the flame propagation velocity.

Evolution of Hydrocarbons Ignition Delay Time Over HTAC (High Temperature Air Combustion) Conditions

In this investigation, ignition processes of the methane and the propane using a high-temperature oxidizer (Toxi > Tai) with a varying oxygen concentration zO2 - the HTAC (High Temperature Air Combustion) conditions, applying a constant-volume bomb (CVB) was investigated. The influence of the initial temperature of the oxidizer Toxi was analyzed and discussed. It is shown that in order to achieve an effective reaction of ignition the oxidizer temperature needn't be maximized. Detailed evolution of ignition delay time with equivalence ratio τig was presented. It can be concluded that increment of oxygen concentration zO2 in oxidizer results in decrease of τig. Trials with propane are characterizing lower values of ignition delay time τig in comparison with methane.

Laminar premixed flame characteristics of hydrogen blended iso-octane–air–nitrogen mixtures

The combined effects of hydrogen addition and nitrogen dilution on laminar flame characteristics for iso-octane–air stoichiometric mixtures were investigated. The spherical expanding flames technique, in a constant volume bomb, was employed. The mole fraction of hydrogen in the iso-octane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (iso-octane–hydrogen–air-diluent) from 0 to 0.4. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The laminar burning velocity strongly decreases with nitrogen dilution and for all dilution rates increases linearly with hydrogen mass percentage. The burned gas Markstein length is reduced with the increase in hydrogen amount in the mixture until it reaches negative values, the opposite effect of nitrogen dilution. From these new experimental data, an experimental correlation to estimate the laminar burning velocity of iso-octane–hydrogen–air–nitrogen mixtures is proposed. The experimental values of laminar burning velocities are found to be in very good agreement with those predicted by the high temperature mechanism developed by Jerzembeck et al. [1].

Laminar flame speed measurements of dimethyl ether in air at pressures up to 10 atm

Laminar flame speed measurements of dimethyl ether/air mixtures were made at 1, 5, and 10 atm with equivalence ratios ranging from 0.7 to 1.6. All experiments were performed in a large cylindrical constant-volume bomb with optical access. A new method for converting flame images into flame radii was used. Results reported in other studies were investigated, and some explanations on the disparities found are presented. A full uncertainty analysis was performed combining precision errors from data scatter with predicted systematic errors. Uncertainties ranging between 4.2% and 8.6% were found depending on the equivalence ratio and initial pressure. Experimental results agreed well with some other spherical flame experiments and counterflow flame measurements, but were found to be much lower than PIV-based stagnation flame results. Also, two spherical flame studies deviated significantly both in magnitude and trend. Critical radii and Peclet numbers, defined by the onset of rapid flame acceleration, were recorded for all high-pressure experiments. Markstein lengths were measured and showed a decreasing trend with increasing equivalence ratio. Three different methods were used to define the laminar flame thickness, and large disparities were found between them. In this study, the modeled temperature gradient method for the definition of flame thickness is preferred over other methods. Modeling was performed with the latest version of a C3 chemical kinetics mechanism. Good agreement is seen between the experimental results and the model at all pressures. Emphasis is placed in this paper on reporting experimental uncertainties, calculated density ratios, flame temperatures, and flame radii ranges used for data analysis, and the results resolve some discrepancies seen in the literature for dimethyl ether flame speeds.

Study on the spray characteristics of methyl esters from waste cooking oil at elevated temperature

In Taiwan, millions of tons of waste cooking oil are produced each year, and less than 20% of it, about 150,000 ton/a, is reclaimed and reused. Most waste oil is flushed down the drain. Utilizing waste cooking oil to make biodiesel not only reduces engine exhaust gas pollution, but also replaces food-derived fuels, and reduces ecologic river pollution. This study employed two-stage transesterification to lower the high viscosity of waste oil, utilized emulsion to reduce the methyl ester NOx pollution, and used methanol to enhance the stability and viscosity of emulsified fuel. To further analyze spray characteristics of fuels, this experiment built a constant volume bomb under high temperature, used high speed photography to analyze spray tip penetration, spray angle, and the Sauter mean diameter (SMD) of fuel droplets, and compared the results with fossil diesel. The experimental results suggested that, two-stage transesterification can significantly lower waste oil viscosity to that which is close to fossil diesel viscosity. At a temperature above 300 °C, waste cooking oil methyl esters had a water content of 20%, spray droplet characteristics were significantly improved, and NOx emission dropped significantly. The optimal fuel ratio suggested in this experiment was waste cooking oil methyl ester 74.5%, methanol 5%, water 20%, and composite surfactant Span–Tween 0.5%.

Ignition of methane and propane in high-temperature oxidizers with various oxygen concentrations

In this investigation, ignition processes of methane (CH 4 > 98%) and propane (C 3H 8 > 95%) using a high-temperature oxidizer ( T oxi > T ai) with a varying oxygen concentration ( z O 2 = 0.05 ÷ 0.21 ), applying two types of experimental installations, viz. a constant–volume bomb (CVB) and a co-flow reactor (CFR) were investigated. The influence of the initial temperature of the oxidizer (for methane T oxi = 960 ÷ 1234 K and for propane T oxi = 803 ÷ 1055 K), the equivalence ratio and oxygen concentration in the oxidizer on ignition of gaseous fuels is analyzed and discussed. It is shown that in order to achieve an effective reaction of ignition (taking into account the minimum value of ignition delay time τ ig and maximal value of the increment of temperature Δ T) the oxidizer temperature need not be maximized. There are optimal values of temperature of the oxidizer (for methane T oxi ≈ 1100 K and for propane T oxi ≈ 950 K) in which the parameters mentioned above reach their extreme values.