H2-air Mixtures Research Articles

Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures

There is worldwide interest in using renewable fuels within the existing infrastructure. Hydrogen and syngas have shown significant potential as renewable fuels, which can be produced from a variety of biomass sources, and used in various transportation and power generation systems, especially as blends with hydrocarbon fuels. In the present study, a reduced mechanism containing 38 species and 74 reactions is developed to examine the ignition behavior of iso-octane/H2 and iso-octane/syngas blends at engine relevant conditions. The mechanism is extensively validated using the shock tube and RCM ignition data, as well as three detailed mechanisms, for iso-octane/air, H2/air and syngas/air mixtures. Simulations are performed to characterize the effects of H2 and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in CHEMKIN software. The effect of H2 (or syngas) is found to be small for blends containing less than 50% H2 (or syngas) by volume. However, for H2 mole fractions above 50%, it increases and decreases the ignition delay at low (T < 900 K) and high temperatures (T > 1000 K), respectively. For H2 fractions above 80%, the ignition is influenced more strongly by H2 chemistry rather than by i-C8H18 chemistry, and does not exhibit the NTC behavior. Nevertheless, the addition of a relatively small amount of i-C8H18 (a low cetane number fuel) can significantly enhance the ignitability of H2-air mixtures at NTC temperatures, which are relevant for HCCI and PCCI dual fuel engines. The CO addition seems to have a negligible effect on the ignition of i-C8H18/H2/air mixtures, indicating that the ignition of i-C8H18/syngas blends is essentially determined by i-C8H18 and H2 oxidation chemistries. The sensitivity and reaction path analysis indicates that i-C8H18 oxidation is initiated with the production of alkyl radical by H abstraction through reaction: i-C8H18 + O2 = C8H17 + HO2. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C8H17 + O2 = C8H17O2) and R8 (C8H17 + O2 = C8H16 + HO2), with the R8 path dominating, and increasing the ignition delay. As the amount of H2 in the blend becomes significant, it opens up another path for the consumption of OH through reaction R36 (H2 + OH = H2O + H), which slows down the ignition process. However, for T > 1100 K, the presence of H2 decreases ignition delay primarily due to reactions R31 (O2 + H = OH + O) and R35 (H2O2 + M = OH + OH + M).

Optimizing the power of enzyme-based membrane-less hydrogen fuel cells for hydrogen-rich H2–air mixtures

The unusual ability of O2-tolerant hydrogenases (H2ase) to produce electricity from a H2–air mixture (when used as the anodic electrocatalyst in a simple, membrane-less fuel cell) is investigated with the aim of establishing a strategy for raising volume power density, the measure of importance for miniature devices. Compacted mesoporous carbon electrodes provide a simple and inexpensive method for obtaining a large increase in productive enzyme loading, greatly increasing current densities and stability. Operated under a 78% H2–22% air mixture at 25 °C, typical current densities at a stationary H2ase anode and bilirubin oxidase cathode are 4.60 ± 0.32 mA cm−2 and 1.23 ± 0.12 mA cm−2, respectively. The power limitation due to low O2 concentration is addressed by re-proportioning the cathode/anode area ratio to balance the cathodic and anodic currents. At room temperature, the maximum power density of the fuel cell with an anode/cathode (A/C) ratio of 1 : 3 (1A/3C) is 1.67 ± 0.24 mW cm−2 (per anode area) or 0.42 ± 0.06 mW cm−2 (per total area). Good prospects for stability are demonstrated by the fact that 90% of the power is retained after continuously working for 24 h, and more than half of the power is retained after one week of non-stop operation. Using an even weaker O2 mixture (89% H2, 11% air) the 1A/3C cell gives over 0.8 mW cm−2 (anode) or 0.2 mW cm−2 (total electrode area). The results demonstrate the feasibility of membrane-less hydrogen–air fuel cells delivering volume power densities well in excess of 1 mW cm−3.

Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts

This article discusses the results of experimental investigation of turbulent boundary layer flashback limits for premixed hydrogen-air flames confined in ducts. A tube burner experiment was set-up to double-check the findings of the channel rig. Unconfined flashback experiments were carried out by stabilizing the flame on top of the pilot burner in free atmosphere. A confined flame configuration was achieved by simply fixing a ceramic ring with a diameter higher by 4 mm on top of the pilot burner. Flashback measurements with unconfined flame holding neatly reproduced literature values for fully premixed, atmospheric H2–air mixtures and turbulent flow. The results of unconfined and confined tube burner experiments were plotted. The results showed that the drastic decrease of wall flashback stability for confined flames was the very same for both, tube and channel.

OH radical and temperature measurements during ignition of H2-air mixtures excited by a repetitively pulsed nanosecond discharge

Ignition delay time, time-resolved temperature, and time-resolved absolute OH concentration are measured in mildly preheated H2–air mixtures (T=473–500K), excited by a repetitive nanosecond pulse discharge in a plasma flow reactor. The measurements are done in decaying plasma after the discharge pulse burst (pulse repetition rate 10–40kHz). Ignition delay increases steeply as the number of pulses in the burst is reduced, exhibiting threshold-like behavior. Temperature and OH concentration during the ignition process are measured by two-line OH LIF, as functions of time delay after the discharge burst. Absolute calibration is done using a near-adiabatic flame. The results show that the temperature at the end of the discharge burst is 700–750K, rising to 1500–1600K within a few ms after the burst, indicative of ignition. During ignition, OH concentration increases by a factor of 20–50, from (0.5–1.1)×1014cm−3 at the end of the burst to peak value of (2.3–2.6)×1015cm−3. Kinetic modeling calculations show good agreement with the experimental results, demonstrating quantitative insight into kinetics of plasma-assisted ignition. Modeling calculations demonstrate that ignition is induced primarily by accumulation of H atoms and gradual temperature rise in the discharge, at temperatures significantly lower than autoignition temperature, by up to 200K.

Evaluation of the one-step hydrogen–oxygen global reaction rate in a non-premixed mixture to predict auto-ignition limits

Evaluation of the hydrogen–oxygen global reaction rate was conducted to predict auto-ignition onset and delay.For this purpose, a global reaction rate was incorporated into a CFD simulation of hot hydrogen that flows in a laminar regime, in a simple geometry of a cylindrical enclosure where ambient air prevails.Auto-ignition was obtained for different inflow temperatures at atmospheric pressure. The results agree with the classical results of H2–O2 explosion limits, referred to as the “peninsula”. Ignition onset was estimated at different pressures within the thermal branch of the peninsula (third limit). Results show that the current global reaction rate adequately represents ignition onset dependency on temperature and equivalence ratio, whilst dependency on pressure is poor. The ignition delay decreases with the increase in temperature; however, at higher temperatures (1000–1200K), it remains almost constant. This may be explained by the fact that a non-premixed mixture is being examined, in which minimal time is needed to form the required conditions for ignition temperature and fuel to oxidizer ratio (FOR).The present study focuses on the transient hydrogen dispersion and examines the flammability limits of non-premixed H2–air mixtures with no external ignition energy.Transient flows, that are often the subject of safety-related scenarios, are characterized by a rapid formulation of flammability zones. The present work allows the prediction of ignition zones directly, rather than predicting them by combining dispersion maps and flammability limits maps.

Stochastic reactor modelling of multi modes combustion with diesel direct injection or hydrogen jet ignition start of combustion

Recent papers [1–5] have proposed two different systems to more efficiently and more rapidly burn the fuel in highly boosted, high compression ratio, directly injected internal combustion engines permitting multi-mode combustion operation. In a first system, a second direct injector is coupled with the standard Diesel direct injector and glow plug. The second direct injector introduces the most of the fuel while the Diesel direct injector only introduces a minimum amount of fuel to control the start of the combustion about top dead centre. The fuel injected before the Diesel ignition injection burns premixed, the fuel injected after the Diesel ignition injection burns diffusion. This design permits combustion premixed gasoline-like if all the fuel is injected before the Diesel ignition injection, diffusion Diesel-like if all the fuel is injected after the Diesel ignition injection (as done in the Westport High Pressure Direct Injection concept [12]), and mixed gasoline/Diesel like injecting the fuel before and after the Diesel ignition injection. The premixed gasoline-like mode is actually a homogeneous charge compression ignition (HCCI)-like mode, where an amount of fuel smaller than the threshold value producing top dead centre auto ignition is then ignited at top dead centre by the Diesel ignition injection in a more robust, stable and repeatable operation unaffected by small changes in properties and composition of the fuel and air mixture. In an alternative design, the glow plug is replaced by a jet ignition devices feed preferably with H2. In this case, a spark ignition ignites the stoichiometric H2-air mixture within the jet ignition pre-chamber. The jets of hot reacting H2-air combusting gases then ignite the main chamber premixed mixture in the gasoline-like operation or create suitable conditions for the fuel subsequently injected to burn diffusion in the Diesel-like operation or perform both duties in the mixed gasoline/Diesel-like operation. A single main chamber direct injector is generally needed (for example with H2, CH4 or C3H8 fuels). With NH3, a second main chamber direct injector with H2 is also used to limit the volume of the jet ignition pre-chamber. In this short communications, the results of detailed chemistry simulations with the SRM (Stochastic Reactor Model) suite, a sophisticated engineering tool combining conventional 1D or 3D fluid dynamics approaches are presented to further support these two engine concepts working with fuels H2, CH4, C3H8, NH3, I-C8H18 and N-C7H16 and adopting two different mechanisms for chemical kinetics. Within the limits of the present simulations (a very accurate chemical kinetic for combustion of I-C8H18 and N-C7H16 but a much less accurate chemical kinetic for the other fuels and especially for NH3, unavailability of variable composition and variable properties multiple injections), the Diesel injection ignition and the hydrogen jet ignition are proved to permit combustion modes leading to indicated thermal efficiencies up to 10% better than the latest Diesels at high loads within the same peak pressure and peak temperature constraints.

Intermittency in premixed turbulent reacting flows

Intermittency in premixed reacting flows is studied using numerical simulations of premixed flames at a range of turbulence intensities. The flames are modeled using a simplified reaction mechanism that represents a stoichiometric H2-air mixture. Intermittency is associated with high probabilities of large fluctuations in flow quantities, and these fluctuations can have substantial effects on the evolution and structure of premixed flames. Intermittency is characterized here using probability density functions (pdfs) and moments of the local enstrophy, pseudo-dissipation rate (strain rate magnitude), and scalar (reactant mass fraction) dissipation rate. Simulations of homogeneous isotropic turbulence with a nonreacting passive scalar are also carried out in order to provide a baseline for analyzing the reacting flow results. In the reacting flow simulations, conditional analyses based on local, instantaneous values of the scalar are used to study variations in the pdfs, moments, and intermittency through the flame. For low intensities, pdfs of the local enstrophy vary substantially through the flame, with greater intermittency near the products. Changes in the pseudo-dissipation pdfs are, however, less pronounced. As the intensity increases, both the enstrophy and pseudo-dissipation pdfs become increasingly independent of position in the flame and are similar to results from the nonreacting simulations. The scalar dissipation intermittency is largest near the reactants and increases at all flame locations with increasing turbulence intensity. For low intensities and in the reaction zone, however, scalar dissipation pdfs approximately follow a Gaussian distribution, indicative of substantially reduced intermittency. Deviations from log-normality are observed in the pdfs of all quantities, even for intensities and flame locations characterized by strong intermittency. The implications of these results for the internal structure of the flame are discussed, and we also propose a connection between reacting flow intermittency and anisotropic vorticity suppression by the flame.

Numerical simulations of hydrogen detonations with detailed chemical kinetics

Time-dependent, multidimensional simulations of unstable propagating detonations were performed using a detailed thermochemical reaction model for a stoichiometric argon-diluted hydrogen–oxygen mixture at low pressures and a hydrogen–air mixture at atmospheric pressure. Detonation cells computed for the low-pressure, dilute H2–O2–Ar systems were regular in shape, and their sizes compared reasonably well with experimental observations. The computed H2–air cells at atmospheric conditions were qualitatively different from those observed in experiments, and their widths range from less than 1mm to nearly 5mm with multilevel hierarchal structures. The effective activation energy of the H2–air mixture, based on constant-volume ignition delay times computed using the detailed thermochemical model, varies between 5 and 40 over the range of post-shock temperatures and pressures in the simulations and is, on average, significantly larger than expected based on the regularity of experimental cellular patterns. Analysis of the simulations suggests that vibrational relaxation of the gas molecules, a process which is ignored when calibrating detailed chemical reaction models, occurs on time scales similar to the ignition delay times for the detonations and may be a source of discrepancy between numerical and experimental results.

Numerical study of combustion initiation in a supersonic flow of H2–air mixture by resonance laser radiation

A comparative analysis of the efficiency of approaches based on the exposure of reacting gas to resonance laser radiation to enhance combustion in a supersonic flow of H2–air mixture is conducted. The kinetic processes responsible for the intensification of chain reactions in premixed and non-premixed H2–air flows upon photodissociation of O2 molecules by 193.3 nm laser radiation, excitation of these molecules to the singlet sigma state by laser photons with 762.346 nm wavelength and heating the mixture by laser radiation are analysed in a detailed manner. It is shown that both photochemical methods, photodissociation and excitation of O2 molecules, are much more effective in shortening the ignition delay length than merely heating the mixture. For the premixed flow, the photodissociation of O2 molecules ensures a slightly higher reduction in the ignition delay than the laser-induced excitation of molecular oxygen to the singlet sigma state. However, in the non-premixed flow the situation is inverted. The analysis shows that both photochemical methods make it possible to raise the efficiency of conversion of reactant chemical energy to thermal energy released during combustion compared with the method of heating the mixtures.

Flame acceleration and transition to detonation in an array of square obstacles

We study flame acceleration and DDT in a two-dimensional staggered array of square obstacles by solving the compressible multidimensional reactive Navier–Stokes equations. The energy release rate for a stoichiometric H2-air mixture is modeled by a one-step Arrhenius kinetics. The space between obstacles is filled with a stoichiometric H2-air mixture at 1 atm and 298 K. Initially, the flow is at rest, and a flame is ignited at the center of the array. Computations show effects of the obstacles as a series of events leading to DDT. During the initial flame acceleration, the speed of the flame depends on the direction of flame propagation since some directions are more obstructed than others. This affects the macroscopic shape of the expanding burned region, which forms concave boundaries in more obstructed directions. As the flame accelerates, shocks form ahead of the flame, reflect from obstacles, and interact with the flame. There are more shock–flame interactions in more obstructed directions, and this leads to a greater flame acceleration and stronger leading shocks. When the shocks become strong enough, their collisions with obstacles ignite the gas mixture, and detonations form. The simulation shows four independent DDT events within a 90-degree sector, all in more obstructed directions. Resulting detonations spread in all directions. Some parts of detonation fronts are quenched by diffractions around obstacles, but they are reignited by collisions of decoupled shocks, or overtaken by other detonations. Thus detonations continue to spread and quickly burn all the material between the obstacles.

Hydrogen release from a high pressure gaseous hydrogen reservoir in case of a small leak

The dynamic blow-down process of a high pressure gaseous hydrogen (GH2) reservoir in case of a small leak is a complex process involving a chain of distinct flow regimes and gas states. This paper presents models to predict the hydrogen concentration and velocity field in the vicinity of a postulated small leak. An isentropic expansion model with a real gas equation of state for normal hydrogen is used to calculate the time dependent gas state in the reservoir and at the leak. The subsequent gas expansion to 0.1 MPa is predicted with a zero-dimensional model. The gas conditions after expansion serve as input to a newly developed integral model for a round free turbulent H2-jet into ambient air. Predictions are made for the blow-down of hydrogen reservoirs with 10, 30 and 100 MPa initial pressure. A normalized hydrogen concentration field in the free jet is presented which allows for a given leak scenario the prediction of the axial and radial range of flammable H2–air mixtures.

The structure and flame propagation regimes in turbulent hydrogen jets

Experiments on flame propagation regimes in a turbulent hydrogen jet with velocity and hydrogen concentration gradients have been performed. Horizontal stationary hydrogen jets released at normal and cryogenic temperatures of 290, 80 and 35K with different nozzle diameters and mass flow rates have been investigated. Sampling probe method and laser PIV techniques have been used to evaluate the distribution of hydrogen concentration and flow velocity. High-speed photography combined with a Background Oriented Schlieren (BOS) system was used for the visual observation of the turbulent flame propagation. In order to investigate different flame propagation regimes the ignition position was changed along the jet axis. It was found that the flame propagates in both directions, up- and downstream of the jet flow if hydrogen concentration is >11%, whereas in case [H2] < 11%, the flame propagates only downstream. This means that at normal temperature the flame is able to accelerate effectively only if the expansion ratio σ of the H2-air mixture is higher than a critical value σ* = 3.75 defined for a closed geometry.

Initiation of diffusion combustion in a supersonic flow of H2–air mixture by electrical-discharge-excited oxygen molecules**This paper was presented at the Third International Symposium on Nonequilibrium Processes, Combustion, and Atmospheric Phenomena (Dagomys, Sochi, Russia, 25–29 June 2007).

The methodology of arranging a stable combustion in a diffusion mode in a supersonic non-premixed H2–air flow at a short distance and at a low air temperature due to excitation of O2 molecules to the a 1Δg and states in the electrical discharge is considered. It is shown that for initiation of combustion in this case, it is sufficient to subject the oxygen molecules to an electrical discharge in a thin (with a thickness of 0.5–1 cm) layer located between hydrogen and air streams. It is shown that the mechanism of combustion enhancement is caused by accelerating the chain reactions due to the abundance of excited oxygen molecules in a shear layer and has a non-thermal character.

A Novel Micro Sliding Rotary Engine

In recent years, there is an effort to develop a portable, autonomous micro power generation system to obtain an order of magnitude improvement in energy density over general alkaline or lithium ion batteries. As hydrocarbon fuels have much higher energy to weight ratios than general batteries, researches to realize micro-engines fueled by such hydrocarbon fuels are carried out in some universities or institutes. The first key problem of the researches is how to get a micro-engine structure suitable for MEMS (Micro-Electro Mechanical Systems) fabrication. This micro-engine structure needs characteristics such as planar geometry, self-valving operation and a minimal number of moving parts and so on. In this paper, a micro sliding rotary type combustion engine structure is presented and described. The intrinsic characteristics of the engine housing curve named of “kindred cardioids curve” are described in details. The structural scheme and cycle process of the micro-engine are discussed. Some performance parameters of the micro engine are theoretically calculated with H2-Air mixture and specified geometry parameters. The primitive calculated results indicate that the sliding rotary combustion engine is workable and effective.

Effect of chemically active additives on the detonation wave velocity and the detonation limits in lean mixtures

On the basis of the Zel’dovich-von Neumann-Doering detonation theory with allowance for heat losses, as well as the theory of chain processes through, the example of the oxidation of hydrogen-rich mixtures in the presence of an inhibitor, it is shown that taking into account inhibition reactions and the trimolecular chain termination has the consequence that there are not only heat losses but also “chemical” losses. It is demonstrated that the presence of only “chemical” losses alone can ensure that there is a concentration limit of detonation and that the combustion wave near the limit is supersonic. Theoretical estimates agree qualitatively with the experimental data on the inhibition of a developed detonation wave in H2-air mixtures with additives of a propane-butane mixture (0.5–4%) at a pressure of 1 atm.

A micro-fuel processor with trench-refilled thick silicon dioxide for thermal isolation fabricated by water-immersion contact photolithography

This paper presents a micro-fuel processor to produce hydrogen for micro-fuel cells. The micro-fuel processor has an original design for a heat source and a reforming chamber to improve thermal efficiency. To improve the mechanical strength as well as to maintain the effectiveness of thermal isolation, the reaction area is suspended by trench-refilled thick SiO2. When the reaction area was heated at 200 °C, the outer silicon frame was at 100 °C, confirming the effectiveness of thermal isolation. To fabricate the trench-refilled thick SiO2 using a contact aligner, water-immersion contact photolithography was used and successfully demonstrated to produce fine patterns with line/space of 2 µm/1 µm (a pitch of 3 µm). The highest temperature could be controlled by tuning the equivalent ratio of H2–air mixture fed into the catalytic micro-combustor. The conversion rate of methanol to hydrogen by steam reforming was lower than 1% due to uneven temperature distribution and channeling effect.

Activation of Chain Processes in Combustible Mixtures by Laser Excitation of Molecular Vibrations of Reactants

The combustion kinetics of H2-air mixtures containing small amounts (<1%) of ozone is analyzed for the case of excitation of asymmetric vibrations of O3 molecules by CO2 laser radiation with a wavelength of ≈ 9.7 µm. It is shown that the irradiation leads to acceleration of the collisional dissociation of O3 molecules, activation of the chain ignition mechanism, and a decrease in the induction period and ignition temperature. The excitation of asymmetric vibrations of O3 molecules by the CO2 laser radiation is 10–103 times more effective than the currently used method of combustion initiation based on local heating of a medium by IR radiation.

Relation of Pt Wire Temperature and Catalytic Heat Release Rate on Pt in Unsteady State of H2-Air Mixture.

The authors have proposed a hydrogen concentration probe using catalytic reaction on Pt wire surface. To use this probe to detect a concentration change in a supersonic mixing layer, the response of the catalytic heat release rate must depend only on concentration change around the probe. Catalytic heat release rate on the Pt wire surface in unsteady state is measured using a constant temperature type hotwire anemometer technique and a shock tube to investigate the relation of the response of the catalytic heat release rate and Pt wire temperature. Catalytic heat release rate begins increasing at the arrival of the shock wave. The increasing rate of the catalytic heat release depends on the Pt wire temperature when the wire temperature is low. However, the dependence is very weak when the wire temperature is over about 680 K. This shows that not the catalytic reaction but molecular transfer from the flow to the Pt wire surface is the controlling step when Pt wire temperature is high enough. As a conclusion, the Pt wire temperature over about 680 K is necessary to use the hydrogen concentration probe in a supersonic mixing layer.

Geometrical study of spark ignition in two dimensions

Spark ignition, as the first step during the combustion in Otto engines, has a profound impact on the further development of the flame kernel. Over the last ten years growing concern for environment protection, including low emission of pollutants has increased the interest in the numerical simulation of ignition phenomena to guarantee successful flame kernel development even for lean mixtures.However, the process of spark ignition in a combustible mixture is not yet fully understood. The use of detailed reaction mechanisms, combined with electrodynamical modelling of the spark, is necessary to optimize ignition of lean mixtures.This work presents simulations of the coupling of flow, chemical reactions and transport with discharge processes in order to investigate the development of a stable flame kernel initiated by an electrical spark. A two-dimensional code to simulate the early stages of flame kernel formation, shortly after the breakdown discharge, has been developed. The model includes Joule heating. The spark plasma channel formed as a consequence of the breakdown is incorporated into the initial conditions. The computations include the initial phase (1–5 µs), which is governed by pressure wave formation, but also the transition to flame propagation. A thorough study of the influence of the electrodes' geometry, i.e. shape and size, and gap width, has been performed for air and a lean H2–air mixture. Also a detailed methane-air mechanism was chosen as another example including combustion.Due to the fast expansion of the plasma channel, together with the geometrical complexity of the electrodes, a complicated flow field develops after the emission of a pressure wave by the expanding channel. Special numerical methods, including artificial viscosity, are required to resolve the complicated flow field during these first 1–5 µs. The heat release through chemical reactions and transport processes is almost negligible during this short phase. The second phase, i.e. the development of a propagating flame and the flame kernel expansion, can last up to several milliseconds and is dominated by diffusive processes and chemical reactions. It has been found that the geometry greatly influences the developing flame kernel and the flow field. As soon as chemical reactions begin to contribute significantly to the heat release, the effect becomes smaller.

Mechanism for electric breakdown in a chemically nonequilibrium system and the influence of the chain oxidation reaction in an H2-air mixture on the breakdown threshold

A mathematical model is developed and a numerical analysis is performed for an electric breakdown in a hydrogen-air mixture with a low concentration of H2. It is shown that, at sufficiently low pressures p<10−2 atm, a small molecular-hydrogen additive (η=5×10−5–5×10−3) decreases the reduced field of an electric breakdown in air by a factor of more than 2 because of the appearance of an additional detachment process associated with the chain hydrogen-oxidation reaction. Detailed calculations are performed for the mean number density of negative oxygen ions [O2−]=103 cm−3 and the hydrogen concentration in air [H2]=0.5, 0.05, and 0.005%. It is found that, for [H2]=0.005%, the breakdown can develop under the action of a geoelectric field of 1.3 V/cm at p≃10−4 atm.