Laminar-burning velocities of hydrogen–air and hydrogen–methane–air mixtures: An experimental study

The laminar burning velocities of hydrogen–air and hydrogen–methane–air mixtures are very important in designing and predicting the progress of combustion and performance of combustion systems where hydrogen is used as fuel. In this work, laminar flame velocities of hydrogen–air and different composition of hydrogen–methane–air mixtures (from 100% hydrogen to 100% methane) have been measured at ambient temperatures for variable equivalence ratios (ER=0.8–3.2$\mathrm{ER}=0.8–3.2$). A modified test rig has been developed from the former Cardiff University ‘Cloud Chamber’ for this experimental study. The rig comprises of a 250 mm length cylindrical stainless steel explosion bomb enclosed at one end with a stainless steel plug which houses an internal stirrer to allow mixing. The other end is sealed with a 120 mm diameter round quartz window. Optical access for filming flame propagation is afforded via two diametrically opposed quartz windows in both sides. Flame speeds are determined within the bomb using a high-speed Schlieren photographic technique. This method is an accurate way to determine the flame–speed and the burning velocities were then derived using a CHEMKIN computer model to provide the expansion ratio. The design of the test facility ensures the flame is laminar which results in a spherical flame which is not affected by buoyancy. The experimental study demonstrated that increasing the hydrogen percentage in the hydrogen–methane mixture brought about an increase in the resultant burning velocity and caused a widening of the flammability limits. This experiments also suggest that a hydrogen–methane mixture (i.e. 30% hydrogen+70% methane) could be a competitive alternative fuel for existing combustion plants.     

Formation of flammable hydrogen–air clouds from hydrogen leakage

The possibilities of the formation of a flammable cloud over the ground in an open atmosphere from the leakage of hydrogen stored at different temperatures are studied. The dispersion of hydrogen in the stable and unstable atmospheric conditions is determined using the Gaussian dispersion model. The efflux of hydrogen from the storage vessel is considered at velocities between 1 m/s and 1500 m/s, the latter corresponding to the upper limit of velocities arising from the choked flow. The dispersion analysis shows that flammable hydrogen–air clouds would not be formed over the ground under unstable atmospheric conditions for all efflux velocities and leakage areas and for the different temperatures of the hydrogen leak. However, under strongly stable atmospheric conditions, such as those associated with clear sky winter nights with low winds and temperature inversion in the planetary boundary layer, a flammable cloud is seen to be formed. This is particularly true for low temperature hydrogen efflux and very low velocities of the efflux.     

Effects of hydrogen concentration on premixed laminar flames of hydrogen–methane–air

The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogen–methane–air flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.     

Duct-vented hydrogen–air deflagrations: The effect of duct length and hydrogen concentration

Experiments on duct-vented explosions of hydrogen–air mixtures in a 12.3 l cylindrical vessel were conducted, and the effects of duct length and hydrogen concentration on the maximum overpressure and flame behavior within and outside the vented enclosure were investigated. The results show that the maximum overpressure in the vessel first increased and then was maintained nearly unchanged with the length of a relief duct increasing to 2 m. For a given duct length, the maximum overpressure first increased and then decreased when hydrogen concentration increased from 20% to 55%. The burn-up in the duct caused the gas mixtures to move in reverse from the duct to vessel, which consequently decreased the venting efficiency. A pressure wave caused by burn-up in the duct was observed, which resulted in a pressure peak in the external pressure–time histories after it traveled outside the duct. The maximum external overpressure first increased and then decreased with an increase in duct length. For a given duct length, the maximum external overpressure increased with an increase in hydrogen concentration.     

Effect of hydrogen concentration on vented explosion overpressures from lean hydrogen–air deflagrations

Experimental data from vented explosion tests using lean hydrogen–air mixtures with concentrations from 12 to 19% vol. are presented. A 63.7-m³ chamber was used for the tests with a vent size of either 2.7 or 5.4 m². The tests were focused on the effect of hydrogen concentration, ignition location, vent size, and obstacles on the pressure development of a propagating flame in a vented enclosure. The dependence of the maximum pressure generated on the experimental parameters was analyzed. It was confirmed that the pressure maxima are caused by pressure transients controlled by the interplay of the maximum flame area, the burning velocity, and the overpressure generated outside of the chamber by an external explosion. A model proposed earlier to estimate the maximum pressure for each of the main pressure transients was evaluated for the various hydrogen concentrations. The effect of the Lewis number on the vented explosion overpressure is discussed.     

Vented hydrogen–air deflagration in a small enclosed volume

Since the rapid development of hydrogen stationary and vehicle fuel cells the last decade, it is of importance to improve the prediction of overpressure generated during an accidental explosion which could occur in a confined part of the system. To this end, small-scale vented hydrogen–air explosions were performed in a transparent cubic enclosure with a volume of 3375 cm³. The flame propagation was followed with a high speed camera and the overpressure inside the enclosure was recorded using high frequency piezoelectric transmitters. The effects of vent area and ignition location on the amplitude of pressure peaks in the enclosed volume were investigated. Indeed, vented deflagration generates several pressures peaks according to the configuration and each peak can be the dominating pressure. The parametric study concerned three ignition locations and five square vent sizes.     

Experimental study on premixed hydrogen/air and hydrogen–methane/air mixtures explosion in 90 degree bend pipeline

Nowadays, hydrogen is being utilized massively in industries as a clean fuel. Displacing of hydrogen due to unique chemical and physical properties has adversely affect on pipeline network, hence increases the potential risk of explosion. This study was carried out to determine the flame propagation of hydrogen/air and hydrogen–methane/air mixtures in pipeline. A 90° pipeline with L/D ratio of 40 was used. Pure hydrogen/air mixture with equivalence ratio, φ = 0.13, 0.17, 0.2, 0.24, 0.27 and 0.30 were used in this work. Different composition of hydrogen–methane–air mixtures were tested in this study i.e. 3%H₂ + 97CH₄, 4%H₂ + 96CH₄, 6%H₂ + 94CH₄ and 8%H₂ + 92CH₄. All mixtures were operated at ambient condition. The results show that bending is the critical part of pipeline and higher concentration of hydrogen can affect on maximum overpressure, flame speed and temperature rise of both pure hydrogen/air and methane-hydrogen/air mixtures.     

Effects of hydrocarbon substitution on atmospheric hydrogen–air flame propagation

In order to evaluate the potential of partial hydrocarbon substitution to improve the safety of hydrogen use in general and the performance of internal combustion engines in particular, the outward propagation and development of surface cellular instability of spark-ignited spherical premixed flames of mixtures of hydrogen, hydrocarbon, and air were experimentally studied at NTP condition in a constant-pressure combustion chamber. With methane, ethylene, and propane being the substituents, the laminar burning velocities, the Markstein lengths, and the propensity of cell formation were experimentally determined, while the laminar burning velocities and the associated flame thicknesses were computed using recent kinetic mechanisms. Results show substantial reduction of laminar burning velocities with hydrocarbon substitution, and support the potential of propane as a suppressant of both diffusional–thermal and hydrodynamic cellular instabilities in hydrogen–air flames. Such a potential, however, was not found for methane and ethylene as substituents.     

Analysis of entropy generation in counter-flow premixed hydrogen–air combustion

In this paper, entropy generation in counter-flow premixed hydrogen–air combustion confined by planar opposing jets is investigated for the first time. The effects of the equivalence ratio and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for reactive flows, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the equivalence ratio and the inlet Reynolds number for all the cases under the present study.     

A mechanistic study of Soret diffusion in hydrogen–air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the structure and propagation speed of the freely-propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen–air mixtures using a detailed reaction mechanism and transport properties. Results show that, except for the conservative freely-propagating planar flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and as such modifies the mixture stoichiometry and flame temperature, which could lead to substantial increase (decrease) of the flame speed for the lean (rich) mixtures respectively. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates of the symmetric, twin, counterflow premixed flames, especially at near-extinction states, can be increased for lean flames but decreased for rich flames, whose active reaction regions are respectively located at, and away from, the stagnation surface. However, such a difference is eliminated for the single counterflow flame stabilized by an opposing cold nitrogen stream, as the active reaction zone up to the state of extinction is always located away from the stagnation surface. Finally, the reaction rate is increased in general for diffusion flames because the bell-shaped temperature distribution localizes the H concentration to the reaction region which has the maximum temperature.     

Explosion characteristics of hydrogen–nitrogen–air mixtures at elevated pressures and temperatures

An experimental study on the combustion characteristics of nitrogen diluted hydrogen was conducted in a constant volume combustion vessel over a wide range of equivalence ratios and dilution ratios at elevated pressures and temperatures. The explosion characteristics such as the explosion pressure, the combustion duration, the maximum rate of pressure rise, the deflagration index and the normalized mass burning rate were derived. The result shows that a short combustion duration and higher normalized mass burning rate were presented with the increase of equivalence ratio. With the increase of initial temperature, the explosion pressure, the maximum rate of pressure rise and the deflagration index were decreased, and a shorter combustion duration and higher normalized mass burning rate were presented. With the increase of initial pressure, the explosion pressure, the maximum rate of pressure rise and the deflagration index increase, a shorter combustion duration and higher normalized mass burning rate were presented. Nitrogen dilution significantly reduces the normalized mass burning rate and the deflagration index and thus the potential of explosion hazards.     

Flame propagation in stratified hydrogen–air mixtures: Spark placement effects

Two-dimensional laminar flame simulations of a forced-ignition event in an initially quiescent mixing layer of hydrogen and air have been carried out at atmospheric pressure using detailed chemistry and effective binary diffusion coefficients. Since control of the ignition location is known to be critical in direct-injection spark-ignition engines, this study primarily investigates the effect of initial spark placement within the flammability limits of hydrogen–air. Displacement and stabilization speeds of the propagating flame fronts have been computed along isocontours of water vapor representing 10% and 25% of the downstream equilibrium concentration. Following the period of spark energy addition the flame kernel is observed to develop into tribrachial flames that subsequently propagate along the stoichiometric line. For all cases of successful ignition, transient spark effects are observed to dissipate within 0.18 ms. Subsequent structure and propagation speed of the flame are not influenced by the transient development phase.     

Numerical investigations of the restabilization of hydrogen–air rotating detonation engines

Based on 3D numerical simulations, the restabilization of hydrogen–air rotating detonation engines (RDEs) from one stable state to another after the operating conditions are changed is investigated. After a sudden change is imposed on the injection stagnation pressure, the transition process is clarified and the transition time, needed by the RDE to stabilize at a new state, is calculated. It is found that the sudden change of the stagnation pressure has an immediate influence on the average axial velocity at the head end of the RDE, which increases abruptly and instantly with the sudden rise of the stagnation pressure. After that, the average axial velocity drops and the average pressure increases gradually at the head end until they reach a new stable state. The average pressure has a bounce and the average axial velocity fluctuates at the head end in the transition process of the sudden decrease of the stagnation pressure. The total transition time increases with the variation range of the stagnation pressure. However, the initial adjusting time is independent of the variation range of the stagnation pressure and it is about twice the cycle period of the detonation wave around the chamber, demonstrating the high stability of the RDE.     

Measurements of the laminar burning velocity of hydrogen–air premixed flames

Experimental and numerical studies on laminar burning velocities of hydrogen–air mixtures were performed at standard pressure and room temperature varying the equivalence ratio from 0.8 to 3.0. The flames were generated using a contoured slot-type nozzle burner (4 mm × 10 mm). Measurements of laminar burning velocity were conducted using particle tracking velocimetry (PTV) combined with Schlieren photography. This technique provides the information of instantaneous local burning velocities in the whole region of the flame front, and laminar burning velocities were determined using the mean value of local burning velocities in the region of non-stretch. Additionally, average laminar burning velocities were determined using the angle method and compared with the data obtained with the PTV method. Numerical calculations were also conducted using detailed reaction mechanisms and transport properties.     

Combustion of hydrogen–air in catalytic micro-combustors made of different material

Micro-combustors have low stability, thus catalyst is applied to improve it. In this experiment, the performances of catalytic micro-combustors made of different materials (quartz glass, alumina ceramic, copper) are compared. Asbestine threads are used as the catalyst supports of Pt, and installed in the combustors. According to the experimental results, the combustors have high stability, they keep working until the extreme equivalence ratio close to 0. The stability limits of homogeneous reaction in the quartz glass and alumina ceramic combustor range from 0.0907 to 8.69 and 0.158 to 7.31 on average, respectively. But the two combustors exhibit obvious hot spots, which are 1058 and 728 K at 0.2 L/min, respectively. Whereas the copper combustor has low and uniform temperature distribution on its surface. Moreover, the heat loss in the quartz glass combustor is 4.13 W higher than in the copper one at 0.2 L/min, which is opposite to the conventional situation that heat loss increases with the wall thermal conductivity. Computational fluid dynamic simulation reveals that the reaction modes inside the combustors differ. The higher wall thermal conductivity makes the heterogeneous reaction dominate, thus induces the temperature distribution and heat loss aforementioned.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.