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.     

An experimental study on performance and emission characteristics of a methane–hydrogen fuelled gasoline engine

In this paper, the performance and emission characteristics of a conventional twin-cylinder, four stroke, spark-ignited (SI) engine that is running with methane–hydrogen blends have been investigated experimentally. The engine was modified to realize hydrogen port injection by installing hydrogen feeding line in the intake manifolds. The experimental results have been demonstrated that the brake specific fuel consumption (BSFC) increased with the increase of hydrogen fraction in fuel blends at low speeds. On the other hand, as hydrogen percentage in the mixture increased, BSFC values decreased at high speeds. Furthermore, brake thermal efficiencies were found to decrease with the increase in percentage of hydrogen added. In addition, it has been found that CO₂, NO_x and HC emissions decrease with increasing hydrogen. However, CO emissions tended to increase with the addition of hydrogen generally increase. It has been showed that hydrogen is a very good choice as a gasoline engine fuel. The data are also very useful for operational changes needed to optimize the hydrogen fuelled SI engine design.     

An experimental study of combustion characteristics of hydrogen–Air pre-mixture on the wall boundary condition

By means of the Schlieren method, the combustion characteristics of hydrogen–air pre-mixture on wall boundary conditions have been studied in a constant volume combustion cell with ignition point located in the upper part of the cell. The results indicated that the restrictive combustion process of the pre-mixture can be divided into four stages: the laminar flame stage, the cellular flame stage, the squish flame stage and the auto-ignition stage. The squish flames appear on both sides of the main flame under most of the experimental conditions. The appearance of squish flames can cause a fluctuation in pressure change rate. The lower the equivalence ratio is, the earlier squish flames appear spatially and the less the auto-ignition is prone to present itself. The lower the initial pressure is, the more difficultly the squish flames present themselves. As the initial pressure drops, the combustion tends toward directly enter into the auto-ignition stage from the laminar flame stage. The higher the initial pressure is, the earlier the auto-ignition presents itself spatially. Squish flames experience two processes of formation and spread. The restrictive effect of the wall could be a factor that impacts on squish flames.     

Experimental study on a spark ignition engine fuelled by methane–hydrogen mixtures

In this study, effects on a spark ignition engine of mixtures of hydrogen and methane have been experimentally considered. This article presents the results of a four-cylinder engine test with mixtures of hydrogen in methane of 0, 10, 20 and 30% by volume. Experiments have been made varying the equivalence ratio. Equivalence ratios have been selected from 0.6 to 1.2. Each fuel has been investigated at 2000 rpm and constant load conditions. The result shows that NO emissions increase, HC, CO and _CO2${\mathrm{CO}}_2$ emission values decrease and brake thermal efficiency (BTE) values increase with increasing hydrogen percentage.     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.     

Numerical study on combustion characteristics of hydrogen addition into methane–air mixture

Understanding of the chemical kinetics and heat transfer mechanism within micro-combustors is essential for the development of stable-combustion technology. Computational Fluid Dynamics (CFD) based numerical simulation has been proven to be an effective approach to analyze the performance of combustion under various conditions. The objective of this paper is to study hydrogen-assisted catalytic combustion of methane. It is proved that methane conversion rate decreases as the inlet velocity increases. The most suitable inlet velocity was 0.2 m/s, while the inlet temperature was 900 K. The ignition temperature will decrease considerably when hydrogen content of the fuel was increased with a fixed value of equivalent ratio, meanwhile, the moment of the ignition temperature advances and methane conversion rate also rises accordingly. This is useful for optimization micro combustion fuel.     

Numerical study of the effect of hydrogen addition on methane–air mixtures combustion

The stoichiometric methane–hydrogen–air freely propagated laminar premixed flames at normal temperature and pressure were calculated by using PREMIX code of CHEMKIN II program with GRI-Mech 3.0 mechanism. The mole fraction profiles and the rate of production of the dominant reactions contributing to the major species and some selected intermediate species in the flames of methane–hydrogen–air were obtained. The rate of production analysis was conducted and the effect of hydrogen addition on the reactions of methane–air mixtures combustion was analyzed by the dominant elementary reactions for specific species. The results showed that the mole fractions of major species CH₄, CO and CO₂ were decreased while their normalized values were increased as hydrogen is added. The rate of production of the dominant reactions contributing to CH₄, CO and CO₂ shows a remarkable increase as hydrogen is added. The role of H₂ in the flame will change from an intermediate species to a reactant when hydrogen fraction in the blends exceeds 20%. The enhancement of combustion with hydrogen addition can be ascribed to the significant increase of H, O and OH in the flame as hydrogen is presented. The decrease of the mole fractions of CH₂O and CH₃CHO with hydrogen addition suggests a potential in the reduction of aldehydes emissions of methane combustion as hydrogen is added. The methane oxidation reaction pathways will move toward the lower carbon reaction pathways when hydrogen is available and this has the potential in reducing the soot formation. Chemical kinetics effect of hydrogen addition has a little influence on NO formation for methane combustion with hydrogen addition.     

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.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Effect of hydrogen on hydrogen–methane turbulent non-premixed flame under MILD condition

Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H₂/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H₂+95%CH₄, 10%H₂+90%CH₄ and 20% H₂+80%CH₄(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release.     

A study on flame interaction between methane/air and nitrogen-diluted hydrogen–air premixed flames

Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H₂ + 70% N₂. Extinction limits are determined for methane/air and (30% H₂ + 70% N₂)/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures’ concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed.     

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.     

Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH₄–7% H₂, 80% CH₄–20% H₂ and 70% CH₄–30% H₂ by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.     

Effects of hydrogen addition on oblique detonations in methane–air mixtures

As a low-carbon fuel, methane has been used in various engines; however, the studies on its application in hypersonic propulsion are few. Here, oblique detonation waves (ODWs) in methane–air mixtures have been simulated to facilitate methane applications in shock-induced combustion ramjets. The shortcoming of using methane in hypersonic air-breathing propulsions has been presented via examining initiation distance of ODWs. Results demonstrate that ODWs are difficult to be initiated in the methane–air mixture, and similar to normal detonations studies before, this leads to a long initiation length; therefore, methane-fueled ODW is only applicable for high flight Mach number (M₀). To broaden the M₀ regime, hydrogen has been added to methane to decrease the initiation length. An increasing in the hydrogen percentage leads to the nonlinear decrease of the initiation length, and the initiation structures also vary simultaneously. To elaborate the physical mechanism of the initiation length variation, a theoretical model of the initiation length for fuel blends has been proposed. Meanwhile, the advantages of methane fuel in ODW-based propulsion have been discussed by analyzing on the effects of hydrogen addition on the total pressure.     

An experimental study of fuel–air mixing section on unstable combustion in a dump combustor

The main objective of this study is effect of the various fuel–air mixing section geometries on the unstable combustion. For the purpose of observing the combustion pressure oscillation and phase difference at each of the dynamic pressure results, the multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section. By using an optically accessible quartz-type combustor, we were able to OH* measurements to characterize the flame structure and heat release oscillation with the use of a high-speed ICCD camera. In this study, we observed two dominant instability frequencies. Lower frequencies were measured around 240–380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were measured around 410–830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter-wave mode in the inlet mixing section. These second instability mode characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling of the combustor and inlet mixing section geometry.     

Experimental study on explosion characteristics of hydrogen–propane mixtures

The explosion characteristics of a hydrogen–propane mixture under different initial temperatures and at different hydrogen addition, equivalence, and dilution ratios were studied using a 20-L apparatus. First, the effects of hydrogen addition and equivalence ratio were studied. The results showed that the maximum explosion pressure and the maximum rate of pressure rise first increased, then decreased from lean to an equivalence ratio of 1.6, and reached the maximum value at the equivalence ratio of 1.2. The maximum explosion pressure and the maximum rate of pressure rise increased with the increasing hydrogen proportion of the mixtures, and the time to the maximum explosion pressure decreased. Furthermore, the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, was explored under nitrogen or carbon dioxide dilution. Carbon dioxide exhibited a stronger dilution effect than nitrogen. For the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, the maximum explosion pressure saliently decreased with the increasing initial temperature in the range of 25 °C-120 °C. The variation trend of experimental results was consistent with theoretical predictions.     

Parametric study on combustion characteristics of virtual HCCI engine fueled with methane–hydrogen blends under low load conditions

Homogeneous charge compression ignition (HCCI) is an alternative combustion strategy employed for automotive systems. It has a higher thermal efficiency with lower nitric oxides and particulate matter emissions that are below current emission requirements. However, owing to difficulties associated with combustion control, HCCI engines have disadvantages in terms of combustion instability, such as low-speed-low-load or high-speed-high-load conditions.This study investigates the effects of different parameters on HCCI engine combustion using numerical methods. The parametric study is carried out at low loads (25% part load), and a reference intake temperature of 550 K is used to preheat the air–fuel mixture. The GRI-3.0 chemical reaction mechanism involving 53 species and 325 reactions is used for 1-D simulations describing the combustion process fueled with methane and hydrogen added methane. By changing the variables, including compression ratio, excess air ratio, and hydrogen content, the combustion behavior is investigated and discussed. The results show that an increase in compression ratio resulted in a faster start of combustion and caused higher in cylinder pressure and heat-release rate. When the excess air ratio was increased, the start of combustion was delayed and lower in-cylinder pressure and heat release rate were observed. The results were similar for varying compression ratios.     

An experimental study on energy generation with a photovoltaic (PV)–solar thermal hybrid system

A hybrid system, composed of a photovoltaic (PV) module and a solar thermal collector is constructed and tested for energy collection at a geographic location of Cyprus. Normally, it is required to install a PV system occupying an area of about 10 m² in order to produce electrical energy; 7 kWh/day, required by a typical household. In this experimental study, we used only two PV modules of area approximately 0.6 m² (i.e., 1.3×0.47 m²) each. PV modules absorb a considerable amount of solar radiation that generate undesirable heat. This thermal energy, however, may be utilized in water pre-heating applications. The proposed hybrid system produces about 2.8 kWh thermal energy daily. Various attachments that are placed over the hybrid modules lead to a total of 11.5% loss in electrical energy generation. This loss, however, represents only 1% of the 7 kWh energy that is consumed by a typical household in northern Cyprus. The pay-back period for the modification is less than 2 years. The low investment cost and the relatively short pay-back period make this hybrid system economically attractive.