Lean Hydrogen-air Mixtures Research Articles

Pathway dynamics to double-cell premixed flames in lean hydrogen–air mixtures

The propagation of premixed flames over lean hydrogen–air mixtures have been found to be bistable in recent studies, considering slender channels with non-negligible conductive heat losses. In particular, two stable configurations conformed by either a circular or a double-cell flame front arise for the same combination of controlling parameters (fuel mixture, channel size and thermal conductivity). Nevertheless, this multiplicity of solutions lacks a satisfactory explanation to predict the formation of either one structure or the other during a particular ignition transient. In this work, detailed analyses are performed over the unsteady evolution of a set of numerical simulations. Specifically, the initial temperature profiles (distribution and peak value) and subsequent expansion of the flow field prescribe the early growth of the flame front leading to different curvatures and sizes of the kernel that control the evolution into each of the canonical structures. This study explores the underlying physics controlling the final-stage bistable behavior. In particular, the local convective effects and interactions between fronts have been identified as the key causes to produce each of the two stably propagating flames. Frequently found division of kernels cannot ensure the formation of double cells, which require additional reorientation dynamics and merging events due to asymmetric seeding of flame fragments or to interaction with neighboring structures.

Are differential diffusion effects of importance when burning hydrogen under elevated pressures and temperatures?

A ratio of turbulent burning velocity to laminar flame speed is well known to be abnormally high in lean hydrogen-air mixtures, with this phenomenon being commonly attributed to differential diffusion effects. Magnitude of such effects is known to be increased by pressure, but a few recent studies have indicated that the effects are mitigated when reactants are preheated. It is not yet known, however, which of these two counteracting trends is of more importance under elevated pressures and temperatures associated with combustion in engines. Accordingly, it is not yet clear whether or not models of differential diffusion effects are required for research and development of future ultra-clean and highly efficient engines that burn hydrogen (the emphasized effects are typically ignored in engineering computations). To clarify the issue, numerical simulations of lean complex-chemistry hydrogen-air strained laminar flames are performed by varying strain rate, pressure 1≤P≤50 bar, temperature 300≤Tu≤900 K, and equivalence ratio (Φ=0.4, 0.55, and 0.7). This simple problem is selected because maximal consumption speeds reached in critically strained (close to extinction) lean hydrogen-air laminar flames are considered to characterize the influence of differential diffusion on turbulent burning rates within the framework of Zel'dovich's leading point concept, which was well supported in recent studies. Computed results show that, even at Tu=900 K, the aforementioned consumption speeds are significantly larger than the unperturbed laminar flame speeds if the mixture is sufficiently lean (the equivalence ratio Φ=0.55 or lower) and pressure is sufficiently high (P≥30 bar). Therefore, differential diffusion effects are expected to be of importance when burning so lean hydrogen-air mixtures in engines and should be properly modeled.

Numerical Simulation of the Effect of Additives on Autoignition of Lean Hydrogen–Air Mixtures

Numerical Simulation of the Effect of Additives on Autoignition of Lean Hydrogen–Air Mixtures

Confined Boundary-Layer Flashback Flame Dynamics in a Turbulent Swirling Flow

Understanding boundary-layer flashback is critical to the design of safe and efficient gas turbines, especially as the addition of highly reactive hydrogen to these devices becomes a prevailing trend for reduction of carbon dioxide emissions. In this work, the boundary-layer flashback of lean hydrogen–air mixtures is studied using large-eddy simulations based on a generic turbulent swirl burner previously investigated experimentally. Simulations at increasing equivalence ratios are used to identify the flashback limits, showing reasonable qualitative agreement with experimental measurements. Flame dynamics during flashback are compared to previous experimental studies, indicating that important physics are captured in the simulations even if the exact flashback limits are not. In particular, a change in the swirl vane angle is shown to dramatically change the flame dynamics, with flame propagation occurring in the direction of swirl at high angles and against the direction of swirl at low angles, consistent with experimental observations. The near-wall nonreacting mean velocity field is shown to be controlled by a temporally stationary flow instability, which is directly responsible for the counterswirl flame flashback exhibited at low swirl angles. Finally, the interaction of local axial flow velocity and flame propagation speed of the leading flame point is investigated with varying swirl angles both before and after the onset of flashback, elucidating the differences in flame dynamics in all four of these cases. In particular, the importance of local flame extinction on flashback limits is emphasized.

Minimum ignition energy of hydrogen-air mixtures at ambient and cryogenic temperatures

The ignition and combustion of hydrogen in air is considered more hazardous compared to other fuels due to the lower minimum ignition energy (MIE) and the wider flammability range. Spark discharge is the most common type of electrostatic ignition hazard. There is a need in validated safety engineering tools to accurately calculate MIE in a wide range of temperatures from atmospheric to cryogenic which are characteristic for hydrogen systems and infrastructure. Current MIE assessment methodologies rely on the availability of experimental data on quenching distance and/or laminar burning velocity and thus are mostly empirical correlations. This prevents their application beyond the limited number of experimental data, i.e. to arbitrary composition of the hydrogen-air mixture at arbitrary temperatures including cryogenic. This work aims at the development of a model able to accurately predict MIE for hydrogen-air mixtures with arbitrary initial composition and temperature. Cantera and Chemkin software are used to calculate the properties and unstretched laminar burning velocity of hydrogen-air mixtures. The flame thickness is found to well represent the critical flame kernel in the suggested model. The model is validated against experimental data on MIE for mixtures at ambient and cryogenic (down to 123 K) temperatures. Results show that the effect of flame stretch and preferential diffusion shall be considered to accurately predict MIE for lean hydrogen-air mixtures, which was not possible for previous models.

Simple criterion of importance of laminar flame instabilities in premixed turbulent combustion of mixtures characterized by low Lewis numbers.

By (i) highlighting the mitigation effect of strain rates on laminar flame instabilities and (ii) comparing peak growth rates of laminar flame instabilities with strain rates generated by small-scale turbulent eddies, a simple criterion of importance of the influence of the instabilities on an increase in premixed flame surface area in turbulent flows is suggested. The criterion implies that, even in lean hydrogen-air mixtures, laminar flame instabilities can significantly affect the flame area only in weak or moderate turbulence (the Karlovitz number defined using laminar flame speed, thermal flame thickness, and Kolmogorov time scale is on the order of 10 or less under room conditions).

Effect of swirler vane angle on the combustion characteristics of premixed lean hydrogen-air mixture in a swirl micro-combustor

This study analyses the influence of swirler vane angle (β) on the combustion characteristics of premixed lean hydrogen-air flame in a swirl micro-combustor via numerical simulations. The results show that lower flammability limit (LFL) under β = 15° is larger than that of β = 0°; and β = 30°, 45° and 60° have very little difference in the LFL, and their LFL are smaller than that of β = 0° Flame anchoring mainly depends on the corner recirculation zone (CRZ) when β is 0° and 15°, and when β exceeds 30°, flame anchoring is chiefly determined by the inner recirculation zone (IRZ). The flame stabilization ability of IRZ is stronger than that of CRZ, but excessive swirling intensity will increase the length of IRZ, which in turn results in a decrease in stabilization ability. Increasing β improves outer wall temperature when inlet velocity is relatively small. Under a larger inlet velocity, an enormous β reduces combustor thermal performance and combustion efficiency due to the downstream movement of flame root and reaction zone. This work helps us to better understand the swirling flame characteristics in micro-combustor and provides guidance for the design and optimization of swirl micro-combustor.

Numerical Simulation of Autoignition Characteristics of Lean Hydrogen–Air Mixtures

Numerical Simulation of Autoignition Characteristics of Lean Hydrogen–Air Mixtures

Suppression of hydrogen-air explosions by isobutene with special molecular structure

Effects of isobutene (i-C4H8) with the unique molecular structure on hydrogen explosions are investigated in a 14 L spherical chamber at 100 kPa and 298 K. The results reveal that for lean hydrogen-air mixtures, the maximum explosion overpressure, maximum pressure rise rate, and deflagration index show a trend from rise to decline. However, for stoichiometric and rich hydrogen-air mixtures, pressure parameters decrease monotonously with increasing i-C4H8. The chemical suppression mechanism of i-C4H8 on hydrogen explosions is revealed in detail by numerical simulation. The relevant analyses show that i-C4H8 can not only change the chain reaction path and cut off the hydrogen-oxygen reaction, but also strengthen the consumption of oxygen and H radicals. This work will provide a novel theoretical foundation for hydrogen explosion prevention and control.

Performance Estimation of a Downsized SI Engine Running with Hydrogen

Hydrogen is a carbon-free fuel that can be produced in many ways starting from different sources. Its use as a fuel in internal combustion engines could be a method of significantly reducing their environmental impact. In spark-ignition (SI) engines, lean hydrogen–air mixtures can be burnt. When a gaseous fuel like hydrogen is port-injected in an SI engine, working with lean mixtures, supercharging becomes very useful in order not to excessively penalize the engine performance. In this work, the performance of a turbocharged PFI spark-ignition engine fueled by hydrogen has been investigated by means of 1-D numerical simulations. The analysis focused on the engine behavior both at full and partial load considering low and medium engine speeds (1500 and 3000 rpm). Equivalence ratios higher than 0.35 have been considered in order to ensure acceptable cycle-to-cycle variations. The constraints that ensure the safety of engine components have also been respected. The results of the analysis provide a guideline able to set up the load control strategy of a SI hydrogen engine based on the variation of the air to fuel ratio, boost pressure, and throttle opening. Furthermore, performance and efficiency of the hydrogen engine have been compared to those of the base gasoline engine. At 1500 and 3000 rpm, except for very low loads, the hydrogen engine load can be regulated by properly combining the equivalence ratio and the boost pressure. At 3000 rpm, the gasoline engine maximum power is not reached but, for each engine load, lean burning allows the hydrogen engine achieving much higher efficiencies than those of the gasoline engine. At full load, the maximum power output decreases from 120 kW to about 97 kW, but the engine efficiency of the hydrogen engine is higher than that of the gasoline one for each full load operating point.

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

AbstractThe premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two‐dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.

Structure and Dynamics of the Combustion Front of a Lean Hydrogen-Air Mixture in a Flow-Through Reactor

Structure and Dynamics of the Combustion Front of a Lean Hydrogen-Air Mixture in a Flow-Through Reactor

FlameFoam: An open source CFD solver for turbulent premixed combustion

Severe accidents in nuclear power plants may cause significant and long-lasting harm to the environment in the case of containment integrity loss. Numerical simulation of phenomena threatening containment integrity, e.g., hydrogen combustion and explosion, is essential for the effective severe accident management.International research projects and benchmarks are organized in order to promote improvement of simulation codes adequacy and accuracy in the severe accident relevant conditions. Results of the most recent benchmarks show that current state-of-the-art simulations were able to mostly reproduce the observable flame speed at the considered conditions, but there are still inadequacies in estimating velocity and its maximum value.Addressing a need for an accessible open source turbulent premixed combustion simulation tool, a new solver is being developed. An OpenFOAM version 7 based combustion solver called flameFoam has been built by implementing a progress variable approach and turbulent flame closure model for premixed combustion simulation. Turbulent flame speed is defined as a function of laminar flame speed and turbulence parameters, and is evaluated using one of three different correlations. Laminar flame speed can be set as a constant or calculated using a laminar flame speed correlation. The basis for the solver are standard compressible OpenFOAM solvers rhoPimpleFoam, buoyantPimpleFoam and chtMultiRegionFoam.The article presents initial public flameFoam version 0.8 and provides examples of its validation. Initial validation and verification of general solver features are represented by the simulations of shock tube and backward facing step cases. Solver capabilities in relation to the current turbulent premixed combustion modeling state-of-the-art are demonstrated with the simulations of ETSON-MITHYGENE benchmark experiments.It is shown that flameFoam version 0.8 is capable of adequate simulation of general flow features, turbulent compressible flow phenomena and turbulent premixed combustion of lean hydrogen-air mixtures at the considered conditions. Solver code is publicly hosted and being developed further.

Experimental and theoretical analysis of carbon driven detonation waves in a heterogeneously premixed Rotating Detonation Engine

Coal dust explosions can be hazardous; however, they can also generate a significant rise in stagnation pressure if adequately harnessed. Rotating detonation combustors seek to take advantage of the stagnation pressure rise phenomenon in a more sustained and controlled manner via confinement to a physical annulus, leading to increased overall thermodynamic efficiency. This investigation presents an analysis of detonations fueled by Carbon Black, a solid particulate consisting of virtually pure carbon molecules and lean Hydrogen-Air mixtures. It is realized that with the addition of Carbon Black, an increase of lean mixture detonability and detonation velocities extending the operating limit over that of a pure hydrogen-air mixture is experienced. For all testing conditions, the total equivalence ratio is held at φ = 1, while the fuel mixture's carbon mass fraction is increased from 0 to 0.7 while the hydrogen is decreased. Detonation wave velocities are extracted from high-speed imaging through applying a Discrete Fourier Transform algorithm to determine changes to the wave speed as Carbon Black particles are introduced. As a result, due to the addition of Carbon Black as an auxiliary fuel source, detonations were formed instead of deflagrations in operating conditions where one would expect deflagrations at the same hydrogen-air equivalence ratios without Carbon Black addition. The detonation formation provides evidence that the coal particles are reacting within the detonation wave in a large enough capacity to support a detonation wave within the annulus. Furthermore, the wave speed is shown to increase with the additional of carbon particles. At a constant global equivalence ratio, the detonation wave velocities were found to decrease with hydrogen's incremental replacement with coal particles. Whereby, through a theoretical comparison of the heat of combustion as computed from the experimentally derived detonation wave velocities, a linear relationship of the two was shown to exist. Therefore, the heat of combustion has the potential to describe an operational limit to sustaining a detonation wave.

Experimental study of lean hydrogen-air mixture combustion in a 12m3 tank

Hydrogen can be released to the nuclear power plant containment atmosphere during a reactor core melt accident, and the potential hydrogen explosion need to be mitigated by all efforts. One of the current hydrogen management methods in pressurized water reactors (PWR) is to prevent detonation with slow combustion using ignitor as designed in AP1000. A HYdrogen MIddle scale Test facility (HYMIT) was setup to study the hydrogen behavior and mitigation efficiency of ignitor. The main component of HYMIT is a steel cylinder tank whose height is 4 m and diameter is 2 m, and the tank volume is about 12 m3. The mixed gas content, gas temperature and pressure, and arrival of flame can be measured with gas analyzer, thermocouple array, piezoelectric pressure sensors, and photodiode array. The current paper studied combustion behavior of hydrogen-air homogeneous mixture in various hydrogen volume ratio up to 10% at different ignition positions, the results showed that for tests with less than 10% hydrogen concentration the hydrogen consumption ratio increased when ignition location descending from top to bottom, and the maximum pressure and temperature increased accordingly, and for 10% hydrogen concentration tests the consumption ratio, the maximum pressure and temperature were not affected by the ignition location,. The apparent flame velocities were estimated with both photodiode sensors and thermal couples and they were in good agreement, the maximum upward apparent flame velocity measured was 2.27 m/s. A minimum actual radius was recommended to estimate the initial quasi-laminar flame velocity using pressure measurement.

Turbulence–Combustion Interactions in Premixed and Non-premixed Flames Generated by Hot Active Turbulent Jets

Direct numerical simulations of turbulent jet ignition (TJI)-assisted combustion of lean hydrogen–air mixtures are performed in a three-dimensional planar jet configuration for various thermo-chemical conditions. TJI is a novel ignition enhancement method which facilitates the combustion of lean and ultra-lean fuel–air mixtures by rapidly and continuously exposing them to high temperature combustion products. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen–air reaction is simulated by a detailed chemical kinetics mechanism. Turbulence–combustion interactions in TJI systems are studied here for different conditions using the flame heat release, temperature, species concentrations, and a newly defined progress variable. Important phenomena such as localized flame extinction/re-ignition and simultaneous existence of premixed/non-premixed flames in TJI-assisted combustion are also investigated.

Self-similar propagation of spherically expanding flames in lean hydrogen–air mixtures

The self-similar propagation of expanding spherical flames in lean hydrogen–air mixtures at elevated pressure was experimentally investigated using a dual-chamber apparatus with the quartz windows that enabled observing a flame radius of up to 110 mm. The flame images of the fully developed cellular structure were recorded, and the largest values of r/rc and Pe for the dual-chamber apparatus were estimated. The acceleration exponent, α, corresponding to the fractal dimension in lean hydrogen–air mixtures at elevated pressure was evaluated by plotting the experimental flame radius as a function of time. In this study, the experimental values of α (α = 1.25–1.43) occur within a wide range of r/rc (r/rc = 2–15). The observed trend of α to increase with a decrease in equivalence ratio, φ, and an increase in initial pressure, Pi, is owing to the effects of the diffusional–thermal and Darrieus–Landau instabilities. The value of α increased with an increase in r/rc and saturated at α = 1.43 at r/rc > 10, and the saturated values of the hydrogen–air flame are within α = 1.4–1.5. The results demonstrated that the flame propagation in lean hydrogen–air mixtures was classified as laminar (r/rc < 1), acceleration (r/rc > 1), transition (1 < r/rc < 10), and self-similar (r/rc > 10) regimes. Consequently, the practical three-dimensional fractal dimensions for accidental hydrogen–air explosions are D3 = 2.24–2.33.

Richtmyer-Meshkov Instability of Laminar Flame

A slow laminar flame which is a thin (less than 1 mm thick) gas layer separating media of different densities is proposed as the interface between gaseous media for laboratory studies of Richtmyer-Meshkov and Rayleigh-Taylor instabilities. The potential of the proposed approach to produce the interface is shown by the example of shock wave interaction with a laminar flame in a lean hydrogen-air mixture (6 vol.% hydrogen). The development of Richtmyer-Meshkov instability at the interface between a heavy (cold) mixture and a light (hot) mixture was recorded by shadowgraphy.

Effects of hydrogen concentration, non-homogenous mixtures and obstacles on vented deflagrations of hydrogen-air mixtures in a 27 m3 chamber

Experimental data from vented hydrogen deflagrations with concentrations from 15% to 21% are presented. The experiments were performed in a 27 m3 cubic chamber with a vent area of either 0.78 or 1.76 m2. Uniform and stratified mixtures were used in the test matrix, and the obstacle configuration also varied to achieve different volumetric blockage ratios. The purpose of this study was to investigate the effect of hydrogen concentration, vent area, non-homogeneous mixtures and obstacles on the explosion venting for lean hydrogen-air mixtures, and to evaluate the performance of an analytical model for calculating the maximum reduced overpressure. The results show that the maximum overpressure measured inside the empty chamber increases from 3 kPa to 28.5 kPa as hydrogen concentration increases from 15.3% to 20.2%. The raised vent location may affect the pressure development during explosion venting, and a third pressure peak caused by the occurrence of the maximum flame surface area could appear in the pressure profile when a large vent is located on the upper part of the side wall. Higher volumetric blockage ratio leads to higher maximum overpressure and flame velocity, and the increment of each pressure peak with the blockage ratio is more pronounced for the back ignition. Furthermore, when comparing with the volume of unburnt gases, the obstacles have a greater effect on the flame. However, the obstructions have a limited effect on the non-homogenous hydrogen deflagration, whose combustion behaviour is governed by the maximum hydrogen concentration in the chamber. Molkov’ best-fit model over-predicts the maximum reduced overpressure measured inside the empty chamber, but the predictions are relatively acceptable for the center ignition.

Filtered Rayleigh scattering measurements of temperature in cellular tubular flames

Steady, premixed tubular flames that produce cellular flames due to low Lewis number can assist in validating diffusional properties for numerical models. The lean hydrogen-air mixture creates conditions of low Lewis number that have shown to result in cellular flames inside tubular burners due to preferential diffusion. Filtered Rayleigh scattering (FRS) is used to achieve two-dimensional temperature measurements across perpendicular planes in the cellular tubular flame. Different FRS strategies, a pulsed Nd:YAG laser with and without Fabry–Perot etalon as well as a continuous wave laser, are contrasted for consistency and uncertainty in the tubular flame measurements. The FRS temperature profiles are found to be consistent with previous spontaneous Raman scattering temperature measurements but differ from previous direct numerical simulation results.