An experimental study on the high frequency oscillatory combustion in tubular flame burners

In this study, high frequency oscillatory combustion in tubular flame burner was experimentally investigated using large scale 8- and 12-in. diameter tubular flame burners. The conditions for the high frequency oscillatory combustion were determined, and the pressure fluctuations were measured, on which spectral analyses were made. The results showed that a smooth laminar tubular flame could be established, however, high frequency combustion sound was emitted from the 8- and 12-in. burners when the air flow rates exceeded 650 and 1200 m³/h, respectively. Pressure fluctuation measurements and spectral analyses showed that high frequency pressure fluctuations occurred simultaneously during the high frequency oscillation. The flame images were also obtained with a high speed video camera. The flame surface was found to be notably corrugated and the symmetry was broken during the oscillation. By assuming that the tubular flame burner is a simple tube, the natural frequencies of the burners were determined using the fundamental theory of the acoustic resonance in a cylindrical cavity, and the experimental peak frequencies in the spectra were compared with the natural frequencies. As a result, it was found that the high frequency oscillations in both the burners were identified as the tangential/radial mode acoustic resonant oscillations. It was further found that the tangential first mode of oscillation, which had an asymmetric structure, preceded the higher modes of oscillation which indicated that the mode dominated the occurrence of the high frequency oscillation. To verify the occurrence of the tangential first mode oscillation, the pressure fluctuations were measured with two pressure sensors installed at opposed locations. The results showed that the phase of the oscillation was 180° difference, and the asymmetric structure confirmed the occurrence of the tangential first mode of oscillation.     

Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion

This paper studied the flame propagation characteristics of heavy oil oxy-fuel combustion in ignition and stable combustion. The results showed that the ignition process could be divided into three stages: the pro-ignition, mid-ignition and end-ignition. The pro-ignition, the fire core generated and evolved into spherical; the mid-ignition, the spherical fire core gradually turn into tapered structure; the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. By calculating the flame propagation velocities, we found that in the same combustion atmosphere, the flame propagation velocity in 29% O2 was higher than that in 21% O2; in the same O₂ concentration, the flame propagation velocity in O₂/N₂ atmosphere was higher than that in O₂/CO₂. During the stable combustion, we observed the local flame structure extinguished, distorted and grew.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

Effect of hydrogen addition on the structure and stabilization of a micro-jet methane diffusion flame

The micro-jet diffusion flame can act as the heat source for the micro power generation systems due to some advantages. The present work investigates the effect of hydrogen addition on the structure and stabilization of micro-jet methane diffusion flame by numerical simulation. The results show that the oval flame becomes more and more circular with the increase of hydrogen addition fraction. The addition of hydrogen remarkably suppresses the increase of the flame height with the inlet velocity. The methane sharply decreases around the outlet of the micro-jet tube due to the high fresh fuel temperature. The intermediate species (e.g., H₂ and CO) increase sharply before the flame front, and they are consumed sharply within the flame front. With the increase of hydrogen addition fraction, the concentration gradients of reactive species increase before the flame front, while the flame temperature decreases. In addition, with the increase of hydrogen addition fraction, the micro-jet flame root shifts toward the tube-wall and downstream direction at the radial and axial directions, respectively, and the addition of hydrogen decreases the anchoring temperature of the micro-jet flame root, which is conductive to improve the flame stabilization. Meanwhile, a large hydrogen addition fraction is detrimental for the flame stabilization in terms of the thermal interaction between the micro-jet flame and tube-wall. However, the positive effects brought by a large hydrogen addition fraction are noticeably larger than the adjunctive negative effects. This study not only provides the guideline for further expanding the operating range of the micro-jet methane diffusion flame but also helps us to gain insights into the mechanism of hydrogen addition on improving the flame stabilization.     

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NO_x profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.     

Direct numerical simulation and analysis of a hydrogen/air swirling premixed flame in a micro combustor

A three dimensional spatially developing hydrogen/air premixed flame in a micro combustor with a moderate Reynolds number and a high swirl number is studied using direct numerical simulation. The inflow mixture is composed of hydrogen and air at an equivalent ratio of 1.0 in the jet core region, and pure air elsewhere. The maximum axial velocity at the inlet is 100 m/s. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. Vortex and turbulence characteristics are examined. Two instabilities, namely Kalvin–Helmholtz instability and centrifugal instability, are responsible for the transition from laminar flow to turbulence. A cone-like vortex breakdown is observed both in the isothermal swirling flow and in the swirling flame. One dimensional premixed laminar flame is studied, the structure of which is compared with that of the multi-dimensional one. Probability density functions of the curvature and tangential strain rate are presented. It is shown that the flame curvature has a near zero mean, and the flame aligns preferentially with extensive strain. Finally, the turbulent premixed flame regime diagram is used to characterize the flame. It is found that most of the flame elements lie in the laminar flame regime and the thin reaction zones regime.     

Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.     

The effect of an obstacle plate on the spontaneous ignition in pressurized hydrogen release: A numerical study

Spontaneous ignition of a pressurized hydrogen release has important implications in the risk assessment of hydrogen installations and design of safety measures. In real accident scenarios, an obstacle may be present close to the release point. Relatively little is known about the effect of such an obstacle on the salient features of highly under-expanded hydrogen jets and its spontaneous ignition.In the present study, the effect of a thin flat obstacle on the spontaneous ignition of a direct pressurized hydrogen release is investigated using a 5th-order WENO scheme and detailed chemistry. The numerical study has revealed that, for the conditions studied, the presence of the obstacle plays an important role in quenching the flame following spontaneous ignition for the release conditions considered.     

Effects of content of hydrogen on the characteristics of co-flow laminar diffusion flame of hydrogen/nitrogen mixture in various flow conditions

Effect of content of hydrogen (H₂) in fuel stream, mole fraction of H₂$\left(X_{H_2}\right)$ in fuel composition, and velocity of fuel and co-flow air $\left(V_{avg}\right)$ on the flame characteristics of a co-flow H₂/N₂ laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with $X_{H_2}$ varying between 0.25 and 1, and $V_{avg}$ between 0.25 and 1 ms^?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either $X_{H_2}$ or $V_{avg}$. Significant effect on the flame temperature and NOx emission are obtained from a higher $X_{H_2}$ in fuel whereas the flame size and heat release are the result of increasing $V_{avg}$. Along with this finding, the role of N₂ and its higher content reducing the flame temperature and NOx emission are presented.     

A numerical study of transient ignition and flame characteristics of diluted hydrogen versus heated air in counterflow

Combined experimental and numerical studies of the transient response of ignition to strained flows require a well-characterized ignition trigger. Laser deposition of a small radical pool provides a reliable method for initiating ignition of mixtures that are near the ignition limit. Two-dimensional direct numerical simulations are used to quantify the sensitivity of ignition kernel formation and subsequent edge-flame propagation to the oxidizer temperature and the initial width and amplitude of O-atom deposition used to trigger ignition in an axisymmetric counterflow of heated air versus ambient hydrogen/nitrogen. The ignition delay and super-equilibrium OH concentration in the nascent ignition kernel are highly sensitive to variations in these initial conditions. The ignition delay decreases as the amplitude of the initial O-atom deposition increases. The spatial distribution and the magnitude of the OH overshoot are governed by multi-dimensional effects. The degree of OH overshoot near the burner centerline increases as the diameter of the initial O-atom deposition region decreases. This result is attributed to preferential diffusion of hydrogen in the highly curved leading portion of the edge flame that is established following thermal runaway. The edge-flame speed and OH overshoot at the leading edge of the edge flame are relatively insensitive to variations in the initial conditions of the ignition. The steady edge-flame speed is approximately twice the corresponding laminar flame speed. The rate at which the edge flame approaches its steady state is insensitive to the initial conditions and depends solely on the diffusion time scale at the edge flame. The edge flame is curved toward the heated oxidizer stream as a result of differences in the chemical kinetics between the leading edge and the trailing diffusion flame. The structure of the highly diluted diffusion flame considered in this study corresponds to Liñán's ‘premixed flame regime’ in which only the oxidizer leaks through the reaction zone such that the flame is located at fuel lean rather than stoichiometric mixture fraction conditions.     

Rapidly mixed combustion of hydrogen/oxygen diluted by N2 and CO2 in a tubular flame combustor

In this study hydrogen flames have been attempted in a rapidly mixed tubular flame combustor for the first time, in which fuel and oxidizer are individually and tangentially injected into a cylindrical combustor to avoid flame flash back. Three different cases were designed to examine the effects of fuel and oxidizer feeding method, diluent property, oxygen content and equivalence ratio on the characteristics of hydrogen flame, including the flame structure, lean extinction limit, flame stability and temperature. The results show that by enhancing mixing rate through feeding system, the range of equivalence ratio for steady tubular flame can be much expanded for the N₂ diluted mixture, however, at the oxygen content of 0.21 (hydrogen/air) the steady tubular flame is achieved only up to equivalence ratio of 0.5; by decreasing oxygen content, the lean extinction limit slightly increases, and the upper limit for steady tubular flame establishment increases significantly, resulting in an expanded tubular flame range. For CO₂ diluted mixture, the stoichiometric combustion has been achieved within oxygen content of 0.1 and 0.25, for which the burned gas temperature is uniformly distributed inside the flame front; as oxygen content is below 0.21, a steady tubular flame can be obtained from the lean to rich limits; and the lean extinction limit increases from 0.17 to 0.4 as oxygen content decreases from 0.21 to 0.1, resulting in a shrunk tubular flame range. Laminar burning velocity, temperature and Damköhler number are calculated to examine the differences between N₂ and CO₂ diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment.     

An experimental study on the effect of hydrogen enrichment on diesel fueled HCCI combustion

This paper experimentally investigates the influence of hydrogen enrichment on the combustion and emission characteristics of a diesel HCCI engine using a modified Cooperative Fuel Research (CFR) engine. Three fuels, n-heptane and two middle distillates with cetane numbers of 46.6 and 36.6, are studied.The results show that hydrogen enrichment retards the combustion phasing and reduces the combustion duration of a diesel HCCI engine. Besides, hydrogen enrichment increases the power output and fuel conversion efficiency, and improves the combustion stability. However, hydrogen enrichment may narrow the operational compression ratio range and increase the knocking tendency. Both the overall indicated specific CO emissions (isCO) and CO emissions per unit burned diesel fuel mass are reduced by hydrogen enrichment. Although hydrogen enrichment decreases the overall indicated specific unburned hydrocarbon emissions (isHC), it does not significantly affect the HC emissions per unit burned diesel fuel mass.     

Remaining uncertainties in the kinetic mechanism of hydrogen combustion

An analysis of the performance of an updated hydrogen combustion mechanism is presented. Particular attention was paid to different channels of reaction between H atoms and HO₂ radicals, to pressure dependence of the recombination of HO₂ radicals, and to the anomalous rate constant of reaction between OH and HO₂ radicals. The contemporary choice of the reaction rate constants is presented with the emphasis on their uncertainties. Then the predictions of ignition, oxidation, flame burning velocities, and flame structure of hydrogen-oxygen-inert mixtures are shown. The modeling range covers ignition experiments from 950 to 2700 K and from subatmospheric pressures up to 87 atm; hydrogen oxidation in a flow reactor at temperatures around 900 K from 0.3 up to 15.7 atm; flame burning velocities in hydrogen-oxygen-inert mixtures from 0.35 up to 4 atm; and hydrogen flame structure at 1 and 10 atm. Comparison of the modeling and experiments is discussed in terms of the range of applicability of the present detailed mechanism. The necessity for analysis of the mechanism to have an exhaustive list of reactions is emphasized.     

Numerical simulation of the laminar hydrogen flame in the presence of a quenching mesh

Recent studies of J.H. Song et al. [1], and S.Y. Yang et al. [2] (see also references therein) have been concentrated on mitigation measures against hydrogen risk. The authors have proposed installation of quenching meshes between compartments or around the essential equipment in order to contain hydrogen flames. Preliminary tests were conducted which demonstrated the possibility of flame extinction using metallic meshes of specific size.Considerable amount of numerical and theoretical work on flame quenching phenomenon has been performed in the second half of the last century and several techniques and models have been proposed to predict the quenching phenomenon of the laminar flame system (see for example [3] and references therein). Most of these models appreciated the importance of heat loss to the surroundings as a primary cause of extinguishment, in particular, the heat transfer by conduction to the containing wall. The supporting simulations predict flame-quenching structure either between parallel plates (quenching distance) or inside a tube of a certain diameter (quenching diameter).In the present study the flame quenching is investigated assuming the laminar hydrogen flame propagating towards a quenching mesh using two-dimensional configuration and the earlier developed models. It is shown that due to a heat loss to a metallic grid the flame can be quenched numerically.     

An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH₄/H₂ proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO₂ concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions.     

An experimental study on combustion characteristics of flames in a coaxial flow with a stagnation point

By using a stagnation-point coaxial flow generated by a lower coaxial burner and an upper quartz plate, an inner (or outer) premixed flame influenced by outer (or inner) oxygen content is experimentally developed to simulate and study double-flame burning structures modified by interactions of flamelets in turbulent combustible flows. In the experiments, fuel-air and oxygen-nitrogen mixtures are therefore introduced into outer (or inner) and inner (or outer) flows, respectively. This experimental arrangement allows either the inner flame or the outer flame to be located at different planes by separately adjusting the compositions and injection velocities of the inner and outer flows. An inner (or outer) planar premixed flame with a small outer (or inner) lifted tail or an inner (or outer) nonplanar premixed flame and an outer (or inner) trumpet-shaped diffusion flame can be developed in the flow field. The lifted tail and the trumpet-shaped diffusion flame are stabilized along the interface between the inner and outer jets in the coaxial flow. The inner (or outer) premixed flame influenced by the outer (or inner) oxygen content may experience transports of mass and thermal diffusion parallel to the flame surface. It endures the flow stretch tangent to the flame surface. Furthermore, in the flow field, the directions of flow convection for both inner and outer flows are the same (both divergent). The combustion characteristics, including extinction, blow off, flashback, the transition from the flat flame to the hat-shaped flame, and the ignition and development of diffusion flame are reported and discussed. Finally, the measurements of flame shape and temperature distribution are involved.     

Effect of initial pressure on laminar combustion characteristics of hydrogen enriched natural gas

Flame propagation of premixed natural gas–hydrogen–air mixtures was studied in a constant volume combustion bomb. Laminar burning velocities and mass burning fluxes were obtained under various hydrogen fractions and equivalence ratios with various initial pressures, while flame stability and their influencing factors (Markstein length, density ratio and flame thickness) were obtained by analyzing the flame images at various hydrogen fractions, initial pressures and equivalence ratios. The results show that hydrogen fraction, initial pressure as well as equivalence ratio have combined influence on both unstretched laminar burning velocity and flame instability. Meanwhile, according to flame propagation pictures taken by the high speed camera, flame stability decreases with the increase of initial pressures; for given equivalence ratio and hydrogen fraction, flame thickness is more sensitive to the variation of the initial pressure than to that of the density ratio.