旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Effects of hydrogen addition on combustion characteristics of a methane fueled MILD model combustor

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NO_x emissions are less than 5 ppm@15%O₂ when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NO_x emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH1 signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor.     

Flame characteristics of methane/air with hydrogen addition in the micro confined combustion space

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio.     

Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.     

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.     

Numerical study of the physical and chemical effects of hydrogen addition on laminar premixed combustion characteristics of methane and ethane

Methane and ethane are taken as the research objects. Using H₂ as diluent, based on Chemkin II/Premix Code and modified detailed chemical reaction mechanism: GRI 3.0*-Mech (introducing three hypothetical substances of FH₂, FO₂ and FN₂), the physical and chemical effects of hydrogen on laminar burning velocities (LBVs), adiabatic flame temperatures (AFTs), net heat release rates (NHRRs) and elementary reactions responsible for temperature changes of two alkanes under different equivalence ratios were analyzed and determined. Results showed that the chemical effect of H₂ promotes the LBVs and ATFs of methane and ethane, while the physical effect decreases the two parameters. In addition, the physical effects of H₂ inhibit the chemical reactions of methane and ethane, resulting in the decrease of NHRRs. The chemical effect of H₂ accelerates the process of chemical reaction and obviously increases the NHRRs. The two most vital elementary reactions that promote the temperature rise of methane and ethane are H + O₂ <=> OH + O and CO + OH <=> H + CO₂. The important reactions responsible for inhibiting the temperature rise are H + CH₃(+M) <=> CH₄(+M) and H + O₂ + H₂O <=> HO₂ + H₂O.     

The effect of hydrogen addition on combustion and emission characteristics of an n-heptane fuelled HCCI engine

The mechanisms of the influence of hydrogen enrichment on the combustion and emission characteristics of an n-heptane fuelled homogeneous charge compression ignition (HCCI) engine was numerically investigated using a multi-zone model. The model calculation successfully captured the most available experimental data. The results show that hydrogen addition retards combustion phasing of an n-heptane fuelled HCCI engine due to the dilution and chemical effects, with the dilution effect being more significant. It is because of the chemical effect that combustion duration is reduced at a constant compression ratio if an appropriate amount of hydrogen is added. As a result of retarded combustion phasing and reduced combustion duration, hydrogen addition increases indicated thermal efficiency at a constant combustion phasing. Hydrogen addition reduces indicated specific unburned hydrocarbon emissions, but slightly increases normalized unburned hydrocarbon emissions that are defined as the emissions per unit burned n-heptane mass. The increase in normalized unburned hydrocarbon emissions is caused by the presence of more remaining hydrocarbons that compete with hydrogen for some key radicals during high temperature combustion stage. At a given hydrogen addition level, N₂O emissions increases with overly retarding combustion phasing, but hydrogen addition moderates this increase in N₂O emissions.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

Effects of hydrogen addition on combustion and flame propagation characteristics of laser ignited methane/air mixtures

In this study, effects of hydrogen addition on combustion and flame propagation characteristics of methane/air mixtures were investigated in a constant volume combustion chamber. Tested gas mixtures are 100% CH₄, 05% H₂ – 95% CH₄, 10% H₂ – 90% CH₄ and 15% H₂ – 85% CH₄, and such mixtures were ignited using a passively Q-switched Nd:YAG laser ignitor which has a pulse energy of 12.3 mJ, pulse duration of 2.4 ns and wave length of 1064 nm. A Schlieren setup coupled with a high-speed camera enabled evaluating flame propagation behavior, while pressure curve analysis provided necessary data for characterization of combustion properties. Additionally, lean flammability limits of gas mixtures were also determined at the test conditions. The unique properties of hydrogen (such as low density, high reactivity, high diffusivity etc) widened lean flammability limit. Rate of pressure rise and measured pressure values increased with hydrogen addition, regardless of the air-fuel equivalence ratio (λ). Lastly, hydrogen addition uniformly affected flame propagation characteristics and flame luminosity. Combustion process became more stable with hydrogen addition.     

Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine

The free-piston engine (FPE) is a new crankless engine, which operates with variable compression ratio, flexible fuel applicability and low pollution potential. A numerical model which couples with dynamic, combustion and gas exchange was established and verified by experiment to simulate the effects of different hydrogen addition on the combustion and emission of a diesel FPE. Results indicate that a small amount of hydrogen addition has a little effect on the combustion process of the FPE. However, when the ratio of hydrogen addition (R_H2) is more than 0.1, the R_H2 gives a positive effect on the peak in-cylinder gas pressure, temperature, and nitric oxide emission of the FPE, while soot emission decreases with the increase of hydrogen addition. Moreover, the larger R_H2 induces a longer ignition delay, shorter rapid combustion period, weaker post-combustion effect, greater heat release rate, and earlier peak heat release rate for the FPE. Nevertheless, the released heat in rapid combustion period is significantly enhanced by the increase of R_H2.     

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.     

Experimental study of the effects of hydrogen addition on the thermoacoustic instability in a variable-length combustor

The current study examined the self-excited thermoacoustic instability of hydrogen/methane premixed flames using a variable-length combustor (300–1100 mm). The global dynamic pressure, heat release rate oscillation, together with the flame dynamics were studied. Results showed that both the hydrogen concentration and the chamber length were critical in determining the acoustic oscillation mode and instability trend. Low-frequency primary acoustic modes (<200 Hz) were mainly excited when the hydrogen concentration was low, whereas primary acoustic modes with relatively higher frequencies (~400 Hz) tended to occur in cases with a high hydrogen proportion (>40%). For primary acoustic modes lower than 200 Hz, the primary oscillation frequency tended to increase linearly with a rising hydrogen proportion. Heat release oscillation and flame dynamics analyses demonstrated that for the flame with large-scale shape deformation, the initial addition of hydrogen would intensify the heat release oscillation. Nevertheless, a further increase in the hydrogen level tended to inhibit the heat release oscillation by weakening the flame shape deformation. Eventually, a sufficient high-level of hydrogen addition would weaken the primary acoustic modes that have similar frequencies.     

Effect of multiple bluff bodies on hydrogen/air combustion characteristics and thermal properties in micro combustor

The bluff body is commonly used to improve micro combustion. The micro combustor with multiple rectangular bluff bodies in a single row was proposed. The effects of bluff bodies on H₂/air combustion characteristics were numerically studied. The temperature distributions, ignition position, combustion efficiency and blow-out limit were investigated via changing the total width and number of bluff bodies. The results show that the combined use of multiple bluff bodies can further expand the blow-out limit of H₂/Air. The effect of high temperature and viscous force on the flow velocity is main factors for the flame morphology. When the total width of bluff bodies is 2 mm, the blow-out limit decreases with the increase of bluff body number. When the total width of bluff bodies is 4 mm and 6 mm, the blow-out limit increases with the increase of the number of bluff bodies. With the increase of inlet velocity, the complete combustion efficiency decreases. The combustion efficiency in the combustor with wider blow-out limit decreases more slowly. It indicates that the combustor with multi-bluff bodies is more suitable for the operation conditions with high flow velocity.     

Experimental analysis of the effects of hydrogen addition on methane combustion

This paper presents gas emissions from turbulent chemical flow inside a model combustor, for different blending ratios of hydrogen–methane composite fuels. Gas emissions such as CO and O₂ from the combustion reaction were obtained using a gas analyzer. NOx emissions were measured with a NOx analyzer. The previously obtained flame temperature distributions were also presented. As the amount of hydrogen in the mixture increases, more hydrogen is involved in the combustion reaction, and more heat is released, and the higher temperature levels are resulted. The results have shown that the combustion efficiency increases and CO emission decreases when the hydrogen content is increased in blending fuel. It is also shown that the hydrogen–methane blending fuels are efficiently used without any important modification in the natural gas burner. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Effects of hydrogen addition on the forced response of H2/CH4 flames in a dual-nozzle swirl-stabilized combustor

The effects of hydrogen addition on the forced response of H₂/CH₄ flames are analyzed in a dual-nozzle swirl-stabilized combustor. The hydrogen volumetric content in the fuel is varied from 0% to 40%. Flame transfer function (FTF) is used to compare the forced response of the flames. The FTF gain featuring the local maximum and minimum values, which occurred commonly in the FTFs under all hydrogen contents, is determined by two different mechanisms: the change in the flame angle and the flame roll-up phenomenon. Among two mechanisms, the flame roll-up phenomenon has a more important role in determining the FTF characteristics. In addition, hydrogen addition attenuates the local maximum gains and decreases the FTF phase slope. The change in the flame roll-up behavior, which is induced by a short and compact flame distribution at high hydrogen contents, is the primary reason of these differences in the FTF.     

Effect of hydrogen addition to intake gas on combustion and exhaust emission characteristics of a diesel engine

The present study experimentally investigated the performance and emission characteristics of the diesel engine with hydrogen added to the intake air at late diesel-fuel injection timings. The diesel-fuel injection timing and the hydrogen fraction in the intake mixture were varied while the available heat produced by diesel-fuel and hydrogen per second of diesel fuel and hydrogen was kept constant at a certain value. NO showed minimum at specific hydrogen fraction. The maximum rate of incylinder pressure rise also showed minimum at 10 vol. % hydrogen fraction. However, it is desirable to set the maximum rate of incylinder pressure rise less than 0.5 MPa/deg. to realize low level of combustion noise and NO emission. We attempt to reduce further NO and smoke emissions by EGR. As the result, in the case of the diesel-fuel injection timing of −2 °. ATDC with 3.9 vol. % hydrogen addition, the smoke emission value was 0%, NO emission was low, the cyclic variation was low, and the maximum rate of incylinder pressure rise was acceptable under a nearly stoichiometric condition without sacrificing indicated thermal efficiency.     

Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.     

Effect of water injection and spark timing on the nitric oxide emission and combustion parameters of a hydrogen fuelled spark ignition engine

One of the main problems with hydrogen fuelled internal combustion engines is the high NO level due to rapid combustion. Use of diluents with the charge and retardation of the spark ignition timing can reduce NO levels in Hydrogen fuelled engines. In this work a single cylinder hydrogen fuelled engine was run at different equivalence ratios at full throttle. NO levels were found to rise after an equivalence ratio of 0.55, maximum value was about 7500 ppm. High reductions in NO emission were not possible without a significant drop in thermal efficiency with retarded spark ignition timings. Drastic drop in NO levels to even as low as 2490 ppm were seen with water injection. In spite of the reduction in heat release rate (HRR) no loss in brake thermal efficiency (BTE) was observed. There was no significant influence on combustion stability or HC levels.     

Numerical analysis of the combustion characteristics of graphitic hydrogen-chlorine synthesis combustor with different intake strategies

This study investigates the effect of intake strategies on the combustion and flows characteristics of hydrogen-chlorine synthesis combustors via numerical methods. A crucial issue of hydrogen-chlorine synthesis combustor is to have a sufficiently low flame height and high conversion efficiency. In this study, the combustion performance of combustors equipped with the annular tube, plum nozzle, and porous-bullet nozzle has been thoroughly analyzed. The temperature distribution and gas flow are analyzed using the method of fluid-solid coupling, which indicates that the combustor with porous-bullet nozzle had the best gas distribution, the maximum HCl mole fraction at outlet is 97.24%, and the lowest flame height is 3.4 m, which is 27.15% lower than the combustor with the annular tube. Furthermore, the nozzle structure has a great influence on the fluid velocity in the recirculation zone of the combustor. Finally, the effect of hydrogen/chlorine equivalence ratio (?) and inlet volume flow rate were analyzed, and it can be concluded that with the increase of inlet volume flow, the high-temperature area inside the combustor gradually increases. As the equivalent ratio increases, the combustor outlet's mole fraction changes with a normal distribution trend. It is the most appropriate when the chlorine gas flow rate is 1,100 m³/h and ? = 1.05. The research can be applied to the field of high-purity hydrogen chlorine production, providing researchers with some solutions.