A modified micro reactor fueled with hydrogen for reducing entropy generation

In order to reduce the entropy generation of premixed hydrogen/air flame in the micro combustion chamber, the combustion chamber of the old micro reactor is modified by gradually varying the diameter of the combustion chamber. Extensive numerical investigations about the entropy generation of premixed hydrogen/air flame in old and modified micro reactors are conducted under various hydrogen mass flow rates, hydrogen/air equivalence ratios, solid materials and inlet/outlet diameter ratios. Results suggest that the modified micro reactor has lower total entropy generation than that of the old micro reactor. This is attributed to that the temperature gradient of the burned gas in the modified micro reactor is lower than that in the old micro reactor after chemical reactions. Finally, the largest descent percentages of total entropy generation are achieved under various conditions, which provide significant reference values for hydrogen energy usage under micro scale combustion.     

零碳燃料对燃气锅炉燃烧过程调控作用

利用小型化模拟炉膛开展了零碳燃料氢气对燃气锅炉燃烧过程调控作用实验研究,研究了掺氢比对炉膛内部预混火焰宏观形态、炉膛温度均匀性、炉膛污染物排放规律的影响,并总结了CO及NO_x的排放规律。实验结果表明：随着预混当量比增加,纯甲烷火焰长度逐渐缩短;对于20%掺氢火焰,随着预混程度的提高,火焰长度降低明显;不同火焰条件下,炉膛温度只由燃烧功率控制;改变燃烧条件时,处于壁面附近位置的温度变化较为平稳,而靠近火焰处温度变化较大;天然气中掺入氢气,燃烧时可以有效降低未燃CO排放;在相同预混程度下,全局当量比减小导致未燃空气增加,热量被稀释,火焰温度降低,热力型NO_x的生成降低;随着掺氢比的增加,燃烧时火焰温度升高,导致热力型NO_x排放增加。     

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。     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

A review of enhancing micro combustion to improve energy conversion performance in micro power system

With the development of microfabrication technology and micro devices, the demand for Power Micro Electro Mechanical System (Power MEMS) is ever-increasing. However, traditional chemical batteries are not suitable for Power MEMS due to their low energy density. The combustion of hydrogen and hydrocarbon fuels offers a more promising alternative to conventional batteries. However, micro combustion faces the problems of flame instability and low combustion efficiency. Therefore, improving flame stabilization and combustion efficiency in micro combustions is necessary. Studies have made considerable progress in these aspects over the last decade. This paper summarized these studies and classified the optimization schemes according to flame stabilization and combustion efficiency. Besides, research on the Field Synergy Principle was discussed. The synergy between the flow field and temperature gradient field in the micro-scale domain will become a key research area in the future. It is proposed to insert porous media in MTES and MTPVS and adopt catalytic combustion. Adding hydrogen to the mixed gas was recommended. The equivalence ratio of the mixed gas in the range of 0.9–1.1 would be best. The equivalence ratio is the ratio of the theoretical requirement of air with complete combustion to the actual supply of air.     

Analysis of exergy losses in laminar premixed flames of methane/hydrogen blends

Combustion is the primary source for exergy loss in power systems such as combustion engines. To elucidate the exergy loss behaviors in combustion and explore the principle for efficiency improvement, the second-law thermodynamic analysis was conducted to analyze the energy conversion characteristics in laminar premixed flames of methane/hydrogen binary fuels. The sources causing exergy losses in laminar premixed flames included five parts, namely heat conduction, mass diffusion, viscous dissipation, chemical reactions and incomplete combustion, respectively. The calculations were conducted at both atmospheric and elevated pressures, with the equivalence ratio varying from 0.6 to 1.5 and the hydrogen blending ratio increasing from 0% to 70%. The results indicated that the total exergy loss firstly increased and then decreased with increased equivalence ratio, and reached the minimum value at the equivalence ratio of 0.9. This was primarily due to the trade-off relation between the decreased exergy loss from entropy generation and the increased exergy loss from incomplete combustion, as equivalence ratio increased. As the hydrogen blending ratio increased from 0% to 70%, the total exergy loss decreased by 2%. Specifically, the exergy loss from heat conduction decreased, primarily due to the decreased flame thickness. Moreover, the reactions with H₂, H and H₂O as reactants were inhibited, leading to decreased the exergy loss from chemical reactions. As pressure increased from 1 atm to 5 atm, the total exergy loss decreased by 1%, because the exergy losses induced by heat conduction and chemical reactions decreased as the flame thickness was reduced. The exergy loss from incomplete combustion also decreased, because elevated pressure inhibited dissociations and decreased the mole fractions of incomplete combustion products.     

Structure of hydrogen triple flames and premixed flames compared

Triple flames consisting of lean, stoichiometric, and rich reaction zones may be produced in stratified mixtures undergoing combustion. Such flames have unique characteristics that differ from premixed flames. The present work offers a direct comparison of the structure and propagation behavior between hydrogen/air triple and premixed flames through a numerical study. Important similarities and differences are highlighted. Premixed flames are generated by spark-igniting initially quiescent homogeneous mixtures of hydrogen and air in a two-dimensional domain. Triple flame results are also generated in a two-dimensional domain by spark-igniting initially quiescent hydrogen/air stratified layers. Detailed flame structure and chemical reactivity information is collected along isocontours of equivalence ratio 0.5, 1.0, and 3.0 in the triple flame for comparison with premixed flames at the same equivalence ratios. Full chemistry and effective binary diffusion coefficients are employed for all computations.     

Effects of hydrogen addition on entropy generation in ultra-lean counter-flow methane-air premixed combustion

In this paper, entropy generation in hydrogen enriched ultra-lean counter-flow methane–air premixed combustion confined by planar opposing jets is investigated for the first time. The effects of the effective equivalence ratio and volume percentage of hydrogen in fuel blends on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, four interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for hydrogen enriched methane–air combustion, are revealed. For a given effective equivalence ratio, the total entropy generation number can be approximated as a linear increasing function of the volume percentage of hydrogen in fuel blends for all cases investigated in the present study.     

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.     

Impact of H2 addition on flame stability and pollutant emissions for an atmospheric kerosene/air swirled flame of laboratory scaled gas turbine

The purposes of this study are to compare the stability domains and the pollutant emissions when combustion occurs with and without addition of H₂ to a kerosene (Jet A1)/air premixed prevaporised mixture injected in a lean gas turbine combustor. Chemiluminescence of CH*, pollutant emissions (NO_x and CO) and pressure fluctuations data are simultaneously collected in order to determine the effects of H₂ addition on the stability of the combustion and on the flame structure for an inlet temperature of 473 K, atmospheric pressure and for a large range of equivalence ratio (from 0.3 to 1). Addition of hydrogen enables keeping stable combustion conditions when, for the same kerosene mass flow, the flame becomes lifted and very unstable. As for pollutant emissions, results show that the equivalence ratio is the key parameter to control NO_x emission even in the situation where the combustion power is increased due to H₂ addition. As H₂ addition strongly increases the flammability limits and the combustion stability domain, stable combustion can occur at leaner equivalence ratio and then decreases CO and NO_x emissions. This is an important fact since no substitution effect takes place in the reduction of NO_x and CO emissions. Study at constant combustion power and equivalence ratio by adjusting hydrogen and kerosene mass flow shows again a decrease in the pollutant emissions. Hydrogen injection in power generation systems using combustion seems to be a promising way in combustion research since due to the combined effects of enlarging combustion stability domain and reducing NO_x emissions by substituting kerosene to the benefit of H₂.     

Experimental study on micro modular combustor for micro-thermophotovoltaic system application

As one of the key components of micro modular thermophotovoltaic power generators, every micro combustor should be able to produce a high and uniform temperature distribution on the surface. In this work, three micro modular combustors with different fuel supply systems were designed and tested. The results indicated that the in-line design with only one fuel supply tube could not equally distribute H₂/air mixture to every combustor. The wall temperatures of the two central combustors were obviously higher than the two side combustors. However, both the in-line design with two fuel supply tubes and the parallel design could equally deliver the fuel/air mixture to every combustor, and an uniform temperature distribution could be obtained for every combustor. The total radiation energy and radiation efficiency of the micro modular combustors were also calculated for various flow speeds and H₂/air equivalence ratios. A radiation efficiency of 27.3% could be achieved when the H₂/air equivalence ratio was 0.8 and the flow speed was 6 m/s.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Effect of initial pressure,temperature and equivalence ratios on laminar combustion characteristics of hydrogen enriched natural gas

In this study, the flame propagation characteristics of premixed natural gas–hydrogen–air mixtures were studied in constant volume combustion bomb by using the high-speed schlieren photography system. The flame radius, laminar flame propagation speed and the flame stretch rate were obtained under different initial pressure, temperature, equivalence ratios and hydrogen fractions. Meanwhile, the flame stability and their influencing factors were obtained by analyzing the Markstein length and the flame propagation schlieren photos under various combustion conditions. The results show that the stretched laminar propagation speed increases with the increase of the initial temperature and hydrogen fraction of the mixture, and will decreases with the increase of the initial pressure. Meanwhile, according to the Markstein length and the flame propagation pictures, the flame stability decreases with the increase of the temperature and hydrogen fraction, and the slight flaws occurred at the early stage; at larger flame radius, the flame stability is more sensitive to the variation of the initial temperature and hydrogen fraction than to that of initial pressure and equivalence ratio.     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the 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.     

Experimental study of propane/air premixed flame dynamics with hydrogen addition in a meso-scale quartz tube

The combustion process of propane/air premixed flame in meso-scale quartz tubes with different hydrogen additions was investigated experimentally to explain the flame-wall interaction mechanism. The ranges of different flame regimes were obtained by changing the flow rates of propane and hydrogen. The effects of hydrogen addition, inlet velocity and equivalence ratio were analyzed. The results show that the hydrogen addition broadens the operation ranges of fast flame regime and slow flame regime significantly. The flame propagation speed is in the same order of the thermal wave speed in solid wall for the slow flames. In fast flame regime, the flame propagation speed has an inverse correlation with the inlet flow velocity irrespective of the equivalence ratio. With the increase of the equivalence ratio, the maximum flame speed in fast flame regime decreases gradually, while the maximum flame speed in slow flame regime increases continually. It indicates that rich fuel condition suppresses the fast flame and promotes the slow flame. In slow flame regime, the output thermal efficiency is dominated by the inlet velocity and equivalence ratio.     

基于详细化学反应动力学的含颗粒甲烷/空气层流火焰模拟研究

本文基于含颗粒甲烷燃烧详细化学反应动力学机理Gri-Mech3.0,采用层流预混火焰速度计算模型计算了含颗粒甲烷空气层流预混燃烧过程。通过对比层流火焰燃烧速度曲线,分析了温度和当量比对燃烧速度的影响,且对计算结果进行敏感性分析。研究结果表明,颗粒存在降低了火焰的燃烧速度,且温度越高,燃烧速度降低的越明显,而燃烧速度变化量随当量比的增加则先增大后减小。     

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.     

Study of flow and combustion characteristics of optimized low swirl premixed nozzle for micro gas turbine

With the micro gas turbine is used more and more widely, the operating conditions become complex and fluctuating which impacts the environmental friendliness of combustor. In this work, a low swirl premixed nozzle (LSPN) is designed based on the original premixed nozzle (OPN) of a 60 kW micro gas turbine, in order to adapt the flexibility of load and operating environment by improving the mixing performance of fuel and air. The flow and combustion characteristics of LSPN and OPN fed by natural gas is numerically studied using the standard k-ε model and the combined Finite Rate Chemistry/Eddy Dissipation Model (FRC/EDM) under various operating conditions. The results show that the mixing performance of air and fuel in LSPN has been improved. The flow unmixedness in LSPN is always smaller than OPN, and it is 41.74% lower at the outlet of the nozzle. While the equivalence ratio decreases, the mean temperature in the combustor decline. Furthermore, under the majority of operating conditions, the emission performance of LSPN is better than OPN.