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Hearing loss is a common disease affecting public health all around the world. In clinic, auditory brainstem response (ABR) has been widely used for the detection of hearing loss based on its convenience and accuracy. The different reference methods directly influence the quality of the ABR waveform which in turn affects the ABR-based diagnosis. Therefore, in this study, a reference electrode standardization technique (REST) was adopted to systematically investigate and evaluate the effect of different reference methods on the quality of ABR waveform in comparison with the conventional average reference (AR) and mean mastoid (MM) methods. In this study, ABR signals induced by click stimulus were acquired via an EEG electrode cap arrays, and those located on the six channels along the midline were compared systemically. The results showed that, when considering the different channels, the ABR in the Cz channel showed the best morphology. Then, the ABR waveforms acquired via the REST method possessed better morphologies with large amplitude ( 0.06 ± 0.02 μV for wave I, 0.07 ± 0.02 μV for wave III, and 0.21 ± 0.04 μV for wave V) when compared with the traditional method. Summarily, we found that the REST and MM methods improved the quality of ABR on both amplitude and morphology under different stimulation rates and levels without changing the latencies of ABR when compared with the conventional AR method, suggesting that the REST and MM methods have the potential to help physicians with high accurate ABR-based clinical diagnosis. Moreover, this study might also provide a theoretic basis of reference methods on the acquisition of electroencephalogram over public health issues.

The numerical simulation was conducted to investigate the coupling effects of H2 and CO2 on the laminar flame speed and reaction pathways of the biogas-hydrogen mixtures. The same proportions of H2 and CO2 were considered in the study. The dilution effects, chemical effects and thermal effects of H2 and CO2 were isolated, while the contributions of different effects were also calculated to identify the dominating factor. The results show that H radical pool plays the important role in the variation of laminar flame speed of biogas-hydrogen mixture. The coupling dilution effects of H2 and CO2 lead to the considerable decrease of H radical and dominate the decreased laminar flame speed. The H2 chemical effects improve R84 (OH + H2 = H2O + H) and R3 (O + H2 = H + OH) quite effectively, which results in the reduced dropping trends of H radical and laminar flame speed from BG60H40 to BG40H60. The CO2 chemical effects suppress the major H radical production reactions of R84, R3, R99 (OH + CO = H + CO2) and R10 (O + CH3 = H + CH2O) more apparently at the fuel-rich condition, while importance of major consumption reactions (R53: H + CH4 = CH3+H2, R52: H + CH3(+M) = CH4(+M) and R58: H + CH2O = HCO + H2) are higher at the fuel-rich condition. This dominates the more evident decrease of H radical and laminar flame speed at the fuel-rich condition. Based on the ROP analysis, R3, R46 (H + HO2 = 2OH), R52, R84, R99 and R158 (2CH3(+M) = C2H6(+M)) have the major contributions on the chemistry transition caused by the coupling effects of H2 and CO2. The decreased contribution of R158 and R99 indicate the weakened CH4-related chemistry while the increased contributions of R3, R46, R52 and R84 indicate the improved H2-related chemistry in the biogas-hydrogen flame. Furthermore, coupling effects of H2 and CO2 or H2/CO2-related chemistry can be monitored by the contribution variations of R84 and R99 on the OH radical concentration.

Electrocardiogram (ECG) is a critical physiological indicator that contains abundant information about human heart activities. However, it is a kind of weak low-frequency signal, which is easy to be interfered by various noises. Therefore, wearable biosensors (WBS) technique is introduced to overcome this challenge. A flexible non-contact electrode is proposed for wearable biosensors (WBS) system, which is made up of flexible printed circuits materials, and can monitor the ECG signals during exercise for a long time. It uses the principle of capacitive coupling to obtain high-quality signals, and reduces the impact of external noise through active shielding; The results showed that the proposed non-contact electrode was equivalent to a medical wet electrode. The correlation coefficient was as high as 99.70 ± 0.30% when the subject was resting, while it was as high as 97.53 ± 1.80% during exercise. High-quality ECG could still be collected at subjects walking at 7 km/h. This study suggested that the proposed flexible non-contact electrode would be a potential tool for wearable biosensors for medical application on long-term monitoring of patients’ health and provide athletes with physiological signal measurements.

The near-wall flame structure and pollutant emissions of the laminar premixed biogas-hydrogen impinging flame were simulated with a detailed chemical mechanism. The spatial distributions of the temperature, critical species, and pollutant emissions near the wall of the laminar premixed biogas–hydrogen impinging flame were obtained and investigated quantitatively. The results show that the cold wall can influence the premixed combustion process in the flame front, which is close to the wall but does not touch the wall, and results in the obviously declined concentrations of OH, H, and O radicals in the premixed combustion zone. After flame quenching, a high CO concentration can be observed near the wall at equivalence ratios (φ) of both 0.8 and 1.2. Compared with that at φ = 1.0, more unburned fuel is allowed to pass through the quenching zone and generate CO after flame quenching near the wall thanks to the suppressed fuel consumption rate near the wall and the excess fuel in the unburned gases at φ = 0.8 and 1.2, respectively. By isolating the formation routes of NO production, it is found that the fast-rising trend of NO concentration near the wall in the post flame region at φ = 0.8 is attributed to the NO transportation from the NNH route primarily, while the prompt NO production accounts for more than 90% of NO generation in the wall vicinity at φ = 1.2. It is thus known that, thanks to the effectively increased surface-to-volume ratio, the premixed combustion process in the downsized chamber will be affected more easily by the amplified cooling effects of the cold wall, which will contribute to the declined combustion efficiency, increased CO emission, and improved prompt NO production.

A numerical model was constructed by Fluent software in this study with the objective to evaluate the model performance of predicting impinging flame heat transfer. The mechanism of methane-air-2step provided by the software and a Chemkin-compatible mechanism are used. With other conditions unchanged, the flame and heat transfer characteristics of the flame were computed and compared. Both mechanisms over predicted the height and maximum temperature of the flame by referring to the experimental data. The highest temperatures were found in small hot spots which are found to influence the impingement heat transfer at large nozzle-plate distance. Overall, the Chemkin mechanism gives a more reasonable temperature field and better predicts flame height and flame structure due to the more species and reactions considered. Both mechanisms can follow the experimental heat flux variation. The Chemkin mechanism better predicts heat transfer quantitatively, while the Fluent mechanism may be preferred by engineers for a qualitative analysis with lower computational cost.

The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H2 and CO2 on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H2 addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO2 proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO2 exerts the stronger suppression on the net reaction rate of H + O2=OH + O than that of H + CH3(+M) = CH4(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H2 addition as the CO2 content is enhanced. For the biogas-hydrogen fuel, the H2 addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO2 content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.

The combustion characteristics of laminar biogas premixed and diffusion flames in the presence of upward decreasing magnetic fields have been investigated in this study. The mechanism of magnet–flame interaction in the literature, in which magnetic fields change the behaviors of laminar flames due to the paramagnetic and diamagnetic properties of the constituent gases, is examined and the results are as follows. The magnetic field has no noticeable effect on premixed flames due to low oxygen concentration of the mixed gas at the injection and the relatively high flow momentum. However, due to the diffusion nature of diffusion flames and paramagnetic property of oxygen in ambient air, oxygen distributions are subjected to the gradient of magnetic flux, thus shortening the height of diffusion flames. Results also show that the flame volume is more strongly varied than flame height. Altered oxygen distributions result in improved combustion and higher flame temperature. In the case of current magnet–flame interaction, the magnetic driving force is combined with gravitational force, and a modified gravity g* as well as gravity modification factor G are derived to characterize the paramagnetism theory of oxygen.

Due to intensive marine activities and the use of low-quality fuel oils, the marine transport accounts for a considerable part of air pollution in the transportation sector. Although ships provide the convenient transportation, they exhaust a large number of hazardous pollutants, especially nitrogen oxides (NOx) and sulfur oxides (SOx), which have a significant side effect on environment and human health. To alleviate the impact of global shipping on the environment, international maritime organization established the more stringent emission regulations from Tier I to Tier III in order to reduce emissions from ships. As a result, various emission reduction technologies need to be developed in order to meet more stringent regulations in the future and reduce the pollutant emissions. Under this background, it is indispensable to examine the existing emission reduction technologies when exploring another possible method to reduce the pollutant emissions. Based on a significant number of related literatures, it is general to utilize the marine alternative fuels to reduce the pollutant emissions. Especially, liquid natural gas (LNG) is considered to be one of the most promising marine alternative fuels due to its economy and environmentally friendly features. This review thus aims to summarize the different emission reduction technologies of marine diesel engines through three reduction paths of fuel optimization, pre-combustion control and exhaust after-treatment. Furthermore, the utilization of LNG in the marine diesel engines are evaluated comprehensively from three aspects of environmental protection, energy structure and economic benefits. At the end, some suggestions on the future research are given based on the extensive review on the state-of-the-art literature.

This work describes an experimental study of the effect of hydrogen addition on the stabilization characteristics of laminar biogas diffusion flame. The focus is to identify and compare various factors influencing the blowoff process. Three compositions of biogas, BG40, BG50 and BG60 were considered and the amount of hydrogen added was varied from 5% to 25% of the biogas by volume. With increasing hydrogen addition, the critical flow velocity beyond which the flame blows off increases faster than the laminar burning velocity (LBV) does, indicating that flame stabilization is not solely dependent on laminar burning velocity. An exponential relationship is observed between LBV and flame propagation speed. Therefore, both flame propagation speed and LBV, together with other factors, contribute to flame stabilization. The reason for no stable lift for either biogas or H2-biogas flame is analyze by Schmidt number calculation, and the results agree with the literature. Also found is that hydrogen added to biogas accelerates the fuel mass diffusion, which may play an important role for stabilization of the nozzle-attached flame. The CO2-C3H8 and BG60 flames were compared to exclude the possible dominant role played by insufficient heat release and/or excessive heat loss due to CO2 present in biogas. Tested on varied-size burners show that flame stabilization depends on burner pore size, where larger diameter allows better flame stability. The universal equation for predicting blowout/blowoff velocity in the literature was found to be invalid for H2-enriched biogas flame and a new scaling law was put forwards.

The quenching distance, wall heat flux and pollutant thermochemical states of the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2 were investigated using the numerical simulation with detailed chemical kinetics and transport parameters. With the 10% H2 addition, the biogas composed of 75% CH4 and 25% CO2 was used as fuel in the study. Results show that PeQ ≈5 is obtained for the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2. The smaller PeQ than that of CH4 flame is resulted from the CO2 increasing the flame thickness, as well as the decelerated flame propagation due to the strong stretch effects. The quenching distance of biogas-hydrogen flame is codetermined by intrinsic characteristics of heat transfer process and effects of equivalence ratio and flame stretch on the flame propagation. The wall heat flux of laminar premixed biogas-hydrogen impinging flame is maximum at ϕ = 1.0, followed by ϕ = 1.2 and 0.8. At ϕ = 0.8 and 1.2, the location difference between the quenching point and peak wall heat flux of premixed biogas-hydrogen impinging flame is ascribed to the oxidization of escaped unburned fuel near the wall, and the worse field synergy of temperatue and velocity near the quenching point. Additionally, it is found that the fast NO accumulation near the wall at ϕ = 0.8 is ascribed to the transportation of NO generated by NNH route and the effective prompt NO production, while that at ϕ = 1.2 is resulted from the prompt NO production primarily. In the wall vicinity, the low temperature shift of NO production are resulted from the much faster characteristic time scale of heat transfer, while the NNH route and prompt route play the dominant roles in this low-temperature shifts of NO production at ϕ = 0.8 and 1.2, respectively. This means the increased importance of prompt and NNH routes on the NO emission in the downsized combustor.

Based on the previous work, this paper presents a thorough review of the laboratory studies of IDF (inverse diffusion flame), introducing its history, its current development and foreseeing the problems to be solved in future. The review is focused on the flow and combustion behaviors during the transition of IDF from laminar to turbulent, which are well documented in the studies of IDF impingement heat transfer, and thus is aimed to provide guidance for expediting future studies of soot formation in turbulent IDF. As a flame, IDF is influenced by the parameters of burner, fuel/oxidizer flows and chemical reactions, which can be overally classified into three types, i.e. geometrical, fluidynamic and combustion parameters. Based on these parameters, this review copes with a number of issues like IDF burner design, features of burner geometry, analysis of flow dynamics, flame structure and characters of thermal field. Firstly, the review focuses the burner geometric features, and presents a full picture of the burner design evolution in the past decades. Then, an attempt will be made to encompass the mixing mechanism between fuel and oxidizer, at either molecular or macroscopic levels. Next, an introduction is made to various flame patterns produced and their thermal and combustion characteristics. After this, the fluid mechanics and particular fluidynamic features of IDF will be fully recognized. Finally, the combustion and usages of various IDFs will be visited, gaining information of both local and global fluidynamic and thermal behaviors of IDF. Through this review work, it is intended to address the investigation need for future challenges in laboratory IDF studies as well as to put IDF into better practical applications.

Numerical studies on the 1-D laminar premixed flame were conducted utilizing the detailed mechanisms to evaluate the validity of several heat release rate (HRR) markers for the single-valued prediction on the HRR in a wide range of conditions. The biogas and methane with hydrogen enrichment are used as the fuel. The peak-to-peak correlation and the peak-to-integrated correlation of considered markers and HRR in premixed flames were established to examine their sensitivity to the equivalence ratio and the fuel composition. For both premixed biogas and methane flames considered, the O × CH 3 product has the almost constant correlation with peak or total HRR at different conditions. Thus the O × CH3 product is suggested as the most reliable indicator for the HRR imaging in the unstretched premixed flames with the unknown equivalence ratio. HCO is demonstrated to be a reliable marker in the premixed combustion, while the OH × CH 2 O, H × CH 2 O and H × HO 2 products are inadequate for the single-valued prediction on the HRR because their correlations with HRR are sensitive to the equivalence ratio, indicating that these markers cannot reflect the HRR change accurately if the local equivalence ratio is affected.

The total emissions (EICO, EINOx, EINO2 and NO2/NOx ratio) of laminar premixed biogas-hydrogen impinging flame were obtained experimentally while a computational model was developed to calculate the impinging flame. The NO amount produced via different routes (thermal, prompt, NNH and N2O routes) were isolated and calculated in the simulation. The effects of separated distance and equivalence ratio on the emission formation were discussed quantitatively. The results are summarized as follows. The lower values of EICO at small and large H are caused by the weakened fuel oxidization and improved CO oxidization, respectively, and the EICO is increased with ϕ owing to the increasingly incomplete fuel oxidization. The NO amounts of thermal, prompt, NNH and N2O routes and their contributions on total NO are affected considerably by the separated distance. The decreased prompt NO with H leads to the initial drop of EINOx at fuel-rich condition. The thermal NO and NNH route dominate the NO formation at ϕ = 0.8 and 1.0, while the contribution of prompt NO is improved significantly at fuel-rich condition. The rising trend of EINOx with ϕ is primarily predominated by the enhanced prompt NO. As H is enhanced, the rising and dropping trends of EINO2, as well as NO2/NOx ratio, are caused by more intensive air entrainment, promoting the NO2 formation, and the extended high temperature region, accelerating the NO formation and the NO2 destruction, respectively. The enhanced EINO2 with ϕ is primarily caused by the increased H atom in the air mixing region to promote the HO2 formation.

The effects of H2 and CO2 on the heat transfer characteristics of the laminar premixed biogas–hydrogen flame were investigated quantitatively. The equivalence ratio was kept to 1.2 so as to obtain the stable flames, and the heat fluxes and flame temperatures of laminar biogas jet flames with different H2 (10–50%) and CO2 (25–50%) volumetric fractions were measured experimentally, and that of the methane and LPG flames were also measured for the comparison purpose. In addition, the total heat transfer rates were calculated based on the experimental data. The results show that the total heat transfer rate of impinging biogas flames can be improved evidently with the hydrogen enrichment, and the optimum hydrogen enrichment is recommended to be αH2 = 0.3 based on this study. With this hydrogen enrichment, the total heat transfer rate can be enhanced by about 20–30% at all tested heating distances. The higher flame temperature and stronger diffusivity caused by H2 addition can lead to the larger temperature gradient and more heating area with the high temperature, respectively, which are the major reasons of its promotion effects. Besides, with the same hydrogen enrichment, the BG75 flame with the less energy input has a better heating performance than the methane flame, while the BG50 flame has the minimum total heat transfer rate. CO2 exerts its influences on the heat transfer characteristics through suppressing the reaction OH + CO = CO2 + H to weaken the combustion and lowering the flame temperature by its larger specific heat capacity and the reduced proportion of the combustible component. However, the larger specific heat capacity of CO2 can also lead to the moderate reduction of flame temperature in the wall jet region, which results in the better heating performance of the BG75 flame.

The effects of equivalence ratio , H 2 and CO 2 on the heat release characteristics of the premixed laminar biogas-hydrogen flame were investigated with the chemical kinetics simulation using the detailed chemical mechanism. The heat release rates, reaction rates and mole fractions of species of the BG50, BG75 and methane flames were calculated at different hydrogen additions (10%–50%) and equivalence ratios (0.8, 1.0, 1.2). The contributions of major elementary reactions were obtained based on the simulation data. The results show that H + O 2 = OH + H is the major endothermic reaction for biogas-hydrogen flame, while H + CH 3 (+M) = CH 4 (+M), O + CH 3 = H + CH 2 O, OH + H 2 = H + H 2 O, O + CH 3 =>H + H 2 + CO and OH + CO = H + CO 2 can always play significant roles in heat release. O + CH 3 = H + CH 2 O and O + CH 3 => H + H 2 + CO can consistently account for a relatively stable proportion of total heat release , meaning that the O × CH 3 product can be an indicator to predict the total heat release. Due to the variation of O 2 concentration, the changes of major exothermic reactions are predominated by the equivalence ratio. Though the global heat release rate is maximum at stoichiometric condition, there exists more heat release in the high temperature zone at rich condition due to the exothermic recombination of radicals. The total heat release can be increased evidently with the H 2 addition which can induce the early heat release and enhance the peak heat release rate, and the significances of OH + H 2 = H + H 2 O and H + OH + M = H 2 O + M on the total heat release are enhanced most evidently . CO 2 exerts influences on heat release characteristics through its dilution/thermal effect and chemical effect. As CO 2 is introduced, the decreasing trend of the global heat release rate is dominated by the dilution/thermal effect, and H + CH 3 ( + M) = CH 4 ( + M) , OH + H 2 = H + H 2 O and 2CH 3 ( + M) = C 2 H 6 ( + M) become increasingly important on the heat release.

This paper presents an experimental study of free flame patterns, impinging flame structures and flame impingement heat transfer characteristics of a partially premixed flame. The partially premixed flame is produced on a double-concentric-pipe burner, with both flow strength and mixedness of its inner and outer jets controlled by digital flowmeters. The aim of the study is, through examination of the effects of inner and outer jet Reynolds numbers (Rei, Reo), equivalence ratios (Φi, Φo) and nozzle-to-plate distance (H), to understand the heat transfer behaviors of such partially premixed flame and further identify the best impinging flame to achieve higher energy usage efficiency. A total of twelve different free flame patterns are observed by varying Rei-Reo-Φi-Φo, and W-shaped flame is the most representative flame under the conditions tested. The partially premixed dual flame is higher in stability than any single component of it, i.e. Bunsen flame or annulus flame. Further, leaner flammability limit is achieved by the dual flame due to mutual interaction which effectively supports leaner inner flame. Both inner and outer flames influence the radial heat flux distribution of W flame, while the contribution of inner flame becomes significant only when the difference in two flame heights is large. The outer flame globally dominates the total heat transfer rate, leading to general decay with increasing H. Comparison of W flame with single Bunsen and annulus flame shows that W flame has the favorable structure for higher energy efficiency due to double-pipe configuration and mutual flame interaction.

Experimental and numerical studies were conducted to investigate the effects of CO2 proportion on the formation processes and overall emissions of CO and NOx of the laminar premixed biogas-hydrogen flames undergoing the flame-wall interaction (FWI). The overall EICO, EINOx, EINO2 and NO2/NOx ratio were obtained. The near-wall combustion processes of the wall-impinging flame at different separated distance (H) were simulated numerically using a computational model, and four major routes (thermal route, prompt route, NNH intermediate route and N2O intermediate route) of the NO production were calculated separately. The results show that the EICO decreases at small H but increases at large H with the increased CO2 proportion, which is dominated by the cooling plate and dilution/chemical effects of CO2, respectively. This indicates that the overall EICO of biogas fuel can be reduced effectively with the enhanced CO2 proportion when the FWI effects are intensive. The overall EINOx is decreased obviously with the increased CO2 proportion owing to the declined temperature and reduced active radicals. With the increased CO2, the contributions of thermal route and N2O route are decreased considerably while the importance of prompt route and NNH route are increased evidently. The strong FWI effects can increase the importance of prompt route while decrease the contribution of thermal route effectively. The EINO2 of biogas-H2 flame is decreased with the increased CO2 proportion. The dilution effects of CO2 can improve the NO2 conversion efficiently but this NO2 conversion process can be suppressed effectively by the intensive FWI effects.

This work presents an experimental study the effects of DC electric field on a laminar premixed flame which impinges on a plate. Positive DC and ground levels of voltage are applied to the plate and burner, respectively. A reverse application of the electric field is also tested. Detailed measurements are undertaken to understand the ionic wind effect on flame impingement heat transfer. Testing shows a favorable effect of positive field on flame stability, as the flame stands leaner combustion. When a sufficiently high voltage is applied, discharge occurs which breaks down the electric field. The results of thermal field testing reveal that the temperature change observed is due to flame shape distortion caused by ionic wind. It is found that ionic wind is majorly physical in nature, inducing the changes in flame stability and temperature through a force. Further, it is mainly the post-combustion part of the flame, instead of reaction zone, bears such force. Under positive field, the burner nozzle is rapidly heated by the flame. However, flame impingement heat transfer has no significant change under either positive or reverse field. It is because heat loss from the post-combustion flame to plate is less altered by DC fields. To further study the ionic wind effect, a co-flow is introduced to the flame, and the standard deviation data of flame temperature indicate that ionic wind insignificantly changes the mixing between flame and ambient air. It confirms that ionic wind is mainly a force effect, with negligible chemical effect in the scope of this work.

Turbulence induced by actuators has been widely utilized as a effective means to enhance jet impingement heat transfer (JIHT). Flame impingement heat transfer (FIHT) has parital similarity to JIHT, and thus it is of interest to investigate the role of an actuator-induced turbulence to flame combustion as well as its FIHT. On a Bunsen burner that is incorporable with a ring-type turbulence actuator, a detailed experimental comparison of the flame appearance, reaction zone structure, thermal field and heat transfer of both laminar and turbulent flames under identical bulk flow conditions and equivalence ratios were conducted. Results show that the turbulent flame has a narrower stable-operation range than the laminar couterpart, and thus turbulence reduces the flame stability. A major difference between two flames exists at the reaction zone, which is identified to be a ‘brush’ structure in the turbulent case, instead of being a perfect cone in the laminar case. The flame brush changes the heat release behavior such that the temperature field of the turbulent flame shows lowered peak temperature while enlarged high-temperature zone. The changed heat release or temperature field reults in distinct local heat fluxes in the impinging flames at the stagnation region and near wall jet region. Consequently, the overall heat transfer rate of the turbulent flame becomes higher than the laminar flame. Accordingly, this study proposes that the actuator turbulence can be utilized as a useful technique to promote FIHT.

Experimental measurements and numerical simulations were both conducted to study effects of unburned gases velocity on the formations and overall emissions of CO and NOx of the laminar premixed biogas-hydrogen impinging flame. Emission indexes (EI) of CO and NOx and NO2/NOx ratio were obtained based on the measured data, while four major routes to produce the NO were isolated and calculated in the simulation. The results show that EICO is decreased with Re at small H, while it is increased with Re at large H. The thermal NO is improved with Re as the flame front is affected strongly by the cold plate, while it is suppressed at larger H due to the reduced residence time and relatively insufficient oxidizer. The N2O route is decreased with Re, while the increased NO amounts of prompt route and NNH route are dominated by the higher flame temperature and more available active radicals. The contributions of thermal route and N2O route are decreased with Re while that of prompt route and NNH route are increased. With the increased Re, EINO2 and NO2/NOx ratio are decreased at small H but increased at large H.

In this study, a double concentric burner burning methane with an annular coaxially-flowing oxidizer was adopted to operate the diffusion flame in lifted flame regime. The effects of coaxial-flow velocity, coaxial-flow composition variation through total and partial replacement of N2, and coaxial-flow oxygen enrichment were experimentally investigated in terms of the resultant changes in the flame stability, and thermal and emission characteristics. Consistent with the triple flame theory, the current stability tests show a linear increase in flame lift height with increasing coaxial-flow velocity and the blowout of lifted flames occurred at constant flame tip height. Replacement of N2 by CO2 in the coaxial-flow deteriorated the flame stability by significantly reducing the threshold coaxial-flow velocity. Due to combustion enhancement that is caused by oxygen enrichment, the threshold coaxial-flow velocity increased and this increase is more significant for the N2-diluted flame than CO2-diluted. Two of the most important NOx formation mechanisms, Zeldovich and Fenimore, were analyzed under the relatively low temperature flame conditions, generally below 1300 °C in this study. Results show that NOx is principally produced via the Fenimore mechanism for both N2- and CO2-diluted flames. NOx productions can be significantly affected by coaxial-flow composition and coaxial-flow velocity. An increase in the velocity of N2-diluted coaxial-flow increases NOx emissions, while a reverse trend occurred, as N2 in the coaxial-flow was replaced or partially replaced by CO2, which is ascribed to the strong combustion-resisting behavior of CO2. For all cases, CO emissions vary in the opposite direction of NOx emissions. Due to the strong thermal and chemical effects of CO2 on combustion in comparison to N2, total or partial replacement of N2 by CO2 results in a steep increase in CO emissions.

Overall pollutants emission from impinging premixed and diffusion flames were measured by investigating its dependence on burner-plate separation distance, fraction of hydrogen in the biogas-hydrogen mixture, Reynolds number and equivalence ratio. Results show that premixed flame is cleaner with both low CO and NO x emissions than diffusion flame, either under open or impingement conditions. Impinging flame emits more CO while less NO x than the corresponding open flame. For both premixed and diffusion flames, CO emission firstly increases at separation distance of H H > 15 mm. While NO x emission monotonically increases with H . NO x emission also increases with fraction of hydrogen, equivalence ratio and Reynolds number, due to its dependence on both temperature and volume of the flame. The lowest CO emission occurs at the slightly rich condition at equivalence ratio of 1.2. This low CO emitting behavior is favorable for cooking applications, where slightly fuel rich flames are adopted.

An experimental study was performed to investigate the heat transfer characteristics of a laminar premixed biogas-hydrogen flame jet impinging upwards normally on a flat plate. The effects of the distance between the nozzle and the impingement plate (H = 5–30 mm), flame jet Reynolds number (Re = 600, 800, 1000) and equivalence ratio (ϕ = 0.8–1.4) were examined, and an empirical correlation to predict the optimized heating distance of impinging laminar premixed flame is obtained based on the experimental data. The results show that the increased velocity of unburnt gas can effectively enhance the local heat flux as well as the total heat transfer rate through the stronger forced convection and more energy input. The enhancement in local heat flux away from the axis gives rise to a large improvement in total transfer rate, indicating that the flame at the rich condition can have a better heating performance due to the diffusive combustion. The total heat transfer rate reaches to its maximum at a certain distance which is influenced by the flame height and equivalence ratio. A nondimensional correlation is obtained to determine the optimized heating distance as a function of Pr, u/SL and ϕ. The correlation indicates that the optimized heating distance has a positive relationship with the unburnt gas velocity u and a negative relationship with the laminar burning velocity SL, and it would increase at either lean or rich condition with the different rising rates.

Effects of stretch and preferential diffusion on tip opening of laminar premixed Bunsen flames of syngas/air mixtures were investigated with experiments and numerical simulation. The reaction zone of the laminar flames at different hydrogen fractions ( X H 2 = 0.6, 0.8), equivalence ratios ( φ = 0.6–1.0) and Reynolds numbers ( Re = 1400, 1800) was obtained by OH-PLIF technique, while the species profile, velocity field, etc. were simulated by STAR-CD. The stretch rates due to the flame curvature and flow strain was calculated separately combined with experimental image and simulation. The results show that the flame tip of laminar premixed Bunsen flame is strongly negative stretched with a strong negative stretch due to flame curvature combined with slight positive stretch due to flow strain. At lean condition, the local preferential diffusion at the flame tip is intensified due to the maximum negative stretch, which promotes the mass diffusion of the fuel at the flame tip and makes the fuel mixture much leaner. This results in the reduction of local combustion intensity and is the main reason for the flame tip opening. The tip opening is promoted with the decrease of equivalence ratio, the increase of hydrogen fraction and the increase of Reynolds number. The strong negative stretch due to flame curvature at flame tip combined with preferential diffusion is the dominant effect of the flame tip opening. While, the slight positive stretch due to flow strain has weak effect.

The overall emissions of CO, NOx and NO2/NOx ratio of laminar premixed biogas-hydrogen flames undergoing the flame-wall interaction (FWI) were measured experimentally while the emission indexes (EI) were calculated. The computational model was employed to calculate the combustion process of the biogas-hydrogen flame, and four major routes to produce the NO were isolated and calculated in the simulations. The effects of H2 addition on the formations and emissions of CO/NO2/NOx of BG75-hydrogen impinging flames were analyzed quantitatively. The results show that, with the H2 addition, the EICO of BG75-hydrogen flame is enhanced steadily at either small separated distance (H) or large H thanks to that the high reactivity of H2 suppresses the CO oxidization efficiently by competing for the OH radical. The H2 addition can improve the NO production via four routes and increase the EINOx of the BG75-hydrogen flame. Although the prompt NO is improved by the H2 addition, its rising trend is decreased considerably as hydrogen addition is larger than 20% owing to the reduced methane proportion and improved premixed combustion. The contribution of prompt NO is decreased with the H2 addition, while that of other routes are increased steadily. The NNH route dominates the increased EINOx at small H while the thermal route dominates the rising trend at large H. EINO2 is enhanced steadily with the H2 addition due to the improved HO2 production in the air mixing region, while the NO2 conversion ratio is improved and suppressed with H2 addition at small and large H, respectively.

In order to investigate oxyfuel combustion characteristics of typical composition of coal gasification syngas connected to CCS systems. Instantaneous flame front structure of turbulent premixed flames of CO/H2/O2/CO2 mixtures which represent syngas oxyfuel combustion was quantitatively studied comparing with CH4/air and syngas/air flames by using a nozzle-type Bunsen burner. Hot-wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Image processing and statistical analyzing were performed using the Matlab Software. Flame surface density, mean progress variable, local curvature radius, mean flame volume, and flame thickness, were obtained. Results show that turbulent premixed flames of syngas possess wrinkled flame front structure which is a general feature of turbulent premixed flames. Flame surface density for the CO/H2/O2/CO2 flame is much larger than that of CO/H2/O2/air and CH4/air flames. This is mainly caused by the smaller flame intrinsic instability scale, which would lead to smaller scales and less flame passivity response to turbulence presented by Markstain length, which reduce the local flame stretch against turbulence vortex. Peak value of Possibility Density Function (PDF) distribution of local curvature radius, R, for CO/H2/O2/CO2 flames is larger than those of CO/H2/O2/air and CH4/air flames at both positive and negative side and the corresponding R of absolute peak PDF is the smallest. This demonstrates that the most frequent scale is the smallest for CO/H2/O2/CO2 flames. Mean flame volume of CO/H2/O2/CO2 flame is smaller than that of CH4/air flame even smaller than that of CO/H2/O2/air flame. This would be due to the lower flame height and smaller flame wrinkles.

The application and fundamental study on turbulent premixed combustion of hydrogen enriched natural gas is reviewed in this paper. Discussions include the combustion characteristics of direct injection engine fueled with hydrogen enriched natural gas, visualization study of direct injection combustion of hydrogen enriched natural gas using a constant volume vessel, and the fundamental study of turbulent premixed combustion of hydrogen enriched natural gas. The effect of additional hydrogen on the combustion process of natural gas engine is investigated from the fundamental view of the interaction between combustion reaction and turbulent flow.

Instantaneous flame front structure of turbulent premixed CH 4 /H 2 /air flames (hydrogen fraction of 0%, 5%, 10% and 20% by mole fraction) was investigated quantitatively using a nozzle-type Bunsen burner. Hot wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Turbulent burning velocity, S T , flame surface density, Σ , and mean flame volume, V f , were calculated by processing the OH-PLIF images. Results show that the flame front structures of the turbulent premixed flames are the wrinkled flame front and it becomes much finer with the increase of turbulence intensity as well as hydrogen fraction. The value of S T /S L significantly increases with the increase of u ′/ S L and it slightly increases with the increase of hydrogen fraction. Flame surface density profile are symmetric and gives its maximum value at about 〈 c 〉 = 0.5. Hydrogen addition slightly enhances the Σ and the tendency is more obvious under higher turbulence intensity. The decrease of Σ with the increase of turbulence intensity is mainly due to the effect of flame volume. The mean flame volume of flame region obviously increases with the increase of turbulence intensity within the experimental range due to the increase in depth of the large scale flame wrinkles and flame height. Hydrogen addition is not a predominant factor within the hydrogen fraction range in this study.

Flame front structure of turbulent premixed CH4/H2/air flames at various hydrogen frac- tions was investigated with OH-PLIF technique. A nozzle-type burner was used to achieve the stabilized turbulent premixed flames. Hot-wire anemometer measurement and OH- PLIF observation were performed to measure the turbulent flow and detect the instanta- neous flame front structure, respectively. The hydrogen fractions of 0%, 5%, 10% and 20% were studied. Results show that the flame front structures of the turbulent premixed flames are wrinkled flame front with small scale convex and concave structures compared to that of the laminar-flame front. The wrinkle intensity of flame front is promoted with the increase of turbulence intensity as well as hydrogen fraction. Hydrogen addition pro- motes the flame intrinsic instability which leads to the active response of laminar flame to turbulence and results in the much more wrinkled flame front structure. The value of ST/SL increases monotonically with the increase of u 0 /SL and hydrogen fraction. The increase of ST/SL with the increase of hydrogen fraction is mainly attributed to the diffusive-thermal instability effects represented by the effective Lewis number, Leeff. A general correlation between ST/SL and u0/SL is provided from the experimental data fitting in the form of ST/