Hydrogen-oxygen Flame Research Articles

Few-Mode Fiber-Based Long-Period Fiber Gratings: A Review

Long-period fiber gratings (LPFGs) are efficient ways to achieve high-order core mode conversion and vortex mode conversion in few-mode fibers (FMFs), which have the benefits of flexible structure, high integration, low insertion loss, and strong wavelength selectivity. With the rapid development of mode division multiplexing (MDM) optical communications, the FMF-LPFGs could have promising applications in the field of MDM optical communications and optical sensors. In this paper, we briefly introduce the principle of optical coupling, theoretical analysis, fabrication techniques, and applications of the FMF-LPFGs in optical communications and optical sensors. The CO2-laser writing technique and arc discharge technique are widely used for the fabrication of FMF-LPFGs and FMF helical long-period gratings (HLPGs). The hydrogen-oxygen flame technique has the advantage on the fabrication of FMF-HLPGs. The mechanical micro-bending and acoustic induction techniques are flexible and effective methods in fundamental researches. The femtosecond laser technique has the advantages for the fabrication of parallel FMF-LPFGs and micro-structure FMF-LPFGs. The mode converters based on the FMF-LPFGs have been demonstrated using different inscription techniques. The FMF-LPFG converters have the advantages such as temperature insensitivity and wavelength tunability. The broadband converters have been demonstrated based on the phase-shift FMF-LPFG, cascaded FMF-LPFGs, and FMF-LPFG operating at dispersion turning point. The FMF-LPFGs and FMF-HLPGs are an excellent option to generate all-fiber orbital angular momentum (OAM) modes in different specialty FMFs, especially the FMF-HLPGs can excite different-order OAM modes directly. As the novel fiber component, the sensing application of FMF-LPFGs and FMF-HLPGs is also briefly introduced.

Cellular instabilities of outwardly propagating spherical hydrogen-oxygen flames using a soap bubble method

Explosion hazards associated with hydrogen-oxygen mixtures remain a significant challenge for the energy industry, despite hydrogen garnering attention as a carbon emissions-free alternative energy source. While several studies have been conducted on hydrogen-air explosions, research on hydrogen-oxygen explosions has been limited. In the current study, we employed a spherical soap bubble method to experimentally investigate the flame propagation behavior of an unconfined hydrogen-oxygen explosion. The experimental values of the laminar burning velocity were in agreement with its calculated values and were notably higher than those observed hydrogen-air mixtures. In the case of hydrogen-oxygen mixtures, a spherical expanding flame is accelerated due to the presence of Darrieus-Landau and diffusive-thermal instabilities. Furthermore, we compared the correlation between the critical Pѐclet number and the critical Karlovitz number with the Markstein number for hydrogen-oxygen mixtures to results obtained with propane-oxygen and methane-oxygen mixtures.

Surface Carbonized Natural Wood via Hydrogen–Oxygen Flame for Electricity Generation from Water Evaporation

Water evaporation-induced power generation technologies based on the interaction between water and nanomaterials have received wide attention over the years. Herein, a natural wood-based water evaporation-induced power generator (EIPG) via the surface carbonization technique under high-temperature hydrogen–oxygen flame was developed. The unique surface carbonization strategy made the internal layer (uncarbonized layer) maintain high hydrophilicity to satisfy the stable and continuous capillary flow for the transport of ions. The internal layer possessed a higher surface charge density, promoting the establishment of the electric double layer (EDL), which ensured high selectivity of the wood channels for positive ions. Meanwhile, the external carbonized layer could effectively adsorb positively charged ions through strong cation−π interactions to strengthen the EDL effect. The carbonized layer also provided an electrical charge transportation channel, lowering the resistance of the device. The device could continuously generate power with an open-circuit voltage (Voc) of ∼170 mV and a short-circuit current (Isc) of ∼730 nA over 24 h, exhibiting excellent stability. Moreover, a higher electric output was generated by the water EIPG through assembling several devices in series or parallel and applied successfully in charging capacitors and powering a commercial digital calculator. This work provides a different strategy for developing an environment-friendly and low-cost water EIPG utilized in harvesting green energy, with no chemical treatment.

On the Critical Condition for Flame Acceleration in Hydrogen-Based Mixtures

The paper presents a novel numerical approach to the quantitative estimation of the concentration limits for flame acceleration in hydrogen-based mixtures. A series of calculations are carried out for hydrogen-air and hydrogen-oxygen flames in channels. The analysis of the obtained numerical results provided the value of 11 ± 0.25 % hydrogen content in the mixture as a lean concentration limit of flame acceleration that agrees well with the available experimental data. Moreover, the basic physical mechanism responsible for the transition from the steady mode of flame propagation to the accelerated one is distinguished. The mechanism is related to flame stretching in the region of interaction with the boundary layer and the competition between the joint increase in burning rate and heat losses. The novel technique for the estimation of concentration limits of flame acceleration presented here can be applied to assess combustion conditions inside combustors of energy and propulsion systems fed with hydrogen. The results are also useful in estimating explosion and fire risks in hydrogen storage, transport, and utilization facilities as parts of hydrogen energy and propulsion systems.

РАЗРАБОТКА СПОСОБА ВОССТАНОВЛЕНИЯ И УПРОЧНЕНИЯ ЛЕМЕХОВ ПЛУГОВ МЕТОДОМ ПАЙКИ МЕТАЛЛОКЕРАМИЧЕСКИХ ПЛАСТИН

The main defect that occurs when using plows is an abrasive wear of its ploughshares, the greatest wear of which is observed in the nose. When renestoring worn plowshares in order to significantly increase their service life, it is advisable to use technical ceramics. A plowshare with soldered ceramic-metal plates was chosen as the investigation object. The proposed investigation methodology included the control of plate adhesion, as well as bench and operational tests of experimental plowshares in comparison with serial ones. It has been established that the proposed type of connection «metal-ceramic plates−ploughshare» has high reliability and can withstand exceeding design loads by 3.8—4.9 times. As a result of the investigation, a new service- life-saving technology has been developed for the restoration and hardening of ploughshares by soldering using a hydrogen-oxygen flame, which allows one to increase the resource by 2-3 times, as well as reduce costs by 30−35% instead of purchasing new serial ploughshares due to using a cheap hydrogen-oxygen gas mixture.

Thermal diffusion coupling mechanism and its application of discrete waveguide

For discrete optical systems integrated into optical fibers, the optical fields of the individual waveguides are coupled and correlated with each other. This paper studies how to adjust the refractive index of discrete waveguides by thermal diffusion, so as to enhance the coupling between discrete waveguides, and also constructs the discrete waveguide thermally diffused model and the thermally diffused coupling model of twin-core and three-core fibers. The multicore fiber is heated different times by a hydrogen-oxygen flame, and the outgoing light field at the end face of the optical fiber is monitored at the same time. Then, the three-dimensional refractive index measurement results of the thermally diffused multicore fiber verify the feasibility of thermal diffusion technology to change the refractive index of discrete waveguides for coupling. Thermal diffusion technology can be used to fabricate multicore fiber couplers. By combining multicore fiber and core-by-core inscribed fiber Bragg grating technology and by using thermal diffusion technology, the single-channel sensing measurement can be realized. The method of changing the refractive index of discrete waveguides through thermal diffusion has the advantages of high integration, high stability, and mass fabrication. The research on the thermal diffusion of discrete waveguides can improve the application potential of multicore fiber sensing systems, and promote the broad application of discrete waveguide structure optical fiber in the fields of optical communication, optical sensing, biomedicine, and artificial intelligence.

U-shaped microfiber sensor coated with PVA nanofibers for the simultaneous measurement of humidity and temperature

In order to measure humidity and temperature simultaneously, a U-shaped microfiber strain sensor coated with polyvinyl alcohol (PVA) nanofibers is proposed in this paper. The PVA nanofibers and U-shaped microfiber are prepared by electrospinning technology and hydrogen-oxygen flame heating technology, respectively. The sensing mechanism is that the radius of U-shaped microfiber will change when the temperature and humidity change, because the expansion and stress-strain properties of the coated PVA nanofibers are tightly related with inside water content which can be greatly influenced by the temperature and humidity in the environment. The temperature and humidity are measured by the wavelength changes of the resonant dips of the sensors. The experimental results show that the average sensitivity of the sensor is 0.21 nm/%RH in the range of 20 %RH to 75 %RH, and the average sensitivity is − 0.19 nm/℃ in the temperature range from 30 ℃ to 50 ℃. Sensors with excellent performance (good repeatability, fast dynamic response, and stability) in this work are expected to play a role in practical applications in many industries.

Effects of hydrogen–oxygen torch igniter combustion to an ABS-GOx hybrid rocket system

A hydrogen-oxygen torch igniter was developed and utilized to study its effects on a solid acrylonitrile butadiene styrene thermoplastic (ABS)-gaseous oxygen (GOx) hybrid rocket motor. The effects thereof are not presently found in literature. An increase of the hydrogen mass flow rate, at constant GOx mass flow rate, led to an increase in the average linear regression rate and a decrease in characteristic ignition time for fuel-lean and stoichiometric hydrogen-oxygen flames. Additionally, extending the duration of hydrogen flow from 10% to 50% of the total burn time led to an increase in the regression rate. Results suggest that the hydrogen-oxygen torch igniter can be used as a reliable starter with reignition capabilities and also for regression rate and ignition delay enhancement that targets the rate-limiting step, pyrolysis, for hybrid rocket combustion at stoichiometric and fuel-lean operating conditions of the torch igniter.

High-sensitivity strain sensor based on a helical-core long-period fiber grating.

A highly sensitive strain sensor based on a helical-core long-period fiber grating (HC-LPFG) is proposed and experimentally investigated. The helical core is fabricated in the common single-mode fiber by using a high-frequency CO2 laser and hydrogen-oxygen flame. This helical shape core of the structure experienced the highly centralized refractive index modulation, which enhances the strain sensitivity and shortens the length of the sensing area to 2 mm. Experimental results indicate that the maximum strain sensitivity of the HC-LPFG reaches -97 pm/µɛ within the measuring range of 0-400 µɛ.

Hydrogen-oxygen flame acceleration in narrow open ended channels

The problem of sudden flame acceleration in narrow channels and subsequent Deflagration to Detonation Transition (DDT) is revisited. The hydrogen–oxygen combustion system is considered both experimentally and numerically. The flame is initiated at one open end of a square narrow channel of different widths (4, 8, 10 and 20 mm) and propagates to another open end. Experimental results show a high sensitivity to the mixture composition: for higher mixture reactivity, an abrupt flame acceleration is reported. In 4 × 4 mm2 channel, when the mixture composition is close to stoichiometry (2:1), the DDT is observed without any evidence of shock waves prior to the detonation transition. Two complementary sub-models are combined to account for the effect of walls on flow and on flame front geometry. A pseudo-spectral numerical scheme is used to integrate the system of equations both in time and in space. The simulation results show how the suggested model reproduces main features of the phenomenon. A simple criterion is suggested for the onset of the flame self-acceleration. There exists a critical flame folding factor for the DDT, which is an invariant of the system. The reported critical folding factor is compared to the theoretical estimation introduced by Gordon et al (2020).

Obtaining pseudo-OH* radiation images from CFD solutions of transcritical flames

The quantitative comparison of experimental data and results from CFD simulations is still an ongoing challenge in the investigation of high pressure combustion in rocket combustion chambers. This is due to the extreme environment which develops in liquid propellant rocket engines, which represent a challenge for experimental data collection. OH* radiation emitted from the flame has often been designated as an indicator of the combustion zone, because of its relative ease of detection with appropriate cameras. A method was developed to compare OH* radiation originating from cryogenic oxygen-hydrogen flames in an experimental combustor with the CFD simulation results. Pseudo-OH* images were obtained from CFD results of two combustors. The method consists in obtaining the path of a ray of light by a reverse ray tracing algorithm and sampling the thermodynamic properties along the path of the ray, simulating the emission and absorption spectra in the wavelength range of interest, in this case of OH* emission during combustion. The spectral radiance is then determined by solving the differential radiative transfer equation. Finally, the total radiance is calculated integrating the spectral radiance. The results obtained applying this method are then compared with former results of two test cases, a laminar and a turbulent flame, and with the related experimental data. An improvement of the comparison with the experimental data was achieved in terms of the prediction of self-absorption, which was underestimated in previous works by a factor of 15, and in terms of radiance near the injection plane, where difference is estimated to be about 40% when including refraction. The method allows for more direct comparison between 3D CFD results and 2D experimental images collected by the optical setup and probes.

Determination of silanol group content on the surface of fumed silica by chemical reaction-headspace gas chromatography

建立了一种基于化学反应-顶空气相色谱测定气相二氧化硅表面硅羟基含量的新方法。实验取气相二氧化硅放入顶空瓶中于105 ℃烘箱中加热2 h去除水分,将甲苯稀释的格氏试剂注入密闭的顶空瓶中,格氏试剂与气相二氧化硅表面硅羟基快速反应产生甲烷(CH4),甲烷量与气相二氧化硅表面硅羟基含量成正比。经过气相色谱-氢火焰离子化检测器测定甲烷,通过外标法定量,根据化学反应方程式计算出样品中羟基含量。同时对反应溶液用量与反应时间等条件进行优化,确定2.0 mL反应溶液,反应15 min为最优的前处理条件。结果表明,硅羟基含量与气相色谱信号值之间存在良好的线性相关性,相关系数为0.9990,相对标准偏差小于3%,本方法的检出限为0.30 mg/g,定量限为1.00 mg/g,开展了4家实验室对5个不同比表面积的样品测试,数据结果的重复性限(r)小于2.5%,再现性限(R)小于6.5%。该方法结合自动化技术,顶空反应操作简单,样品量和试剂用量少,准确性高,重复性好,优于酸碱滴定法,适用于快速检测气相二氧化硅表面硅羟基的含量,解决了硅羟基利用传统方法难以准确测定的难题。该方法的建立对我国二氧化硅产业硅羟基检测标准的制定和产业技术优化,均具有重要的理论和现实意义。

Review of Helical Long-Period Fiber Gratings

In this paper, comprehensive remarks are given that focus on the main fabrications and wide applications of helical long-period fiber gratings (HLPGs). Firstly, the techniques of fabricating HLPGs by CO2 laser, hydrogen–oxygen flame heating, and arc discharge are summarized. Furthermore, the applications of HLPGs are investigated, i.e., orbital angular momentum (OAM) mode converters, all-fiber band-rejection filters, and sensors for measuring physical perturbation of torsion, strain, temperature, curvature, and surrounding refractive index (SRI). Furthermore, several long-period fiber gratings (LPFGs) of near-HLPG structures with periodic refractive index change along the azimuthal direction are introduced. Lastly, the prospects and key challenges for HLPGs are discussed.

Initiation characteristics of spark-ignited premixed hydrogen-oxygen flames with diluents of argon/nitrogen/carbon dioxide

The spark-ignition features of premixed H2 under Ar–O2 atmospheres are essential for the development of high-efficiency and zero-emission Argon Power Cycle engines. Therefore, constant-volume combustion vessel experiments are conducted at an initial temperature of 287 K, initial pressures of 0.1, 0.3, and 0.5 MPa, with diluents of Ar/N2/CO2. Excess oxygen ratio, defined as the ratio of O2 supplied to the O2 needed for stoichiometric combustion, ranges from 1.0 to 4.7. Dilution ratios, defined as the molar fractions of diluents in the diluent-O2 mixture, were separately 70%, 79%, and 88%. Results show that both high dilution ratios and high excess oxygen ratios lengthen the flame-initiation periods. With excess oxygen ratio of 3.3, when Ar dilution ratio increases from 79% to 88%, the flame-initiation period rises by a factor of five to ten. With Ar diluent ratio of 79%, when excess oxygen ratio increases from 2.5 to 4.7, the flame-initiation period rises from under 10 ms to over 60 ms. Both the application of Ar as the diluent and high initial pressures extend the flammability limits. With Ar dilution ratio of 79% and pressure no less than 0.3 MPa, the flammability limit on excess oxygen ratio is over 4.7. Furthermore, Ar can additionally shorten the flame-initiation periods. At 0.1 MPa, excess oxygen ratio of 2.5, when diluents are respectively Ar, N2, and CO2, the flame-initiation periods are 10, 19, and 41 ms. The research reveals that a spark-ignition Argon Power Cycle engine running with ultra-lean and highly diluted mixtures may be feasible.

The Taylor–Green vortex as a benchmark for high-fidelity combustion simulations using low-Mach solvers

Verification and validation are crucial steps for the development of any numerical model. While suitable processes have been established for commercial Computational Fluid Dynamics (CFD) codes, more difficult challenges must be faced for high-fidelity solvers. Benchmarks have been proposed in a series of dedicated conferences for non-reacting configurations. However, to our knowledge, no suitable approach has been proposed regarding turbulent reacting flows. The purpose of this article is to present a full verification and validation chain for high-resolution codes employed to simulate turbulent reacting flows, first for Direct Numerical Simulation (DNS) of combustion in the limit of low Mach numbers. The selected configuration builds on top of the Taylor–Green vortex. Verification takes place by comparison with the analytical solution in two dimensions. Validation of the single-component flow is ensured by comparisons with published results obtained with a pseudo-spectral code. Mixing without reaction is then considered, before computing finally a hydrogen-oxygen flame interacting with a 3-D Taylor-Green vortex. Three low-Mach number DNS solvers have been used for this study, demonstrating that the final accuracy of the simulations is of the order of 1% for all quantities considered. All data-sets are publicly available under [1]. The performance of the codes is finally discussed, both in terms of single-node results and regarding parallel efficiency.

Surface plasmon resonance humidity sensor based on twisted long period fiber grating coated with tungsten disulfide film

A novel surface plasmon resonance (SPR) sensor based on tungsten disulfide (WS2) for relative humidity (RH) sensing is proposed. A spiral twisted long period fiber grating (ST-LPFG) is fabricated by twisting the single-mode fiber (SMF) by hydrogen oxygen flame heating. A layer of gold film is deposited on the surface of the ST-LPFG, and then a layer of WS2 film is deposited on the surface of gold film by natural evaporation. The results showed that the strength of the evanescent field on the surface is enhanced by the WS2 film, which enhances the interaction between the SPR wave and the external matters. These features significantly improve the sensitivity of the proposed humidity sensor. The experimental results show that the sensitivity of the humidity sensor based on WS2 film coated ST-LPFG is up to 37.3 pm/% RH. At the same time, the humidity sensor based on WS2 nanomaterials has good response characteristics. The sensing scheme proposed in this paper provides a promising platform for humidity application.

Propagation of hydrogen–oxygen flames in Hele-Shaw cells

Flame propagation in Hele-Shaw cells with a micro-sized gap was experimentally investigated. The evolution of flame front morphology was recorded via Schlieren photographs as the hydrogen-oxygen (H2–O2) mixture was ignited at ambient temperature and pressure. By varying gap size, two different regimes of flame propagation are identified: 1) the non-accelerating flame in narrow gaps; 2) the self-accelerating flame in relatively wide gaps. For the former, the initial flame front is globally circular, and subsequently evolves into branches separated from the surface, exhibiting dendritic-growth and fingering shapes. In the latter regimes, the flame front exhibits a cellular structure and accelerates nearly sonic speed due to hydrodynamic instabilities. It is found that the flame acceleration depends non-monotonically on the gap size due to the competing mechanisms of viscosity friction and heat loss through the walls. The effect of equivalence ratio on the non-accelerating flame is studied to identify the mechanism controlling the local extinction flame.

Strain, torsion and refractive index sensors based on helical long period fibre grating inscribed in small-core fibre for structural condition monitoring

This study is intended to develop long period fibre grating sensors for potential applications in environmental and durability monitoring of coastal structures. High-quality helical long period fibre gratings (HLPFGs) are inscribed in different types of small-core single mode fibre (SMF) by use of hydrogen-oxygen flame heating technique. A detailed investigation of the effect of core diameter on their transmission spectrum and optimum length of the HLPFG has been pursued. A longer length is required to achieve the same coupling attenuation in a smaller-core SMF than that of a larger-core fibre. The strain, torsion and refractive index (RI) properties of the HLPFG is investigated experimentally to develop a high-sensitivity sensor. The experimental results show that the strain sensitivity could be enhanced by means of employing a larger-core diameter SMF. Moreover, the HLFPGs are also sensitive to the torsion and external RI. Hence, such HLFPGs have great potential for sensing applications.

Helical Long Period Fiber Grating Inscribed in Elliptical Core Polarization-Maintaining Fiber

A high-quality helical long period fiber grating (HLPFG) in elliptical core polarization-maintaining fiber (PMF) was experimentally demonstrated by using the hydrogen-oxygen flame heating technique. Such a single HLPFG with a length of 6.5 mm could be used to generate orbital angular momentum (OAM) modes, i.e., OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±2</sub> . The secret of the successful generation of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±2</sub> mode is that the elliptical core PMF has two-fold symmetry in the cross-section. The resonance dip of the coupling attenuation and wavelength was strongly dependent on the length and helical pitch of the HLPFG. The HLPFG also exhibited a higher strain sensitivity of 30.7 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . The proposed method opens a new way to generate a higher-order OAM mode by using multiple-fold symmetry fiber.

Numerical study of the pseudo-boiling phenomenon in the transcritical liquid oxygen/gaseous hydrogen flame

The interactions and effects of turbulent mixing, pseudo-boiling phenomena, and chemical reaction heat release on the combustion of cryogenic liquid oxygen and gaseous hydrogen under supercritical pressure conditions are investigated using RANS simulations. Comparisons of the present numerical simulation results with available experimental data reveal a reasonably good prediction of a supercritical axial shear hydrogen-oxygen flame using the standard k-ε turbulence model and the eddy dissipation concept combustion model with a 23 reaction steps kinetics for H2-O2 reaction. The present simulation qualitatively reproduced oxygen injection and its reaction with the co-flowing hydrogen, which is characterized by rapid flame expansion, downstream flame propagation, and expansion induced flow recirculation. Several turbulence models were used for numerical simulations. It is shown that the selection of an appropriate turbulence model for transcritical reacting flows is crucial and far more important than for subcritical reacting flows. It is indicated that the pseudo-boiling phenomena is the main reason for the considerable differences between the turbulence models in a transcritical flame. Also, it is demonstrated that the liquid oxygen core disappears faster in a non-reacting flow than in a reacting flow. The shear layer in the non-reacting flow is much stronger than reacting case; providing a large transfer of energy from the outer layer to the inner layer. At the supercritical injection conditions, the difference between the turbulence models is much less than the transcritical injection conditions.