Hydrogen Microjet Research Articles

A Large Eddy Simulation Study on Hydrogen Microjets in Hot Vitiated Crossflow

Abstract In this paper, the results of a large eddy simulation (LES) study on hydrogen microjets (dj ~ 0.5 mm) injected into a hot (1600 K) vitiated crossflow at different angles?namely, normal (90°) and inclined jet (30°), are presented. The goal is to explore the effects of injection angle on coherent turbulent structure formations, flame-vortex interactions, and wall heat flux contributions. The LES identifies the presence of the horseshoe vortex, the shear layer vortices (SLV), and the counter-rotating vortex pair (CVP), along with the shedding of spanwise-symmetry hairpin vortices in both the normal and inclined jets. The structures in the latter, however, appear more convoluted. In the near field, SLV-induced flow is crucial for mixing and flame propagation on the windward side of the jet, stabilized by autoignition near the jet exits. A flame-shear layer offset is observed here. In the far field, CVP dominates hot vitiated crossflow entrainment, driving flame propagation and heat and species transfer to the leeward side near the injection wall. In the wake region, combustion remains closer to the normal jet exit due to a stronger recirculation zone, resulting in higher wall heat fluxes. The mean wall heat flux values of both the normal and inclined jets decrease and approach each other with moving away from the jet exit in the streamwise direction. The present LES study results are compared to experimental data from the literature by considering instantaneous hydroxide (OH) fields and mean wall heat fluxes.

On the features of diffusion combustion of hydrogen microjets

This paper presents the results of an experimental investigation of the diffusion combustion scenario of a round hydrogen microjet flowing from a cylindrical thin-walled micronozzle with a diameter of 200 μm when ignited near and at any distance from the nozzle exit. A review of the literature on the research topic is presented and a comparison is performed in dimensionless parameters based on the Reynolds number. It has been established that stabilization of the torch during the expiration of a round microjet of hydrogen is associated with heating of the nozzle by the laminar region of the flame. A hysteresis of the process of diffusion combustion of round hydrogen microjets was discovered depending on the region of ignition of the microjet (laminar or turbulent region of the torch) and on changes in the jet velocity.

DIFFUSION COMBUSTION OF A HYDROGEN MICROJET, OUTFLOWING FROM A CURVLINEAR CHANNEL

The presented work examines the combustion of a hydrogen microjet flowing from a curved channel with a round micronozzle. Jet flows that are generated using rectilinear and curved channels differ in that in the second case, Dean vortices make a noticeable contribution to the formation of the jet and its combustion. The interaction of the latter with Kelvin–Helmholtz vortices, the formation of which is typical for flows with a velocity shift, causes changes in combustion characteristics. They include spatial distortions of the laminar flame zone near the nozzle exit, the area of turbulent combustion downstream, as well as the turbulent flame in the conditions of its separation from the nozzle exit and the cessation of laminar combustion in the initial section of the flow. The results of these studies provide an opportunity to better understand the combustion features of hydrogen microjets under conditions of their hydrodynamic instability.

Scenarios of combustion of plane hydrogen microjets at sub- and supersonic velocities

Experimental data on diffusion combustion of plane hydrogen microjets emanating from a slotted micronozzle at subsonic and supersonic velocities are reported. Four scenarios of diffusion combustion of the hydrogen microjets are presented. As is found, the stabilization of subsonic combustion of the hydrogen microjet is related to the origination of a “bottle-neck” configuration of flame while in supersonic conditions the flow pattern is dominated by supersonic cells. Hysteresis of the diffusion combustion of plane hydrogen microjet is found depending on its ignition far from or close to the nozzle exit and velocity variation.

Regimes of Combustion of Hydrogen Microjets

Regimes of Combustion of Hydrogen Microjets

Study of the Regimes of Diffusive Combustion of a Hydrogen Microjet

The paper presents the results of experimental studies of scenarios of diffusion combustion of a microjet of hydrogen flowing out of a cylindrical micronozzle with a diameter of 200 μm. The hydrogen microjet is ignited near the nozzle exit and at a distance. The experimental data are compared with the data previously obtained, and the results are expressed in dimensionless parameters (in terms of Reynolds numbers). It is established that the stabilization of flame during the outflow of a circular micro jet of hydrogen is associated with the presence of heating of the nozzle by a spherical flame region covering the nozzle section. The hysteresis of the diffusion combustion process of circular hydrogen microjets is shown depending on the ignition location of the microjet (near or far from the nozzle exit) and the change in the flow rate (growth or decrease).

Numerical Study on the Effect of Wall Thickness on the Combustion Characteristics of Non-premixed Hydrogen Micro-jet Flame

The effect of wall thickness on the combustion characteristics of nonpremixed hydrogen micro-jet flames was studied by two-dimensional numerical computation with a detailed reaction kinetics for the development of hydrogen micro-burner. The hydrogen jet diffusion flames achieved by micro-tubes with the same inner diameter and length but different wall thicknesses were investigated. The heat exchange between solid tube and gases were included in the numerical computation. The distributions of flame temperature, OH radicals, details of thermal interaction, and combustion efficiency were analyzed for comparison. It was found that the temperature distribution, flame shape, and heat recirculation are changed with the fuel flow velocity, and they are affected by wall thickness. The mechanism of wall thickness on the combustion characteristics of hydrogen jet diffusion flame was clarified. Finally, an interesting numerical experiment was conducted to give a further explanation of the effect of heat recirculation and to provide guidance of the thermal management of the micro-burner.

Combustion of Round Hydrogen Microjet in Concurrent Flow

Combustion of Round Hydrogen Microjet in Concurrent Flow

Enhancement of Chemiluminescence Intensity of S2* in Non-premixed Hydrogen Microjet Flame in the Photometric Detector for Sulfur Detection.

A transparent quartz rod (q) placed vertically on top of a non-premixed hydrogen microjet flame in a flame photometric detector (qFPD) was developed and evaluated for sulfur detection. The microjet flame burned around the quartz rod because of Coanda effect, forming an extended downstream flame zone with a relatively low temperature between 550 and 650 °C, which is favorable to the formation of S2*. The emission intensity of S2* and the signal-to-noise ratio (SNR) of sulfur response were enhanced 2.6- and 2.1-fold, respectively. It was found that the quartz rod of diameter 4 mm with a tip shape of semicircle placed 6 mm above the nozzle yielded the highest SNR. The limits of detection (LOD) for seven kinds of tested sulfur-containing compounds of qFPD were 0.3-0.5 pg S s-1, which is 5-7 times better than that of commercially available FPD detectors (LOD: 1.6-2.8 pg S s-1). The selectivity of sulfur over carbon was 105 on qFPD when the SNR for the mass flow rate of S and C atoms was ∼3 times. It was the first time that a quartz rod was used vertically on top of a microjet hydrogen-rich flame in FPD to enhance the chemiluminescence of S2* and improve the LOD down to 0.3-0.5 pg S s-1.

Effect of inert and reactive gas additives to hydrogen and air on blow-off of flame at hydrogen release from microleakage

This paper presents experimental data on the effect of inert (He, N2, Ar, CO2) and reactive (СН4, O2, CF3Br, (CH3O)3PO) gas additives to hydrogen and ambient air on flame blow-off during diffusion combustion of a hydrogen microjet emanating from a micronozzle into air. It is found that the addition of the above gases to hydrogen narrows the microjet velocity range of stable combustion of hydrogen. For hydrogen with the additives studied, the critical velocity of flame blow-off depends primarily on the mean molecular weight of the combustible mixture. When the gases are added to the air surrounding the diffusion flame, the critical velocity of flame blow-off is determined by their inhibiting effect on the chemical reactions of hydrogen oxidation. The results can be used to determine the minimum concentrations of flame-suppressing additives required to prevent the ignition and combustion of hydrogen in its accidental releases.

Diffusion Combustion of a Round Hydrogen Microjet at Sub- and Supersonic Jet Velocity

The experimental research results on diffusion combustion of a round hydrogen microjet flowing from a slit micronozzle at subsonic and supersonic speeds are presented. For the first time, four scenarios of diffusion combustion of a round hydrogen microjet have been identified, including supersonic combustion in the presence of supersonic cells both in air and in hydrogen. It has been found that flame stabilization for a subsonic microjet velocity of hydrogen is associated with the presence of a “bottleneck flame region” leading to the nozzle choking phenomenon, and flame stabilization for a supersonic microjet flow is associated with the presence of supersonic cells. A hysteresis of the diffusion combustion process of a plane microjet of hydrogen is found depending on the method of ignition of the microjet (near or far from the nozzle exit) and the direction of change in the rate of its outflow (growth or decrease).

Combustion of round hydrogen microjet in a concurrent flow

Experimental data on the diffusion combustion of round hydrogen microjet in concurrent coaxial flows are presented. The effects on combustion of the concurrent flow of air and that of air premixed with nanopowder of TiO2 are of interest. The hydrogen microjet is emanated from a round micronozzle which is surrounded by a coaxial slit to produce the concurrent flow. Combustion events found in the present conditions are similar to those observed in the previous studies on diffusion combustion of hydrogen microjets at subsonic and supersonic velocities. In a subsonic range, the so-called “bottleneck-flame region” is generated close to the nozzle exit, while in high-speed conditions the flame separates from the nozzle. At increasing velocity of both the hydrogen microjet and the concurrent flow, the “bottleneck-flame region” is still found and the combustion becomes more intense. The “bottleneck-flame region” is suppressed at the microjet velocity approaching transonic values.

Combustion of round hydrogen microjet in a cocurrent air flow with nanopowder

In the present paper we show our recent experimental data on diffusion combustion of a round hydrogen microjet in a cocurrent flow of air mixed with nanoparticles of TiO2. The hydrogen microjet is emanated from a round micronozzle which is surrounded by a coaxial slit to produce the cocurrent flow. Combustion events found in the present conditions are similar to those observed in the previous studies on diffusion combustion of hydrogen microjets at subsonic and supersonic velocities. In a subsonic range, the so-called “bottleneck-flame region” is generated close to the nozzle exit, while in high-speed conditions the flame separates from the nozzle. At increasing velocity of both the hydrogen microjet and the cocurrent flow, the “bottleneck-flame region” is still found and the combustion becomes more intense. The “bottleneck-flame region” is suppressed at the microjet velocity approaching transonic values.

Combustion of round hydrogen microjet in a cocurrent air flow

Experimental data on combustion of a round hydrogen microjet in a cocurrent coaxial air jet are presented. As is found, the combustion of microjet is dominated by the generation of “bottleneck-flame region” that is in agreement with our previous observations on the diffusion combustion of subsonic hydrogen microjets. Transformation of the spherical “bottleneck-flame region” to a cylindrical one is observed. At supersonic combustion of the round hydrogen microjet, supersonic cells are generated both in the microjet and in the cocurrent coaxial air flow. A feature of the combustion process is the flame lift-off from the nozzle exit.

Combustion of a plane hydrogen microjet at subsonic and supersonic speeds

In this paper, we presented the results of experimental studies of the diffusion combustion of a plain hydrogen microjet flowing from a slit micronozzle at subsonic and supersonic speeds. For the first time, four scenarios of diffusion combustion of a plain hydrogen microjet including supersonic combustion in the presence of supersonic cells in both air and hydrogen are presented. The stabilization of the subsonic combustion of a hydrogen microjet was established to be due to the presence of a «bottleneck flame region» while the stabilization of the supersonic combustion of a microjet was found to be associated with the presence of supersonic cells. The observed hyster­esis of diffusion combustion of a plain hydrogen microjet depends on both the method of igniting the microjet (near or far from the nozzle exit) and the direction of change in the rate of its outflow (growth or reduction).

Combustion of a Plane Hydrogen Microjet at Subsonic and Supersonic Speeds

In this paper, we presented the results of experimental studies of the diffusion combustion of a plain hydrogen microjet flowing from a slit micronozzle at subsonic and supersonic speeds. For the first time, four scenarios of diffusion combustion of a plain hydrogen microjet including supersonic combustion in the presence of supersonic cells in both air and hydrogen are presented. The stabilization of the subsonic combustion of a hydrogen microjet was established to be due to the presence of a «bottleneck flame region» while the stabilization of the supersonic combustion of a microjet was found to be associated with the presence of supersonic cells. The observed hyster­esis of diffusion combustion of a plain hydrogen microjet depends on both the method of igniting the microjet (near or far from the nozzle exit) and the direction of change in the rate of its outflow (growth or reduction).

Features of the Round Hydrogen Microjet Combustion in a Coaxial Air Jet

Results of experimental studies of features of the round hydrogen microjet combustion in a coaxial air jet are presented in this work. It is shown that the combustion scenario is connected with existence of the «bottleneck flame region». This fact correlates with the similar scenarios of the diffusion hydrogen microjet combustion at subsonic efflux velocity investigated by us earlier. It is revealed that the spherical shape of the “bottleneck flame region” is transformed to a cylindrical shape. It is found that the round hydrogen microjet combustion in a coaxial air jet at supersonic efflux velocity is accompanied by existence of supersonic cells both in a hydrogen microjet and in a wake of coaxial air jet. Round hydrogen microjet supersonic combustion in a coaxial air jet is connected with a flame separation from a nozzle exit.

Особенности горения круглой микроструи водорода в спутной струе смеси воздуха и нанопорошка

Results of the experimental studies of a round hydrogen microjet combustion in a coaxinal jet of air and SiO2 nanopouder mixture efflux from a slot-hole (multinozzle) coaxial nozzle at subsonic and supersonic efflux velocity are presented. It is shown that scenarios of a hydrogen microjet combustion in a coaxinal jet of air and nanopouder mixture are similar to scenarios of diffusion combustion of a hydrogen microjet at subsonic and supersonic efflux velocity. Existence of “bottleneck flame region” is revealed at subsonic and a flame separation from a nozzle exit at supersonic efflux velocity. It is found that with efflux velocity growth of jets it is possible to observe intensification of the luminescence of the flame at the boundary between the jet of hydrogen and air / nanopowder mixture and existence of «bottleneck flame region» as in a laminar coaxial jet and in a flame of a hydrogen microjet combustion. On the other hand, it is possible to observe actually disappearance of the «bottleneck flame region» at approach to transonic efflux velocity.

Изучение пределов устойчивого горения диффузионного пламени микроструи водорода, истекающей из круглого микросопла, при введении в водород или воздух инертных и реагирующих газов

The effect addition of inert (He, N2, Ar, CO2) and reacting (СН4, O2, CF3Br, (CH3O)PO) gases in a hydrogen or in coflowing air stream on lift-off diffusion flame conditions of hydrogen micro jet effluxed from the round micro nozzle was experimentally studied. Using the schlieren technique a critical hydrogen flow rate was established at which the flame of the hydrogen microjet detaches from the nozzle when introducing additives of the studied gases into both air and hydrogen. It has been established that the addition of all the studied gases to the hydrogen leads to a decrease in the velocity range of its microjet at which flame stabilization is possible, regardless of whether the gases introduced into hydrogen are inert or reactive. It is shown that in the case of the addition of various gases to the hydrogen, the main factor determining the critical flow rate at flame lift-off from the micro nozzle is the average molecular weight of the H2 gas mixture with additives. At addition of the studied gases into the coflowing air, the critical flow rate of H2 is determined by their affect on the chemical reactions of hydrogen oxidation (inhibition effectiveness), as well as by a decrease in oxygen concentration due to dilution of air by additives. The data obtained are of interest to hydrogen energy in terms of determining the limits of sustainable combustion of the hydrogen microjet, as well as determining the minimum phlegmatizing concentrations of additives of inhibitors and fire suppressant in the air, preventing the ignition and explosion of hydrogen in case of emergency at its leak.

Взаимодействие круглой микроструи воздуха с коаксиальной (спутной) струей воздуха на сверхзвуковой скорости их истечения

Results of experimental studies of the round hydrogen microjet interaction with a coaxial air for supersonic speed jets efflux are presented in this work. It is shown that interaction of round and coaxial air jets at their supersonic efflux leads to appearance of a train small/large-scale and, on the contrary, large/small-scale supersonic cells on a resultant jet with growth and reduction of speed of its efflux, respectively. It is found that the result of the round and coaxial air jets interaction at their supersonic efflux does not depend on geometrical parameters of a round and coaxial exit nozzle.