Self-ignition Of Hydrogen Research Articles

Experiment study on the pressure and flame characteristics induced by high-pressure hydrogen spontaneous ignition

A series of experiments were conducted to study the pressure and combustion characteristics of the high-pressure hydrogen during the occurrence of spontaneous ignition and the conversion from spontaneous ignition to a jet fire and explosion. Different initial conditions including release pressure (4–10 MPa), tube diameter (10/15 mm), and tube length (0.3/0.7/1.2/1.7/2.2/3 m) were tested. The variation of the pressure and flame signal inside and outside of the tube and the development of the jet flame were recorded. The experimental results revealed that the minimum ignition pressure required for self-ignition of hydrogen at different tube diameters decreased first and then increased with the extension of tubes. The minimum ignition pressure for tubes diameters of 10 mm and 15 mm is no more than 4 MPa and the length of the tubes is L = 1.7 m. The minimum release pressure required for spontaneous ignition of a tube D = 15 mm is always lower than that of a tube D = 10 mm at the same tube length. When the spontaneous ignition occurred, it did not absolutely trigger the jet fire. The transition from spontaneous ignition to a jet fire must go through the specific stages.

Numerical study on supersonic boundary-layer transition and wall skin friction reduction induced by fuel wall-jet combustion

In this study, the effect of wall-jet combustion on boundary layer transition and skin friction reduction was numerically investigated. To effectively capture the characteristics of boundary layer flow, the transition k−kl−w model was employed as the turbulence model and laminar finite-rate model was chosen as the combustion model. This numerical method was firstly validated by two sets of experimental results in open domain. After that, the research on wall-jet combustion was conducted and the numerical results showed that when injecting hydrogen in different directions, both the hydrogen self-ignition location and the skin friction on the wall were not altered significantly. When the injection angle to the airflow direction was increased to 30°, the intensity of combustion was insufficient and the skin-friction coefficient would be increased. Meanwhile, boundary layer transition occurred in a relatively smaller Reynolds number at this condition. The variation of the wall-jet height would have a greater impact on both boundary layer transition and the skin friction. More components can diffuse to the lower wall as the height of the wall-jet was enlarged, which can make the process of boundary layer transition be postponed and the skin friction be reduced as well. Furthermore, greater skin-friction reduction would be achieved downstream the self-ignition location when the wall was adiabatic, while the original Reynolds numbers for boundary layer transition is the smallest if the wall temperature is set to 600 K. Finally, in order to simulate the effect of the back pressure on the combustion flow field, another injector is added near the exit at the upper wall to produce air-throttling flow jet. The results showed that altering back pressure nearly has little influence on boundary layer transition and the skin friction decreases with more air-throttling flow rate.

Spontaneous ignition of high-pressure hydrogen and boundary layer characteristics in tubes

Hydrogen is efficient and environmentally friendly, but the danger of self-ignition resulted from the leakage of high-pressure hydrogen cannot be ignored. In this work, the self-ignition of high-pressure hydrogen released through different conditions was studied. 700-mm-length tubes with different diameters were adopted in our experiments. It summarized the characteristics of shock waves' attenuation and evolution process of hydrogen jets in tubes. In addition, effects of the boundary layer on the leading shock waves, the contact surface, and expansion waves were discussed. Results indicate that minimum pressure when self-ignition occurs for 15-mm-diameter tube is similar to 10-mm-diameter. And they have closely velocity of shock waves. Simulations show that the greater the release pressure, the more ignition products of hydrogen. Higher release pressure and smaller diameter can create a thicker boundary layer in micro shock tubes, and the boundary layer can lead to a change in the velocity of shock waves’ structures.

Experimental and numerical study on the high-pressure hydrogen jet and explosion induced by sudden released into the air through tubes

Experiment and simulation were conducted to study the development process of jet fire and explosion induced by the high-pressure hydrogen spontaneous ignition. The pressurized hydrogen was released under different tube length and release pressure in the experiment. The overpressure generated by the leading shock and the light signal of the flame with different conditions were recorded. The propagation of the external jet flame was captured by a high-speed camera. Different parameters including velocity, temperature, and concentration distribution were analyzed based on the simulated results. It is revealed that the jet fire and explosion, which are induced by hydrogen self-ignition, must go through a series of specific stages of development, and this process has no significant correlation with pipe length and release pressure. This process consists of several stages, including the formation of the combustion zone downstream of the Mach disk; the flame chases the top of the jet along the shear layer; the flow velocity at the top of the jet decreases causing the ring vortex to disappear and the combustible gas to accumulate; the shear layer flame propagates to the top of the jet causing the explosion. Besides, the explosion will inevitably occur along with the jet fire, which is caused by the spontaneous ignition induced by the sudden release of high-pressure hydrogen and is developed in the open space.

Experimental study on pressure dynamics and self-ignition of pressurized hydrogen flowing into the L-shaped tubes

To investigate the effects of varying right-angle corner locations inside the L-shaped tube on self-ignition induced by high-pressure hydrogen release, a series of experiments were carried out on L-shaped tubes with different right-angle corner locations and a straight tube was adopted for comparison. It is found that compared with the straight tube, the propagation of shock wave in the L-shaped tubes becomes more complicated due to the existence of reflected shock wave. The pressure profiles detected by pressure transducers before the right-angle corner will undergo secondary rapid rise. The varying right-angle corner locations inside the L-shaped tube have a significant influence on self-ignition of hydrogen. The closer the right-angle corner is to the burst disk, the lower the critical pressure that causes hydrogen self-ignition is. Three possible mechanisms of self-ignition inside the L-shaped tubes are discussed. Nevertheless, different right-angle corner locations have no obvious effects on development process of out-tube jet flame, only the velocity of flame tip and the length and width of jet flame have slight differences.

Experimental Investigation of the Self-Ignition and Jet Flame of Hydrogen Jets Released under Different Conditions

Hydrogen is a promising clean energy source and an important chemical raw material. To use hydrogen energy more safely, a high-pressure hydrogen-release platform for hydrogen self-ignition and for generating hydrogen jet flames under different experimental conditions was investigated in this study. The associated experimental analysis was based on the theory of high-pressure hydrogen tube diffusion. We found that the higher the initial release pressure, the greater the intensity of the leading shock. When the initial release pressure was high and the leading shock intensity was strong, hydrogen was more likely to ignite spontaneously inside the tube. The higher the initial release pressure, the faster the average propagation speed of the shock in the same pipe length. The time during which a stable leading shock was formed inside the tube may be related to the initial release pressure. It was found that flame combustion intensified after the passage of air through a Mach disk, and a stable flame was formed more easily at the jet boundary layer away from the orifice axis. The maximum speed of the flame tip and the flame decay speed were very high. Moreover, the flame length and the diameter of the ball flame first increased and then decreased.

Pressure dynamics, self-ignition, and flame propagation of hydrogen jet discharged under high pressure

The self-ignition of hydrogen released from a high-pressure tank using extension tubes (2200 mm) with different diameters was studied. The processes of flame transition at a nozzle and jet flame development were characterized using a high-speed camera. The results indicated that the intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube than that in a 15-mm-diameter tube. The pressure in a 15-mm-diameter tube was weaker than that in the 10-mm-diameter tube at the initial stage; however, it became higher in the later stage. Spontaneous ignition was more likely to happen in a 15-mm-diameter tube. The formation of a stabilized flame at the tube exit and Mach disk were observed during the transition of the flame to a jet fire. The stabilized flame showed a triangular shape because of the influence of a Prandtl–Meyer flow when a hydrogen jet entered a suddenly expanding environment. The formation and separation of a spherical flame were recorded during jet flame development. Large vortexes were formed in front of the flame because of the Kelvin–Helmholtz instability, which resulted in the separation of the spherical flame. The vortexes stopped rotating until the separated flame disappeared.

ВЗРЫВОПОДОБНЫЕ ГЕОФИЗИЧЕСКИЕ ЯВЛЕНИЯ В АТМОСФЕРЕ ЗЕМЛИ

It was shown that the nature of such geophysical phenomena as the ozone layer, ozone “holes” and silvery clouds are directly related to the process of self-ignition and chain combustion of hydrogen in the atmosphere, similar to what occurs in the explosion of explosive clouds. This indicates the absence of a direct relationship between human activities and the formation of ozone “holes”.

EVALUATION OF EXPLOSIVE RISK IN THE PROCESS STORAGE OF WELDED NUCLEAR FUEL

EVALUATION OF EXPLOSIVE RISK IN THE PROCESS STORAGE OF WELDED NUCLEAR FUEL

Reliable and safe thermal coupling of generation-IV VHTR to a hydrogen fuel production complex

This paper presents a conceptual design of a hybrid, Heat Pipes-Thermosyphon Intermediate Heat Exchanger (HPTIHEX) for passive and reliable thermal coupling of the GEV-IV, helium-cooled and graphite moderated, Very High Temperature Reactor (VHTR) to a complex for the co-production hydrogen fuel. A VHTR could generate up to ∼600MWth at an exit temperature of 900–1300K. In addition to electricity generation at high thermal efficiency (>48%), these small-modular reactors provide high-temperature thermal energy for the co-production of hydrogen fuel using thermochemical processes. For safety considerations, however, the VHTR needs to be separated from the hydrogen production complex by 110–140m. This relatively long distance is a challenge to thermally coupling the VHTR and the hydrogen complex, while reducing heat losses and the temperature drop. In addition, the thermal coupling needs to effectively suppress the propagation of a hypothetical explosion from the hydrogen complex to the VHTR. The proposed HPTIHEX design would facilitate a reliable and safe operation of the VHTR for electricity generation and the co-production of hydrogen and would effectively suppress the propagation of an explosion due to a self-ignition of hydrogen to the VHTR.

Mechanism of self-ignition of pressurized hydrogen flowing into the channel through rupturing diaphragm

The paper considers numerically the mechanism of hydrogen self-ignition occurring when hydrogen flows into the channel through gradually rupturing diaphragm. The analysis focuses on the near-critical self-ignition regimes arising at initial hydrogen pressure ∼60 atm that is marginal for self-ignition occurrence for selected channel geometry. Numerical simulation reproduces two types of ignition kernels observed experimentally. Both types of kernels arise on the contact surface between the released hydrogen and air heated behind the outrunning shock wave. In the initial stage the self-ignition kernels arise on the side contact surface interacting with channel walls and compression waves reflected from the walls. However, because of oxygen deficit these ignition kernels are extinguished. In the subsequent stage the self-ignition kernel arises in the region where the side contact surface penetrates into the boundary layer heated behind the shock wave. The further evolution of the ignition kernel is provided by the vortical structures evolving at the side contact surface and defining the mixing of hydrogen with air as well as the sustained supply of the mixture into the ignition kernel.

Self-ignition of hydrogen–nitrogen mixtures during high-pressure release into air

This paper demonstrates experimental and numerical study on spontaneous ignition of H2–N2 mixtures during high-pressure release into air through the tubes of various diameters and lengths. The mixtures included 5% and 10% (vol.) N2 addition to hydrogen being at initial pressure in range of 4.3–15.9 MPa. As a point of reference pure hydrogen release experiments were performed with use of the same experimental stand, experimental procedure and extension tubes. The results showed that N2 addition may increase the initial pressure necessary to self-ignite the mixture as much as 2.12 or 2.85 – times for 5% and 10% N2 addition, respectively. Additionally, simulations were performed with use of Cantera code (0-D) based on the ideal shock tube assumption and with the modified KIVA3V code (2-D) to establish the main factors responsible for ignition and sustained combustion during the release.

An experimental study on the mechanism of self-ignition of high-pressure hydrogen

In the present study, the self-ignition of high-pressure hydrogen released in atmospheric air through a diaphragm is visualized under various test conditions. The experimental results indicate that the hydrogen that jets through the rupturing diaphragm is mixed with the heated air near the tube wall. The self-ignition event originated from this mixing. The self-ignition was strongly dependent on the strength of an incident shock wave generated at the diaphragm rupture. As a result, a cylindrical flame that formed after the self-ignition shows a tendency to become longer as it propagates in the downstream direction. The head velocities of the hydrogen-air mixture and the cylindrical flame are consistent with that of a contact surface calculated from the measured shock speed. A modified self-ignition mechanism is proposed based on the experimental observations.

Effects of diaphragm rupturing conditions on self-ignition of high-pressure hydrogen

In the present work, self-ignition of high-pressure hydrogen that is released to air through a ruptured diaphragm is studied experimentally under various test conditions. Diaphragms with several thicknesses and scores are ruptured by high-pressure hydrogen, and the formation of a shock wave in the tube is experimentally observed. The behavior of the ruptured diaphragm and the subsequent self-ignition is photographed using a high-speed camera, while the self-emission intensity from a flame is measured using a photomultiplier. The experimental results show that the diaphragm rupturing condition is one of the main factors affecting the self-ignition phenomena. In addition, the self-ignition causes a ring-shaped flame that travels downstream, along a boundary between the hydrogen jet and the shock-heated air.

Self-ignition of hydrogen jet discharged under high pressure into a perforated channel

The self-ignition of a hydrogen jet during spontaneous release from a high-pressure chamber into a perforated channel was investigated experimentally. The experiments were devoted to the investigation of the possibility of preventing the self-ignition of hydrogen by using the perforated channel. Two lateral orifices with diameters of 2–6 mm were located in a side surface of a rectangular cross-section channel. The length of the channel was 180 mm and its sides were 2 and 10 mm. The initial hydrogen pressure varied from 3 to 9 MPa. Maps of the possibility of hydrogen self-ignition depending on the Mach number of the incident shock wave, the distance between the position of the orifices and the rupture diaphragm, and the orifice’s cross-sectional area are presented. Using the perforated channel as a pressure-relief device can be effective under certain conditions.

Slotted Channel As A Gasdynamical Source Of Ignition And Flame Stabilization In The Supersonic Combustion Chamber

The original scheme of flame stabilization in the channel at close to cocurrent fuel supply for the fuel combustion at a high supersonic speed has been designed. Such solution provides high temperature of a stream in a zone of fuel-air mixture formation. Computational and experimental investigations of self-ignition and combustion of hydrogen were carried out in the model of combustor chamber with slotted channel (gasdynamical source of ignition) at Mach numbers 3.7 and 5.8 at the entrance. Tests have been performed in hot-shot wind tunnel IT-302M of ITAM SB RAS in a mode of the attached pipe. Numerical study has been performed on the basis of solving the full averaged Navier-Stokes equations, supplemented k-Q SST turbulence model. Configuration of the slotted channel has been designed with two variants of exit nozzle: with and without geometrical throat. It has been established that at the channel entrance two vortexes with high temperature have been appeared. Temperature has been keeping high in the channel with geometrical throat and at blocking of the slotted channel without throat. It was found that uniform subsonic stream in the channel with geometrical throat has been realized. The stream in the slotted channel without geometrical throat keeps supersonic but Mach number was lower than in the main channel. The structure of the flow at the slotted channel exit is significantly differs for this both cases.

EFFECT OF WALL THERMAL CONDUCTIVITY ON HYDROGEN-ASSISTED CATALYTIC IGNITION CHARACTERISTICS OF PROPANE-AIR AT MICRO-SCALES IN DIFFERENT FEEDING MODES

Effect of wall thermal conductivity on hydrogen self-ignition and hydrogen-assisted ignition of propane-air mixtures in different feeding modes from ambient cold-start conditions were investigated numerically with chemical kinetic model in Pt/γ-Al2O3 catalytic micro-combustors. For the steady and transient state, effect of wall thermal conductivity on self-ignition characteristics of lean hydrogen-air mixtures was presented, and hydrogenassisted combustion of propane-air mixtures was investigated numerically in the co-feed mode and the sequential feed mode. The computational results indicate the large thermal inertia of the micro-combustor solid structure leads to slow temperature dynamics, and transient response is dominated by the thermal inertia. The heat localization in poorly conducting walls leads to fast ignition and shorter steady state time. In general, the concentration of hydrogen required for propane ignition increased with increasing wall thermal conductivity, decreasing inlet velocity, and decreasing inlet equivalence ratio of propane-air mixtures. In the co-feed mode, the combustion characteristics of hydrogen-assisted propane qualitatively resemble the selectively preheating initial portion of the combustion chamber wall. In the sequential feed mode, the time taken to reach steady state, the hydrogen cut-off time, the propane ignition time and the cumulative propane emissions increased with increasing wall thermal conductivity; the ignition characteristics are similar to partially preheating the initial segment for low and moderate wall thermal conductivity values (0.5 and 20 W/m·K); however, the ignition characteristics are close to completely heating the micro-combustor wall for high wall thermal conductivity values (200 W/m·K).

Self-ignition and explosion of a 13-MPa pressurized unsteady hydrogen jet under atmospheric conditions

The process of unsteady high-pressure hydrogen release observed on local vessel rupture is investigated experimentally. The flow structure during the formation of a supersonic free hydrogen jet is studied. The obtained data correlates well with the numerical simulation of sonic underexpanded hydrogen jet released into the atmosphere. In particular, it has been confirmed that the Mach disk fluctuates in the case of unsteady outflow of underexpanded hydrogen jet. It has been shown that in order to provide diffusion self-ignition, it is necessary to focus an air shock wave, formed after the vessel rupture, within the studied initial hydrogen tank pressure range. Lab and ground tests on self-ignition and explosion of hydrogen that outflows through a 15-mm opening when the flow interacts with a 37-mm-radius hemispherical cavity are described. A comparison of the diffusion combustion dynamics of releasing hydrogen in self-ignition and induced ignition scenarios is made. The observed visible flame front velocities of 150–220 m/s can result in dangerous blast waves in the case of large-scale hydrogen release. Measures to reduce the risk of explosion of unsteady high-pressure hydrogen jets releasing into an encumbered space are proposed.

Hydrogen-assisted catalytic ignition characteristics of propane–air with a chemical kinetic model in a Pt/γ-Al2O3micro-combustor in different feeding modes

From ambient cold-start conditions, the hydrogen self-ignition and hydrogen-assisted ignition of propane–air mixtures with chemical kinetic model in different feeding modes were numerically investigated in Pt/γ-Al2O3catalytic micro-combustors.

Some Effects of Hydrogen Self-Ignition and Combustion in Supersonic Flow

<p>Results are presented of computational and experimental investigations of the influence of temperature and flow composition on the hydrogen combustion kinetics for a coaxial fuel supersonic flow. Depending on the flow parameters, combustion is shown to occur with an intense heat release governed by the speed of chemical reactions, or a diffusion combustion with heat release governed by mixing. The computational results are in good agreements of with laboratory data and portrays many important features of supersonic combustion. The influence of the gas temperature and composition on the diffusion combustion of a circular hydrogen jet in supersonic coaxial flow at the over expanded exhaust regimes is investigated. It is found that at low flow temperatures (Т<sub>2 </sub>~ 900 K) and in the absence of water vapors in the oxidizer gas composition, the speed of chemical reactions is the determining factor for combustion. An increase in the flow temperature (Т<sub>2</sub> > 1200 K) causes a reduction of the induction time of the reactive mixture, because the mixing of fuel with oxidizer decreases, and a “sluggish” diffusion combustion of non-mixed gases is observed. The presence of water vapor and active radicals in the gas ensures the self-ignition from the start of the mixing, and the diffusion combustion mode is limited by mixing of the hydrogen jet with the coaxial flow (similar to the case with high initial temperatures of the air stream). In the case of the delay combustion process the maximum pressure level on the wall is 10% more than that in the combustion mode with ignition at the start of mixing. A sluggish combustion regime may lead to an incomplete hydrogen burnout.</p>