Experimental research on unconfined hydrogen explosion of different gas scale and the overpressure prediction method

In this research, unconfined hydrogen experiments are performed in 1 m³ and 27 m³ gas scale with gas concentration varying from lean-burn to rich burn. The results show that the flame travels fastest upwards and slowest downwards, which makes the flame shape irregularly spherical. The critical flame scales for the extra acceleration in the upward direction and for the deceleration in the downward direction are both smaller in 1 m³ gas scale. The acceleration exponent α is higher in the upward direction. With the gas scale increasing, the value of α increases gradually. For φ = 0.8, 1.0 and 1.5, the equivalent flame radius and the explosion overpressure in different gas scales overlap before the film rupture. According to the wrinkled laminar flame assumption and the self-similar theory, an overpressure prediction model is proposed based on the wrinkling factor Ξ_Δ. The predicted results agree well with the experimental data before the film rupture.     

Experimental investigation on the dynamic responses of vented hydrogen explosion in a 40-foot container

To investigate the structural dynamics of a container subjected to a vented hydrogen explosion, 48 field tests were conducted in a 40-foot container with roof vents and an end vent. The effects of the hydrogen concentration, ignition position, and obstacles on the evolution of the dynamic responses were investigated. Three stages were generally observed for displacements: (1) At the stage of the vent rupture, the displacement could be approximated as a quasi-static response, and there was a linear relationship between the peaks of positive overpressure and displacement. (2) Structural deformation appeared as reciprocating vibration at the stage of Helmholtz oscillation. (3) The structure exhibited relatively weak irregular fluctuation when high-frequency acoustic oscillation occurred. Two types of the structural acceleration with low and high amplitudes resulting from Helmholtz oscillation and acoustic oscillation, respectively, were clearly observed. For the end-vented explosion, multiple peaks were observed for the displacement at the quasi-static stage due to the rupture, discharge, and external explosion. Moreover, the displacement was sensitive to hydrogen concentration, whereas the number of obstacles and the ignition position had significant influences on the peak acceleration for roof venting. This work conducted the fundamental explanation for the evolution law of structural responses induced by vented hydrogen explosions from the perspective of structural dynamics and enriched the experimental accumulation in a large-scale container with congestion in this field.     

Investigation on unconfined hydrogen cloud explosion with external turbulence

Fully understanding the coupling mechanism between the enhancement of explosion overpressure and flame acceleration is a prerequisite for assessing hydrogen cloud explosion overpressure. In this research, unconfined fan-stirred hydrogen explosion experiments were performed to study the effects of flame instability and external turbulence on flame propagation and overpressure characteristics. The results showed that the combination of the external turbulence and the flame instability could result in great flame acceleration and explosion overpressure enhancement. For the intensity of external turbulence considered in this study, the combustion regimes were all in the flamelet zone. With the increase of the external turbulence intensity, the flame gradually got accelerated, and the explosion overpressure got enhanced. A theoretical prediction model for the upper and lower limit of the maximum overpressure was proposed, which fully accounted for flame instability, external turbulence, and flame-induced turbulence. It provides a conservative evaluation for hydrogen cloud explosion.     

Experimental study on explosion characteristics of syngas with different ignition positions and hydrogen fraction

The influence of different ignition positions and hydrogen volume fractions on the explosion characteristics of syngas is studied in a rectangular half-open tube. Three ignition positions were set at the axis of the tube, which are 0 mm, 600 mm and 1100 mm away from the closed end, respectively. A range of hydrogen volume fraction (φ) from 10% to 90% were concerned. Experimental results show that different ignition positions and hydrogen volume fraction have important influence on flame propagation structure. When ignited at 600 mm from the closed end on the tube axis, distorted tulip flame forms when flame propagates to the closed end. The formations of the tulip flame and the distorted tulip flame are accompanied by a change in the direction of the flame front propagation. The flame propagation structure and pressure are largely affected by the ignition position and the hydrogen volume fraction. At the same ignition position, flame propagation speed increases with the growing of hydrogen volume fraction. And the pressure oscillates more severe as the ignition location is closer to the open end. And pressure oscillations bring two different forms. The first form is that the pressure has a periodic oscillation. The amplitude of the pressure oscillation gradually increases. It takes several cycles from the start of the oscillation to the peak. For the second form, the pressure reaches the peak of the oscillation in the first cycle of the start to the oscillation.     

Experimental and theoretical investigation on hydrogen cloud explosion subjected to external turbulence

This work is to experimentally and theoretically explore the hydrogen cloud explosion subjected to external turbulence. In the experiments, the flame characteristics and explosion pressure are obtained using high-speed camera and pressure sensor. In the theoretical calculation, the peak explosion pressure is obtained using LM, LMIET and TM method. The results indicated that most flame characteristics in the experiments are located in the zone of wrinkled flamelets. The explosion-related parameters including flame propagation velocity, peak explosion pressure and peak rate of pressure rise continue to increase as the gear level increases from G0 to G3, increase firstly and then decrease as the equivalence ratio increases from Φ = 0.5 to Φ = 3.0. Due to ignoring flame acceleration propagation induced by flame instabilities, external turbulence and flame-induced turbulence, the peak explosion pressure obtained using experimental method is significantly larger than that obtained using LM method. Owing to considering the limit value of flame wrinkling level induced flame instabilities and flame-induced turbulence, the peak explosion pressure obtained using experimental method is significantly lower than that obtained using LMIET and TM method.     

Experimental study on spark ignition of non-premixed hydrogen jets

The leaks of pressurized hydrogen can be ignited if an ignition source is within a certain distance from the source of the leaks, and jet fires or explosions may take place. In this paper, a high speed camera was used to investigate the ignition kernel development, ignition probability and flame propagation along the axis of hydrogen jets, which leaked from a 3-mm-internal-diameter nozzle and were ignited by an electric spark. Experimental results indicate that for successful ignition events, the ignition delay time increases with an increase of the distance between the nozzle and the electrode. Ignitable zone of the hydrogen jets is underestimated if using the predicted hydrogen concentration along the jets centerline. The average rate of downstream flame decreases but that of the upstream flame increases with the electrode going far from the nozzle.     

Experimental and numerical investigation of hydrogen gas auto-ignition

This paper describes hydrogen self-ignition as a result of the formation of a shock wave in front of a high-pressure hydrogen gas propagating in the tube and in the semi-confined space, for which the numerical and experimental investigation was done. An increase in the temperature behind the shock wave leads to the ignition on the contact surface of the mixture of combustible gas with air. The required condition of combustible self-ignition is to maintain the high temperature in the mixture for a time long enough for inflammation to take place. Experimental technique was based on a high-pressure chamber inflating with hydrogen, burst disk failure and pressurized hydrogen discharge into tube of round or rectangular cross section filled with air. Two numerical models involving the gas-dynamic transport of a viscous gas, the detailed kinetics of hydrogen oxidation, turbulence model, and heat exchange were used for calculations of the hydrogen self-ignition both in semi-confined space and a tube.     

Numerical assessment of hydrogen explosion consequences in a mine tunnel

The aim of the work is a numerical estimation of the conditional probability of damage to the mine personnel during an accidental explosion of a hydrogen-air mixture. The methodology for determining the parameters of the gas-dynamic process of the explosion of a hydrogen-air cloud in an open and closed space, taking into account chemical interaction and space clutter, is presented. A computational method based on a probit analysis for determining the damage probability fields of a person exposed to the explosion shock wave has been developed. To automate the computational process, the tabular dependence “probit-function-damage probability” is replaced by a piecewise cubic spline. Numerical studies of the influence of the drift working space clutter by an electric locomotive on the distribution of the overpressure of the gaseous medium and the conditional probability of the eardrums rupture and lethal damage to personnel in the accidental zone of the coal mine have been carried out. It was obtained that the closed nature of the working space and its blockage significantly changes the shape and size of the danger zone and requires consideration by an expert at the stage of deciding on the safety level at the mine. The scientific novelty of the method proposed in the work is in taking into account in the mathematical model of the movement of a multi-component chemically reacting gas mixture the effect of compressibility of flow, complex terrain (space clutter with equipment), three-dimensional nature of the gas-air mixture dispersion process. The model allows obtaining the space-time distributions of the shock-impulse load of the blast wave that is necessary for determining the non-stationary three-dimensional fields of the conditional probability of damage to the staff on the basis of probit analysis. The developed computational method allows analyzing and forecasting in time and space the conditional probability of damage of varying degrees of severity of personnel who are exposed to an explosive shock wave as an indicator of the safety level of a coal mine.     

A simple and effective approach for evaluating unconfined hydrogen/air cloud explosions

The topic of hydrogen safety assessment has been focused by many researchers. The overpressure evaluation of vapor cloud explosion (VCE), is an important issue for both designing and evaluating on chemical plants, as well as buildings. Unknown flame radius history limits the original acoustic approximation model's application. The objective of this work is to develop an achievable model for hydrogen/air deflagration assessment in engineering applications, and the model should have high computational efficiency. A tentative scheme that starts from flame/piston speed history solving was adopted, and the flame/piston radius and acceleration history will be obtained subsequently. Thus, the overpressure history for far field could be gotten based on the acoustic approximation model. A simplified scheme was employed for the region inside the flame cloud. The model proposed in this paper could be solved in several seconds, because there are no differential equations but only algebraic equations. The model was verified by hydrogen/air deflagration tests from small scale to large scale. Compared with the experimental data, the model appeared well agreements in the medium and large scale cases. In the small scale cases, the model obtained acceptable solutions.     

Experimental study on the explosion behaviors of premixed syngas-air mixtures in ducts

Explosion characteristics of premixed syngas-air mixtures at room temperature and atmospheric pressure were experimentally reported when the explosion flame propagates in ducts with various heights (H) and lengths (L). The discussion was based on flame morphology and pressure dynamics. The ratio of L/H and the ratio of H₂/CO had a significant effect on the explosion flame behaviors as the explosion occurred in ducts. The structure of the explosion flame changes more drastically, as both the L/H ratio is large. The ratio of L/H affected the flame tip dynamics after the flame reached the duct wall, and the time of flame reaching the duct walls is divinable. For a given duct height, the shorter the duct length is, the faster flame propagates, and the maximum flame tip speed was higher as the duct length was small. For a given duct length, flame tip dynamics showed a nearly same development tendency, but the shorter the duct height, the faster the flame propagated. The venting pressure affected the overpressure dynamics, and the venting pressure increased with the increase of the L/H ratio and the H₂/CO. For a given duct height, the overpressure reached the maximum value almost at the same time, and the longer duct length resulted in the greater maximum overpressure. Finally, for a given duct length, the higher duct height caused the higher maximum overpressure.     

Inerting effect of carbon dioxide on confined hydrogen explosion

Taking maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse as index, this paper is aimed at evaluating the inerting effects of carbon dioxide on confined hydrogen explosion by varying initial pressure, carbon dioxide addition and equivalence ratio. The results indicated that under enhancing hydrodynamic instability, the stronger flame destabilization occurs with the increase of initial pressure. At Φ = 0.8 and Φ = 1.0, the destabilization effect of thermodiffusive instability continues to increase with the increase of carbon dioxide addition. At all equivalence ratios, the destabilization effect of hydrodynamic instability decreases monotonously with the increase of carbon dioxide addition. All of maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse reach the peak value at Φ = 1.5, and decrease significantly with increasing carbon dioxide addition. The inerting effect of carbon dioxide could be attributed to the reduction of thermal diffusivity, flame temperature and active radicals. The chemical effect of carbon dioxide reaches the peak value at Φ = 1.0. With the increase of carbon dioxide addition, the chemical effect continues to decrease at Φ = 0.8 and Φ = 1.0, and increase monotonously at Φ = 2.5.     

Experimental study on the propagation characteristics of hydrogen/methane/air premixed flames in a narrow channel

This work is focused on the explosion characteristics of premixed gas containing different volume fractions of hydrogen in a narrow channel (1000 mm × 50 mm × 10 mm) under the circumstance of stoichiometric ratio. The ignition positions were set in the closed end and the middle of the pipeline respectively. The results showed that when the gas was ignited at the pipeline closed end, the propagating flame was tulip structure for different premixed gas. When the hydrogen volume fraction was less than 40%, the flame propagation speed increased significantly with the rise of hydrogen volume fraction, and the overpressure peak also appeared obviously in advance. However, when the volume fraction of hydrogen was more than 40%, the increase of flame propagation speed and the overpressure peak occurrence time varied slightly. Furthermore, when the ignition position was placed in the middle of the pipeline, the flame propagation speed propagating to the opening end was much faster than that propagating to the closing end, and there was no tulip shape when the flame propagates to the opening end. The flame propagating to the closed end appeared tulip shape under the influence of airflow, and high-frequency flame oscillation occurred during the propagation. This work shows that the hydrogen volume fraction and ignition position significantly affected the flame structure, flame front speed, and explosion overpressure.     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.     

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (${\mathrm{X}}_{{\mathrm{H}}_2}$) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for ${\mathrm{X}}_{{\mathrm{H}}_2}$ less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for ${\mathrm{X}}_{{\mathrm{H}}_2}$ = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.     

Safety studies on high-pressure hydrogen vehicle refuelling stations: Releases into a simulated high-pressure dispensing area

If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogen with the same degree of confidence, and with comparable risk, as conventional liquid and gaseous fuels. The hazards associated with jet releases from leaks in a vehicle-refuelling environment must be considered if hydrogen is stored and used as a high-pressure gas since a jet release in a confined or congested area can create an explosion hazard. As there was insufficient knowledge of the explosion hazards, a study was initiated to gain a better understanding of the potential explosion hazard consequences associated with high-pressure leaks from hydrogen vehicle refuelling systems. This paper describes the experiments with a dummy vehicle and dispenser units to represent refuelling station congestion. Experiments with ignition of premixed 5.4 m × 6.0 m × 2.5 m hydrogen–air clouds and hydrogen jet releases up to 40 MPa (400 bar) pressure are described. The results are discussed in terms of the conditions leading to the greatest overpressures and overall conclusions are made from these.     

An experimental study on ignition timing of hydrogen Wankel rotary engine

The hydrogen-fueled Wanke rotary engine is a promising power system that has both high power and eco-friendly properties. This work investigated the effect of ignition timing on a dual-spark plugs synchronous-ignition hydrogen-fueled Wankel rotary engine under low speed, part load and lean combustion. The results show that with delaying the ignition timing, CA0-10 is shortened first and then lengthened and CA10-90 is consistently shortened. When the CA50 is located between 35 and 40°CA ATDC, the maximum brake torque can be realized. Besides, the selection of ignition timing needs to consider the “trade-off” relationship between the combustion phase and corresponding in-cylinder pressure. The maximum brake torque ignition timing is between 5 and 10°CA ATDC. And there is also a “trade-off” relationship between stability and thermal load when ignition timing is selected. In addition, HC and NO emissions will not become the problem limiting the power performance of hydrogen-fueled Wankel rotary engine under this operating condition.     

Experimental research on combined effect of obstacle and local spraying water fog on hydrogen/air premixed explosion

In order to reveal the mechanism of water fog explosion suppression and research the combined effect of water fog and obstacle on hydrogen/air deflagration, multiple sets of experiments were set up. The results show that the instability of thermal diffusion under lean combustion conditions is the main influencing factor of hydrogen/air flame surface instability, and the existence of water fog will aggravate the hydrogen/air flame surface instability. When obstacle is not considered, 8 μm, 15 μm, 30 μm water fog can significantly reduce the flame velocity and explosion overpressure of hydrogen/air, 45 μm fine water fog plays the opposite role. When considering the relative position of the water fog release position and the obstacle, the 8 μm, 15 μm, 30 μm water fog has almost no suppression effect when released near the obstacle, but a significant suppression effect occur, when using the 45 μm water fog. In the field of theoretical research, the research results not only provide an experimental basis for the fine water fog to reduce the consequences of hydrogen explosion accidents, and the optimal diameter range used by the water fog, but also provide experimental reference for the numerical simulation of hydrogen/air explosion suppression in semi-open space, and promote the development of hydrogen explosion suppression theory. In terms of engineering applications, this study can provide a theoretical basis for the layout of fire fighting equipment in the engine room of nuclear power plants or hydrogen-powered ships.     

Effects of hydrogen concentration and film thickness on the vented explosion in a small obstructed rectangular container

The explosion venting is an effective way to reduce hydrogen-air explosion hazards, but the explosion venting has been less touched in an obstructed container. The present study mainly focused on the effects of hydrogen concentration and film thickness on the explosion venting in a small obstructed rectangular container. High speed schlieren photography was employed to obtain the flame fine structure and velocity. Pressure transducers were used to measure the overpressure nearby the obstacle. The experimental results show that the obstacle has a significant effect on the flame shape, tip speed and overpressure. In the process of flame evolution, the flame surface becomes more wrinkled with time after the tulip flame. Compared with the cases without the obstacle, the flame surface becomes more distorted and wrinkled downstream of the obstacle under the influence of obstacle enhanced turbulence and flow instability. Upstream of the obstacle, the lower part of the flame surface becomes concave while the upper part shows convex. The pressure histories show that the maximum overpressure increases with the hydrogen concentration in the range of 11.8%–23.7%. Two main pressure peaks were observed for all hydrogen concentrations in the presence of the obstacle. The Helmholtz oscillations appear after the second pressure peak and its duration increases slightly when the hydrogen concentration increases. The combined effect of the obstacle and hydrogen concentration on the second peak overpressure is more significant than on the first peak overpressure. Moreover, the maximum overpressure shows a monotonic increase with the film thickness.     

Experimental research on hydrogen/air explosion inhibition by the ultrafine water mist

This experimental study focused on the inhibition of ultrafine water mist on hydrogen explosion inside the closed vessel. The inhibition law and mechanism were studied through changes of explosion intensity, flame propagation velocity and temperature under different mist concentrations. Results indicate that flame propagation and pressure rise inside the closed vessel were corresponding. Explosion intensity was reduced after adding mist, which was mainly manifested in the reductions of explosion pressure and flame propagation velocity. Flame was accelerated to extinguish and the inhibition effect was enhanced with increasing mist concentration. However, the explosion prussure did not present obvious reduction as the mist concentration reached a certain value. Besides, it indicates that the absoption heat effect of ultrafine water mist was an important factor on hydrogen explosion inhibition by the reductions of flame temperature and propagation velocity. The inhibition effect was mainly attributed to the combination effect of physical and chemical inhibitions.