Scaling effects of vented deflagrations for near lean flammability limit hydrogen-air mixtures in large scale rectangular volumes

Combustion-generated overpressures in nuclear containment buildings during a severe accident may be relieved by venting gases to adjacent compartments through relief panels or existing openings to avoid compromising a containment breach. Experimental studies on the dynamics of vented hydrogen-air combustion were extensively performed using vessels varied in shape and size at the Canadian Nuclear Laboratories. In this paper, the scaling effects are examined for near lean flammability hydrogen-air mixtures (6–12 vol.% H₂) with tests performed in rectangular volumes (25, 57 and 120 m³) with a scaled vent area (A_v/V^2/3) of 0.02–0.05 under both initially quiescent and fan-induced turbulent conditions. This study has found that the maximum peak overpressure of all quiescent tests are dominated by the acoustic coupled effect for the hydrogen concentration greater than 8 vol.%, and the acoustic effect becomes insignificant under turbulent conditions. The measured peak over-pressures are generally over-predicted for the quiescent tests and better predicted for the turbulent tests by the well-known Bradley–Mitcheson and Molkov correlations.     

Simulating vented hydrogen deflagrations: Improved modelling in the CFD tool FLACS-hydrogen

This paper describes validation of the computational fluid dynamics tool FLACS-Hydrogen. The validation study focuses on concentration and pressure data from vented deflagration experiments performed in 20-foot shipping containers as part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA), funded by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU). The paper presents results for tests involving inhomogeneous hydrogen-air clouds generated from realistic releases performed during the HySEA project. For both experiments and simulations, the peak overpressures obtained for the stratified mixtures are higher than those measured for lean homogeneous mixtures with the same amount of hydrogen. Using an in-house version of FLACS-Hydrogen with the numerical solver Flacs3 and improved physics models results in significantly improved predictions of the peak overpressures, compared to the predictions by the standard Flacs2 solver. The paper includes suggestions for further improvements to the model system.     

Effect of igniter type and number of igniters on vented deflagrations for near lean flammability limit hydrogen-air mixtures in a large scale rectangular volume

This paper examines the effect of igniter type (glow plug vs. spark igniter) and number of igniters on the dynamics of vented combustion under both initially quiescent and fan-induced turbulent conditions. These experiments are a subset of many series of tests performed in a 120 m³ large scale vented combustion test facility at the Canadian Nuclear Laboratories using near lean flammability hydrogen-air mixtures (8–12% H₂). One of the objectives of these studies was to have a better understanding of the effectiveness of deliberate ignition for mitigation of hydrogen during a postulated accident and to provide data for code validation. The test results of the current study show that the two types of ignition sources have no significant influence on the maximum combustion overpressure except that the initial burning rate is slightly faster using the spark igniter. Under either the quiescent or turbulent conditions, the maximum combustion overpressure always increases with an increase in the number of igniters, but under the current experimental conditions, the turbulent combustion overpressure with one igniter is always higher than quiescent combustion with multiple igniters.     

Research on mechanism and influence factors of the overpressure development in vented hydrogen deflagrations based on numerical simulation

Venting of hydrogen-air deflagrations is a complex process, and many issues remain to be investigated. In order to analyze the mechanism and influence factors of the overpressure development during vented hydrogen deflagrations, a commercial code FLACS was used and the capability of the code was validated by previous experimental data. Based on the experimental and numerical results, the effect of concentration, ignition location and vent area on the vented overpressure was discussed in detail. It was confirmed that in the condition of the large vent area, three overpressure peaks are formed at the moments of the vent failure, the external explosion and the occurrence of the maximum flame surface area in the vessel, which are marked as P_burst, P_ext and P_mfa. The overpressure peak P_ext is corresponded to the formation of the external pressure caused by the external explosion, and peaks P_ext in BWI conditions are larger than those in CI conditions. The relationship between overpressure and vent area match the power law with negative exponent, while the larger vent area may lead to the stronger effect of the external explosion on the internal overpressure. Moreover, the differences in magnitude between P_ext and P_mfa were discussed.     

Experimental studies on vented deflagrations in a low strength enclosure

This paper describes an experimental programme on vented hydrogen deflagrations, which formed part of the Hyindoor project, carried out for the EU Fuel Cells and Hydrogen Joint Undertaking. The purpose of this study was to investigate the validity of analytical models used to calculate overpressures following a low concentration hydrogen deflagration. Other aspects of safety were also investigated, such as lateral flame length resulting from explosion venting. The experimental programme included the investigation of vented hydrogen deflagrations from a 31 m³ enclosure with a maximum internal overpressure target of 10 kPa (100 mbar). The explosion relief was provided by lightly covered openings in the roof or sidewalls. Uniform and stratified initial hydrogen distributions were included in the test matrix and the location of the ignition source was also varied. The maximum hydrogen concentration used within the enclosure was 14% v/v. The hydrogen concentration profile within the enclosure was measured, as were the internal and external pressures. Infrared video images were obtained of the gases vented during the deflagrations. Findings show that the analytical models were generally conservative for overpressure predictions. Flame lengths were found to be far less than suggested by some guidance. Along with the findings, the methodology, test conditions and corresponding results are presented.     

Blind-prediction: Estimating the consequences of vented hydrogen deflagrations for homogeneous mixtures in 20-foot ISO containers

This paper summarises the results from a blind-prediction study for consequence models used for estimating the reduced explosion pressure and structural response in vented hydrogen deflagrations. The work is part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA). The scenarios selected for the blind-prediction entailed vented explosions with homogeneous hydrogen-air mixtures in a 20-foot ISO (International Organization for Standardization) container. The test program included two configurations and six experiments, i.e. three repeated tests for each scenario. The comparison between experimental results and model predictions reveals reasonable agreement for some of the models, and significant discrepancies for others. The results from the first blind-prediction study in the HySEA project should motivate developers to improve and validate their models, as well as to update documentation and guidelines for users of the models.     

External overpressure of vented hydrogen-air explosion in the tube

Vented explosion experiments involving hydrogen-air mixtures are performed in a 2 m-long cylindrical tube under the influences of the hydrogen concentration and vent burst pressure. Photos of the external flame shot by a high-speed camera show that the jet flame was expelled outside the vessel, and the relation between the flame propagation and external overpressure is summarized. The internal peak overpressure increases and then decreases with increasing hydrogen concentration. In contrast, the external peak overpressure exhibits the opposite correlation in comparison with the internal peak overpressure. The variations in the pressure peaks of the internal pressure curves are also discussed. When the hydrogen concentration is lower than 40 vol %, the second pressure peak plays a more dominant role than the other pressure peaks. However, when the hydrogen concentration is higher than 40 vol %, the third pressure peak becomes more dominant.     

Modelling the mitigation of lean hydrogen deflagrations in a vented cylindrical rig with water fog

The wide flammability range of hydrogen–air mixtures means that the generation and presence of significant quantities of hydrogen in a confined space will always present some likelihood that a deflagration or explosion might occur. Very fine water mist fogs have been suggested as a possible method of mitigating the overpressure rise should a hydrogen–air deflagration occur.     

Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 2: Numerical study

Safety studies for hydrogen retail stations involve identification of possible accidental scenarios, modelling of consequences and measures to mitigate associated hazards with it. Accidental release of hydrogen during its handling and storage can lead to formation of ignitable mixture in a very short time. Ignition of such a mixture can lead to generation of overpressure affecting structure and people. Understanding of the possible overpressures generated is critical in designing the system safe from explosion hazards. In the present study, the worst-case scenario where high-pressure hydrogen storage cylinders are enveloped by a premixed hydrogen-air cloud is numerically simulated. The computational domain mimics the setup for premixed hydrogen cloud in a mock hydrogen cylinder storage congestion environment experimentally studied by Shirvill et al. [1]. Large Eddy Simulations (LES) are performed using OpenFOAM CFD toolbox solver. The Flame Surface Wrinkling Model in LES context is used for modelling deflagrations [2]. Numerical simulation results are compared against experiments. Simulations are able to predict experimental flame arrival and overpressure reasonably well. The effects of ignition location, congestion and confining walls on the turbulent deflagrations in particular on explosion overpressure are discussed. It was concluded that explosion overpressure increases with increase in confinement.     

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.     

Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media

Rich and ultrarich combustion of methane, ethane, and propane inside inert porous media is studied experimentally and numerically to examine the suitability of the concept for hydrogen production. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.5, for a filtration velocity of 12 cm/s. Two-temperature numerical model based on comprehensive heat transfer and chemical mechanisms is found to be in a good qualitative agreement with experimental data. Partial oxidation products of methane, ethane, and propane (H₂, CO, and C₂ hydrocarbons) are dominant for ultrarich superadiabatic combustion. The maximum hydrogen yield is close to 50% for all fuels, and carbon monoxide yield is close to 80%.     

Modelling the mitigation of hydrogen deflagrations in a vented cylindrical rig with water fog and nitrogen dilution

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of mitigating the overpressure rise, should a hydrogen deflagration in a vented enclosure occur. A numerical CFD gas explosion code (FLACS) has been used to simulate the pressure-time curves and the rate of pressure rise generated following the ignition of different hydrogen–oxygen–nitrogen mixtures in a small scale vented cylindrical explosion rig. This has allowed the potential mitigating effect of nitrogen dilution (reduced oxygen) and very fine water fog, used both alone and in combination, to be explored and permitted their direct comparison with corresponding experimental test data.     

掺氢对微尺度空间内预混层流火焰转捩爆燃特性的影响

针对基于燃烧的微小型动力装置存在燃烧效率低、火焰传播速度慢的问题,设计了一个可视化的、特征间距仅为0.45 mm的微尺度定容燃烧室,实验比较了0～1的掺氢比例下,丙烷/氢气/空气预混火焰在该燃烧室内的传播以及加速过程.实验发现没有掺氢时,丙烷/空气预混火焰需要在0.25 MPa初始压力下才能够传播;当掺氢比例为0.2时...     

Hydrogen non-premixed combustion in enclosure with one vent and sustained release: Numerical experiments

Numerical experiments are performed to understand different regimes of hydrogen non-premixed combustion in an enclosure with passive ventilation through one horizontal or vertical vent located at the top of a wall. The Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) model with a reduced chemical reaction mechanism is described in detail. The model is based on the renormalization group (RNG) k-ε turbulence model, the eddy dissipation concept (EDC) model for simulation of combustion coupled with the 18-step reduced chemical mechanism (8 species), and the in-situ adaptive tabulation (ISAT) algorithm that accelerates the reacting flow calculations by two to three orders of magnitude. The analysis of temperature and species (hydroxyl, hydrogen, oxygen, water) concentrations in time, as well as the velocity through the vent, shed a light on regimes and dynamics of indoor hydrogen fires. A well-ventilated fire is simulated in the enclosure at a lower release flow rate and complete combustion of hydrogen within the enclosure. Fire becomes under-ventilated at higher release flow rates with two different modes observed. The first mode is the external flame stabilised at the enclosure vent at moderate release rates, and the second mode is the self-extinction of combustion inside and outside the enclosure at higher hydrogen release rates. The simulations demonstrated a complex reacting flow dynamics in the enclosure that leads to formation of the external flame or the self-extinction. The air intake into the enclosure at later stages of the process through the whole vent area is a characteristic feature of the self-extinction regime. This air intake is due to faster cooling of hot combustion products by sustained colder hydrogen leak compared to the generation of hot products by the ceasing chemical reactions inside the enclosure and hydrogen supply. In general, an increase of hydrogen sustained release flow rate will change fire regime from the well-ventilated combustion within the enclosure, through the external flame stabilised at the vent, and finally to the self-extinction of combustion throughout the domain.     

Coal dust gasification in the filtration combustion mode with syngas production

A new method for the gasification of fine solid fuel was proposed and worked out, by partial oxidation in a flow of gaseous oxidant with filtration of the suspended fuel through an inert porous matrix. In this case, the solid fuel gasification was carrying out similar to the filtration combustion of gases. The gasification of fine solid coal allows one to produce a combustible gas rich in H₂ and CO was studied. A possibility of pulverized coal gasification in a fixed bed reactor with production of gaseous products containing up to 13% by volume of hydrogen and carbon monoxide was shown experimentally.     

Numerical investigation of the combustion characteristics of hydrogen and oxygen in a microthruster

Recent e?orts have developed hydrogen and oxygen propellant-based (HOP) microthruster systems. In this study, a novel microthruster was investigated numerically. The combustion and mixing characteristics of hydrogen and oxygen were investigated numerically according to a variety of inlet flow rates and equivalence ratios in a 3-dimensional microthruster. Results demonstrated that when the equivalence ratio equalled 2, the temperature was the highest. When the equivalence ratio equalled 4, the specific impulse peaked. With identical equivalence ratios, as the flow rate increases, the temperature and specific impulse increased, and as the flow rates increases up to a certain value, the specific impulse changed more slowly. Computational results showed that the proposed microthruster could achieve steady combustion at various mass flow rates in all tested conditions. This study describes the relationship of the mixing performance and combustion characteristics with the equivalence ratio and flow rate, which provides a guideline for the design of novel microthrusters.     

Setting a best practice for determining the EGR rate in hydrogen internal combustion engines

Exhaust gas recirculation (EGR) is an effective way to reduce NO_x-emissions and increase the efficiency of hydrogen fueled internal combustion engines. Knowledge of the exact amount of EGR is crucial to understand the effects of EGR. As the exhaust gas flow is pulsating and chemically aggressive, the flow rate is typically not measured directly and has to be derived from other quantities. For hydrocarbon fuels, the EGR rate is generally calculated from a molar CO₂ balance, but for hydrogen engines this obviously cannot be used as there are no CO₂ emissions, and consequently no standard practice has been established. This work considers three methods to calculate the amount of EGR in a hydrogen engine. The first one is based upon a volume balance in the mixing section of exhaust gases and fresh air. The second and third method uses a molar balance of O₂ and H₂O respectively in this mixing section. The three methods are developed and tested for their accuracy with an error analysis. Additionally, the methods are applied to an experimental dataset gathered on a single cylinder hydrogen engine. Both the theoretical analysis and the experimental results confirm the method based on an O₂ molar balance as the most accurate one. The least practical method is the one based on an H₂O balance as it requires additional relative humidity sensors and is less accurate than the others.     

Gasification of low-melting hydrocarbon material in the airflow heated by hydrogen combustion

The airflow heated by combustion of hydrogen–oxygen mixture to a temperature ranging from 300 to 1500 K is used for the gasification of a low-melting hydrocarbon material – polypropylene (PP) in a flow-type reactor. The yield of PP gasification products is shown to increase with the airflow temperature. At temperatures up to 1200 K, the yield of PP gasification products attains 19.7 g/s and the ratio of the mass flow rates of air and PP gasification products attains a value of 1.8. The presence of oxygen in the airflow allows increasing the yield of the gasification products due to the subsequent combustion of PP which ensures the gasification process at low airflow temperatures. Only short-time ignition with help of hydrogen–oxygen mixture combustion is required at the airflow temperature of 300 K. At these conditions, the yield of PP gasification products attains a value of 8.5 g/s and the ratio of the mass flow rates of air and PP gasification products is 5.0. An analytical model is proposed for calculating the characteristics of the PP gasification process with PP combustion taken into account. The results of calculations agree with the experimental data within an accuracy of 2%–4%. Further estimates indicate that the minimum possible ratio of the mass flow rates of air and PP gasification products can attain 0.34 with an increase in the airflow temperature to 3000 K.     

Impacts of low temperature combustion and diesel post injection on the in-cylinder production of hydrogen in a lean-burn compression ignition engine

Strategies were investigated for increased in-cylinder formation of hydrogen. The use of low intake oxygen with a post injection was proposed. An intake oxygen sweep was conducted on a lean-burn compression ignition engine by adjusting of the exhaust gas recirculation rate. The results revealed that the yield of hydrogen increased exponentially when the intake oxygen was reduced to achieve low temperature combustion. Further tests showed that low temperature combustion operation consistently produced more hydrogen than high temperature combustion for similar air-to-fuel ratios.To increase the hydrogen yield further, a post injection timing sweep was carried out with low temperature combustion operation. Increased yields of hydrogen were obtained, up to 0.76% by volume, when then the post injection timing was advanced from 70 to 20° crank angle after top dead centre. At the same time, the indicated NO_X emissions reduced to 0.013 g/kW·hr and the smoke emissions were 0.14 FSN. Thus, the tests established that the combination of low temperature combustion, low intake oxygen, and an early post injection produced a high yield of hydrogen with simultaneously ultra-low NO_X and smoke emissions. The main drawback of this strategy was the increased formation of methane, up to 3015 ppm by volume. However, further analysis showed that the hydrogen to methane ratio actually increased under low temperature combustion operation.