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In this article, the effect of different boundary conditions and different thermal and physical properties of walls and gas on flame characteristics and stability of hydrogen–air mixture are investigated using an analytical method. This method solves the gas–wall energy equation, and the hydrogen mass conservation equations. The jump conditions are obtained by integrating the energy and mass equation into a small control volume around the flame. For validation of this model, the temperature distribution on the outer surface of the wall is compared with experimental data that show the maximum relative error of 3.5% for Q = 400 mL/min and 4.9% for Q = 200 mL/min. The maximum variation of gas temperature is nearly 6.5 times of wall temperature variation. The wall can be considered one‐dimensional for conventional wall materials with K > 10. For the existence of combustion inside the chamber, when the value of K is greater than 10, the Péclet number should also be considered greater than 10. In a constant equivalence ratio, increasing the medium temperature increases flame stability. [ABSTRACT FROM AUTHOR]

Numerical simulations of autoignition of lean (6% H2), stoichiometric, and rich (90% H2) hydrogen–air mixtures have been performed to examine the influence of third-body efficiency (chaperon efficiency, CE) on the value of ignition delay, τ. The temperature ranges explored in the computations are 850–1000 K for P0 = 1 bar and 1000–1200 K for P0 = 6 bar. By using a detailed kinetic mechanism, it has been found that the sensitivity of ignition delay to CE is the highest for the reaction step H + O2 + M = HO2 + M, which can lead to a variation in τ by a factor of 2 to 3. A pressure increase or deviation from stoichiometry reduces the sensitivity. The influence of CE is qualitatively different and weaker for the reaction step OH + OH + M = H2O2 + M. [ABSTRACT FROM AUTHOR]

Thermal ignition of fuel-air mixtures due to hot particles can pose security risks and be hazardous under various circumstances, leading to their auto-ignition. The ignition characteristics of hydrogen-air mixtures with hot particles were investigated by performing 2-D numerical simulations using a detailed H2-air kinetic model. The simulations were performed with hot particles of various sizes and shapes with hydraulic diameters of 2, 4, and 6 mm. The effect of particle surface temperature on the ignition characteristics was studied by performing simulations at various particle surface temperatures ranging from 1000 to 1200 K. The results showed that for different particle shapes and sizes, the ignition delay depends strongly on the particle temperature. The location of the ignition point depends on the particle shape and temperature. A particle with a temperature of 1200 K ignites at the front stagnation point. The behavior significantly differs at 1000 K as the ignition location is shifted to a different point due to a competition between reaction kinetics and flow around the hot particles. Spherical particle showed the highest heat release rate compared to other particle shapes.

Numerical simulations were performed to study the effect of composition gradient and obstacle arrangement on detonation initiation in fuel-rich hydrogen-air mixtures. A third-order WENO method with adaptive mesh refinement (AMR) was used to solve the unsteady, fully-compressible, reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model (CDM). An obstructed channel at a blockage ratio of 0.3 filled with non-uniform hydrogen-air mixtures of an average H 2 concentration 50 vol% was considered. The results show that unilateral obstacle arrangement is more conducive to flame acceleration (FA) and DDT than bilateral obstacle arrangement for both homogeneous and inhomogeneous mixtures. For inhomogeneous mixtures, the flame accelerates faster, and the detonation onset time is shorter when obstacles are placed on the sidewall with a hydrogen concentration closer to the stoichiometric ratio than the those when obstacles are placed on the other sidewall. However, the placement of unilateral obstacles on either the upper or lower sidewall does not significantly affect the DDT run-up distance. Besides, for fuel-rich inhomogeneous hydrogen-air mixtures, detonation tends to be initiated in the regions of high H 2 concentration. Idealized models were introduced to simplify the intricate interactions of the reaction waves and flow fields to better understand the connection between H 2 concentration distribution and detonation initiation. The analysis suggests that the minimum shock strength required for detonation initiation decreases with increasing equivalence ratio. This is supported by a kinetics analysis with detailed reaction model, which shows that ignition delay increases with decreasing equivalence ratio when detonation is about to be initiated. • Simulating flame acceleration (FA) and DDT using high-order numerical method. • Studying effects of obstacle arrangement and inhomogeneity of H 2 -air on FA and DDT. • Analyzing through simplified detonation onset models and detail chemical kinetics. • Providing valuable insights on detonation initiation physics in non-uniform mixtures. [ABSTRACT FROM AUTHOR]

This study focused on using swirling flow as a method to stabilize the detonation behavior and shorten the deflagration-to-detonation transition (DDT) distance of pulse detonation engines; additionaly, the study investigates the effect of swirling flow on flame propagation behavior and DDT distance. A swirling flow in which the circumferential velocity in the radial direction gradually increased from the center toward the wall and decreased near the wall was confirmed. Laser doppler velocimetry (LDV) confirmed that the swirl number is smaller in the axial velocity distribution when the number of swirling injector couplings is small. The DDT shortening ratio obtained using ten swirling injector couplings is larger than that obtained using five couplings. These results indicate that injector-induced swirling flow can increase the burning velocity and is an effective method to shorten the DDT distance. • Swirling flow used to stabilize detonation behavior and shorten DDT distance. • Circumferential velocity distribution across pipe section measured using a particle image velocimetry. • Swirl number of pipe swirling flow measured using a laser Doppler velocimeter. • The swirling flow increased the flame propagation velocity and reduced the DDT distance. [ABSTRACT FROM AUTHOR]

Accurate prediction of blast loading from accidental hydrogen-air cloud explosion is critical for the planning, design, and operation of the hydrogen industry. This study proposes an efficient and accurate prediction model of explosion loads from hydrogen-air cloud explosions in vented silos. Based on the Bayesian regularization and data augmentation, an artificial neural network (ANN) model is generated and trained with the data collected from computational fluid dynamics (CFD) simulations using FLACS. The influences of various parameters including the silo dimensions (diameter and height), the ventilation size (width and length), and the ventilation panel activation pressures are considered. Analysis shows that the R2 value of the ANN model predictions and CFD simulation results exceeds 0.99. The developed ANN model could provide almost instantaneous prediction of the peak overpressure, the rise rate of overpressure, and the impulse for constructing the blast loading time history, which can be used in the design analysis and safety assessment of structures exposed to hydrogen explosions. Additionally, the model interpretability was conducted to analyse the importance and contribution of the input variables, which also verifies the reliability of the prediction model. • A comprehensive ANN model with interpretability was developed to predict Hydrogen-Air explosion loads. • Over 400 data sets for ANN training are generated via CFD simulation using FLACS Hydrogen Modular. • The performance of the developed ANN model closely aligns with CFD simulation results. • The developed model combines the advantages of simulations, empirical methods, and neural networks. [ABSTRACT FROM AUTHOR]

Numerical simulations are conducted to investigate the effect of wall roughness on flame acceleration (FA), deflagration-to-detonation transition (DDT), and detonation propagation in a narrow channel filled with stoichiometric hydrogen-air mixture. The wall roughness is determined by the element height h relative to the pipe diameter d and can be described using a dimensionless number Ra = 2h/d. A high-order numerical algorithm is employed to solve the Navier–Stokes equations on an adaptive mesh. The results show that the roughness enhances the effect of boundary layer and promotes FA and DDT. In channels with small roughness (Ra <0.1), detonation is not observed. Flame instabilities are caused by the interaction between the flame surfaces and reflected waves from the sidewalls, wrinkling of the flame front, leading to additional flame acceleration, and the production of intense pressure waves. In comparison, in channels with large roughness (Ra >0.1), vortices, shears, and even turbulence are produced in the cavity-like regions as leading shock passes over the elements. The mechanisms of the final detonation transition in the rough narrow channel are thought to be the formation of local hot spots arising from the multiple interactions of shocks with the roughness elements and viscous heating of unburned gas in the highly turbulent boundary layer. • A high-order numerical method with adaptive mesh refinement was used to solve the FA and DDT in a narrow rough channel. • The role of small roughness elements in the development of boundary layer and detonation initiation was discussed. • The formation mechanism of galloping detonation in the rough channel was explored. [ABSTRACT FROM AUTHOR]

A numerical study of two-dimensional cellular detonation of a hydrogen–air mixture in channels of various widths was carried out, and data were obtained on the process of formation of a cellular structure and on the number, shape, and size of cells depending on the channel width. Based on the results of calculations, classification of two-dimensional cellular detonation structures was carried out. This study was carried out at the facilities of the Joint Supercomputer Center, Russian Academy of Sciences. The flow model of a multicomponent reacting mixture was implemented using a customized opensource solver of the OpenFoam framework based on the Kurganov–Tadmor scheme. [ABSTRACT FROM AUTHOR]

Numerical modeling of the interaction of cellular detonation in a hydrogen-air mixture with several systems of porous filters covering part of the channel was carried out. The main regimes of detonation propagation and the critical conditions for detonation failure in the filter systems were obtained for each system. A map of detonation regimes was constructed, from which it follows that with an increase in the concentration of particles in the filters, it is possible to increase the distance between the filters or reduce the number of filters in the system necessary to successfully suppress detonation. A comparison of various filter systems in terms of blockage ratio and the total surface area of particles was made, from which it was concluded that the system of two filters was the most efficient to suppress detonation. • Concentration limits for different filter systems were obtained. • Maps of detonation modes for three filter systems were built. • Comparison of the efficiency of different filter systems for DW attenuation was made. • Increasing the concentration allows to reduce the number or thickness of filters. • System of two filters is the most effective for attenuating the DW. [ABSTRACT FROM AUTHOR]

The results of numerical simulation of the propagation of a detonation wave in a layer of a combustible gas mixture bounded on one side by a wall and, on the other, by a volume filled with an inert gas are analyzed. The calculations are performed using the GasDynamicsTool package in a two-dimensional formulation for a model gas with the parameters corresponding to a stoichiometric hydrogen–air mixture. The evolution of the cellular structure of a detonation wave during its propagation along a semi-confined layer is analyzed. It is shown that with an increase in the layer thickness, the distance at which the detonation decays exponentially increases. The problem of mitigating an explosion by creating an intermediate inert gas layer separating the combustible mixture from the protected surface is considered. It is established that the detonation wave is effectively quenched when the thickness of the inert layer corresponds to the size of the detonation cell. [ABSTRACT FROM AUTHOR]

The results of numerical study of the interaction of the gaseous detonation with the regular obstacle consisting of checkered elements with circular cross section are presented. The aim of this work is to identify and generalize the parameters affected the attenuation of a cellular detonation propagating in a premixed stoichiometric hydrogen-air mixture. Studies of such problems are aimed at studying issues related to explosion and fire safety in the operation of highly efficient gaseous fuels, which are currently very widespread. As a result, the dependencies of the leading shock wave propagation velocity on the obstacle geometry are obtained. The contribution of each of the considered parameters to the detonation wave velocity deficit is estimated. Maps of detonation suppression and re-initiation modes for varying barrier parameters are obtained. • There are three flow modes appearing as the detonation wave passes through an obstacle. • Detonation re-initiation is missing as the leading shock wave speed reaches 0.43 of the Chapman-Jouguet detonation speed D CJ. • The vertical distance between obstacles is an independent parameter influencing the detonation attenuation. • The horizontal distance isn't an independent parameter, but affects other significant parameters of the obstacle geometry. • The obstacle diameter influences the discharge capacity of the whole obstacle and total area of the reflecting surface. [ABSTRACT FROM AUTHOR]

The results of calculation of the detonation propagation in a porous medium for hydrogen-air mixture are presented. The porous medium was specified explicitly and consisted of sets of individual obstacles in the form of solid walls or the sets of finite-size plates. Various modes of detonation propagation depending on obstacle parameters are obtained: propagation in a cellular mode, stationary propagation with destruction of the cellular structure of the detonation front, propagation of a monotonically attenuating detonation wave with destruction of the cellular structure of the front. The possibility of reducing the detonation propagation velocity by replacing solid plates with finite-size ones was shown. The effect of the geometrical parameters of the plates and the step of it installation on the degree of detonation attenuation was estimated. It was determined that an increase in the number of plates leads to a stronger attenuation of the detonation. • Channels narrower the detonation cell destroy the cellular structure. • Finite-size plates decelerates the detonation more than solid walls. • The more the plates, the better the detonation is attenuated. • Number of plates has a stronger effect on detonation than its specific volume. [ABSTRACT FROM AUTHOR]

The interaction of a detonation wave in a hydrogen-air mixture with a series of clouds of inert particles with diameters of 1, 10, and 100 μm is calculated. The concentration and geometric limits of detonation are determined. It was found that an increase in the particle diameter leads to an increase in the concentration limit and to decrease in the geometric limit of detonation. It has been found that an increase in the number of clouds in a series of clouds makes it possible to increase the length of the gaps between them for the successful failure of detonation. In addition, the longer the clouds of inert particles, the greater the maximum distance between them and the smaller their number is necessary to quench the detonation. • The smaller the inert particles the smaller the concentration limit. • The smaller the inert particles the greater the geometric limit. • Friction and heat exchange are the determinant factors in determining the concentration limit. • Mainly friction is the determinant factors in determining the geometric limit. • The greater the number of clouds the greater the gap between them. [ABSTRACT FROM AUTHOR]

The ignition temperatures and effective activation energies of mixtures of 5–40% H2–air over metallic Rh and stoichiometric mixtures (30–70% H2 + 70–30% C2H6 (and C2H4)) + air over metallic Rh and Pd are experimentally determined at pressures of 1 to 2 atm over the temperature range 20–300°C. It is shown that, for the investigated mixtures, metallic Rh is more effective than Pd, and the effective activation energies of ignition depend not only on the nature of the catalyst but also on the chemical nature of the hydrocarbon in the mixture. The data obtained indicate that catalytic ignition is initiated only by an exothermic surface reaction of hydrogen oxidation on the catalyst; the hydrocarbon on the surface is consumed in reactions involving intermediate products of hydrogen oxidation that do not lead to chain branching; and then combustion propagates into the volume. It is established that in an untreated reactor, the ignition temperature of the mixture of 70% H2 + 30% methane with air above the surface of palladium at a pressure of 1.75 atm is 310°C; and above the surface of rhodium, 105°C. In a reactor treated with ignition, the ignition temperature of a mixture of 70% H2 + 30% methane with air above the surface of palladium at a pressure of 1.75 atm is 270°C; and above the surface of rhodium, 62°C. The result obtained indicates the potential of using a rhodium catalyst to significantly lower the ignition temperature of fuels based on methane and hydrogen mixtures. [ABSTRACT FROM AUTHOR]

The expediency of using a Busemann diffuser in a scramjet engine with detonation combustion of hydrogen-air mixtures is studied. A method is developed for calculating an air intake that provides spontaneous initiation of oblique detonation in a hydrogen-air mixture under conditions of rarefied atmosphere at hypersonic freestream Mach numbers. The geometry of the Laval nozzle (convergent-expanding) with the Busemann diffuser for the free flow Mach number 9 is determined. It is shown that at an altitude of about 40 km, the detonation combustion of the hydrogen-air mixture in this nozzle can provide more than 0.4 tons of thrust and more than 35% efficiency. The mathematical model is based on unsteady two-dimensional Euler equations for axisymmetric multicomponent reacting gas flow. To simulate chemical transformations, the detailed kinetic scheme is used that takes into account 33 non-equilibrium reactions for nine components of the mixture. The heat capacity, enthalpy, and entropy of a mixture are calculated using the reduced Gibbs energy of the gas components. Numerical modeling is carried out according to the modified Godunov scheme of the second order of approximation in spatial variables. • Busemann diffuser provides spontaneous initiation of oblique detonation in H 2 +air. • The method for calculating a Busemann diffuser (BD) for a scramjet is developed. • The geometry of the BD for a freestream Mach number of 9 is determined. • At an altitude of 40 km, the efficiency of the scramjet with BD is more than 35%. [ABSTRACT FROM AUTHOR]

The energy efficiency of a supersonic ramjet engine with detonation combustion of a hydrogen-air mixture in an axisymmetric Laval nozzle is considered. First of all, within the framework of the one-dimensional (1D) theory, it is shown that extreme combustion corresponding to the Chapman-Jouguet (C.-J.) deflagration, like constant pressure combustion, surpasses detonation in energy efficiency. However, this advantage becomes insignificant with an increasing the free-flow Mach number. Based on the results of the 1D theory, an axisymmetric Laval nozzle is constructed, which provides steady-state detonation combustion of a stoichiometric hydrogen-air mixture at an altitude of about 40 km and with freestream Mach numbers 9 and 12. The efficiency of detonation combustion in this case is about 35% instead of the maximum 65% according to the 1D theory. • In terms of efficiency, detonation reaches deflagration at a high flight Mach number. • In 1D-theory a stationary combustion in Laval nozzle has a maximum flight Mach number. • 1D theory gives overestimated values of the efficiency and expansion of the diffuser. • The detonation slope expands the limits of its applicability in terms of speed and height. • The steady-state detonation in the Laval nozzle can provide an efficiency of about 35%. [ABSTRACT FROM AUTHOR]

Understanding flame acceleration (FA) and deflagration-to-detonation transition (DDT) is important for combustion applications. However, DDT is a high-speed, nonlinear and complex process and the exact underlying mechanisms remain unclear, especially for narrow channels with a width smaller than detonation cell size. In this paper, numerical simulations were carried out to study the FA and DDT in a stoichiometric hydrogen-air mixture in narrow channels with width on the order of magnitude comparable to laminar flame thickness. The fully-compressible reactive Navier-Stokes equations were solved by using a high-order numerical method. Hydrogen-air combustion was taken into account using a calibrated simplified chemical-diffusion model. Four different channel widths, i.e., d = 2.0 mm (∼6 δ l), 1.0 mm (∼3 δ l), 0.5 mm (∼1.5 δ l), 0.25 mm (∼0.7 δ l), were considered in the simulations to explore the size effect, where δ l is the laminar flame thickness. The results show that the FA is mainly determined by boundary layer effects for all the channels considered. In the early stages, the flame keeps accelerating due to the stretching effect of boundary layer that leads to a significant increase in flame surface area. In the later stages of acceleration, an ultra-fast flame develops in the boundary layer ahead of the flame front due to the viscous heating effect associated with wall friction. It was found that the DDT mechanism is essentially consistent in all the channels. Detonation arises from the contact of two neighboring flame fronts in the shocked region that is vulnerable to a mild temperature gradient or fluctuation. The contact of the two high-temperature reaction fronts instantly causes a very high concentration of heat flux and consequently a detonation wave emerges from the contact spot. The detonation propagation after DDT, however, depends on the channel width. When d ≥ 1.5 δ l , a quasi-steady or a single-head detonation forms and sustains until the end of the combustion. For the case of d < 1.5 δ l , the detonation decays into a high-speed flame. When the channel width is sufficiently small, e.g., d ∼0.7 δ l , detonation initiation can occur occasionally, but soon decouples into a shock and a flame. • Flame acceleration and DDT in narrow channels with diameters comparable to laminar flame thickness were studied. • Flame acceleration is mainly determined by boundary-layer effects. • Detonation arises from the local contact of neighboring high-temperature flames in shocked region. • Channel width has a significant effect on flame acceleration, DDT, and detonation propagation. [ABSTRACT FROM AUTHOR]

Numerical simulations were performed to study flame acceleration and deflagration-to-detonation transition (DDT) in hydrogen-air mixture in a channel with a two-dimensional array of cylindrical obstacles. A high-order numerical algorithm with adaptive mesh refinement was applied to solve reactive Navier-Stokes equations. The effect of obstacle layout was examined by considering three layouts at a fixed blockage ratio of 0.5: staggered, inline-concentrated, and inline-scattered. Three blockage ratios, 0.33, 0.5, and 0.67, were used for the case of staggered obstacles to explore the influence of blockage ratio. The results show that both obstacle layout and blockage ratio have significant effects on flame acceleration and DDT occurrence, although the basic mechanism of detonation initiation is consistent for all the cases involving shock focusing. In the staggered case, the head-on collisions of flame and pressure waves with obstacles greatly promote the growth of flame surface area and thus lead to the fastest flame acceleration and shortest detonation onset time. In the inline-concentrated case, flame propagates slower than that in the staggered case due to smaller flame surface area. However, compared to the zigzag path in the staggered case, the straight passages parallel to flame propagation direction in the inline-concentrated case are more conducive to producing strong shock focusing and thus result in the shortest DDT distance. In the inline-scattered case, the straight passage along the centerline of channel facilitates the early acceleration of flame, but it has the slowest flame propagation in lateral directions and thereby the longest DDT time and distance. For the staggered obstacles at different blockage ratios, flame acceleration rate increases with increasing blockage ratio. The occurrence of DDT is hindered by the most congested obstacles, because the shock focusing is insufficiently strong to initiate detonation after passing through the excessively narrow gaps. • Flame acceleration and DDT in a channel with an array of cylindrical obstacles are studied. • Staggered obstacles promote flame acceleration and transition to detonation. • Obstacle located on sidewall is not conducive to flame entrainment into its wake. • Highly congested obstacles hinder detonation initiation via weakening shock focusing. [ABSTRACT FROM AUTHOR]

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent. • Premixed hydrogen-air flame dynamics after planar ignition is studied. • The flame lips are produced due to the stretching effect of nonuniform flow. • The distorted tulip flames can be evolved from an initially planar flame. • Influence of geometry of ignition region on flame evolution is analyzed. [ABSTRACT FROM AUTHOR]

In relation to the grave consequences arising after accidents at nuclear power plants (NPPs) caused or accompanied by explosions and ignition of hydrogen–air mixtures, the need for a more thorough approach to study the accompanying physicochemical processes has emerged. This paper presents a brief analysis of these situations in the light of prevention of hydrogen explosions at NPPs and the use of catalysts based on platinum and palladium for the utilization of hydrogen. The interaction of a hydrogen–air flame with the surface of a bulk catalyst is experimentally investigated, and the efficiencies of the selected catalysts are compared with respect to the heterogeneous reaction of hydrogen oxidation. The dependence of the experimental results on the design and location of the functional zones of the reactor is analyzed. [ABSTRACT FROM AUTHOR]

• The flame wrinkling characteristics were investigated in a set of spherically expanding hydrogen-air combustion experiments. • The temporal and spatial evolutions of the wrinkling factor are consistent with the experimental findings on the flame morphology and flame propagation speed. • The performance of a simple algebraic correlation developed for the wrinkling factor is adequate. • Small-scale tests are beneficial in predicting the flame acceleration in a larger scale. When modelling the combustion of hydrogen in such a large domain as a nuclear reactor containment building, flame wrinkling needs be quantified in the changing conditions of the combustion process. In addition to turbulence, intrinsic instabilities and buoyancy is known to contribute to the wrinkling of the flame surface and thus increase the total area of the flame front leading to flame acceleration. The flame surface wrinkling factor represents the magnification of the flame surface area due to the flame wrinkles. This study examines the feasibility of evaluating the development of the wrinkling factor from the time evolution of the pressure commonly measured in combustion experiments. Several published quiescent hydrogen combustion experiments with spherically propagating flames are analyzed to study effects of the experimental scale, hydrogen content, pressure, and temperature. The values obtained for the wrinkling factor are within the scatter of the ones determined from the directly measured flame propagation speed and exhibit similar dependence on the distance and hydrogen content. The characteristics of the wrinkling factor are utilized to formulate an algebraic correlation for the flame wrinkling factor for spherically expanding flames in lean and stochiometric quiescent premixed hydrogen-air mixtures. The parameters controlling flame cellularity are concluded to be the propagation distance, the laminar burning velocity, and the density difference between the unburned and burned gas mixtures. The form of the correlation is based on the fractal behavior of the flame front and the model parameter, the characteristic time in the fully developed self-similar acceleration stage, was determined from small-scale combustion experiments. In the analyzed cases, the performance of the correlation is adequate and within the experimental uncertainties which suggest that cellular flame fronts can be considered behaving as fractal surfaces and the small-scale tests are beneficial in predicting the flame front acceleration in a larger scale. [ABSTRACT FROM AUTHOR]

We experimentally investigated the promotion of deflagration-to-detonation transition (DDT) in hydrogen-air mixtures contained in a tube in which straight-shaped rods were installed as obstacles. In the experiments, the number of obstacle rods, their spacing, their blockage ratio, and the equivalence ratio of the hydrogen-air mixture were varied as the governing parameters. The obstacle rods had a spacing of 10 or 20 mm and a blockage ratio of 0.32, 0.41, or 0.51. As a result of an optimization of the obstacle-rod conditions, when fourteen rods, whose blockage ratio was 0.32, were installed in a tube with a spacing of 20 mm and with a hydrogen-air mixture with equivalence ratios from 0.8 to 1.2, the run-up distance to the DDT was shortened to approximately 20 times the tube diameter.

Numerical simulations were conducted to study the flame acceleration and deflagration-to-detonation transition (DDT) in a channel with a bank of aligned obstacles. The multidimensional, fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusion model for stoichiometric hydrogen-air mixture were solved using a high-order numerical algorithm. The results in the case with circular obstacles show that occurrence of DDT arises from shock focusing at flame front. Higher blockage ratio leads to faster flame acceleration and facilitates the formation of choked flame. Heat losses can significantly attenuate flame acceleration and delay detonation initiation in case of DDT, and even hinder occurrence of DDT for high blockage ratio. To understand the effect of shape of obstacle on the flame acceleration and DDT, circular, square, left triangular, and right triangular shapes, always maintaining the same formal blockage ratio, were simulated. The simulations indicate that there are significant differences in flame acceleration and DDT among these differently shaped obstacles, although the basic mechanism for detonation initiation is similar in all of the cases studied and involves interactions of shock wave with flame front and reactivity gradient. Square obstacles produce the largest growth rate of flame surface area in the flame-acceleration stage and thus result in the fastest flame acceleration and shortest detonation onset time compared to circular or triangular obstacles. Circular obstacles produce the weakest flame-surface growth, and thus they have the least effect on promoting flame acceleration and transition to detonation. A closer analysis shows that the sharp angles of the triangular obstacles, when compared to the circular obstacles, are more conducive to flame stretching and convolution and thus facilitate flame acceleration and DDT. The results also show that the effect of obstacle shape does not correlate with obstacle area. [ABSTRACT FROM AUTHOR]

A closed mathematical model of continuous spin detonation with the chemical kinetics equation correlated with the second law of thermodynamics is developed for a hydrogen–air mixture within the framework of the quasi-three-dimensional unsteady gas-dynamic formulation. The model takes into account the reverse influence of the oscillation processes in the combustor on the injection system of the mixture components. For comparisons with experimental data, the numerical simulations are performed for the geometric parameters of the flow-type annular combustor with an outer diameter of 306 mm used in the experiments. For the flow rates of the mixture varied in the interval 73.1–171.3 kg/(s ⋅ m 2 ), the one-wave, two-wave, and three-wave regimes of continuous spin detonation are calculated, the flow structure is analyzed, the specific impulses are determined, and comparisons with experimental data are performed. It is shown that the use of a simplified single-stage kinetic scheme of hydrogen oxidation, which was used in some investigations, for simulating continuous spin detonation leads to results that differ from the experimental data by several times. [ABSTRACT FROM AUTHOR]

Oblique detonation wave (ODW) structures are studied widely in recent years, but most of them are solved by the Euler equations without considering viscosity and then effects of boundary layer. In this study, the Navier-Stokes Equations are used to simulate the wedge-induced ODWs in hydrogen-air mixtures, and the two types of ODW transition structures at different incident Mach number M i are analyzed to clarify the effects of viscosity and hence the boundary layer. Results show that the effect of boundary layer on ODW structures should be classified by the types of ODW transition patterns. As for the smooth transition pattern of ODW at high Mach numbers, the effect of boundary layer can be neglected, but for the abrupt transition pattern of ODW at low Mach numbers, the effect of boundary layer is large and it changes the ODW structure greatly. Resulting from the interaction of shock and boundary layer, a recirculation zone is formed within the viscous ODW layer at M i = 7, which leads to the phenomenon that the straight oblique shock wave evolves into two sections, with the downstream one having a larger shock angle. Additionally, the corresponding transition position moves upstream, and the initiation length becomes only one third of that in inviscid ODW. The great importance of considering viscosity in ODW simulations and future designs of combustor of oblique detonation engine has been addressed. • Effects of boundary layer are important in abrupt transition of ODW. • Shock-boundary layer interaction in abrupt transition of viscous ODW. • Significantly shorter transition length in viscous ODW. • Different evolution process in viscous ODW. • Great importance of considering viscosity in ODW simulations and ODE designs. [ABSTRACT FROM AUTHOR]

The identification of the chain origin of gas combustion at atmospheric and elevated pressure has opened up new aspects of the theory and wide-ranging possibilities for effectively managing the processes of combustion, explosion, and detonation by controlling the branching and loss rates of active intermediate particles, such as atoms and radicals, by using inhibitors and promoters. In this paper, we briefly describe methods for preventing fires and explosions of methane-air mixtures, including those in coal mines, ignition and explosion of hydrogen-air mixtures, and the transition of combustion to detonation in the current ramjet engine model. The results of interdepartmental tests and experimental data illustrating the destruction of a stationary detonation wave by the small impurities of the lower hydrocarbons are given. [ABSTRACT FROM AUTHOR]

In this paper, the ignition of a hydrogen-air mixture by a jet of burned hot gas (often termed reignition in the field of explosion protection) is studied in a two-chamber experimental configuration. This experiment allows systematic creation of hot gas jets emerging from a thin, long nozzle and them to impinge into an unburned, cold hydrogen-air mixture, possibly causing (re)ignition. In an extensive parametric study, hot exhaust gas jets originating from different nozzle diameters (0.6 mm-1.2 mm) and lengths (25 mm and 70 mm), as well as different pressure ratios along the nozzle (up to 5.5) were tested for their ability to reignite the hydrogen-air mixture. It was investigated how often reignition occurred within 10 tests under each condition. Dependencies between the occurrence of reignition and the mentioned nozzle and jet parameters were determined. It was found that the outcome (reignition or no reignition) can differ for nominally identical experimental parameters, which demonstrates the parametric sensitivity of the process. The statistical frequency of reignition occurrence depends on the nozzle diameter as well as on its length. For pressure ratios above the critical value, reignition occurred more often for the longer nozzle (l = 70 mm) at a given diameter. Some of these findings, notably the dependence on the nozzle length, pose a challenge to conventional models. [ABSTRACT FROM AUTHOR]

Summary: To further reveal the pre‐ignition characteristics of hydrogen internal combustion engine, the effect of hot surface characteristic parameters on the ignition characteristics of hydrogen‐air mixture was investigated in this research. Based on the prototype of the constant volume combustion bomb with an overhead glow plug, the duration from the heating of the hot surface to the combustion of hydrogen‐air mixture, the so‐called heating duration, was firstly researched under different fuel‐air equivalence ratio, initial temperature, initial pressure, hot surface temperature, and hot surface area, and the influence of each factor on the heating duration was analyzed. The results show that the order of the effect of each factor on the hot surface ignition is as follows: hot surface temperature > initial pressure of hydrogen‐air mixture > equivalent ratio > initial temperature of hydrogen‐air mixture > hot surface area. The influence of the hot surface characteristic parameters on the heating duration was further analyzed in detail. On this basis, the relationship among the critical ignition temperature, the heating duration and the hot surface area was researched and established. The results show that the heating duration is the only major factor affecting the critical ignition temperature. Finally, the research results were applied to analysis the pre‐ignition in hydrogen internal combustion engine. [ABSTRACT FROM AUTHOR]

• The explosion characteristics of the hydrogen-air mixture near a concrete barrier wall were captured. • The CESE solver based on the immersed boundary method was utilized and coupled with the LS-DYNA® structural FEM solver. • The reduced chemical kinetic model was used to describe the hydrogen-air explosion. • The numerical model was verified with existing experimental data and compared with the conventional TNT equivalent method. • TNT equivalent method is just reliable to capture the 1st peak overpressure in front of the barrier wall. • CESE approach with the reduced chemical kinetic model accurately predicts all the explosion characteristics in front of and behind the barrier wall. Barriers are introduced as an efficient method for the protection of infrastructures and people from overpressure effects owing to accidental gaseous explosions. Therefore, capturing the pressure wave propagation in front of and behind the barrier plays an important role in designing a protective wall and hazard analysis of pipeline explosion accidents. To this end and to track the explosion characteristics of the hydrogen-air mixture near a concrete barrier wall, a new numerical approach in the compressible CESE solver based on the immersed boundary method (IBM) was utilized and coupled with the LS-DYNA® structural FEM solver in which the finite-rate chemistry model was in touch with the fluid-structure interaction (FSI). For the wall, the Concrete Damage Model (MAT_72R3) was employed as the constitutive material model. Existing experimental results were utilized to verify the 3D numerical model. Furthermore, the obtained numerical simulation results were compared with the conventional numerical approach called the TNT equivalent method using the MMALE algorithm in the open literature to show its drawbacks in predicting the pressure wave propagation in front of and particularly behind the wall surface. The results showed that by incorporating the finite-rate chemistry model in the CESE IBM FSI solver, all the explosion characteristics related to the hydrogen-air mixture detonation in front of and behind the wall surface can be accurately predicted since it involves detonation wave/structure interaction, chemical reaction, fluid motion, and structural deformation. Therefore, it is strongly recommended to use the CESE IBM FSI solver in conjunction with appropriate chemical kinetics as an alternative method instead of the conventional method to accurately model the gaseous mixture explosion process in a relatively complex environment and solve more practical applications in explosion and safety industries. [ABSTRACT FROM AUTHOR]

The problem of initiation and stabilization of detonation combustion of a hydrogen-air mixture injected into an axisymmetric channel with a finite-length central body in a flow with a Mach number M = 5-9 is solved. It is numerically demonstrated that the presence of the central body both in a convergent-divergent nozzle and in an expanding channel leads to stabilization of detonation combustion of a stoichiometric hydrogen-air mixture at free-stream Mach numbers M > 7. Various channel configurations that ensure different values of thrust generated by detonation combustion of a stoichiometric hydrogen-air mixture are compared. [ABSTRACT FROM AUTHOR]

A large eddy simulation is performed to study the deflagration to detonation transition phenomenon in an obstructed channel containing premixed stoichiometric hydrogen–air mixture. Two-dimensional filtered reactive Navier–Stokes equations are solved utilizing the artificially thickened flame approach (ATF) for modeling sub-grid scale combustion. To include the effect of induction time, a 27-step detailed mechanism is utilized along with an in situ adaptive tabulation (ISAT) method to reduce the computational cost due to the detailed chemistry. The results show that in the slow flame propagation regime, the flame–vortex interaction and the resulting flame folding and wrinkling are the main mechanisms for the increase of the flame surface and consequently acceleration of the flame. Furthermore, at high speed, the major mechanisms responsible for flame propagation are repeated reflected shock–flame interactions and the resulting baroclinic vorticity. These interactions intensify the rate of heat release and maintain the turbulence and flame speed at high level. During the flame acceleration, it is seen that the turbulent flame enters the ‘thickened reaction zones’ regime. Therefore, it is necessary to utilize the chemistry based combustion model with detailed chemical kinetics to properly capture the salient features of the fast deflagration propagation. [ABSTRACT FROM AUTHOR]

A large-scale continuous detonation combustor (CDC) has been designed, fabricated and tested to study the effect of different design elements on the operation process and CDC propulsion performance. It has been shown experimentally that widening of the air-inlet slit in the annular combustion chamber from 2 to 15 mm leads to a decrease in the number of detonation waves (DWs) simultaneously circulating in the combustor from four to one and, finally, to transition to the operation mode with intermittent (pulse) longitudinal reaction waves resembling pulse detonations. The number of DWs and the thrust produced by the CDC can be increased by installing a shaped obstacle at the CDC exit nozzle providing the blockage of the combustor cross section. The maximum net thrust produced by the CDC attained 6 kN at the total mass flow rate of fuel components of 7.5 kg/s, whereas the maximum fuel-based specific impulse attained ∼3000 s. [ABSTRACT FROM AUTHOR]

To verify the predictability of a computational technology developed at the Semenov Institute of Chemical Physics, Russian Academy of Sciences, three-dimensional calculations of the operation process in a hydrogen-air continuous detonation combustor (CDC) of the Lavrent'ev Institute of Hydrodynamics, Siberian Branch, Russian Academy of Sciences have been conducted with the reproduction of the geometrical dimensions of all the elements of the experimental combustor and the main operating conditions. The calculation results are in good agreement with the experiment data on all the measured characteristics. The problem of the applicability of a planar two-dimensional approximation with periodic boundary conditions to the simulation of the physicochemical processes in an annular CDC has been specifically studied. It has been shown that the distributions of density, temperature, Mach number, and axial velocity component in the different sections of the combustor are substantially three-dimensional, whereas the static pressure distribution approaches a two-dimensional pattern with increasing distance from the bottom of the CDC. The three-dimensional calculations have shown that the conventional assumption of a supersonic discharge at the outlet of a two-dimensional computational domain is not always correct: extensive zones of a subsonic discharge of detonation products can exist in the outlet section. [ABSTRACT FROM AUTHOR]

The feasibility of steady detonation combustion of a hydrogen-air mixture entering at a supersonic velocity in an axisymmetric convergent-divergent nozzle with a central coaxial cylinder is considered. The problem of the nozzle starting and the initiation of detonation combustion is numerically solved with account for the interaction of the outflowing gas with the external supersonic flow. The modeling is based on the gasdynamic Euler equations for an axisymmetric flow. The calculations are carried out using the Godunov scheme on a fine fixed grid which allows one to study in detail the interaction of an oblique shock wave formed in the diffuser with the nozzle axis. It is shown that a central coaxial cylinder ensures the starting with the formation of supersonic flow throughout the entire nozzle and stable detonation combustion of a stoichiometric hydrogen-air mixture in the divergent section of the nozzle. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents 2D simulations of the ignition of a lean premixed –air flame by a nanosecond spark discharge between two-point electrodes at atmospheric pressure and at an initial temperature of 1000K. As a first step, it was assumed that thermal and chemical effects of the nanosecond spark discharge are the same in a lean –air mixture as in air. Comparing different models for the gas heating, we have shown that the fraction of the discharge energy going to gas heating for nanosecond spark discharges is in the range 20–30%, in agreement with literature values. The gas heating was found to start during the nanosecond voltage pulse as soon as the nanosecond spark channel is formed and the discharge energy is deposited non-uniformly in air with hot spots close to electrode tips. For the chemical effect, results show that in air, the atomic oxygen radical is mostly produced after the nanosecond voltage pulse due to the dissociative quenching of electronically excited . The maximum of the dissociation level of molecular oxygen is in the range 30%-60 in the plasma channel. Then, thermal and chemical effects of the nanosecond spark discharge on the ignition of a lean premixed –air flame have been studied separately. In both cases, a flame ignition was observed. Results show that thermal and chemical effects of the nanosecond spark on the ignition are of the same order with a slightly higher ignition efficiency for the chemical effect. Finally, we have taken into account simultaneously thermal and chemical effects and a synergistic activation has been observed. [Copyright &y& Elsevier]

This paper presents data on the lower and upper flammability limits of hydrogen–air mixtures at elevated temperature and pressure. A 5-L explosion vessel, an ignition system, and a transient pressure measurement sub-system were used in this study. Through a series of experiments carried out, the lower and upper flammability limits of hydrogen–air mixtures at different initial pressures and temperatures have been studied and the influence of initial temperature and pressure on the lower and upper flammability limits of hydrogen–air mixtures has been analysed and discussed. It was found that the decrement of the LFLs of hydrogen–air with the initial temperature from 21 to 90 °C at the initial pressure of 0.1 MPa is less than 1%, the decrement of the LFLs with the initial temperature from 21 to 90 °C at 0.2 MPa is less than 1%, the decrement of the LFLs with the initial temperature from 21 to 90°Cat 0.3 MPa is less than 0.66%, and the decrement of the LFLs with the initial temperature from 21 to 90 °C at 0.4 MPa is less than 0.25%. The lower flammability limits of hydrogen–air mixtures at the pressures of 0.1 and 0.4 MPa are 4 and 1.25%(V/V), respectively. The upper flammability limits of the hydrogen–air mixtures increase with the initial pressure and temperature. The upper flammability limit of the hydrogen–air mixtures at 90 °C and 0.4 MPa reaches 93%(V/V) which is much higher than that (76%(V/V)) at 21 °C and 0.1 MPa. [ABSTRACT FROM AUTHOR]

Processes of continuous spin detonation and pulsed detonation, as well as combustion of a hydrogen-air mixture in an annular combustor 306 mm in diameter in the regime of air ejection are studied experimentally. The specific flow rates of hydrogen are 0.6-9.8 kg/(s ·m). It is found that the greatest specific impulses of thrust generated by the combustor are reached in the case of continuous spin detonation. On the average, they are greater than the corresponding values by a factor of 1.5 in the case of burning the mixture in streamwise detonation waves, by a factor of 2 in the case of conventional combustion (by a factor of 3 at the maximum thrust impulse of 2200 m/s), and by a factor of 10 in the case of exhaustion of cold hydrogen. A change in the specific flow rate of hydrogen beginning from ≈1.2 kg/(s·m) corresponding to the maximum thrust impulse decreases its value, and this decrease is more profound as the detonation limits in terms of the specific flow rate of hydrogen are approached. The maximum reactive thrust (83 N) is developed in the examined detonation chamber near the upper limit at the specific flow rate of hydrogen equal to 3 kg/(s·m). [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen is a promising fuel and is expected to replace hydrocarbon fuels for its significant potentials to reduce the pollutants and greenhouse gases. It is very important to investigate Minimum ignition energy (MIE) on safety standards and ignition process of hydrogen–air mixtures. Even though the formation of flame kernels in quiescent hydrogen–air mixtures has been researched numerically and experimentally, the details of ignition mechanism have never been satisfactorily explained. In this study, the spark ignition of hydrogen–air mixture is investigated by using detailed chemical kinetics and considering the heat loss to the electrode. The purpose of this study is emphasized in the effects of the energy supply procedure, the radius of the spark channel, electrode size and electrode gap distance on the MIE. In addition, the effects of mixture temperature, electrode gap distance and electrode size on relationship between the equivalence ratio and the MIE are examined. [Copyright &y& Elsevier]

Results of an experimental study of continuous and pulsed detonation of a hydrogen-air mixture in an annular flow-type combustor 306 mm in diameter with an expanding channel in the air ejection mode are reported. By varying the free-cross-sectional area of the slot for air supply, the number and cross-sectional area of the orifices of the fuel injectors, the size of the fuel receiver, and the initial pressure of the fuel, the domain of existence of detonation regimes in the coordinates 'air ejection slot size versus the specific flow rate of hydrogen' is determined. An optimal air ejection slot width for the combustor and fuel used is found (10-12 mm); deviations from this slot width to either side reduce the domain of existence of detonation regimes. A necessity of making a step in the air supply path is found. It is also shows that there exists an optimal geometry of injector orifices, which expands the domain of existence of detonation regimes. Rough mixing of hydrogen with air, as well as too rapid mixing, makes the domain of detonation existence narrower. The following sequence of processes is found to occur as the hydrogen flow rate is increased: combustion transforms to longitudinal pulsed detonation, then to continuous spin detonation, then to pulsed detonation again, and finally to usual combustion. Experiments of long-time operation of the combustor without cooling are performed. [ABSTRACT FROM AUTHOR]

Results of an experimental study of continuous detonation of a hydrogen-air mixture in a flow-type annular combustor 306 mm in diameter in the regime of air self-ejection are reported. The regime of pulsed detonation is also obtained. Stable regimes of continuous detonation with one and two transverse detonation waves having velocities D = 1.48−1.16 km/sec are observed in experiments. The frequency of the pulsed detonation wave is ≈1.4 kHz. The known condition for the continuous detonation regime (good mixing for the formation of a detonable layer) is validated. The size of the slot for air ejection, providing a necessary flow rate for detonation and a necessary ratio of the species in the mixture, is determined. Some methods for estimating the air flow rate are presented. [ABSTRACT FROM AUTHOR]

Abstract: The hazard associated with flame acceleration to supersonic speeds in hydrogen mixtures is discussed. A set of approximate models for evaluation of the run-up distances to supersonic flames in relatively smooth tubes and tubes with obstacles is presented. The model for smooth tubes is based on general relationships between the flame area, turbulent burning velocity, and the flame speed combined with an approximate description for the boundary layer thickness ahead of an accelerated flame. The unknown constants of the model are evaluated using experimental data. This model is then supplemented with the model for the minimum run-up distance for FA in tubes with obstacles developed earlier. On the basis of these two models, solutions for the determination of the critical run-up distances for FA and deflagration to detonation transition in tubes and channels for various hydrogen mixtures, initial temperature and pressure, tube size and tube roughness are presented. [Copyright &y& Elsevier]

Abstract: Laser-induced spark ignition of lean hydrogen–air mixtures was experimentally investigated using nanosecond pulses generated by Q-switched Nd:YAG laser (wavelength 1064nm) at initial pressure of 3MPa and temperature 323K in a constant volume combustion chamber. Laser ignition has several advantages over conventional ignition systems especially in internal combustion engines, hence it is necessary to characterise the combustion phenomena from start of plasma formation to end of combustion. In the present experimental investigation, the formation of laser plasma by spontaneous emission technique and subsequently developing flame kernel was measured. Initially, the plasma propagates towards the incoming laser. This backward moving plasma (towards the focusing lens) grows much faster than the forward moving plasma (along the direction of laser). A piezoelectric pressure transducer was used to measure the pressure rise in the combustion chamber. Hydrogen–air mixtures were also ignited using a spark plug under identical experimental conditions and results are compared with the laser ignition ones. [Copyright &y& Elsevier]

A principal possibility of approximate reconstruction of the chemical composition in combustion of a hydrogen-air mixture at the end of the scramjet duct from incomplete experimental data (measured concentration of OH radicals and temperature) under the assumption of detailed chemical equilibrium of exchange reactions is analyzed. A closed algebraic system of equations including the concentration of OH radicals and the temperature as parameters is derived in this approximation. A code for solving this system numerically is developed for approximate determination of reaction completeness from the measured temperature and concentration of OH radicals. The model was tested by results of exact thermodynamic calculations of the Jouguet state and overdriven waves in a stoichiometric hydrogen-air mixture and various hydrogen-oxygen mixtures. In the range of pressures of 0.2 to 500 atm and temperatures of 2500 to 3500 K, this method allows the molecular weight and heat release to be reconstructed with accuracy sufficient for gas-dynamic calculations. [ABSTRACT FROM AUTHOR]

The possibility of increasing the laminar flame velocity in a hydrogen-air mixture by excitation of O2 molecules into the a 1Δ g singlet state. The presence of 10% of O2( a 1Δ g ) molecules in oxygen is demonstrated to result in noticeable (up to 50%) enhancement of mixture burning. The temperature of combustion products and also the concentrations of H2O, NO, and other constituents increase. The greatest effect of O2( a 1Δ g ) molecules is manifested in combustion of lean mixtures; the least pronounced effect is observed in rich mixtures. These effects are caused by intensification of the chain mechanism in the presence of a super-equilibrium amount of excited O2( a 1Δ g ) molecules in a hydrogen-air mixture. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the electrostatic ignition of hydrogen–air mixtures, the density of OH radicals and the gas temperature are measured in an incipient spark-ignited hydrogen–air flame using laser-induced predissociation fluorescence (LIPF). The assessment of the electrostatic hazard of hydrogen is necessary for developing hydrogen-based energy systems in which hydrogen is used in fuel cells. The spark discharge occurs across a 2-mm gap with pulse duration approximately 10 ns. First, a hydrogen (50%)–air mixture is ignited by spark discharge with , where E is the spark energy and is the minimum ignition energy. In this mixture, OH density decreases after spark discharge. It is at and at , where t is the postdischarge time. On the other hand, the gas temperature increases after spark discharge. It is 900 K at and 1400 K at . Next, a stoichiometric (hydrogen (30%)–air) mixture is ignited by spark discharge with . In this mixture, OH density is approximately constant at for 150 μs after spark discharge, and the gas temperature increases from 1000 K () to 1800 K (). [Copyright &y& Elsevier]

Abstract: A two-phase thermo-siphon loop for removing nuclear heating and maintaining the stable liquid level in the moderator cell was adopted for the cold neutron source (CNS) of the China advanced research reactor (CARR). The moderator is liquid hydrogen. The two-phase thermo-siphon loop consists of the crescent-shape moderator cell, the moderator transfer tube, and the condenser. The hydrogen is supplied from the buffer tank to the condenser. The main feature of the loop is that the moderator cell is covered by the helium sub-cooling system. The cold helium gas from the helium refrigerator is firstly introduced into the helium sub-cooling system and then flows up through the tube covering the moderator transfer tube into the condenser. The main part of this system is installed in the CNS vertical channel made of aluminum alloy 6061 T6 (Al-6061–T6) of 6mm in thickness, 270mm in outer diameter and about 6m in height. For confirming the safety of the CNS channel, the combustion tests using a tube compatible with the CNS channel were carried out using the hydrogen–air mixture under which air is introduced into the tube at 1 atmosphere, and then hydrogen gas is supplied from the gas cylinder up to the test pressures. And maximum test pressure is 0.14MPaG. This condition is involved with the maximum design basis accident of the CARR–CNS. The peak pressure due to combustion was 1.09MPa, and the design pressure of the CNS channel is 3MPa. The safety of the CNS was thus verified even if the maximum design basis accident occurs. The pressure and stress distributions along the axial direction and the displacement of the tube were also measured. [Copyright &y& Elsevier]