Russian Federation

High resolution numerical simulations used to study the mechanism of deflagration-to-detonation transition (DDT). The computations solved two-dimensional time-dependent reactive Navier-Stokes equations including the effects of compressibility molecular diffusion thermal conduction viscosity and detailed chemical kinetics for the reactive species with subsequent chain branching production of radicals and energy release. It is shown that from the beginning the flame accelerates exponentially producing shock waves far ahead. On the next stage the flame acceleration decreases and the shocks are formed close ahead of the flame front. The final stage is the actual transition to detonation. During the second stage a compressed unreacted mixture of increased density enters the flame producing a high pressure pulse which enhances reaction rate and the heat release in the reaction zone with a positive feedback coupling between the pressure pulse and the reaction rate. As a result the peak of the pressure pulse grows exponentially steepens into a strong shock which is coupled with the reaction zone forming the overdriven detonation. This new mechanism of DDT is different from the Zel’dovich’s gradient mechanism. The temperature gradients which appear in the form of hot spots and the like are not suitable to initiate detonation.

The objectives of the presented experimental work were investigation of hydrogen release distribution and combustion modelling possible emergency situation at industry scale. Results of large scale experiments on distribution and combustion in an open and congested area are presented. The mass of hydrogen in experiments varied from 50g to 1000g with release rate from 180 to 220 g/s. Qualitative characteristics of high momentum hydrogen jet releases distribution and subsequent combustion were obtained. It is shown that obstacles slow down jet speed promote combustible mixture formation in a large volume and accelerate combustion process. The maximum overpressure in experiments with additional congested area reached ΔР = 0.4 atm. Using partial confinement of congested area turbulent combustion regime with the maximum overpressure more than 10 atm. was obtained.

Any experimental study of catalysts and catalytic recombining devices for removal of hydrogen gas from industrial environments is known to carry a risk of ignition of hydrogen. Experiments conducted in an atmosphere with a high concentration of hydrogen present a particular danger. Here a technique is reported that allows conducting such experiments with relative safety. This technique has been developed and applied by the company ‘Russian Energy Technologies’ for the last five years without any significant incident.<br/>A “Gas stream method” for testing and analysis of the characteristics of a catalyst for hydrogen/oxygen recombination is proposed. Tests with a variety of catalysts in a passive recombining device were carried out in a climatic chamber (86 l in volume) with a hydrogen/air mixture containing up to 20% (v/v) hydrogen flowing through it. The balance equation for hydrogen and oxygen flows entering reacting and exiting the chamber led to a formula for calculating the efficiency of a catalyst or a catalytic device under stationary conditions.<br/>Fluctuations in local temperatures of the catalyst and other parts of the chamber along with variation in the concentration of hydrogen gave the authors an insight into the thermal regime of an active catalyst. This enabled them to develop new catalysts for removal of hydrogen from the environment using industrial recombining devices.

Formic acid is a liquid organic hydrogen carrier giving hydrogen on demand using catalysts. Metal complexes are known to be used as efficient catalysts for the hydrogen production from formic acid decomposition. Their performance could be better than those of supported catalysts with metal nanoparticles. However difficulties to separate metal complexes from the reaction mixture limit their industrial applications. This problem can be resolved by supporting metal complexes on the surface of different supports which may additionally provide some surface sites for the formic acid activation. The review analyzes the literature on the application of supported metal complexes in the hydrogen production from formic acid. It shows that the catalytic activity of some stable Ru and Ir supported metal complexes may exceed the activity of homogeneous metal complexes used for deposition. Non-noble metal-based complexes containing Fe demonstrated sufficiently high performance in the reaction; however they can be poisoned by water present in formic acid. The proposed review could be useful for development of novel catalysts for the hydrogen production.

Report presents the preliminary experimental results on hydrogen subsonic leakage in a closed vessel under the well-controlled boundary/initial conditions. Formation of hydrogen-air gas mixture cloud was studied for a transient (10 min) upward hydrogen leakage which was followed by subsequent evolution (15 min) of explosive cloud. Low-intensity ( 0.46⋅10−3 m3/sec) hydrogen release was performed via circular (diameter 0.014 m) orifice located in the bottom part of a horizontal cylindrical vessel ( ≈4 m3). A spatially distributed net of the 24 hydrogen sensors and 24 temperature sensors was used to permanently track the time dependence of the hydrogen concentration and temperature fields in vessel. Analysis of the simultaneous experimental records for the different spatial points permits to delineate the basic flow patterns and stages of hydrogen subsonic release in closed vessel in contrast to hydrogen jet release in open environment. The quantitative data were obtained for the averaged speeds of explosive cloud envelop (50% fraction of the Lower Flammability Limit (LFL)) propagation in the vertical and horizontal directions. The obtained data will be used as an experimental basis for development of the guidelines for an indoors allocation of the hydrogen sensors. Data can be also used as a new benchmark case for the reactive Computational Fluid Dynamics codes validation.

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The “Magnesium group” of international experts contributing to IEA Task 32 “Hydrogen Based Energy Storage” recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures kinetics and thermodynamics of the systems based on MgH₂ nanostructuring new Mg-based compounds and novel composites and catalysis in the Mg based H storage systems. Finally thermal energy storage and upscaled H storage systems accommodating MgH₂ are presented.

Hydrogen technologies such as hydrogen fuelled vehicles and refuelling stations are being tested in practice in a number of projects (e.g. HyFleet-Cute and Whistler project) giving valuable information on the reliability and maintenance requirements. In order to establish refuelling stations the permitting authorities request qualitative and quantitative risk assessments to show the safety and acceptability in terms of failure frequencies and respective consequences. For new technologies not all statistical data can be established or are available in good quality causing assumptions and extrapolations to be made. Therefore the risk assessment results contain varying degrees of uncertainty as some components are well established while others are not. The paper describes a methodology to evaluate the degree of uncertainty in data for hydrogen applications based on the bias concept of the total probability and the NUSAP concept to quantify uncertainties of new not fully qualified hydrogen technologies and implications to risk management.

With regard to the goals of the European HySafe Network research facilities are essential for the experimental investigation of relevant phenomena for testing devices and safety concepts as well as for the generation of validation data for the various numerical codes and models. The integrating activity ‘Integration of Experimental Facilities (IEF)’ has provided basic support for jointly performed experimental work within HySafe. Even beyond the funding period of the NoE HySafe in the 6th Framework Programme IEF represents a long lasting effort for reaching sustainable integration of the experimental research capacities and expertise of the partners from different research fields. In order to achieve a high standard in the quality of experimental data provided by the partners emphasis was put on the know-how transfer between the partners. The strategy for reaching the objectives consisted of two parts. On the one hand a documentation of the experimental capacities has been prepared and analysed. On the other hand a communication base has been established by means of biannual workshops on experimental issues. A total of 8 well received workshops has been organised covering topics from measurement technologies to safety issues. Based on the information presented by the partners a working document on best practice including the joint experimental knowledge of all partners with regard to experiments and instrumentation was created. Preserving the character of a working document it was implemented in the IEF wiki website which was set up in order to provide a central communication platform. The paper gives an overview of the IEF network activities over the last 5 years.

The report summarizes experimental results on the mechanisms and kinetics of hydrogen-air flammable gas cloud formation and evolution due to foreseeable (less than 10-3 kg/sec) hydrogen leaks into confined spaces with different shapes sizes and boundary conditions. The goals were - 1) to obtain qualitative information on the basic gas-dynamic patterns of flammable cloud formation at different leak velocities (between 935 and 905 m/sec) for a fixed leak flowrate and 2) to collect quantitative data on spatial and temporal characteristics of the revealed patterns. Data acquisition was performed using a spatially distributed reconfigurable net of 24 hydrogen gauges with short response time. This experimental innovation permits to study spatial features of flammable cloud evolution in detail which previously was attainable only from CFD computations. Two qualitatively different gas dynamic patterns were documented for the same leak flowrate. In one limiting case (sufficiently low speed of leak) the overall gas-dynamic pattern can be described by the well-known “filling box” model. In another limited case (high velocity of leak) it is proposed to describe the peculiarities of gas-dynamic behavior of flammable cloud by the term of a “fading up box” model. From the safety view point the “fading up box” case is more hazardous than the “filling box” case. Differences in macroscopic and kinetic behavior which are essential for safety provision are presented. Empirical non-dimensional criterion for discrimination of the two revealed basic patterns for hydrogen leaks into confined spaces with comparable length scale is proposed. The importance of the revealed “fading up box” gas-dynamic pattern is discussed for development of an advanced hydrogen gauges system design and safety criteria.

Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean complex-chemistry hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed to perform a priori assessment of predictive capabilities of the flamelet approach for evaluating mean species concentrations. For this purpose dependencies of mole fractions and rates of production of various species on a combustion progress variable c obtained from the laminar flame are averaged adopting either the actual Probability Density Function (PDF) P (c)  extracted from the DNS data or a common presumed β-function PDF. On the one hand the results quantitatively validate the flamelet approach for the mean mole fractions of all species including radicals but only if the actual PDF P (c) is adopted. The use of the β-function PDF yields substantially worse results for the radicals’ concentrations. These findings put modeling the PDF P (c) on the forefront of the research agenda. On the other hand the mean rate of product creation and turbulent burning velocity are poorly predicted even adopting the actual PDF. These results imply that in order to evaluate the mean species concentrations the flamelet approach could be coupled with another model that predicts the mean rate and turbulent burning velocity better. Accordingly the flamelet approach could be implemented as post-processing of numerical data yielded by that model. Based on the aforementioned findings and implications a new approach to building a presumed PDF is developed. The key features of the approach consist in (i) adopting a re-normalized flamelet PDF for intermediate values of c and (ii) directly using the mean rate of product creation to calibrate the presumed PDF. Capabilities of the newly developed PDF for predicting mean species concentrations are quantitively validated for all species including radicals.