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Consideration is given to a promising approach to heat treatment of biofuel, viz. oxidative torrefaction in a layer of mineral filler. The key features of the proposed technique are the uniformity of heating up the specimen and the possibility of controlling oxidizing processes in the specimen by changing the height of the mineral filler. Based on design-theoretical analysis, an investigation has been made of the thermal and kinetic processes accompanying torrefaction in a layer of mineral filler. It is shown that the presence of the mineral layer ensures a high degree of uniformity for heating up a biofuel specimen in the process of torrefaction. The influence of the mineral filler layer on the diffusion flows of atmospheric oxygen and volatile compounds released during heating up the specimen is shown.

The paper is devoted to the numerical analysis of the reactive mixture explosion during the accidents at a launch site. The particular scenario concerns the explosion in the unconfined volume after accidental fuel release into the atmosphere. It is shown that the development of intrinsic flame instability results in the generation of compression waves, which occur to be intensified via the mechanism of thermo-acoustic instability. In turn, intensification of the compression waves leads to the formation of strong shock waves and even detonation in the case of a highly reactive mixture. The leading role in this process belongs to the transversal waves generated in the folds of the developed flame surface and propagated along the flame surface. It is the localization of the compression wave inside the reaction zone that causes amplification of the compression wave due to energy release.

Reducing highway hazards through the creation of modern lighting systems is an important practical task that currently has some difficulties. The rapid development of renewable energy makes it possible to use off-the-shelf solutions to create autonomous lighting complexes for unregulated pedestrian crossings. The analysis of peculiarities of design, constructive structure and operation of autonomous lighting complexes, which are located in Kharkiv region and use renewable energy sources and energy storage systems to power lighting devices and flashing signal lights, is carried out. The analysis results showed that the lighting complexes under investigation are not capable of supplying sufficient energy for themselves and of functioning properly at low insolation and low ambient temperatures. The reasons for the unstable operation of autonomous lighting complexes have been identified, with the main one being the insufficient accuracy of taking into account the actual climatic operating conditions of facilities when forming design solutions. Measures have been developed to improve the efficiency of the autonomous lighting complexes under study, involving the use of off-the-shelf technical solutions based on modern tools and technologies. A criterion for sufficiency of the developed measures to solve the problem of autonomous power supply of lighting complexes for highways is proposed. The sign of the resulting capacity of the complex over the calculation time period is taken as a sufficiency criterion. The practical use of the developed criterion confirmed the feasibility of the proposed measures to improve the operational efficiency of the investigated autonomous lighting complexes for highways. Calculations have shown that it is not sufficient to use solar panels alone to supply highway lighting complexes located in regions with unstable levels of insolation. The feasibility of hybrid power supply systems for autonomous lighting complexes that use additional power sources such as wind turbines and systems with electromechanical converters in addition to solar panels has been quantitatively confirmed.

A problem formulation is proposed for simulating lean premixed hydrogen–air combustion in a closed volume with numerically generated turbulence. Two-dimensional simulations of flame propagation from a small ignition kernel showed that the following sequence of regimes is observed with increasing turbulence intensity and decreasing mixture strength: a corrugated flame with a continuously connected front, a regime with a local loss of front connectivity, and flame extinction through disintegration of a growing kernel into fragments. The transition from steady self-sustained combustion to extinction corresponds to changes in mixture and turbulence parameters leading to a stronger influence of turbulent velocity field on local flame structure. The proposed approach to numerical simulation of the transition regime characterized by loss of front connectivity and fragmentation of the flame under the action of turbulent eddies can be used to evaluate the effect of turbulence intensity on flammability limits.

The paper analyses numerically the scenarios of ignition kernels formation in the shock tube in the intermediate temperature range. Three modes of mild ignition are distinguished among which are: ignition related to the shear heating in the developed boundary layer, ignition due to the shear heating in the recirculation zone behind the reflected shock and ignition in the central region of the tube due to the axial compression. All three modes define short ignition delays at low temperatures compared to the ideal shock-tube values. Moreover, it is shown that the gas-dynamic mechanisms responsible for additional local heating of the test mixture provide close rates of heating. As a result, all the data on ignition delays fall in a certain range defined by the mixture composition and pressure, independent on the particular mode of mild ignition.

The paper is aimed at numerical study of one of the most hazardous events at a launch place: open space explosion of fuel air mixtures due to accidental loss of containments. A mechanism of transition to detonation in the process of unconfined flame propagation is proposed. It is shown that the detonation onset takes place as a result of local exponential growth of unstable short-wavelength perturbations. Exactly the same mechanism is known to be responsible for the self-similar flame acceleration, however the detonation can arise only in the case of extremely high reaction rate. High reaction rate defines a coupling of the accelerating flamelets with diverging shock waves that leads to the detonation onset.

The physical mechanisms controlling the development of a flame prior to transition to detonation in a channel filled with a combustible gaseous mixture are systemized and analyzed. The features of the development of the accelerated flame in a channel are demonstrated with the results of numerical simulation for smooth and obstructed channels. This analysis makes it possible to formulate the criteria for a detonation onset induced by the development of an accelerated flame. Estimations on the basis of the proposed criteria predict with reasonable accuracy the limits of detonation initiation in hydrogen-based combustible mixtures.

The structure of the flow and temperature field behind a reflected shock wave in a shock tube is studied by numerical modeling. Two possible scenarios of the formation of the regions of elevated temperature, which are the potential ignition kernels of the gaseous mixture under the study, are demonstrated. Both scenarios are assessed as equally probable; however, depending on the parameters of the flow and diameter of the tube, one of the scenarios may be implemented earlier than the other, which is demonstrated by the example of a hydrogen–air mixture. The scenario with the ignition on the axis is the most probable one for the case of high temperatures and narrow channels, while ignition in the region of the boundary layer is implemented with a higher probability at moderate temperatures in wide channels.

This work presents the review and classification of the scenarios of the development of the final stage of flame acceleration in channels and tubes including the transition to detonation on the flame front and formation of the conditions for kernel ignition ahead of the front. An approach to the numerical assessment of the detonability of combustible gaseous mixtures is formulated based on the classification of high-speed combustion modes proposed by the authors, and the general possibility of the practical application of such an approach is shown. The quantitative estimates of the critical conditions for the stable detonation formation as a result of flame acceleration and upon the transmission of the detonation wave, which are in agreement with the available published experimental data, are presented.

The problem of the detonation formation as a result of unconfined flame propagation is solved numerically. The mechanism of detonation formation is distinguished. It is related to the local formation of shock waves du- ring the linear stage of development of flame front perturbations formed on the surface of the expanding flame front. General criteria of the establishment of the conditions for the detonation transition via the proposed mechanism are formulated.

We consider a new promising approach to the combustion of water-saturated hydrocarbon fuels in the form of foamed emulsions, stable combustion of which can sometimes be maintained at a water content above 90% mass %. Theoretical analysis and numerical simulations demonstrated the basic physical mechanism determining these broad combustibility limits, which consists in the natural spatial separation of the zones of fuel combustion and water evaporation. During combustion, the foam predominantly breaks up into emulsion drops. Nonstationary regimes of combustion are determined by the foam structure. The obtained data define applicability of the proposed concept of combustion of water-saturated foamed hydrocarbon emulsions.

In the paper, the propagation of the detonation wave in the hydrogen-oxygen microfoam on the aqueous base is considered. Microfoam represents a two-phase system containing micron-sized gas bubbles filled with the hydrogen-oxygen mixture. These bubbles are dispersed in the water solution of surfactant (sodium dodecyl sulfate). The dependencies of detonation speed on the equivalence ratio and on the water content in the foam are obtained with the use of high-speed filming. It is found that the detonation speed slightly increases with the decrease in water content in the foam. Based on the pressure measurements, it is established that the detonation propagation in the foam is driven by relatively weak shock waves, which by themselves are not able to induce ignition of the hydrogen-oxygen mixture. To substantiate the fact of detonation existence in the microfoam a hypothesis of the cumulative collapse of gas bubbles under the shock wave action is proposed. The estimation for detonation speed in microfoam is analytically derived on the basis of simple phenomenological representations.

The paper analyses in details and describes the process of ignition kernel formation and subsequent detonation onset behind the shock wave propagating in the shock tube. To get the overall pattern of the process a series of one- and two-dimensional calculations are carried out with the use of a dissipation-free numerical technique. It is shown that one of the leading roles in the process of ignition kernel formation belongs to the non-steady flow dynamics establishing behind the shock wave. The development of the boundary layer determines both the temperature re-distribution in the near-wall region and the conditions for gas-dynamic acceleration of the flow. With an account of thermal runaway, the most intensively heated region corresponds to the area between the inner margin of the boundary layer and the contact surface separating driver gas and the test mixture. After localized ignition takes place the forming reaction wave propagates out from the ignition epicenter. Reaction wave propagates behind the outrunning compression wave through the already reacting mixture. Shock-induced compression of the test mixture provides conditions for the self-sustained acceleration of the combustion wave, and finally, the detonation onset takes place.

This paper is devoted to the analysis of the effect of gas injection on the efficiency of gaseous fuel burning. A coaxial burner with a perforated inner wall is presented in which the methane–air acceleration is observed. With the use of numerical analysis, it is demonstrated that the flame acceleration is related to the flow separation from the inner wall that, in turn, leads to the reduction in heat losses to the wall as well as to vortex formation and reduction in momentum losses. On the basis of the obtained results, a new technology of efficient burning of gaseous fuels can be proposed with the use of gas and/or liquid fuel injection.

Development of the combustion process in the gaseous mixtures of near-limit composition is of great interest for fundamental aspects of combustion theory and fire-safety applications. The dynamics of ultra-lean gaseous flames in near-limit mixtures is governed by many effects, such as buoyancy, preferential diffusion, radiation, and instability development. Though ultra-lean combustion was extensively studied in microgravity conditions, the influence of gravity on the ultra-lean flame structure and stability is still poorly understood. The paper is devoted to deepening the knowledge of ultra-lean flame dynamics in hydrogen-air mixtures under terrestrial gravity conditions. The spatial structures of the flame developing under the effect of buoyancy forces are investigated employing detailed numerical analysis. Different modes of near-limit flame evolution are observed depending on the mixture concentration. In particular, we registered and described three distinct spatial structures: individual kernels tending to extinguish in leanest compounds, complex multi-kernel structures in marginal compositions, and stable cap-shaped flames in more chemically active mixtures. We apply the flame-bubble analogy to interpret flame dynamics. On this basis, the diagram in the Re-Fr plane is developed. That allows classifying the emerging flame structures and determine flame stability. Additionally, different ignition modes are studied, and the mechanisms determining the impact of ignition mode on the flammability limits are distinguished. Obtained results provide useful insights into the processes of flame quenching and development in near-limit hydrogen-air mixtures under real gravity conditions and can be applied in the design of contemporary fire-safety systems.

Combustible aqueous foams and foamed emulsions represent prospective energy carriers. This paper is devoted to the overview of model assumptions required for numerical simulations of combustion and detonation processes in aqueous foams. The basic mathematical model is proposed and used for the analysis of the combustion development in the wet aqueous foam containing bubbles filled with reactive gas. The numerical results agree with the recent experimental data on combustion and detonation in aqueous foams containing premixed hydrogen–oxygen. The obtained results allowed for distinguishing the mechanisms of flame acceleration, transition to detonation, detonation propagation, and decay.

The paper considers trans- and supersonic flames propagating through the channels filled with hydrogen-oxidizer mixtures. High-resolution numerical analysis of the flame front propagating in the so-called “chocked” regime allowed us to formulate possible mechanisms of further flame acceleration and successive onset of detonation. It is shown that the following criteria should be satisfied for further flame acceleration after “chocked” flame regime establishment: 1) the reaction rate should rise with pressure; 2) the effect of external gasdynamical and acoustic fields should be reduced to minimum. According to the formulated criteria all of the experimentally obtained regimes of supersonic combustion and onset of detonation can be described generally in terms of “chocked” flame stability towards chemical and gasdynamical factors. The first criterion is satisfied in near stoichiometric hydrogen–oxygen and hydrogen–air mixtures at normal and higher ambient pressures due to peculiarities of the hydrogen oxidation kinetics. The second criterion is satisfied in smooth channels and can be temporary satisfied in obstructed channels of specific widths and blockage ratios. In case of smooth channel filled with highly active mixture subsequent flame acceleration results in a deflagration-to-detonation transition. In case of lower chemical activity of the combustible mixture flame propagation continues in a quasi-steady supersonic regime. In this case oscillations of the flame velocity provoke the non-steady flow perturbations and compression waves emergence that can cause formation of auto-ignition kernels on the surface of contact discontinuity or on the obstacles surfaces.

The paper is devoted to the analysis of non-steady combustion and detonation in aqueous solutions foamed with the hydrogen-oxygen gaseous mixture. The processes of flame acceleration and transition to detonation are studied experimentally and numerically. Based on complex analysis, the effect of cumulative compression of the reactive gaseous mixture inside collapsing bubbles is proposed to be the mechanism of detonation formation and its further self- sustained propagation. Flame propagation through the foam leads to the compression waves generation. Under the action of compression waves, the gas bubble collapses, and states with high energy densities are achieved inside the bubble. As a result, the ignition inside the bubble takes place that represents a basic mechanism of detonation formation and propagation.

The concept of detonation wave initiation in the local volume of a fuel–gas mixture containing suspended chemically neutral microparticles heated by radiant energy from an external source is proposed. Mechanisms of initiation of the combustion and detonation waves in a region of accumulation of the radiation- heated microparticles have been studied by numerical simulation methods. Criteria that determine geometric dimensions of a region of the two-phase medium, which are necessary for the initiation of detonation waves, are formulated.

One of the perspective types of combined fuels is the oil-in-water emulsion foamed with the hydrogen-oxygen mixture. In this paper, the process of combustion of such a system obtained via hydrogen-oxygen mixture bubbling through the heptane-in-water emulsion is considered experimentally for the first time. The main goal of this paper is to study different regimes of foamed emulsion combustion at varying composition of hydrogen-oxygen mixture inside bubbles and heptane content in the emulsion. Flame speed is measured with the use of high-speed filming. It is shown that the use of hydrogen allows increasing the speed of flame propagation through the foamed emulsion, and at certain conditions even detonation onset becomes possible. The detonation in the foamed emulsion is determined by the effect of energy focusing during the collapse of gaseous bubbles. When using a lean hydrogen-oxygen mixture, the dependence of the total burning rate of the foamed emulsion on heptane concentration has a maximum. It is found that when using the stoichiometric hydrogen-oxygen mixture, the flame speed decreases monotonically with the increase in heptane concentration. In the case of a weakly reactive compound of the foamed emulsion, the regime of spinning detonation is possible.

Paper presents a comprehensive study of contemporary opportunities in the numerical analysis of transient combustion regimes on the example of the topical problem of hydrogen combustion. The elementary processes determining the development of combustion under different conditions are considered. In view of this analysis, it is concluded that the most appropriate for numerical modeling of classic deflagration inside confined space is the low-dissipation numerical schemes of low (second) order of approximation. For near-critical conditions where distinct mechanisms determine the combustion development one is able to utilize the simplifications such as low Mach number approximation.

The gas-dynamical mechanism of ignition kernels formation in shock tubes is formulated on the basis of numerical analysis of the flow pattern developed behind the shock wave propagating in the rectangular tube filled with a reactive gaseous mixture.

The concept of using the waves generated by gaseous fuel detonation for microparticles acceleration during their implantation onto a solid substrate is analyzed. The dynamics of microparticles of the 1.0–100 μm size has been numerically simulated in a flow behind the shock wave, which is formed as a result of detonation-wave flowing out from a channel into a chemically neutral gas and then interacts with a substrate situated at some distance from the channel exit. It is established that the efficiency of implantation by a detonation-wave impulse decreases for microparticles with sizes on the order of or below 10 μm (4–15 μg). Based on detailed analysis of the implantation-process dynamics, it is proposed to use profiled substrates for increasing the efficiency of implantation of small particles.

Torrefaction is one of the most efficient techniques for improvement of biomass thermophysical properties. One of the possible methods of torrefaction is the biomass heating inside a quiescent mineral layer in the presence of air. Mineral filler prevents excessive oxidation of biomass and favors more uniform temperature distribution inside the reactor. One can effectively control the properties of biofuel via varying the parameters of torrefaction. The main goal of this work is to study the pyrolysis characteristics of torrefied pine pellets. Thermogravimetric analysis and differential scanning calorimeter were used to study the effect of different torrefaction conditions (including the torrefaction duration, the height and type of mineral filler, the presence of an inhibitor of oxidation reactions) on the pyrolysis behavior of the torrefied biomass. It is shown that the reactivity of fuel decreases with the decrease in height of the mineral layer as well as with the increase in torrefaction duration. In turn, the use of an inhibitor of oxidation reactions (sodium bicarbonate) favors the increase in biofuel reactivity. A correlation between the biofuel reactivity properties and the conditions of torrefaction inside a quiescent mineral layer is obtained as result of the study.

The torrefaction of pine pellets in a quiescent mineral layer is considered experimentally and theoretically. In essence, this process represents an oxidative torrefaction of biomass since it is carried out in the presence of air. The main role of mineral filler is to limit the oxygen access to the biomass from the ambient environment. In this paper, the influence of the type of mineral filler (ash, bentonite, talc and calcite) on the properties of torrefied biomass is investigated at a varying height of the mineral layer, temperature and duration of torrefaction. The pyrolytic behavior of torrefied biomass is analyzed via thermogravimetry and differential scanning calorimetry measurements. To analyze individual components of biomass the Fourier transform mid-infrared spectroscopy is used. It is shown that it is inefficient to use the ash as mineral filler while the most optimal fillers are bentonite and talc with the particles size within the range of 15–25 μ m .

The paper analyzes numerically the hydrodynamic processes developing inside the fused quartz under the action of intense laser impulses. Depending on laser intensity two basic regimes are obtained and described in details. At low intensities, the slow regime driven by the heat transfer is observed. Herewith, the fracture took place in the heated region before the phase transition. At higher intensities, the high-speed propagation regime is established characterized by the fracture events exactly at the interfacial boundary between the hot plasma and condensed phase. The propagation of absorption wave coupled with the fracture wave is limited by the value of sound speed in the hot plasma, which determines the expansion of plasma into the spallation region ahead of the absorption front. The proposed model of the process agrees well with the recent experimental data, in particular with the characteristic velocity scales.

Hydrogen-oxygen flame acceleration and the transition from deflagration to detonation (DDT) in channels with no-slip walls are studied using high resolution simulations of 3D reactive Navier-Stokes ...

A procedure has been proposed for the synthesis of monoazacrown ethers by reaction of N-benzyldiethanolamine with oligo(ethylene glycol) bis-p-toluenesulfonates in a two-phase system aromatic hydrocarbon-50% aqueous alkali, followed by removal of the benzyl group by catalytic hydrogenolysis. The maximal yields of N-benzylaza-12-crown-4, -18-crown-6, and -21-crown-7 were achieved by adding 4–10 equiv of LiCl, BaBr2, and CsCl, respectively, to the reaction mixture, which probably indicated template effect.

A procedure has been proposed for the synthesis of monoazacrown ethers by reaction of N-benzyldiethanolamine with oligo(ethylene glycol) bis-p-toluenesulfonates in a two-phase system aromatic hydrocarbon-50% aqueous alkali, followed by removal of the benzyl group by catalytic hydrogenolysis. The maximal yields of N-benzylaza-12-crown-4, -18-crown-6, and -21-crown-7 were achieved by adding 4–10 equiv of LiCl, BaBr2, and CsCl, respectively, to the reaction mixture, which probably indicated template effect.

The basic question raised in the paper concerns the origins of exothermal reaction kernels and the mechanisms of detonation onset behind the reflected shock wave in shock-tube experiments. Using the conventional experimental technique, it is obtained that in the certain diapason of conditions behind the reflected shocks a so-called "mild ignition" arises which is characterized by the detonation formation from the kernel distant from the end-wall. The results of 2-D and 3-D simulations of the flow evolution behind the incident and reflected shocks allow formulation of the following scenario of ignition kernels formation. Initial stage during and after the diaphragm rupture is characterized by a set of non-steady gasdynamical processes. As a result, the flow behind the incident shock occurs to be saturated with temperature perturbations. Further evolution of these perturbations provides generating of the shear stresses in the flow accompanied with intensification of velocity and temperature perturbations. After reflection the shock wave interacts with the formed kernels of higher temperature and more pronounced kernels arise on the background of reactivity profile determined by moving reflected shock. Exothermal reaction starts inside such kernels and propagates into the ambient medium as a spontaneous ignition wave with minimum initial speed equal to the reflected shock wave speed.

The paper considers gas-dynamical processes evolving as a result of laser action in fused quartz. A conventional approach is used to construct a model for equation of state which provides an adequate description of the silica state at high densities of energy typical for local optical silica damage. Shock-wave processes generated in the medium due to the local laser energy deposition are calculated using fully conservative numerical technique. The obtained results provide relatively accurate description of the process in a wide range of parameters and allow further research to get clear interpretation of high-speed propagation of the laser absorbing front through the silica optical fiber.

The present study discusses capabilities of dissipation-free CABARET numerical method application to unsteady reactive gasdynamic flows modeling. In framework of present research the method was adopted for reactive flows governed by real gas equation of state and applied for several typical problems of unsteady gas dynamics and combustion modeling such as ignition and detonation initiation by localized energy sources. Solutions were thoroughly analyzed and compared with that derived by using of the modified Euler-Lagrange method of "coarse" particles. Obtained results allowed us to distinguish range of phenomena where artificial effects of numerical approach may counterfeit their physical nature and to develop guidelines for numerical approach selection appropriate for unsteady reactive gasdynamic flows numerical modeling.

This paper discusses capabilities of the novel dissipation-free CABARET numerical algorithm to solve the range of complex non-stationary combustion problems. On the basis of detailed analysis of the obtained results and comparison with the data derived with the classic low-order coarse particles method it was shown that reactive flow evolution process may be strongly influenced by the artificial effects introduced by the numerical algorithm, numerical dissipation in particular. Revealed peculiarities of the flame propagation dynamics regimes taking place in considered tests allowed us to propose a number of requirements which should be taken into account when choosing numerical procedure suitable for modelling combustion processes in real technical environment.

The origins of different supersonic combustion regimes in channels are discussed. It is shown that the evolution of supersonic flames including the formation of quasi-steady supersonic flames, transition to detonation directly at the flame front and auto-ignition inside the kernels at some distance ahead the flame front are fully determined by the same gasdynamical effects and peculiarities of combustion kinetics. Three basic criteria are formulated aiming to establish correlation between the initial state of the gaseous combustible mixture and the possibility of deflagration-to-detonation transition. Formulated criteria are in an adequate agreement with the data on combustion of hydrogen-based mixtures.

Features of the unsteady flames propagating in channels filled with gaseous combustible mixtures are studied numerically. The analysis is based on the model treating the flame as a moving energy source. It is shown that the crucial role in flame dynamics and its structure evolution belongs to the compression waves emitted by non-steady flame itself. The compression waves establish flow pattern, temperature and pressure fields near the flame front, which in turn determine the features of flame evolution on the different stages of its propagation.

We propose a new conceptual approach for direct detonation initiation in the gaseous mixtures seeded with micro particles via the radiative heating from the external energy source. The basic mechanisms of energy absorption, ignition and detonation formation are analyzed numerically on the example of hydrogen-oxygen mixture. Obtained data is very promising and allows us to formulate conditions for the source power to ignite detonation in certain system geometry.

The aim of the paper was to analyze the structure and the stability of the chocked flames to understand the origins of different possible combustion modes, including quasi-stable supersonic flames and deflagration-to-detonation transition. By means of numerical analysis, it is shown that the chocked flame structure and its stability are defined by two basic mechanisms: compression of the fresh mixture ahead of the flame front and compression of the reacting mixture inside it. The first mechanism provides burning velocity increase; the second one can either accelerate or decelerate reaction depending on the pressure-dependent reaction behavior in the observed pressure range and depending on the rate of compression. In case of reaction intensification with rising pressure, a detonation forms on the leading edge of the flame front. Otherwise, the flame propagates in a quasi-stable supersonic regime consisting of consequential stages of deceleration and re-acceleration of the flame. On the deceleration stage, the compressed fresh mixture priorly chocked by the supersonic flow near the flame tip flows downstream generating the compression wave ahead. The new contact surface between this packet of compressed mixture and the fresh mixture ahead of the flame front can become the kernel of the exothermal reaction, evolving in a new deflagration wave or even detonation.