Deflagration Combustion Research Articles

Pressure Gain Combustion for Gas Turbines: Analysis of a Fully Coupled Engine Model

Abstract The ‘Shockless Explosion Combustion’ (SEC) concept for gas turbine combustors, introduced in 2014, approximates constant volume combustion (CVC) by harnessing acoustic confinement of autoigniting gas packets. The resulting pressure waves simultaneously transmit combustion energy to a turbine plenum and facilitate the combustor's recharging against an average pressure gain. Challenges in actualizing an SEC-driven gas turbine include i) the creation of charge stratifications for nearly homogeneous autoignition, ii) protecting the turbo components from combustion-induced pressure fluctuations, iii) providing evidence that efficiency gains comparable to those of CVC over deflagrative combustion can be realized, and iv) designing an effective one-way intake valve. This work addresses challenges i)-iii) utilizing computational engine models incorporating a quasi-one-dimensional combustor, zero- and two-dimensional compressor and turbine plena, and quasi-stationary turbo components. Two SEC operational modes are identified which fire at roughly one and two times the combustor's acoustic frequencies. Results for SEC-driven gas turbines with compressor pressure ratios of 6:1 and 20:1 reveal 1.5-fold mean pressure gains across the combustors. Assuming ideally efficient compressors and turbines, efficiency gains over engines with deflagration-based combustors of 30% and 18% are realized, respectively. With absolute values of 52% and 66%, the obtained efficiencies are close to the theoretical Humphrey cycle efficiencies of 54% and 65% for the mentioned pre-compression ratios. Detailed thermodynamic cycle analyses for individual gas parcels suggest that there is room for further efficiency gains through optimized plenum and combustor designs.

Large-Eddy Simulation Study of Flow and Combustion Dynamics in a Full-Scale Hydrogen-Air Rotating Detonation Combustor-Stator Integrated System

Abstract A first-of-its-kind large-eddy simulation (LES) study is conducted to numerically investigate the combustion dynamics as well as aero-thermal phenomena in a full-scale non-premixed hydrogen-air rotating detonation engine (RDE) integrated with nozzle guide vanes (NGV) acting as the turbine stator. The LES framework incorporates hydrogen-air detailed chemical kinetics and adaptive mesh refinement. A comparative analysis is carried out for two operating conditions with different fuel/air mass flow rates but global equivalence ratio of unity. The LES model is validated against experimental data with respect to detonation wave speed/height, wave dynamics, and axial static pressure distribution. Numerical results indicate significant deflagrative combustion occurring in the fill region near the inner wall due to formation of recirculation zones in the injection near-field driven by the backward facing step. The leading detonation wave is found to be trailed by an azimuthal reflected-shock combustion wave, which consumes unburned vitiated reactants that leak through the main detonation wave. The detonation wave characteristics do not change appreciably with the presence of NGV. The exit flow is found to be nearly fully subsonic and supersonic for the low and high mass flux conditions, respectively. The presence of NGV renders the flow more axial, and significantly impacts the exit Mach number and total pressure. The low mass flux operating point, despite exhibiting more deflagrative losses within the combustor, yields overall lower pressure drop from plenum to exhaust, mainly attributed to lower pressure drop across the injectors.

Deflagration Dynamics of Methane–Air Mixtures in Closed Vessels at Elevated Temperatures

In this paper, we explore the deflagration combustion of methane–air mixtures through both experimental and numerical analyses. The key parameters defining deflagration combustion dynamics include maximum explosion pressure (Pmax), maximum rate of explosion pressure rise (dP/dt)max, deflagration index (KG), and laminar burning velocity (SU). Understanding these parameters enhances the process of safety design across the energy sector, where light-emissive fuels play a crucial role in energy transformation. However, most knowledge on these parameters comes from experiments under standard conditions (P = 1 bar, T = 293.15 K), with limited data on light hydrocarbon fuels at elevated temperatures. Our study provides new insights into methane–air mixture deflagration dynamics at temperatures ranging from 293 to 348 K, addressing a gap in the current process industry knowledge, especially in gas and chemical engineering. We also conduct a comparative analysis of predictive models for the laminar burning velocity of methane mixtures in air, including the Manton, Lewis, and von Elbe, Bradley and Mitcheson, and Dahoe models, alongside various chemical kinetic mechanisms based on experimental findings. Notably, despite their simplicity, the Bradley and Dahoe models exhibit a satisfactory predictive accuracy when compared with numerical simulations from three chemical kinetic models using Cantera v. 3.0.0 code. The findings of this study enrich the fundamental combustion data for methane mixtures at elevated temperatures, vital for advancing research on natural gas as an efficient “bridge fuel” in energy transition.

Research on the Combustion Mode and Thrust Performance of Rotating Detonation Scrarmjet Engines

Decades of research in hypersonic science and engineering have shown that scramjet engines are the preferred choice for powering hypersonic vehicles. In these engines, oblique shock combustion and rotating detonation combustion are two common combustion modes. This article primarily focuses on the numerical simulation and theoretical analysis of these two combustion modes. The research indicates that as the combustible mixture injected into the combustion chamber increases, the combustion mode transitions from deflagration combustion to self-sustaining detonation combustion and then to forced detonation combustion to maintain stable combustion in the chamber. Under self-sustaining detonation combustion mode, as the amount of combustible mixture injected into the chamber increases, the number of detonation waves gradually increases from one to multiple and the angle between the detonation waves and the inflow changes from acute to right angles. In the forced detonation combustion mode, the minimum shock wave intensity required to initiate detonation combustion can be obtained by drawing a tangent from the Rayleigh line. The structure generating oblique shock waves in the combustion chamber needs to match the shock wave intensity. In the detonation combustion mode, both the exit total temperature and specific impulse of the combustion chamber increase with the increase in Mach number at the inlet, while the pressure ratio decreases. This article provides a reference for the design of the combustion chamber in supersonic combustion ramjet engines.

Initiation process of non-premixed continuous rotating detonation wave through Schlieren visualization

High-speed schlieren visualization on the initiation process of non-premixed air and hydrogen continuous rotating detonation (CRD) is carried out in a well-designed rounded-rectangle hollow combustor in this study. The CRD initiation processes of pre-detonator ignition and spark plug ignition are comprehensively revealed by synchronous high-speed schlieren images and high-frequency dynamic pressure results. The results show that the CRD undergoes three processes after ignition, including initiation process, adjustment process and stable propagation process. The CRD initiation process of pre-detonator ignition involves the upstream-propagation phase of leading shock wave and reaction zone and the coupling phase of circumferentially-propagating shock wave and reaction zone, while that of spark plug ignition includes the upstream-propagation phase of reaction zone, and the coupling phase of circumferentially-propagating pressure wave and reaction zone. The formation of deflagration combustion flowfield and re-establishment of combustible mixture layer after ignition are important prerequisites for CRD initiation. The strong interaction between circumferentially-propagating shock wave and post-shock wave reaction conduces to the rapid initiation of CRD by pre-detonator ignition, while the pressure wave gradually interacts with reaction leading to the detonation initiation by spark plug ignition. The CRD initiation time and adjustment time of pre-detonator ignition are shorter than those of spark plug ignition. This study provides detailed insight into the physics of CRD initiation process of pre-detonator and spark plug ignition, which can deepen the understanding of the initiation mechanism and provide practical guidance for the optimal design of CRDE ignitor.

NOx formation processes in rotating detonation engines

High-fidelity simulations of RDEs with H2-Air-NOx chemistry are employed to study NOx emissions in such devices. Discrete injection of gaseous hydrogen fuel and continuous injection of air oxidizer is used at various mass flow rate conditions in several 3D RDE simulations to understand resulting NOx production behaviors. Simulations are also performed for two different injector configurations, one in which air is injected axially into the detonation chamber [Axial Air Inlet (AAI)] and one in which air is injected radially [Radial Air Inlet (RAI)]. It is seen that the AAI RDE produces much less NOx than the RAI RDE, mainly due to the weaker waves seen in this system as a result of parasitic combustion losses from product gas recirculation. Parasitic combustion does lead to NOx formation in its own right, but the emissions levels from this process are negligible compared to emissions stemming directly from detonation processes. In regards to detonation strength in particular, it is generally seen that detonation strength increases with increasing mass flow rate, in turn increasing peak pressure, peak heat release and NOx emissions levels. Nevertheless, even the highest recorded NOx levels at the combustor exit in this study remain on the same order of magnitude as compared to gas turbine exhaust emissions levels, supporting the claim that significant differences between detonative and deflagrative combustion do not necessarily lead to significant differences in NOx levels. Overall, this study provides greater understanding into the behaviors of NOx formation in RDEs and how these behaviors are affected by changes in operating parameters.

Effects of swirling inflow on the stability and combustion mode of rotating detonations

In this study, a novel approach for enhancing the stability of rotating detonation waves (RDWs) with the use of a swirling inflow strategy is presented. A series of numerical simulations are carried out by solving the two-dimensional reactive Navier–Stokes equations. The effects of the swirling angle on the stability of the RDWs and the combustion mode are analyzed. The results show that the formation of the burnt gas bumps is suppressed by the implementation of a swirling inflow. The swirling inflow also contributes to an increased homogeneity of the reactant within the fuel refill zone. As a result, a remarkable enhancement of the stability of the RDWs in terms of their oscillations in heights and inclined angles is achieved without an apparent compromise of the heights of the RDWs. The propagation speeds of the RDWs are controllable within a wide range approximately from 81% to 114% of the Chapman–Jouguet detonation speed by adjusting the swirling angle. Moreover, the oscillations in the instantaneous fuel consumption rates of both detonative and deflagrative combustion are dominated by the oscillation in the height of the RDW; hence, the swirling inflow reduces the oscillations in these two fuel consumption rates and subsequently the detonation fraction. Consequently, the smoothness of the performance output in terms of specific impulses can be significantly improved with a reduced standard deviation of oscillation up to 84% by the implementation of swirling inflows, and the averaged specific impulse only encounters a small deficit of no more than 7.4%.

Computational Analysis of Shrouded Ejector Effect on Starting Vortex and Combustion Efficiency in Pulse Detonation Combustor with Different Fuels

ABSTRACT This research paper deals with the starting vortex of detonation combustion wave flow field in pulse detonation combustor (PDC). In order to achieve the potential pressure gain from PDC the shrouded ejectors are placed at exit section of detonation tube. Therefore, influence of shrouded ejector position plays a great role on leading vortex and reduction of NOx formation. Furthermore, n-butane (C4H10) and hydrogen (H2) fuel are used for stoichiometric (= 1) fuel-air mixture condition to simulate the thermal and prompt NOx formation at pulse time of 0.022s to 0.030s. The LES turbulence model is used to simulate the unsteady flow field in CFD based fluent platform. The one step irreversible chemical kinetics model analyzed the details of exothermic chemical reaction mechanism inside the combustor. The non-dimensional value of /d = 1.5 geometry position of shrouded ejector is better for leading vortex. So far, pickup detonation wave speed of 2306 ms−1 is obtained at shortest pulse time of 0.028s and this magnitude is higher than C-J velocity. The minimum pollutant number of 0.0000341 is obtained from hydrogen-air detonation, this magnitude is lesser compared to n-butane (C4H10)-air combustion. Furthermore, the maximum combustion efficiency of 85% is obtained from hydrogen-air mixture in detonation combustion process, which is comparatively higher than deflagration combustion process.

Combustion and detonation of paste fuel of rocket engine

Purpose. Confirmation of the possibility of using a paste fuel based on ammonium perchlorate in a rocket engine and identifying the characteristics of its combustion. Methodology. Previous experimental studies on the burning of paste fuel in a constant pressure bomb determined the burning rate at different pressures. The stability of deflagration combustion without transition to the detonation mode was confirmed. An explosion occurred during the fire test of the engine model on paste-like fuel. The analysis of the causes of the explosion made it possible to put forward a hypothesis about the enrichment of the paste fuel with ammonium perchlorate, which created the prerequisites for its detonation. The conducted additional experiments showed a change in the combustion mode when enriching paste fuel with ammonium perchlorate. Findings. Theoretical and experimental studies have shown the possibility of obtaining detonation fuel based on the enrichment of paste fuel with ammonium perchlorate. It has been proven that, under certain production conditions, paste fuels can detonate, which opens up a new way of using such fuels for rocket engines. The conditions for the transition of the burning mode of pasty fuel from deflagration to detonation combustion are determined. The speed of the engine element during the explosion was evaluated and it was shown that during explosive combustion due to the large area and, accordingly, the mass flow, it is not possible to obtain a pressure value that could ensure the movement parameters registered in this engine design. Originality. Another criterion is established of engine operability when designing an engine on paste fuel. The effect of enrichment of pasty fuel with ammonium perchlorate during its flow through the supply system at the time of start-up was revealed. Practical value. The given information makes it possible to improve the design of the engine on paste fuel and to modernize the stand for its tests.

Influence of initiation conditions of methane-air mixture on the explosion parameters in the degasation pipeline

Purpose. Increasing the safety of personnel at underground works in case of the threat of gas explosions, including explosion of degassing pipelines by taking into account the influence of the place of initiation of the explosion and the mode of combustion when calculating the parameters of the explosion. Mathematical modeling of the process of ignition and combustion of the methane-air mixture in the degassing pipeline. Research methods. nalysis and generalization of theoretical studies, numerical simulation of gas-dynamic detonation combustion of a gas-air mixture, including a model of chemical combustion kinetics, which, on the whole, allows one to trace the dynamics of formation of shock air waves in conditions of mine workings. Mathematic processing of the results of the experiment by the method of least squares. When solving the nonlinear regression equations, linearization was used by the method of logarithm. Results. It is established that the combustion dynamics of the gas-air mixture has a decisive influence on the formation of UVB: in the deflagration combustion mode of the gas-air mixture, with the initiation at the edge of the cloud, the length of the blast wave is increased 3.6 times in the direction opposite to combustion, and the wave amplitude by 10% spread of flame. In the detonation mode, the effect of a directed explosion is observed, in which the amplitude of the shock air wave is opposite to the motion of the detonation wave 5 times less than in the direct shock air wave. The model of an instantaneous volumetric explosion, in comparison with combustion models, yields an underestimated amplitude value. Scientific novelty. As a result of numerical modeling of the ignition and combustion of the sealed section of the mine atmosphere, regularities in the formation and propagation of shock air waves, the dependence of their parameters on the combustion regime and the location of the initiation of the gas-air mixture were obtained. Practical value. The conducted research allows to improve the method of calculating the resistance of degassing pipelines and their elements to explosive load.

Numerical Analysis on Detonation Wave and Combustion Efficiency of Pulse Detonation Combustor With U-Shape Combustor

Abstract The research work is carried out for deflagration and detonation combustion processes at different equivalence ratios of hydrogen–air mixtures in a pulse detonation combustor (PDC). Furthermore, the U-shape channel curvature radius and thickness effect on detonation wave propagation are also investigated. This numerical simulation has been done using a SIMPLE algorithm with the finite volume discretization method and laminar finite rate chemistry for volumetric reaction in the Ansys Fluent platform. The numerical result shows that the U-bend radius of R = 3.5 cm can enhance the faster deflagration-to-detonation transition. So far, the fully developed detonation wave was found near the curvature area of the detonation tube having a width of W = 8 cm. This enhanced detonation wave velocity reaches 2775 m/s, which is higher than the C-J detonation velocity. Furthermore, the entropy generation has been analyzed in two modes of the combustion process. The entropy generation number of 0.76 and 0.7 is obtained from the deflagration and detonation combustion processes. However, the entropy production rate is less in the detonation combustion process, but thermal entropy generation is more in the deflagration combustion process with a magnitude of 3.5 kJ/kg K for an equivalence ratio of φ = 1.5. A combustion efficiency of 78% is found in the detonation combustion process, which is comparatively higher than the deflagration process.

Application of detonation in impulse systems of energy engineering and mining industry

Nowadays the demand for energy needs is constantly growing. Given the fact that most of this demand is provided by non-renewable energy sources, another way of development remains relevant is more efficient use of available resources. One of the areas of effective energy use is swapping deflagration combustion to the detonation process. Due to the rapidity of the detonation, the entropy of the reaction products is lower than that of deflagration combustion. In addition, there are only “fast” reactions, so the molar mass of the products is the minimum possible for a specific fuel mixture. The article considers a special case of the application of the detonation process in a new way of control the thrust vector of rocket engines, shows its effectiveness and features of use. Numerical modeling of the injection of detonation products into the supersonic part of nozzle at different angles was carried out, and the change in relative lateral force over time was determined for each case. It is shown that the control system with discrete control pulses can be used to maneuver the upper stage of the rocket and to destroy hard mineral rocks, to compact soils and materials in compressed conditions.

Application of liquid hydrogen as a fuel for future passenger aircraft

The paper briefly reviews a recent initiative on the application of hydrogen as a fuel for future commercial aircraft. Special attention is paid to the benefits of using liquid hydrogen (LH2) to power aircraft engines. A comparison of LH2 to other fuels is presented, as well as a comparison of LH2 hazard to hazard imposed by typical jet fuel. Major attention is focused on the combustion of hydrogen benefits in turbine engines with classic deflagrative combustion chamber and engines utilizing detonative combustion of the hydrogen-air mixture. Benefits and problems with the utilization of LH2 are discussed in this paper.

Numerical Analysis of Combustion Dynamics in a Full-Scale Rotating Detonation Rocket Engine using Large Eddy Simulations

Abstract Large eddy simulations (LES) using detailed chemistry and leveraging adaptive mesh refinement (AMR) are performed to gain insights into the combustion dynamics within a full-scale methane-oxygen non-premixed rotating detonation rocket engine (RDRE) employing impinging discrete injection schemes. In particular, a comparative analysis of two operating conditions corresponding to the same global equivalence ratio but different mass flow rates is carried out to investigate the resultant impact on detonation wave characteristics and RDRE global performance. Multiple co-rotating detonation waves with spatially-distributed wave structure and preferential alignment with the inner wall of the annulus (due to asymmetry in fuel distribution) are encountered under both conditions. Both cases exhibit pre-detonation deflagrative burning in the fill region, while one of the cases shows higher susceptibility to backflow into the feed plenums due to lower plenum pressures. Heat release analysis shows that the thrust obtained from the RDRE is closely linked to the distribution of total heat release between detonative and deflagrative combustion. On the other hand, combustion efficiency is associated with the fraction of heat release occurring in fuel-rich versus fuel-lean regions within the RDRE.

Ignition and deflagration-to-detonation transition modes in ethylene/air mixtures behind a reflected shock

Dynamics of ethylene autoignition and deflagration-to-detonation transition (DDT) are first numerically investigated in a one-dimensional shock tube using a reduced chemistry including 10 species and 10 reactions. Different combustion modes are investigated through considering various premixed gas equivalence ratios (0.2 − 2.0) and incident shock wave Mach numbers (1.8–3.2). Four ignition and DDT modes are observed from the studied cases, i.e., no ignition, deflagration combustion, detonation after reflected shock, and deflagration behind the incident shock. For detonation development behind the reflected shock, three autoignition hot spots are formed. The first one occurs at the wall surface after the re-compression of the reflected shock and contact surface, which further develops to a reaction shock because of “the explosion in the explosion” regime. The other two are off the wall, respectively, caused by the reflected shock/rarefaction wave interaction and reaction induction in the compressed mixture. The last hot spot develops to a reaction wave and couples with the reflected shock after a DDT process, which eventually leads to detonation combustion. For deflagration development behind the reflected shock, the wave interactions, wall surface autoignition hot spot as well as its induction of reaction shock are qualitatively similar to the mode of detonation after incident shock reflection, before the reflected shock/rarefaction wave collision point. However, only one hot spot is induced after the collision, which also develops to a reaction wave but cannot catch up with the reflected shock. For deflagration behind the incident shock, deflagration combustion is induced by the incident shock compression whereas detonation occurs after the shock reflection. The chemical timescale increases after the reflected shock/contact surface collision, whereas decreases behind the incident and reflected shocks, as well as after the reflected shock/rarefaction wave interaction. Therefore, mixture reactivity behind the reflected shock is weakened by the contact surface, but is intensified by the rarefaction wave. The multi-dimensionality characteristics, including reflected shock/boundary layer interactions, reflected shock bifurcation, destabilization, and detonation, are further present in a two-dimensional configuration. Planar autoignition occurs because of reflected shock compression and detonation combustion is formed first in the central region due to the collision of the reflected shock wave/reflected compression wave. The left and right bifurcations of the separation region in the wall boundary layer are then sequentially ignited.

Режимы взрывного горения при аварийных взрывах газовых облаков в открытом пространстве

Emergency explosions of steam clouds in the open space occur in the deflagration combustion mode. Destructive force of the explosive waves is mainly determined by the rate of combustion in the steam cloud. Therefore, the issue of explosive combustion rate is the key one for predicting explosion parameters. To form the waves of destructive force, it is required that the combustion rate of the substance in the cloud increase by 30 or more times compared to laminar. The main and generally recognized mechanism of combustion intensification is turbulization of the process as a result of interaction of the gas flow field with various obstacles located in the area of the exploding cloud. In the work, the analysis focuses on the combustion processes in the obstacles with continuously changing blocking of space. Under such conditions, the combustion is not structured, it smoothly changes its characteristics, and not jerks at the locations of blocking barriers. That is, explosive combustion can be considered as a classic turbulent combustion of a homogeneous mixed mixture. The work gives preference to the analysis of works, in which the turbulent combustion rate is presented as allowing a change in the scale of turbulence. The results of these works are presented in the form of functions ff(U¢/Sl, l/d) of the ratio of the pulsation component of turbulence to the laminar combustion rate, and the ratio of the integral scale of turbulence to the thickness of the laminar flame. The work gives a comparison of the turbulent combustion velocity depending on the U¢/SL ratio for three values l/d = 100, 1000, 10 000. On the basis of the turbulent combustion modes diagram, the zones of applicability of various methods for determining the turbulent combustion rate are shown. The paper expresses preference for the Peters theory as the most universal and giving a realistic value of the turbulent combustion rate at l/d >> 1.

Experimental Study on Propagation Characteristics of Kerosene/Air RDE with Different Diameters

A series of experimental tests were carried out in order to study the propagation characteristics of a liquid kerosene rotating detonation engine (RDE) with different diameters. Distinguished characteristics of spatial and temporal instability were found in the large-scale RDE due to the uneven circumferential distribution of kerosene supply pressure. As for the two counter-waves detonation system, the 500 mm-diameter RDE maintains a higher detonation wave velocity due to its longer injection recovery time. However, the 220 mm-diameter RDE can obtain a larger equivalence ratio operation range, higher specific thrust, and higher specific impulse. In addition, compared with the deflagration combustion, the detonation combustion has higher chamber pressure and thrust under the same operational conditions.

A Diagnostic Approach to Assess the Effect of Temperature Stratification on the Combustion Modes of Gasoline Surrogates

ABSTRACT Thermal stratification may switch the combustion mode from deflagration to spontaneous (auto-ignition) in spark-ignition engines leading to knock. Despite the available numerical and experimental works, an analytical and systematic method on the role of local thermal stratification on the combustion mode is still missing. Particularly, the effects of heat diffusion before ignition in negative temperature coefficient (NTC) chemistry of gasoline surrogates are typically ignored. In this study, an a-priori diagnostics tool is provided to separate the deflagration and auto-ignition combustion modes by considering the diffusion effects, based on two parameters: stratification wavelength () and amplitude (). The diagnostics tool is an extension of Zeldovich’s theory to transient problems by solving the diffusion equation and considering the flame and ignition timescales. It is found that 1) The theory is valid against one-dimensional numerical simulations under different average temperatures and pressures. 2) NTC chemistry promotes spontaneous ignition at both low and high pressures. 3) In the presence of NTC chemistry, the transition region between the two combustion modes is broadened. A third blended mode is observed (spontaneous ignition assisted flame) with front speeds . 4) Finally, estimation of the knock propensity from the generated maps is related to the surrogates’ octane sensitivities.

Numerical investigation on reacting shock-bubble interaction at a low Mach limit

We investigate the deflagration combustion in the reacting shock-bubble interaction at M = 1.34 using a novel adaptive mesh refinement combustion solver with comprehensive H2/O2 chemistry. The numerical results are compared with an experiment by Haehn et al [1]. The Richtmyer-Meshkov instability dominates the shock-bubble interaction, and the shock focusing in the heavy bubble induces ignition. By following the initial experimental setup published by Haehn et al [1]. and adopting the axisymmetric assumption, we successfully reproduce most of the flow features observed in the experiment both qualitatively and quantitatively, including the bubble morphology evolution and the corresponding chemiluminescence images. The fuel consumption rate is nonmonotonic because of unsteady flame propagation, and it also depends on interfacial instabilities. The deflagration waves increase transverse bubble diameter, mildly decrease the total vorticity, and promote mixing by more than 150% because of the thermal effects. The mixing promotion is approximately 88% related to the diffusivity and 12% related to other mechanisms after ignition. A new shock focusing mechanism is observed due to the secondary refracted shock. During shock focusing, Mach reflection occurs and transits from the bifurcated type to the single type. This transition causes two ignitions: the first occurs in the spiral hot spot entrained by the jet vortex, and the second arises from the hot spot caused by the triple point collision. After the second ignition, the newborn flame is a deflagration at the beginning but is unstable and tends to transit to a detonation as a consequence of shock-flame interactions. Nevertheless, the deflagration-to-detonation transition fails, and the stable combustion mode is deflagration.

Экспериментальные исследования горения сферических водородно-воздушных смесей в открытом пространстве при воздействии на них замедляющих и ускоряющих факторов

It is shown that the experimental studies of flame propagation speed, the maximum pressure of the explosion of hydrogen-air mixtures were carried out in free space under the influence of slowing down and accelerating factors, such as power and type of ignition source, the effect of flame acceleration in the pipe, the effect of obstacles on the flame propagation path, the volume and composition of the combustible mixture when the content of the oxidizing agent or phlegmatizer changes in it. The dependence was established allowing to estimate the volume of a hydrogen-air mixture, during the combustion of which one can expect the burning speed that is close to the speed of sound in the initial mixture, and at which the transition of deflagration combustion to detonation is possible. When phlegmatizing hydrogen-air and oxygen-hydrogen mixtures with inert gases, such as nitrogen for example, the effect of reducing the burning speed and explosion pressure is achieved. The processes and conditions for the transition of deflagration combustion to detonation of free volumes of hydrogen-air mixtures and mixtures enriched and depleted of oxygen, depending on the power of the ignition source, volume, composition of the mixture and its turbulence, were studied. It is noted that as the content of the inert gas in the mixture increases, these indicators decrease, and the profile of the pressure wave changes qualitatively. Turbulization of hydrogen-air and hydrogen-oxygen mixtures with dilution using nitrogen to certain limits by turbulators leads to an increase in the values of the apparent burning speed and the maximum speed of explosion burning, up to the transition to detonation. Irrigation of combustible mixtures with atomized water leads to the same effect.