An over-expanded (Atkinson cycle) reciprocating internal combustion engine is a crucial component of hybrid and range-extended electric vehicles. The realization of the over-expansion cycle with high efficiency in novel power devices with a compact, simple design, such as cycloidal rotary engines, is a promising alternative. The cycloidal rotary engine is an internal combustion engine that operates four strokes without a reciprocating mechanism and valve train. The over-expansion cycle is implemented in the cycloidal rotary engine by asymmetrical intake and exhaust port arrangements, resulting in open and closure timings. This study aimed to establish inherent characteristics of the over-expansion cycle in gasoline-fueled rotary engines with spark ignition. A numerical simulation of the gas exchange and combustion process in a wide range of over-expansion ratios was conducted. It was concluded that the over-expansion cycle is realized without typical piston engine methods such as variable valve timing and multilink mechanism. The thermal conversion efficiency is a peak function of the over-expansion ratio. In the over-expansion range of 1.1–1.2, the thermal conversion efficiency of the cycloidal rotary engine increased up to 32.7–34.5%. The benefits of fuel economy and carbon oxide emission can also be achieved within the specified range of over-expansion ratio.

Under the background of carbon neutral, hydrogen-fueled Wankel rotary engine (WRE) with high power density and compact layout has the potential to become a substitute for current fossil engines. Ammonia as a carbon-free renewable energy is considered alternative energy used in vehicles, however, its application on WRE is lacking. Therefore, the present work aims to investigate the performance of ammonia-enriched hydrogen-fueled WRE to analyze the feasibility of ammonia applied in hydrogen-fueled WRE. The main conclusions are as follows: Under test conditions, the maximum brake torque and thermal efficiency are obtained at about 30% and 40% volume fraction of ammonia in fuel, and compared with pure hydrogen conditions, achieving relative improvements of 21% and 29%, respectively. Slightly rich combustion is recommended to increase power and decrease NO emissions with an acceptable economic loss. In addition, ammonia enrichment can prevent the occurrence of abnormal combustion.

Upstream propagation of filtration combustion wave in a one-layer porous burner is studied experimentally using thermal imaging and high-speed filming. The burner is the one-layer packed bed of spheres modeling porous media and providing optical access to the flame. This design allowed us to study macroscopic propagation of the combustion wave as well as non-stationary flame behavior at the pore scale. Experimental results demonstrate that upstream propagating combustion wave forms non-uniform porous medium temperature distribution in transverse direction which consists of hot areas separated by low-temperature regions. Macroscopic flame propagation can be accompanied by oscillations of some flame fragments at the pore scale. The mechanism of these pulsations is identical to the flames with repetitive extinction and ignition in narrow channels with external heating. Experimental results suggest that large scale evolution of the thermal wave affects the non-stationary behavior of the flame fragments at the pore scale and vice versa. The interplay between pore-scale and macro-scale processes manifests itself in the multiple repetitions of some patterns of the combustion wave propagation. Two most typical and common patterns are described and discussed. These patterns are step-wise transition of the flame fragment into the upstream pore and transition through the series of flame oscillations with repetitive extinction and ignition. It is shown that highly simplified one-dimensional model describes flame propagation in externally heated variable cross-section channel with heat conducting walls is capable to describe main features of these two patterns. Parametric study allows to predict the effect of problem parameters on the regions of existing of one or another pattern of upstream flame propagation.Novelty and Significance StatementThis paper presents new experimental data on temperature characteristics of upstream propagating filtration combustion wave in a one-layer porous burner. For the first time the patterns of combustion wave propagation uniting macro-scale processes of the thermal wave propagation and oscillations of the flame fragments at the pore scale are described on the basis of experimental data and numerical modeling. Results obtained for the one-layer burner can apparently be generalized to the case of combustion in three-dimensional porous media and packed beds. These results are significant because extend fundamental knowledge on porous media combustion by information on pore- and macro- scale flame dynamics as well as on the interplay of these scales. Experimental results can also be useful for validation of pore-resolved numerical simulations actively developed in the last decade.

Micro -flow reactors and porous media burners are prone to instability in the form of flame with repetitive extinction and ignition (FREI) near the region of a temperature gradient. Although significant progress has been made in this field, there is still a lack of understanding regarding the nature of such stable -to -unstable transition. Some studies reported a smooth and continuous instability transition in microchannels, interpreting this phenomenon as a supercritical bifurcation. Meanwhile, others have observed that the transition is subcritical, with rapid, stepwise evolution of the stable flame to FREI. This study aims to rigorously determine the transition character theoretically and numerically using a two-dimensional model with a precise resolution of the bifurcation parameter (flow velocity) near the critical point and gradual wall temperature ramp. The results indicate that the transition is a subcritical bifurcation with strong hysteresis. However, the amplitude of the limit cycle born in the bifurcation point could be small compared to FREI-like oscillations. Moreover, further development of the instability depends significantly on the channel width. In relatively wide channels, the stable regime transforms immediately into the extinction/ignition one with rapid growth in amplitude. In moderate channels, complex transitional dynamics were observed with an evident pulsating regime, which evolved into FREI through mixed -mode oscillations. In narrow channels, the amplitude of transitional pulsating flame becomes comparable to the fully -developed extinction/ignition cycles, and the bifurcation diagram has a continuous character with no significant jumps or discontinuities. Such qualitative variation of transition character provides a possible explanation for contradictory interpretations presented in the literature.

This research presents a novel port parametric modeling technique using three-dimensional computational fluid dynamics for the design and optimization of intake and exhaust phases in side-ported Wankel rotary engines (WREs). Definitions for the port phases encompass parameters such as port start opening, port full opening, port start closing, and port full closing timings. The four port phase control arcs are obtained by translating and rotating the rotor flank to satisfy the high control accuracy. Further, the shape of the port is further smoothed and varied by four auxiliary circular arcs. Moreover, the influence of port full closing timing and the size of auxiliary circular arcs (R1, and R3) on the intake characteristics is studied. The results show that the novel method can flexibly and effectively control the phases and shapes. The early port full closing timing reduces fluid backflow and improves volumetric efficiency (VE) but increases intake loss (IL). The small size of R1 facilitates to increase the VE and reduce IL. A larger or smaller size of R3 is not conducive to reducing IL, and the smaller size of R3 improves the VE. The novel generation method proposed in this paper provides a theoretical basis to optimize the design of various sizes of side-ported WREs and guidance for practical manufacturing.

Using syngas as a primary fuel for internal combustion engines with spark ignition is a bridge technology for transitioning from carbon-based to hydrogen-based energy sources. There are two different ways of syngas delivery into combustion chamber: port injection or direct injection. In this study, the combination of these methods is considered for integration benefits of each injection technology and promotion of syngas utilization in rotary engines. In the novel concept of syngas dual injection, part of the fuel is delivered by a port injector during the induction stroke, while another part is directly injected during the compression stroke. The ratio of fuel mass delivered by port and direct injection is changed within the range of 0.0 (direct injection only) to 1.0 (port injection only) and chosen as a key parameter of concentration stratification. This study aims to investigate the optimal fuel injection strategy with numerical simulation of mixture formation, ignition, and combustion processes. It was shown that dual injection strongly influences turbulence intensity and mixture heterogeneity. The turbulence lengthscales generally increase with dual injection ratio growth. The direct injection case demonstrates maximum performance characteristics. However, the case with a small portion of directly injected fuel (10%–25%) increases fuel conversion efficiency by 5%–7% and decrease fuel consumption by 4%–6% with lower level of CO emission. The results show that the dual injection strategy provides flexibility in controlling the combustion process and emission reduction.

Ni-Al intermetallic alloys have superior high-temperature properties such as excellent oxidation resistance and high yield strength, making them particularly attractive as materials for the fabrication of porous components of energy conversion and combustion devices. In this study, the cyclic oxidation resistance of highly permeable Ni-Al-Cr alloys has been investigated. The porous alloys with a structure of irregular porous scaffold comprised of welded mm-sized spheroidal strut elements forming a net of mm-sized in-terconnected pore channels (porosity of 0.55-0.60, specific surface area below 10-3 m2/g) were manu-factured by the combustion synthesis method using mu m-sized Ni, Al, and Cr powders of commercial purity as starting reagents. The surface area of alloys exposed to oxidation was calculated with an accuracy of 6% using quantitative 2D stereology. The cyclic oxidation tests were conducted in dry artificial air at 1150 degrees C for 100 h. The Ni-Al-Cr alloys, compared to Ni-Al alloys, exhibited a twofold increased oxidation resistance because of decreased scale spallation. The porous alloys were also tested in a combustion environment. The lifetime prediction showed that Ni-Al-Cr radiant burners could be used for > 10,000 h at a temperature of 1000 degrees C, which makes them relevant for application in domestic combustion appliances such as water heating boilers.(c) 2022 Elsevier B.V. All rights reserved.

Advanced high-temperature oxidation resistance is a crucial characteristic of metallic materials in porous burners. Extreme combustion conditions could lead to oxidation-affected erosion of porous media at a long-time period of burner operation. In this paper, we numerically simulated oxide scale growth at a porous radiant burner fabricated by Ni-Al intermetallic alloy using the combustion synthesis method, focusing on the structure degradation caused by periodic oxide scale spallation. A three-dimensional geometrical model of a porous intermetallic scaffold was obtained by scanning the porous burner using the X-ray CT technique. The surface erosion was modeled by the surface reconstruction based on calculated values of spalled oxide layer thickness. The simulation revealed that the submerged flame results in non-uniform distribution of the temperature at the solid surface. Such non-isothermal conditions lead to a two-times thicker oxide scale at the external surface of the burner. Thin struts of the intermetallic scaffold are prone to oxidation-affected erosion first, which forms discontinues and further fragmentation. The porous scaffold could lose about 50% of initial weight before fragmentation under conditions with intense oxide spallation. In such large structural degradation, the average flow velocity could reduce by a factor of 3, leading to changing of flame stabilization region.

The flames inside porous burners with low filtration velocity are prone to oscillatory behavior similar to unstable combustion in narrow channels but accompanied by complicated dynamics due to the irregularity of porous structure, flow field, and thermal gradient formed as a result of heat recuperation. This work is a numerical study within the framework of the pore-scale simulation approach devoted to investigating the flame oscillation mechanisms and characteristics in a two-dimensional pseudo-irregular packed bed of particles generated using the Voronoi tessellation algorithm with constant pore size. The results demonstrate that unstable flame oscillations like flames with repetitive extinction and ignition (FREI) lead to the velocity pulsation in adjacent pore channels, where the stable flame fragments are exposed to the hydrodynamically-driven fluctuations. In the case of the flow rate close to the stability threshold, the flow pulses can reach a magnitude enough to shift the stable flame in the upstream quenching zone, promoting its oscillations with repetitive extinction and ignition. Results of parametric simulation with a variation of the particle diameter and flow velocity were summarized in the regime diagram revealing the transition between stable normal flame, FREI, and stable weak flame with the velocity reduction. The pulsating transitional regime was detected between the stable normal flame and the FREI. The amplitude and intensity of such pulsations increase monotonically with the flow rate reduction in a narrow range. Oscillations of the unstable flame and hydrodynamically-driven flame pulsations are similar in dynamics but different in their physical mechanisms. However, in irregularly packed beds of particles, both these phenomena unavoidably coexist and interfere significantly with each other due to the connectivity of the pore channels.

Novel design of one-layer porous burner for experimental studies of the flame front dynamics at pore scale level is proposed. Results of the experimental and numerical studies of non-stationary flame front behavior at pore scale are discussed and compared at a qualitative level. The results demonstrate that upstream propagation of the combustion wave can be accompanied by the oscillations of two different types. The first type is the FREI-like pulsations, which mechanism is identical to the flames with repetitive extinction and ignition in narrow channels with external heating. The second type is the small-amplitude oscillations, which, apparently, have the same nature as the transitional oscillatory combustion mode between the steady flame and fully-developed FREI pulsations. This type of unstable flame behavior differs by existing of the flame over the entire oscillation period without extinction. It is found that these two types of flame oscillations clearly separates by oscillation frequencies and amplitudes. Typical frequencies of FREI-like pulsations are about tens of hertz, while for small amplitude oscillations it has an order of hundreds of hertz. It was shown that both types of oscillations can coexist inside the porous media at the same time but at different pore channels. Moreover, the mixed oscillations representing by fully developed FREI-like pulsations alternating with small-amplitude oscillations are frequently observed.

The syngas-fueled rotary engine can be considered as a promising solution for small-scale decentralized power generation due to high specific power, engine simplicity, smoothness and a low tendency for abnormal combustion. Since the decisive feature of syngas refers to feedstock flexibility, in this study we consider syngas with various compositions, produced from biomass, coal and natural gas, as a primary fuel for rotary engines. It was shown that rotary engine performance, efficiency and emission are mostly affected by the balance between combustion promoters and diluents in the air-syngas mixture. Particularly, high hydrogen concentration and lower inert content result in maximum power, heat release and thermal efficiency for stoichiometric and lean conditions. For the rotary engine with a homogeneous charge of the fuel, the combustion process is characterized by the weak influence of turbulence, while the laminar flame speed is an influential parameter in combustion behavior. The high magnitude of specific fuel consumption makes it difficult for engine operation at lean conditions. Although, the lean mixture cannot provide enough high flame temperature for NOx formation. The CO2 emission is affected by the hydrogen to carbon ratio. The opposite trend is observed between NOx and CO emission pathways due to oxygen availability. The power generation system based on the syngas-fueled rotary engine with syngas produced by not only well-known technology such as gasification or steam reforming but novel concepts (non-catalytic partial oxidation) can provide sustainability to the energy sector and reduce consumption of fossil fuels..

A recent surge of interest is focused on developing radiant burners that may replace traditional open flame burners because of their ability to interchangeably use a wide range of fuels with improved emissions and the overall efficiency of combustion appliances. This research focuses on identifying conditions under which a radiant burner with a thin‐layered radial porous emitter operates in the internal combustion mode with increased radiation efficiency alongside decreased noise and NOx emission. The way the fresh mixture distributes over the cylindrical cavity of the emitter is the main factor affecting the burner performance. Two strategies for fuel‐air mixture supply were considered: (i) co-axial feed of turbulent flow through the open end of the cylindrical emitter using a divergent annular nozzle; (ii) distributed radial feed of laminar flow through a thin‐layered conical porous insert located inside the cavity of the emitter. The burner performance (radiation efficiency, NOX, and noise emissions) was experimentally studied in the ranges of thermal power of 4.0–9.1 kW and equivalence ratio of 0.5–1.0, which corresponds to the range of the emitter temperature from 500 to 1190 K. It was established that the distributed radial feed of fresh mixture dramatically reduced the noise emission, markedly improved the uniformity of the emitter temperature, and slightly increased the radiation efficiency of the burner. The improved combustion performance makes the thin‐layered radial porous burners relevant for application in domestic heating appliances.

The trend towards a transition from fossil to renewable energy makes urgent the development of fuel-flexible burners that provides interchangeability between natural gas and a wide range of renewable gases. This study examines the fuel interchangeability potential of a novel annular cylindrical burner made of a Ni-Al intermetallic with advanced high-temperature properties. The burner is designed to operate in the internal combustion mode when the reactions are aerodynamically stabilized in the volume of the cylindrical cavity. The stability limits, radiation efficiency, and CO/NOX emissions of the burner were studied within the firing rate range of 160-420 kW/m(2) and the equivalence ratio range of 0.5-1.0. Experiments were carried out for the natural gas and its blends with CO2, H-2 and H-2-CO, whose flame speeds differ by a factor of three. The addition of H-2 and CO to natural gas was found to reduce emissions and expands the radiant operation mode to a lean region without loss in radiation efficiency. The addition of CO2 to natural gas also reduces NOX, but CO emission increases, radiation efficiency decreases and the infrared range is narrowed. For all studied fuels, within the ranges of firing rates of 260-420 kW/m2 and equivalence ratios of 0.70-0.95, the burner operates in stable radiant mode with a radiation efficiency up to 30-45%, NOX emission of 10-50 ppm, and CO emission of 0-40 ppm. In general, the results confirm the fuel flexibility of the new radiant burner to be much the same as for other radiant burners; however, the radiation efficiency is substantially higher than that for cylindrical radiant burners of other designs.

During the last years the thin-layered radial porous burners are of growing interest, primarily due to low pressure loss, advanced flame stability and high radiant output. The aim of this study is to investigate the flame stabilization phenomena in radial flow burner using a pore-scale approach with direct simulation of the realistic three-dimensional porous structure and interstitial flow with thermal interaction between the fluid and solid phases including surface radiation. The stabilization mechanisms are investigated with focus on the detailed inner flame structure and its aerodynamical and thermal interaction with the porous shell. The results demonstrate that three characteristic combustion regimes can be distinguished, namely, the internal, submerged and surface-stabilized flames that form depending on the mixture composition and the flow rate. The general principles governing change of regimes are discussed. A key factor of the internal flame stabilization within the burner cavity is a kinematic balance between flow and burning velocity. When the reaction zone submerges the porous shell, the heat recuperation becomes an important factor of flame stabilization. In the surface-stabilized regime, the large velocity gradients lead to the highly stretched and curved flames that aerodynamically anchor at the external surface of the shell analogously with the perforated plates and bluff bodies but have wrinkled shape due to the nonuniformity of the flow field within irregular porous structure. A characteristic diagram of regime change is proposed. It is discussed that for porous burners with size of structural elements of the same magnitude as shell thickness, application of the quasi-homogeneous assumption is restricted in contrast to the pore-scale simulation approach that considers as a prospective technique for studying combustion phenomena.

The oxidation state of the mantle plays an important role in many chemical and physical processes, including magma genesis, the speciation of volatiles, metasomatism and the evolution of the Earth’s atmosphere. We report the first data on the redox state of the subcontinental lithospheric mantle (SCLM) beneath the Komsomolskaya–Magnitnaya kimberlite pipe (KM), Upper Muna field, central Siberian craton. The oxygen fugacity of the KM peridotites ranges from −2.6 to 0.3 logarithmic units relative to the fayalite–magnetite–quartz buffer (∆logfO2 (FMQ)) at depths of 120–220 km. The enriched KM peridotites are more oxidized (−1.0–0.3 ∆logfO2 (FMQ)) than the depleted ones (from −1.4 to −2.6 ∆logfO2 (FMQ)). The oxygen fugacity of some enriched samples may reflect equilibrium with carbonate or carbonate-bearing melts at depths >170 km. A comparison of well-studied coeval Udachnaya and KM peridotites revealed similar redox conditions in the SCLM of the Siberian craton beneath these pipes. Nevertheless, Udachnaya peridotites show wider variations in oxygen fugacity (−4.95–0.23 ∆logfO2 (FMQ)). This indicates the presence of more reduced mantle domains in the Udachnaya SCLM. In turn, the established difference in the redox conditions is a good explanation for the lower amounts of resorbed diamonds in the Udachnaya pipe (12%) in comparison with the KM kimberlites (33%). The obtained results advocate a lateral heterogeneity in the oxidation state of the Siberian SCLM.

The results of a numerical investigation of the influence of inhomogeneous dielectric media on the impedance of a system of two parallel linear antennas randomly positioned near the interface are analyzed. Relying on a comparative analysis, the possibility of optimization of the dimensions and configuration of the antenna system for radiowave diagnostics of dielectric media in the microwave range is shown.

To gain better insight into the thermal state and composition of the lithospheric mantle beneath the Upper Muna kimberlite field (Siberian craton), a suite of 323 clinopyroxene xenocrysts and 10 mantle xenoliths from the Komsomolskaya-Magnitnaya (KM) pipe have been studied. We selected 188 clinopyroxene grains suitable for precise pressure (P)-temperature (T) estimation using single-clinopyroxene thermobarometry. The majority of P-T points lie along a narrow, elongated field in P-T space with a cluster of high-T and high-P points above 1300 °C, which deviates from the main P-T trend. The latter points may record a thermal event associated with kimberlite magmatism (a “stepped” or “kinked” geotherm). In order to eliminate these factors, the steady-state mantle paleogeotherm for the KM pipe at the time of initiation of kimberlite magmatism (Late Devonian–Early Carboniferous) was constrained by numerical fitting of P-T points below T = 1200 °C. The obtained mantle paleogeotherm is similar to the one from the nearby Novinka pipe, corresponding to a ~34–35 mW/m2 surface heat flux, 225–230 km lithospheric thickness, and 110–120 thick “diamond window” for the Upper Muna field. Coarse peridotite xenoliths are consistent in their P-T estimates with the steady-state mantle paleogeotherm derived from clinopyroxene xenocrysts, whereas porphyroclastic ones plot within the cluster of high-T and high-P clinopyroxene xenocrysts. Discrimination using Cr2O3 demonstrates that peridotitic clinopyroxene xenocrysts are prevalent (89%) among all studied 323 xenocrysts, suggesting that the Upper Muna mantle is predominantly composed of peridotites. Clinopyroxene-poor or -free peridotitic rocks such as harzburgites and dunites may be evident at depths of 140–180 km in the Upper Muna mantle. Judging solely from the thermal considerations and the thickness of the lithosphere, the KM and Novinka pipes should have excellent diamond potential. However, all pipes in the Upper Muna field have low diamond grades (<0.9, in carats/ton), although the lithosphere thickness is almost similar to the values obtained for the high-grade Udachnaya and Mir pipes from the Daldyn and Mirny fields, respectively. Therefore, other factors have affected the diamond grade of the Upper Muna kimberlite field.

The hydrogen-fueled range extender based on the rotary engine is a promising solution for battery electric vehicles due to key advantages such as: high efficiency, small packaging dimensions and low weight within the framework of the near zero-emission concept. Although rotary engine has been examined over the past years, the detailed knowledge of hydrogen combustion in the rotary engine with direct injection system has not been investigated properly. In this paper, the rotary range extender was numerically studied with a focus on the influence of the direct multiple injection strategies on hydrogen distribution and combustion. The result demonstrates that due to large flame speed and diffusion length the engines fueled by pure hydrogen differ significantly from the engines with conventional fuel. The process is characterized by the weak influence of turbulence on the flame during almost the whole combustion stroke except for the late stages when 70% of the fuel is already burnt. Hence the difference in the injection strategies lead to different local hydrogen distribution that has effects on combustion rate and performance characteristics significantly. Fuel stratification leads to the formation of subregions with various hydrogen concentration. The lean mixture cannot provide enough high temperature, whereas the rich mixture is characterized by a lack of oxygen. The total NOx formation can be twice lower in comparison with premixed combustion at the same averaged equivalence ratio. The direct multiple injection strategy allows the use of a higher averaged equivalence ratio up to 0.8 which results in larger power density in combination with low NOx emission.

The porous media combustion phenomenon was numerically studied with focus on the heat transfer effect in propagating combustion wave for lean and ultra-rich mixture compositions using the approach when the three-dimensional porous structure and interstitial flow are simulated directly at pore scale with explicit consideration of the thermal interaction between fluid and solid phases including the detailed chemical kinetics model and solid-to-solid radiation. The results demonstrate that irregularity of the porous structure leads to a large spatial variation of the process parameters and the flow inhomogeneity. The most intensive heat sources placed within the cavities where the flow is developed. During the process, the interface heat transfer and radiation contribute to heat recuperation mechanism that leads to thermal non-equilibrium in the combustion wave. The heat is transferred through the bed via radiation layer-by-layer due to restricted visibility of the particles. The numerical data about a local variation of the process parameters were presented. The data of this type can be used for modification of the volume-averaged models within the context of spatial variation consideration. It was shown that there is a correlation between the heat release rate, the interface heat flux, the radiative heat flux, and its root mean square values.