Enhancing flame stability has been the primary focus of numerous investigations into porous media burners. One potential solution involves the tailoring of porous media. Recent research indicates that graded topologies might significantly improve stability compared to traditional two-section designs. This study introduces a novel method for fabricating metallic porous burners with tailored structures based on combustion synthesis technology. Manipulating the mass ratio between powder reactants allows for the direct synthesis of tailored materials with a desirable profile of pore size. Using this technique, both step and graded porous burners have been synthesized, yielding average pore sizes ranging from 0.5 to 2.5 mm. These burner designs were tested for hydrogen utilization near the lean limit. Experiments with the two-section design exhibited a wide stable operating range, achieving a turn-down ratio of over 13.8 and maintaining stable performance under ultra-lean conditions, with an equivalence ratio as low as 0.215. In contrast, the graded design exhibited a lower stability range. Analysis of the measured temperature profiles suggests that, in the context of hydrogen combustion, strong stabilization arises from the local flame anchoring phenomenon in the coarse-pored region, rather than from an overall enhancement of the burning rate due to heat recuperation, which diminishes the benefits of graded morphology. These observations are consistent with recent pore-level simulations that have highlighted the dominant role of stretch-induced non-equidiffusion effects in porous media hydrogen combustion.

Porous Ni-Al intermetallics are attractive materials for high-temperature applications due to their excellent oxidation resistance and thermal stability. However, the high cost of nickel motivates the development of more economical porous intermetallics produced by combustion synthesis. This work establishes the processing window for the combustion synthesis of macroporous Ni–Fe–Al alloys by systematically investigating the effect of replacing Ni with Fe while maintaining a constant aluminum content. Stable, self-propagating combustion, producing homogeneous porous alloys, was achieved for Fe substitution levels up to 80%. Without preheating, self-propagating combustion was possible only up to 9% Fe substitution. Increasing Fe content beyond this required progressively higher initial temperatures to compensate for reduced reaction exothermicity and maintain stable combustion. X-ray diffraction revealed a gradual transition from Ni–Al to Fe–Al intermetallic phases with increasing Fe substitution, and no ternary phases were detected. The average scaffold strut size ranged from 0.25 to 2.21 mm, depending on alloy composition and synthesis conditions. For a fixed alloy composition, the scaffold strut size could be tailored from 0.26 to 0.57 mm by adjusting the initial reaction temperature. These results define the compositional and thermal conditions required for the combustion synthesis of porous Ni–Fe–Al intermetallics and demonstrate a practical strategy for producing cost-effective porous materials with controllable structures for high-temperature applications.

This study introduces a novel Ni-Al-based intermetallic porous burner, fabricated using combustion synthesis technology, which features fine-and coarse-pored sections joined in situ, with pore channels measuring 0.65 mm and 3.25 mm, respectively. Combustion experiments demonstrated a wide stable operating achieving a turn-down ratio of over 13.8. The burner was reconstructed using X-ray CT, and a pore simulation was conducted to analyze the evolution of combustion characteristics as the flow rate increases. At high flow rates, the flame anchors to the struts of the coarse-pored downstream section, resembling bluff-body stabilization mechanism. In contrast, at low velocities, the flame stabilizes at the interface due balance between heat loss and gain. As the flow rate increases, the development of the flame surface enhances the overall burning rate. Meanwhile, the consumption flame speed remains nearly constant across the range due to the counterbalance between preheating and preferential diffusion.

A promising approach to improve the high-temperature properties of Ni-Al intermetallics is macroalloying with chromium and microalloying with a small amount of rare-earth elements (REE). For powder manufacturing processes such as the combustion synthesis technique, macro- and microalloying can be achieved by employing Cr & REE containing masteralloy powders as one of the starting reagents along with nickel and aluminum powders. This study investigated a process of dysprosium deposition onto the particles of commercial Ni-Cr powder aimed to produce the Ni-Cr-Dy masteralloy powder. The process involves pretreatment of the particle surface by low-energy high-current electron beam (LEHCEB) and uniform magnetron deposition of a thin dysprosium film. The possibility of embedding the dysprosium film into the particle's surface using an additional stage of LEHCEB treatment has been considered. It has been observed that during LEHCEB modification, a change in the particle size distribution, smoothing of the surface, and the formation of an intermetallic Dy3Ni phase in the subsurface layer occurs. A fraction of the obtained Ni-Cr-Dy powders were added to the starting powder blend for the combustion synthesis of Ni-Al-Cr-Dy porous alloys. It has been demonstrated that dysprosium uniformly distributes throughout the intermetallic scaffold of porous alloys and is localized within the Ni3Al phase in the form of enriched regions and separate inclusions.

Self-propagating high-temperature synthesis (SHS) is a powder processing technique used to effectively produce porous Ni-Al-based intermetallics that are characterized by a coarse structure composed of millimeter-sized strut elements. These porous intermetallics possess a low specific surface area, which enhances their longevity and reliability in various high-temperature environments. Modifying the chemical composition of Ni-Al intermetallics by adding a small amount (0.05-0.10 at%) of rare earth elements (REE) is one of the straightforward methods used to enhance the oxidation resistance of cast intermetallics that have never been applied for porous alloys made by the SHS technique. This study examines the cyclic oxidation resistance of porous Ni-Al intermetallics with minor REE content, obtained by applying Al-3.7wt%Dy and Al-4wt%Y masteralloy powders as an additive to the base nickel and aluminum powder mixture. This microalloying strategy enables over two hundred REE carrier particles to participate in the self-assembly of each millimeter-sized strut of the intermetallic scaffold, representing more than 1 % of the total particles involved in the synthesis. The base Ni-Al and microalloyed NiAl-Dy and Ni-Al-Y porous alloys were oxidized cyclically at 1150 degrees C for 100 h. The structures of initial and oxidized alloys were studied using stereological methods and XRD, SEM, and EDS analysis. Microalloying was found to slightly coarsen the porous scaffold by enhancing the formation and merging of the liquid phase within the combustion wave due to the decreased melting temperatures of the Ni-REE eutectics. The results show that microalloying with masteralloy powders allowed a uniform distribution of REE throughout the scaffold of porous intermetallic, resulting in a two-to-five times reduction in the oxidation rate constant, approaching the minimum value of 2 center dot 10-11 g2/cm4/s.

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.

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.

This research focuses on identifying conditions under which porous intermetallics with a welded granular structure comprised of mm-sized spheroidal alloy elements can be directly obtained by combustion synthesis using μm-sized metal powders as starting reagents. The heat loss of reacting medium to a molding template used to shape a powder mixture of reagents significantly reduce the size of the alloy elements in manufactured material, especially near the mixture/template interface. One possible solution to this problem is to employ a separating layer between the powder mixture and the molding template. The effect of separating layers, both chemically inert (ceramic-based) and active (cellulose-based), on the size of Ni-Al elements has been experimentally studied. It was found that notable structure modification can be achieved by using cellulose-based separating layers. The carbon-containing products released during cellulose pyrolysis interact with the reacting medium and promote the coalescence of metal melts. Changing the thickness of the cellulose-based separating layer is an easy way to control the porous structure and permeation properties of combustion synthesized Ni-Al-based intermetallics, which makes them relevant for practical application.

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.

Infrared heaters based on radiant burners are widely used in industry for thermal processing of materials such as rolled steel, textiles, food and other products. A new configuration of a gas-fired infrared heater is proposed and has been experimentally studied. The heater is constructed using an annular cylindrical radiant burner mounted inside a stainless steel conical reflector. The combustion of premixed natural gas with air is stabilized in the cavity of the cylindrical burner which is made of porous Ni-Al intermetallic. The reflector not only refocuses the omnidirectional radiant flux of the burner, but also participates in heat exchange with flue gases and emits an additional IR flux. The radiant heat output and NOx emission of the IR-heater are experimentally studied in two combustion modes, i.e. operation with and without preliminary heating of combustion air by recuperating flue gas heat. The operating conditions providing a 70-75% radiation efficiency and NOx emission of about 75 ppm in the power range of 700-5700 W are discussed. For evaluating the potential application of a new IR-heater, the obtained results in terms of the relative NOx emission per unit of produced infrared heat are compared with those of typical radiant tubes and electric IR heaters. (C) 2020 Elsevier Ltd. All rights reserved.

Porous Ni–Al–CGO cermet (CGO = Сe 0.9 Gd 0.1 O 2 ) for use in solid oxide fuel cells was fabricated by thermal explosion (volume reaction) in Ni–Al–CGO powder compacts in different heat sink conditions. Temperature profiles of thermal explosion were recorded and analyzed as a function of green composition. Phase composition of resultant porous materials was found to depend on the CGO content of green mixture and temperature of vacuum annealing. Starting and final materials were characterized by XRD, SEM, and EDS. Synthesized uniform cermets with a porosity of 50–60% can be recommended for use as a support for solid oxide fuel cells with Ni/CGO anode.

The use of Ni-Al alloys proves to be highly relevant in advanced radiant burner production. The aim of this study is an in-situ fabrication of macro-porous B2-L1(2) Ni-Al alloys by self-propagating high-temperature synthesis for a specified burner configuration. It has been revealed that the use of small amounts of special additives dramatically affects the synthesis of coarse-porous alloys. High-speed imaging and high-speed spectral pyrometry have demonstrated that a combustion wave consists of a set of short-lived superadiabatic drops of a melt, the temperature of which exceeds the liquidus temperature for a short time. In the formation of melt drops, up to 10(7) nickel and aluminum particles are involved. This makes it possible to synthesize porous alloys with Ni-Al elements of up to several millimeters in diameter, using Ni and Al powders with the size of fewer than 10 mm. The porosity structure of such alloys has been characterized in the paper. It has also been demonstrated that the structure can be controlled by changing initial synthesis conditions such as reaction mixture composition, an additive type and its amount, mixture relative density, and a preheating value. In addition, the article describes the conditions that allow obtaining alloys with Ni-Al elements in the range of 500-2500 mm in size. As for the phase composition of synthesized alloys, it has been examined by X-ray diffraction. Besides B2-L1(2) phases, the alloys may contain some martensite L1(0), which disappears if the alloys are annealed for an hour at a temperature of 1100 degrees C. To evaluate mechanical properties of the synthesized and annealed alloys, a compression test has been applied. The impact of the alloys phase composition on possible operation modes of Ni-Al radiant burners has been discussed in the work. (C) 2019 Elsevier B.V. All rights reserved.

A promising type of radiation burners are hollow cylindrical burners made of materials with high thermal conductivity. In such burners, an internal combustion mode is possible when the combustion takes place in the cavity space of the burner, which allows obtaining radiation efficiency of up to 60%. The aim of the present work is to experimentally define the impact of the cylindrical burner porous structure on NOX/CO emission values and radiation efficiency with respect to equivalence ratio and firing rate. It has been found that NOx emission is <15 ppm if the equivalence ratio is <0.7. The findings indicate that the lower the firing rate and the smaller the size of the structural elements of the burner, the higher the radiation efficiency and emission of CO is. To gain a further understanding of the combustion conditions influence on the radiation efficiency, the analysis of possible efficiency with the assumption of equality the temperatures of flue gases and the burner surface has been carried out. The results suggest that effective heat exchange processes between combustion products and emitter are realized in cylindrical burners, which allows to achieve radiation efficiency close to the maximum possible in all considering operating conditions.

This paper discusses the phase composition of macroporous Ni-Al alloys obtained by self-propagating high-temperature synthesis. The alloys have been synthesized in a nonstationary combustion mode. The combustion wave consists of super adiabatic foci, and macroporous structure is realized under the action of capillary hydrodynamic effects. The influence of reaction mixture composition of aluminium in the range of 13.5-31.5 wt.% is also discussed. The specific requirements to obtain the single phase B2 NiAl and LI2 Ni3Al as well as biphase gas permeable alloys with the average size of structure elements in the range of 1.2-3.15 mm are described in the paper.

This paper presents experimentally studied environmental and radiation parameters of hollow cylindrical burners during operation with LPG-air fuel mixture. Two combustion modes have been examined - an external combustion mode where the flame anchored near the outer surface of the burner, and an internal combustion mode when the combustion takes place in the inner cavity of the burner. The dependences of CO/NOx emissions and radiation efficiency on a firing rate in the range of 160-420 kW/m(2), air-fuel equivalence ratio in the range of 1.0 - 1.4, as well as the porous structure of a burner have been analyzed. An influence of a flow deflector installed in front of the burner inlet in order to distribute the flow over the inner cavity of the cylindrical burner on environmental characteristics of the burner is discussed. The necessary condition that ensures CO emission below 50 ppm, NOx emission below 20 ppm and radiation efficiency in the range of 45-55 % is described in the paper.

This paper presents experimentally studied temperature and radiation parameters of a gas-fired luminous radiant heater based on a conical aluminium recuperator and a cylindrical Ni-Al burner. Two operation modes have been investigated: (i) with air preheating by means of heat recuperation when the air is supplied into the burner through the hollow recuperator, and (ii) without air preheating when the fuel mixture is directly supplied to the burner. The dependences of radiation efficiency on a burner firing rate in the range of 100-340 kW/m2 are analyzed. The specific requirements to obtain the radiation efficiency up to 70% are discussed.