Low Pressure Combustion Research Articles

Assessment of the thermodynamic feasibility of a new configuration for enhancing the performance of an Allam cycle using the reheating method

This study presents a new arrangement of the reheating Allam cycle and investigates the efficacies of key parameters on the net specific work and thermal and exergy efficiencies of the proposed cycle. The performance of the optimized reheating Allam cycle is compared to several oxy-combustion cycles, and the results show that the inlet temperature of the low-pressure turbine, followed by the inlet temperature of the high-pressure turbine, has a remarkable efficacy on the thermodynamic performance of the reheating Allam cycle. In contrast, the minimum temperature of the proposed system has the lowest impact on its performance. The study identifies that the low-pressure combustion chamber accounts for the biggest share of the exergy destruction of the proposed reheating Allam cycle, representing 20.68 % of the overall input exergy. The net specific work of the optimized reheating Allam cycle is evaluated at up to 596.3 kJ/kg, with thermal and exergy efficiencies obtained at 52.37 % and 46.70 %, respectively. These values indicate significant performance improvements compared to other oxy-combustion cycles. This study supplies valuable perceptions into the design of the reheating Allam cycle and suggests that this configuration has the potential to be a promising alternative to other cycles in terms of its high efficiency and low environmental impact.

Combustion behavior of NMP-001, a nitromethane-based green rocket propellant

Hydrazine’s carcinogenic and toxic nature makes its use in space propulsion expensive and necessitates careful launch campaign planning, which may cause delays. One already established alternative is to use energetic salt-based ionic solution propellants. However, the costs remain high because these salts are costly and highly regulated energetic materials. Additionally, propellants based on these energetic salts produce much higher temperatures during decomposition and combustion, as is the case for hydrazine. Consequently, expensive refractory metal-based alloys and high-temperature oxidation-resistant catalytic reactors must be used, which further drives up the costs. Alternatively, hydrogen peroxide may be used, though only in applications with a low specific impulse requirement. This study explores an alternative approach: using nitromethane, a commonly available laboratory solvent. NMP-001 is a nitromethane-based monopropellant that contains additives that allow for low-pressure combustion and reduce sensitivity to mechanical stimuli. Multiple combustion chamber configurations with varying characteristic lengths were tested in rocket combustion experiments. With the 11.5 m L∗-configuration, a 15.1 bar low-pressure limit could be determined. The 7.3 and 5.0 m L∗-configurations yielded a 16.2 and 28.3 bar low-pressure combustion limits, respectively. These outcomes are significantly below the 30 bar limit reported in the literature. Despite using only a simple heat-sink combustion chamber with a non-sophisticated injector element and a non-optimized propellant formulation, combustion was 83 to 95 % efficient with a C* ranged between 1133 to 1302 m/s. These figures are higher than for hydrogen peroxide and are comparable to the theoretical performance of state-of-the-art green liquid monopropellants.

Low air pressure self-sustaining combustion performances of 3D direct writing Al/CuO film

The application and optimization of nano-composite energetic materials in the extreme and particular environment have become a subject of wide concern. Additionally, the design and selection of metastable intermixed composites (MICs) films in low pressure environment is the key problem to explore its future application. Here, 90 wt% high loading Al/CuO nanothermite films were prepared by a simple 3D direct writing approach. Four films with different thicknesses (66, 128, 152, 182 μm) were designed and their combustion properties were tested respectively. The low pressure combustion results show that the self-sustained burning rate of the film (thicker than 66 μm) can be adjusted and controlled in various air pressure environments (101–60 kPa). The self-sustained burning rate of film is affected by both thickness (66–182 μm) and air pressure (101–60 kPa). The lower the air pressure, the lower the burning rate. The greater the thickness, the higher the burning rate. The 66 μm film self-sustainedly burns at atmospheric pressure and extinguishes when the pressure was significantly reduced. The self-sustained combustion rates of the 128 and 152 μm film decreases with decreasing pressure until the pressure drops to 75 and 60 kPa respectively. The 182 μm film keeps self-sustained combustion in a manner of deflagration which is almost unaffected by air pressure, even when the pressure drops to 10 kPa. Simultaneously, it is found that the ignition temperature of the film is almost not affected by pressure. In addition, it is verified that the film has performances of excellent hydrophobic and combustion through air–water interface. These can cope with complex environmental humidity.

Study of Low-Pressure Combustion and Luminescence Properties of Different Magnesium/Nitrate Pyrotechnics

ABSTRACT To explore the effect of pressure on the combustion and luminescence properties of magnesium/nitrate pyrotechnics, four sets of zero-oxygen-equilibrium magnesium/sodium nitrate (N), magnesium/potassium nitrate (K), magnesium/barium nitrate (B), and magnesium/strontium nitrate (S) pyrotechnic samples were subjected to TG-DSC (thermogravimetric analysis-differential scanning calorimetry) testing and combustion tests at atmospheric pressure and 1 kPa. The combustion experiments showed that the flame height increased at 1 kPa due to the rapid diffusion of gas products. Moreover, the degree and rate of oxidation of magnesium are limited by the scarcity of oxygen in the environment, resulting in a decrease in both combustion temperature and luminous intensity at low pressure. Thermal analysis tests showed that B and S have higher combustion temperatures and better luminescence performance at 1 kPa because of their higher low-pressure reactions than N and K. In addition, the Mg/Sr(NO3)2 red light tracer has the best luminescence performance at low pressure because it still has a higher concentration of visible light emitter SrOH at 1 kPa. These scientific findings provide useful insights into the low-pressure combustion behavior of pyrotechnics and support the growing demand for military and civilian applications.

Study of Combustion Characteristics of Magnesium/Strontium Nitrate and Magnesium/Sodium Nitrate Pyrotechnics Under Low Pressure Environment

ABSTRACT To explore the effect of pressure on the combustion and luminescence properties of magnesium/nitrate pyrotechnics, combustion tests and thermal analysis of magnesium/sodium nitrate (N) and magnesium/strontium nitrate (S) pyrotechnic samples with zero oxygen balance were conducted under the pressure range of 101kPa-1kPa in this study. The results show that the flame height of N increases continuously as the pressure decreases, and the flames gradually change from a steady-state convergent flame to a turbulent evanescent flame. However, due to the different diffusion rates of the gas-phase combustion products, the diffusion of S at low pressure is lower and the flame still maintains a stable laminar flow. Although the light intensity and burning speed of both N and S satisfy Vier’s law, the luminous intensity of S is less sensitive to pressure and still has good luminous performance at 1 kPa. Thermal analysis test results show that N and S can react almost completely at atmospheric pressure, and their enthalpies of reaction are 6.75 kJ/g and 6.07 kJ/g, respectively. The difference in the thermal decomposition histories of strontium nitrate and sodium nitrate at 1 kPa results in a reaction enthalpy of 4.24 kJ/g for S and only 2.45 kJ/g for N. This indicates that the reaction degree of S at low pressure is higher than that of N. The SEM-EDS test results further confirm this opinion. These scientific findings provide useful insights into the low-pressure combustion behavior of pyrotechnics and support the growing need for military and civilian applications.

Advances in Computational Combustion Research in China: A Brief Review

Computational combustion modeling was introduced in China in the early 1980s. Specifically, after the first computational fluid dynamics (CFD) lecture in China given by Brian Spalding, Professor of Heat Transfer and Head of the Computational Fluid Dynamics Unit at Imperial College, London, under the invitation from Professor Wei-Cheng Fan of University of Science and Technology of China, Anhui, China, in 1984. This paper presents the development history of computational combustion research led by Fan in China. During the early stages of CFD development in the 1980s and 1990s, numerous innovative models, methods, and computational tools were proposed and developed by Fan's research group to advance computational combustion research in China. Throughout the evolution of computational combustion technologies, the State Key Laboratory of Fire Science, founded and led by Fan, has played a leading role in the development and deployment of these technologies in China. Recent progress in low-pressure combustion and battery fire research and combustion modeling technologies are described.

Carbene Addition Isomers of C70 formed in the Flame of Low-Pressure Combustion.

In the flames during low-pressure combustion, not only a rich variety of fullerenes but also many reactive intermediates can be produced (e.g., carbene, CH2) that are short-lived and cannot be stabilized directly under normal circumstances. These intermediates can be captured by fullerene carbon cages for stabilization. In this paper, three C71H2 isomers were synthesized in situ in low-pressure benzene-acetylene-oxygen diffusion flame combustion. The results, which were unambiguously characterized by single-crystal X-ray diffraction, show that the three isomers are carbene addition products of D5h-C70 on different sites. The relative energies and stability of different C71H2 isomers are revealed by Ultraviolet-Visible (UV-Vis) absorption spectroscopy, in combination with theoretical calculations, in this work. Both the in situ capture and theoretical study of these C71H2 isomers in low-pressure combustion will provide more information regarding carbene additions to other fullerenes or other carbon clusters at high temperatures.

Study of Combustion Characteristics of Magnesium/Sodium Nitrate Pyrotechnics Under Sub-Atmospheric Pressure

ABSTRACT In order to study the combustion characteristics and reaction mechanism of luminescent pyrotechnics under sub-atmospheric pressure conditions, low-pressure combustion tests and thermal analysis were performed for magnesium/sodium nitrate pyrotechnics with zero oxygen equilibrium in the pressure range of 1 kPa~101 kPa. The results reveal that when the pressure drops, the flame height rises, and the flame transitions from a steady-state convergent to a turbulent evanescent flame. Reduced oxygen concentration at low pressure reduces the flame’s high temperature response zone, as well as burning speed and luminous intensity. Furthermore, both the light intensity and the burning rate comply with Vier’s law. The weight loss increased from 16.3% to 36.7% when the pressure was reduced from atmospheric pressure to 1 kPa, the enthalpy change of reaction decreased from 6.75 kJ/g to 2.45 kJ/g, and the characteristic peak of magnesium oxide at 500 nm in the flame spectrum disappeared at 1 kPa, indicating that the reduction of pressure would affect the degree of reaction of pyrotechnics, leading to the decrease of magnesium oxide content and the decrease of heat release. These scientific findings provide useful insights into the low-pressure combustion behavior of pyrotechnics and support the growing demands for military and civilian applications.

Capturing nonclassical C70 with double heptagons in low-pressure combustion.

A double-heptagon-containing C70H6 (dihept-C70H6) was isolated and unambiguously characterized in the soot of low-pressure combustion, which shares the identical heptagonal cage as dihept-C70Cl6 previously identified in the products of carbon arc, and thus represents the first nonclassical fullerene isolable in both carbon arc and combustion.

Molecular diffusion and phase stability in high-pressure combustion

The increased modeling complexity needed to simulate high-pressure combustion in rocket engines results in significant computational costs–costs which are not always justified for engineering applications. Multicomponent diffusion computations are at least 40% more expensive than the constant Lewis number diffusion assumption for the simplest hydrogen/oxygen combustion; the computational penalty increases rapidly with the number of species under consideration. The higher-fidelity diffusion modeling is justified in cryogenic fuel combustion if it affects the phase-stability of the propellants. We investigate the impact of the mixture averaged, multicomponent, and constant Lewis number diffusion models of a laminar counterflow flame at trans- and supercritical conditions for typical rocket propellants, namely: hydrogen, methane, and kerosene. Using vapor liquid equilibrium (VLE) theory, we show that, even in the limit cases, the mass diffusion model has a limited effect on the phase stability and pseudo-phase change of the propellants; the majority of the differential diffusion errors are located in the high temperature/ideal gas regions of the flame. The differential diffusion error due to real fluid thermodynamics is at most about 20% of the differential diffusion error which occurs in the ideal gas region of the flame. As a result, the impact of the differential diffusion remains similar to low-pressure combustion conditions, thus supporting the use of engineering-level simplifications for the simulation of these complex high-pressure reactive flows.

Mineralogical Diversity of Ca2SiO4-Bearing Combustion Metamorphic Rocks in the Hatrurim Basin: Implications for Storage and Partitioning of Elements in Oil Shale Clinkering

This is the first attempt to provide a general mineralogical and geochemical survey of natural Ca2SiO4-bearing combustion metamorphic (CM) rocks produced by annealing and decarbonation of bioproductive Maastrichtian oil shales in the Hatrurim Basin (Negev Desert, Israel). We present a synthesis of data collected for fifteen years on thirty nine minerals existing as fairly large grains suitable for analytical examination. The Hatrurim Ca2SiO4-bearing CM rocks, which are natural analogs of industrial cement clinker, have been studied comprehensively, with a focus on several key issues: major- and trace-element compositions of the rocks and their sedimentary precursors; mineral chemistry of rock-forming phases; accessory mineralogy; incorporation of heavy metals and other trace elements into different phases of clinker-like natural assemblages; role of trace elements in stabilization/destabilization of Ca2SiO4 polymorphic modifications; mineralogical diversity of Ca2SiO4-bearing CM rocks and trace element partitioning during high-temperature–low-pressure anhydrous sintering. The reported results have implications for mineral formation and element partitioning during high-temperature–low-pressure combustion metamorphism of trace element-loaded bituminous marine chalky sediments (“oil shales”) as well as for the joint effect of multiple elements on the properties and hydration behavior of crystalline phases in industrial cement clinkers.

Preliminary evaluation of CAES system concept with partial oxidation gas turbine technology

This paper presents the concept of compressed air energy storage (CAES) system with Partial Oxidation Gas Turbine (POGT) technology. In this type of turbine fuel is oxidized with an excess air coefficient λ of less than 1.0. Therefore, a given value of the turbine inlet temperature (TIT) can be reached at significantly lower air consumption. POGT products are electric power and combustible process gas that can be utilised in various ways. In this paper there is assumed a bottoming process, which consists of low pressure combustion and a secondary turboexpander. The system is considered to be functioning in power-to-power mode. However, POGT creates also other opportunities that can help improve flexibility of energy market. As the paper presents an initial study of possibilities the proposed system has been compared against the existing plant in Huntorf (Germany). In this way main differences as well as advantages and limitations of the proposed solution are compared against the existing conventional first generation CAES facility. The study is focused mainly on key thermodynamic parameters and potential performance improvement of the CAES process. The results revealed considerable round-trip efficiency gains.

An Unconventional Hydrofullerene C66H4 with Symmetric Heptagons Retrieved in Low-Pressure Combustion.

The combustion has long been applied for industrial synthesis of carbon materials such as fullerenes as well as carbon particles (known as carbon black), but the components and structures of the carbon soot are far from being clarified. Herein, we retrieve an unprecedented hydrofullerene C66H4 from a soot of a low-pressure combustion of benzene-acetylene-oxygen. Unambiguously characterized by single-crystal X-ray diffraction, the C66H4 renders a nonclassical geometry incorporating two heptagons and two pairs of fused pentagons in a C2 v symmetry. The common vertexes of the fused pentagons are bonded with four hydrogen atoms to convert the hydrogen-linking carbon atoms from sp2 to sp3 hybridization, which together with the adjacent heptagons essentially releases the sp2-bond strains on the abutting-pentagon sites of the diheptagonal fused pentagon C66 (dihept-C66). DFT computations suggest the possibility for an in situ hydrogenation process leading to stabilization of the dihept-C66. In addition, the experiments have been carried out to study heptagon-dependent properties of dihept-C66H4, indicating the key responsibility of the heptagon for changing hydrocarbon activity and electronic properties. The present work with the unprecedented double-heptagon-containing hydrofullerene successfully isolated and identified as one of the low-pressure combustion products shows that the heptagon is a new building block for constructing fullerene products in addition to pentagons and hexagons in low-pressure combustion systems.

Investigation of Effect of Biomass Torrefaction Temperature on Volatile Energy Recovery Through Combustion

Biomass torrefaction is a mild pyrolysis thermal treatment process carried out at temperatures between 200 and 300 °C under inert conditions to improve fuel properties of parent biomass. Torrefaction yields a higher energy per unit mass product but releases noncondensable and condensable gases, signifying energy and mass losses. The condensable gases (volatiles) can result in tar formation on condensing, hence, system blockage. Fortunately, the hydrocarbon composition of volatiles represents a possible auxiliary energy source for feedstock drying and/or torrefaction process. The present study designed a low-pressure volatile burner for torrefaction of pine wood chips and investigated energy recovery from volatiles through clean co-combustion with natural gas (NG). The research studied the effects of torrefaction pretreatment temperatures on the amount of energy recovered for various combustion air flow rates. For all test conditions, blue flames and low emissions at the combustor exit consistently signified clean and complete premixed combustion. Torrefaction temperature at 283–292 °C had relatively low volatile energy recovery, mainly attributed to higher moisture content evolution and low molecular weight of volatiles evolved. At the lowest torrefaction pretreatment temperature, small amount of volatiles was generated with more energy recovered. Energy conservation evaluation on the torrefaction reactor indicated that about 27% of total energy carried by the exiting volatiles and gases has been recovered by the co-fire of NG and volatiles at the lowest temperature, while around 19% of the total energy was recovered at the intermediate and highest torrefaction temperatures, respectively. The energy recovered represents about 23–45% of the energy associated with NG combustion in the internal burner of the torrefaction reactor, signifying that the volatiles energy can supplement significant amount of the energy required for torrefaction.

Verifying the Prediction Result Reliability Using k-ε, Eddy Dissipation, and Discrete Transfer Models Applied on Methane Combustion Using a Prototype Low-Pressure Burner

Verifying the Prediction Result Reliability Using k-ε, Eddy Dissipation, and Discrete Transfer Models Applied on Methane Combustion Using a Prototype Low-Pressure Burner

Pyrolysis-gas chromatography mass spectroscopy and pipe combustor analysis of the ignition and mass regression characteristics of a sulfur doped polysulfide ramjet fuel

The autoignition and low pressure combustion characteristics of five candidate polysulfide (LP-33) based ramjet solid fuel formulations are evaluated using pyrolysis-gas chromatography mass spectroscopy and a subscale pipe combustor. The polysulfide based fuels are oxidatively cured using p-quinone dioxime and manganese dioxide. The fuel formulations differ by the amount of elemental sulfur that is added to alter the decomposition temperature, mass burning rate, and to modify the mass fraction of low ignition temperature sulfur based decomposition gasses. The objectives of this research are to demonstrate that the autoignition temperature and the mass regression rate of the candidate fuels can be tailored as function of the sulfur doping fraction. The results of the analysis show that the mass fraction of low autoignition temperature decomposition gasses that include carbon disulfide and hydrogen disulfide increase as the mass fraction of elemental sulfur increases in the fuel formulation. Pipe combustor test results demonstrate repeatable autoignition temperatures less than 400 °C at ambient pressure for all formulations. The results also show that the primary effect of sulfur doping is to lower the mass burning rate of the fuel in direct correlation to the Shore-A hardness of the material which increases the time to ignition with no appreciable change to the mean autoignition temperature.

The influence of flash-boiling on spray-targeting and fuel film formation

The variety of fuels used in GDI engines is constantly increasing. This affects the combustion process because the spray formation and the mixture preparation is greatly influenced by the fuel properties. In particular, the reduction of the saturation temperature is promoting the so-called flash-boiling effect, when the bulk liquid or a component of the fuel mixture becomes superheated during the injection into the low-pressure combustion chamber.This study reveals the enormous influence of flash-boiling on the spray/wall interaction and the consequent deposition of fuel wall films. This is of major importance because the formation of wall films must be avoided, since they are the main route to soot particle emissions.In order to increase the understanding of the wall film formation the spray/wall interaction is investigated under conditions representative of a homogeneously charged gasoline engine. Gasoline and iso-octane are injected using a modern six-hole nozzle. Emphasis is placed on the influence of the initial fuel temperature. Using high-speed imaging and infrared thermography, the spray propagation, the spray-targeting and the evaporation duration of the wall films are analysed.It is found that increasing the fuel temperature does not lead to a constant reduction of the wall film problem. Rather, the flash-boiling-induced ‘contraction’ of the spray leads to an accumulation of the wall film mass in a small area. This delays the evaporation process to a great extent and can lead to increased soot particle emissions. From the results, a critical value of the degree of superheat is identified and the findings transferred to various single component fuels, in order to avoid undesired fuel film accumulation.

New insights into methane-oxygen ion chemistry

External electric fields may reduce emissions and improve combustion efficiency by active control of combustion processes. In-depth, quantitative understanding of ion chemistry in flames enables predictive models to describe the effect of external electric fields on combustion plasma. This study presents detailed cation profile measurements in low-pressure, burner-stabilized, methane/oxygen/argon flames. A quadrupole molecular beam mass spectrometer (MBMS) coupled to a low-pressure (P=30Torr) combustion chamber was utilized to measure ion signals as a function of height above the burner. Lean, stoichiometric and rich flames were examined to evaluate the dependence of ion chemistry on flame stoichiometry. Additionally, for the first time, cataloging of flame cations is performed using a high mass resolution time-of-flight mass spectrometer (TOF-MS) to distinguish ions with the same nominal mass. In the lean and stoichiometric flames, the dominant ions were H3O+, CH3O2+, C2H7O+, C2H3O+ and CH5O+, whereas large signals were measured for H3O+, C3H3+ and C2H3O+ in the rich flame. The spatial distribution of cations was compared with results from numerical simulations constrained by thermocouple-measured flame temperatures. Across all flames, the predicted H3O+ decay rate was noticeably faster than observed experimentally. Sensitivity analysis showed that the mole fraction of H3O+ is most sensitive to the rate of chemi-ionization CH+O↔CHO++E−. To our knowledge, this work represents the first detailed measurements of positive ions in canonical low-pressure methane flames.

20811 ジェットエンジン用気体燃料インジェクタの低圧環境におけるメタン保炎性能試験

Fundamental research on re-ignition and flame-holding of methane fuel jet engine at high altitudes were performed. Effects of equivalence ratio on the limit of combustor inlet-flow-velocity for flame-holding at low pressures were investigated. A gaseous fuel injector was installed into a low-pressure combustion wind tunnel. The combustor pressure was constant at 80 kPa. Total equivalence ratio was varied from 0.4 to 1.3. Fuel flow rate was automatically controlled according to air flow rate as keeping the total equivalence ratio constant. At the total equivalence ratio of 1.0, the limit of combustor inlet-flow-velocity for flame-holding took the maximum value. The local maximum of the limit of combustor inlet-flowvelocity for flame-holding appeared at the total equivalence ratio of 0.5. The limit of combustor inlet-flow-velocity for flame-holding at the total equivalence ratio of 0.8 was increased by means of addition of a little amount of air into methane.

Lean burn limit and time to light characteristics of laser ignition in gas turbines

This work details a study of laser ignition in a low pressure combustion test rig, representative of an industrial gas turbine (SGT-400, Siemens Industrial Turbomachinery Ltd.) and for the first time investigates the effect of air mass flow rate on combustion characteristics at air/fuel ratios at the lean burn limit. Both the lean burn limit and time taken to light are essential in determining the suitability of a specified air/fuel ratio, especially in multi-chamber ignition applications. Through extension of the lean burn limit and reduction of the time taken to light, the operating window for ignition with regards to the air/fuel ratio can be increased, leading to greater reliability and repeatability of ignition. Ignition of a natural gas and air mixture at atmospheric pressure was conducted using both a standard high energy igniter and a laser ignition system utilizing a Q-switched Nd:YAG laser source operating at 1064nm wavelength. A detailed comparison of the lean burn limit and time taken to light for standard ignition and laser ignition is presented.