Combustion Regime Research Articles

Anisotropy of Reynolds Stresses and Their Dissipation Rates in Lean H2-Air Premixed Flames in Different Combustion Regimes

The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions.

A residence time-based concept for modeling and analyzing the impact of diffusion on auto-ignition processes with thermal stratification

A meticulous definition of the residence time (also referred to as the fluid age) in the context of auto-ignition in a thermally stratified gas has demonstrated that the age can be considered as a level-set function, which allows the tracking of a self-ignition front propagating. In order to avoid the difficulty of defining the boundary and initial conditions for this age, which can be seen as fluid birth, a normalized age is introduced. Meticulous derivation of the equation revealed additional terms related to the scalar dissipation rates and the gradient in the composition space of the auto-ignition delay. To validate this model equation, two canonical configurations were proposed: one representing a hot spot self-igniting with significant thermal diffusion and the other where a self-ignition front propagates at an almost constant speed. The analysis, based on the normalized age of the particles, reveals the impact of thermal diffusion on the acceleration or deceleration of particle aging. In certain instances, a particle rejuvenation mechanism through thermal diffusion has been identified. Furthermore, this work demonstrates the capacity of the normalized residence time field to map transitions between auto-ignition and laminar flame. Finally, the different cases studied were classified in an auto-ignition/diffusion diagram based on three non-dimensional characteristic numbers, which highlighted five combustion regimes.

Flow field and emission characterization of a novel enclosed jet-in-hot-coflow canonical burner

The jet-in-hot-coflow is a canonical combustion setup, which has been used in several studies to study Flameless/MILD combustion and auto-ignition of fuels. However, the NOx and CO emission measurements from these combustion setups were not possible due to the entrainment of laboratory air and a lack of a well-defined physical system limit. These limitations have been overcome by a new enclosed jet-in-hot-coflow setup. The combustor was operated by injecting a mixture of CH4-Air in the central jet, and the coflow comprised of hot products from CH4-Air combustion in burners upstream. The coflow composition was further controlled by adding diluents such as N2 and CO2. Measurements were done using stereoscopic particle image velocimetry, suction probe gas analysis, thermocouples, and chemiluminescence imaging. Increasing central jet velocity and equivalence ratio led to lower NOx and a reaction zone that enlarged and shifted downstream. The reduction in NOx emission was attributed to the returning mechanism. Adding CO2 and N2 as diluents in the coflow resulted in a longer combustion zone and reduced temperatures in the combustion chamber, leading to decreased NOx production and increased reburning. These experiments provide relevant flowfield and emissions data for modelers and help characterize combustion regimes such as Flameless/MILD.

Repeated Tree Inventories of Pine Forests in South Florida's Big Cypress National Preserve.

The natural forest ecosystems of South Florida, USA, support a high biodiversity of plant and animal species and provide valuable ecosystem services. However, these ecosystems remain poorly represented in global studies, primarily due to a paucity of standardized data. Here, we present previously unpublished data from 332 censuses of 54 permanent 1-ha tree inventory plots in the Racoon Point area of Big Cypress National Preserve, Florida, USA, including a total of nearly 100,000 measurements (diameter or height) of > 17,000 individual living trees and palms (with additional measurements of nearly 6000 dead pine snags) collected sporadically over a 19-year period (1993-2012). These data, which were originally collected as part of a project to investigate tree responses to different experimental burning regimes, provide unique insight into the diversity, composition, structure, and dynamics of South Florida's unique and endangered pine forest ecosystems. Data files include the species identity, size (dbh = diameter at breast height), and location of all trees ≥ 5 cm dbh in 54 individual tree plots. Additional data are provided about heights of palm trees, and the location and burn history of each plot. These data are freely available for noncommercial scientific use under a Creative Commons license; users are encouraged to cite this paper when using the data.

Burning and flame extinction patterns of PMMA under external heating and oxygen deficiency

The paper presents an experimental study investigating the impact of under-oxygenated conditions on the burning characteristics and flame extinction of horizontal transparent poly(methyl-methacrylate) (PMMA) slabs subjected to external heating. Small-sized solids are exposed to various constant levels of radiant heat flux in a nitrogen-diluted environment at ambient pressure. The study aims to understand the multivariable dependence of PMMA combustion on oxygen and heating levels. The analysis goes beyond the study of global and local variables to provide an understanding of the mechanisms involved, revealing a notable effect on burning characteristics and flame extinction mechanisms by blow-off. The results provide a detailed assessment of the surface energy balance and flame bulk properties. The results demonstrate good repeatability up to a certain oxygen limit, beyond which the flame combustion regime exhibits stochastic behaviour. The study of heat transfer mechanisms has revealed that flame radiation and external heating play a predominant role in the heating of horizontal solid fuels, In contrast, within the flame, convection is the main contributor to solid heating, rather than flame radiation. Solid characteristics show linear trends proportional to oxygen depletion, while gas phase characteristics show monotonic trends strongly influenced by the changing combustion regime. The surface temperature of the solid is highly sensitive to the test conditions that affect the energy balance. Dimensionless parameters are developed and data are correlated with existing literature. The analysis demonstrates that the linear or monotonic trends are maintained regardless of external heating. Finally, models are proposed and validated to predict both the extinction limit and the burning rate independently of irradiance and oxygen levels.

Assessing turbulence–flame interaction of thermo-diffusive lean premixed H2/air flames towards distributed burning regime

Simultaneous laser-induced fluorescence of OH radicals and particle image velocimetry, and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements are used to investigate the effects of Karlovitz number (by varying the equivalence ratio) and residence time (by varying the axial measurement location) on the internal flame structures of lean premixed hydrogen/air turbulent jet flames. The turbulent flow fields, instantaneous macroscopic flame structures, and thermochemical states of a set of lean premixed hydrogen/air turbulent flames with varying initial equivalence ratio of 0.3, 0.4, and 0.45 at a constant bulk velocity of 100m/s are discussed. With an increasing equivalence ratio, the Karlovitz number derived from the turbulent flow fields decreases rapidly from 7690 to 260 and 100, determined at a downstream location of x/D=7. At the highest Karlovitz number (i.e., the lowest equivalence ratio), a distributed burning is observed in the jet flame as turbulent transport dominates over molecular mixing, and the effects of differential diffusion and flame curvature are suppressed. With decreasing Karlovitz number, intense burning regions characterized by elevated local equivalence ratio, high water mole fraction, and super-adiabatic flame temperatures are observed in association with positive flame curvature. The same combustion diagnostics are applied to another lean premixed hydrogen/air turbulent flame with an initial equivalence ratio of 0.4 and a bulk velocity of 200m/s at selected downstream locations of x/D=3.5, 7, 10.5, and 14 to assess the effects of developing turbulence and residence time. Corresponding Karlovitz numbers are 680, 730, 775, and 690, as the local turbulent intensity changes along downstream locations. While flame structures reveal characteristics towards distributed burning at lower x/D, a locally intense burning region appears at higher x/D together with positive curvature. This is mainly because the turbulence develops with increasing x/D and the flame surface is more disturbed and curved by turbulent eddies with increasing turbulence length scales. The highly diffusive hydrogen is locally concentrated by positively curved flame surfaces, resulting in fuel-rich burning regions at a higher temperature. This indicates that, as velocity fluctuations increase in axial direction, turbulence can also promote thermo-diffusive instabilities to a certain extent, increasing the mixture inhomogeneity in temperature space by interacting with the differential diffusion effect of hydrogen at atmospheric pressure.Novelty and significanceThe novelty of the current work is the focus on turbulence–chemistry interactions of premixed hydrogen/air jet flames at high turbulence and ultra lean conditions. Comprehensive experimental results including the turbulent flow fields (particle image velocimetry), instantaneous flame structures (laser-induced fluorescence of OH radicals) and internal thermochemical states (quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering imaging) of hydrogen/air jet flames are analyzed and discussed. The data sets with well-characterized boundary conditions and high measurement accuracy, are crucial for validation and development of numerical simulation models.

Learning from each other: Cross-cutting diagnostic development activities between magnetic and inertial confinement fusion (invited).

Inertial Confinement Fusion and Magnetic Confinement Fusion (ICF and MCF) follow different paths toward goals that are largely common. In this paper, the claim is made that progress can be accelerated by learning from each other across the two fields. Examples of successful cross-community knowledge transfer are presented that highlight the gains from working together, specifically in the areas of high-resolution x-ray imaging spectroscopy and neutron spectrometry. Opportunities for near- and mid-term collaboration are identified, including in chemical vapor deposition diamond detector technology, using gamma rays to monitor fusion gain, handling neutron-induced backgrounds, developing radiation hard technology, and collecting fundamental supporting data needed for diagnostic analysis. Fusion research is rapidly moving into the igniting and burning regimes, posing new opportunities and challenges for ICF and MCF diagnostics. This includes new physics to probe, such as alpha heating; increasingly harsher environmental conditions; and (in the slightly longer term) the need for new plant monitoring diagnostics. Substantial overlap is expected in all of these emerging areas, where joint development across the two subfields as well as between public and private researchers can be expected to speed up advancement for all.

Radiative heat recuperation in combustion of layered solid fuel

In this work, we investigate the properties and stability of the combustion waves propagating in a system of layered solid fuel material in which the neighbouring fuel sheets can recuperate heat via thermal electromagnetic radiation. The analysis is undertaken within the flame sheet model, which allows to determine the combustion wave speed and structure and derive the dispersion relation characterising the stability of the reaction front. The proper choice of parameters leads to superadiabatic combustion temperature and significant increase of the combustion wave speed. In addition, the stable steady regimes of combustion can be substantially extended in the space of parameters. Such an enhancement of stability and ability to control the characteristics of the combustion wave propagation can be very attractive for the combustion synthesis of new layered materials, similarly to the chemical furnace technology proposed earlier.

High-fidelity numerical simulation of longitudinal thermoacoustic instability in a high-pressure subscale rocket combustor

A detailed analysis of thermoacoustic combustion instability in a subscale rocket combustor at high pressure is reported in this paper using the high-fidelity large-eddy simulation (LES). Self-sustained longitudinal combustion instability is observed in the experiments for this combustor configuration. The computational setup follows the past experimental study of a rig called the Continuously Variable Resonance Combustor (CVRC). In this combustor, both stable combustion and unstable combustion dynamics have been observed by varying the oxidizer injector length. A combustor configuration with an oxidizer injector length of 12 cm is chosen for this study based on the experimental evidence of longitudinal combustion instability. An autonomous meshing using the modified cut-cell Cartesian grid generation approach, coupled with on-the-fly Adaptive Mesh Refinement (AMR) is employed in this study. Chemical reactions during turbulent combustion in this configuration are modeled by solving the species transport equations with a detailed chemistry solver using a kinetic mechanism with 21 species and 84 steps. Similar to the findings of the experiments, we observe a self-sustained combustion instability in the present study, which is characterized by a limit cycle behavior of the acoustic fluctuations. The spectral analysis of these acoustic fluctuations shows good agreement with experimental data for the frequency of the three dominant modes. We further analyze the features of time-averaged and instantaneous reacting flow to study the effects of combustion instability on the flame holding dynamics, vortex shedding, mixing, and combustion regime due to flame movement along the longitudinal direction of the combustor during a limit cycle. These phenomena are effectively captured through the integration of AMR with complex chemistry in the present study. A particular focus of the study is on understanding the role of minor species (OH, HO2, and CH2O) in the physical and state-space in sustaining the flame during the combustion instability. Additionally, the physical mechanisms responsible for the production and dissipation of enstrophy are examined to demonstrate that their contribution can create significant fluctuations in the reacting flow field, which can assist in sustaining the combustion instability.

Regional polarity in the laminar premixed flame considering the combined effect of positive and negative charge

The study of flame charge is helpful in analyzing the combustion regime, which is significant for optimizing energy conversion in the combustion. In this article, a method to calculate the charge density considering polarity (CDCP) for flame is presented. The charge in the flame local area is obtained by combining the effect of charged particles with positive and negative polarity. The CDCP in different zones of the premixed methane flame with three equivalence ratios of 0.9, 1.0, and 1.3 is measured in the Langmuir probe measurement experiment. The experiment result found that the regional polarity is shown in the premixed flame. Subsequently, the premixed methane flame was simulated using Fluent, and then the simulation distribution of CDCP in the premixed methane flame was obtained. The CDCP and polarity across the reactant, preheat, reaction, and product zones of the premixed flame are analyzed based on the experiment and simulation results. The CDCP of the reaction zone is greater than 0 C/m3 and reaches a peak, showing obvious positive polarity. The CDCP of the reactant zone is less than 0 C/m3, showing negative polarity. The CDCP changes rapidly from less than 0 C/m3 to greater than 0 C/m3 with the approach of the reactant zone to the preheating and reaction zones. The CDCP in the product zone gradually decreases with the increase of the distance from the reaction zone and finally less than and close to 0 C/m3, showing weak negative polarity. Finally, the cause of regional polarity formation is discussed based on the net production rate of charged particles and the velocity distribution of the flow field in the simulation results. The distribution of positive ions and electrons in the premixed flame is different due to the effect of the flow field and the different diffusion properties of charged particles. Thus, the regional polarity is exhibited in the premixed flame.

Comprehensive integral simulation of first 3 weeks of severe accident at Fukushima Daiichi Unit 1. Part II – Simulation of the in-vessel and ex-vessel phases

This paper is the second part of the summary presenting the results of IBRAE work over last 12 years on severe accident investigation at Unit 1 of Fukushima Daiichi with the SOCRAT code. The paper supplements the part I that was published previously, and is dedicated to the description of one-through integral simulation results of the 3 weeks of accident unfolding starting from the tsunami arrival. The integral simulation encompasses the different processes at in-vessel and ex-vessel phases of the accident including the processes in reactor building. The important issues discussed are the hypothesis on the induced leak from a steam line, the operation of isolation condenser, the possible early combustion of hydrogen in the reactor building room that houses the drywell head, as well as the time, conditions and regime of hydrogen combustion in the reactor hall, the flowrate of alternative water injection, and the depth of concrete ablation. The main criterion for checking the adequacy of assumptions and the simulation results is the agreement with factual information available including the measurement data and the results of inverse calculations of the source term. The shortcomings of current simulation with the SOCRAT code are briefly considered as a perspective for improving the realism of results in the next studies.

The Effect of Aluminum in Its Mixtures With Ammonium Nitrate on the Ignition of Burning and Its Transition to the Convective Burning Regime

The Effect of Aluminum in Its Mixtures With Ammonium Nitrate on the Ignition of Burning and Its Transition to the Convective Burning Regime

Jet ignition characteristics of ammonia-hydrogen passive pre-chamber: Emphasis on equivalence ratio and hydrogen/ammonia ratio

Ammonia-hydrogen blend fuels offer the potential for zero-carbon emissions in internal combustion (IC) engines. To improve combustion performance, considerable attention has been focused on pre-chamber jet ignition technologies. Passive pre-chamber featuring a simple structure can be installed in the spark plug hole without modifying the cylinder head. However, the jet ignition characteristics of ammonia-hydrogen passive pre-chambers are not fully understood, and there is a lack of visualized evidence on how critical parameters (e.g., equivalence ratio and hydrogen/ammonia ratio) impact ignition processes. This work investigated the effects of equivalence ratio and hydrogen/ammonia ratio on ammonia-hydrogen jet ignition characteristics in a rapid compression machine with passive pre-chamber. High-speed photography and instantaneous pressure acquisition were employed to capture detailed ignition and combustion evolutions. Results show that under low hydrogen/ammonia ratio conditions, there exists a combustion regime with noticeable fluctuations in jet penetration, indicating degenerate ignition performance. As the hydrogen/ammonia ratio increases from 0 % to 20 %, the ignition delay time of mixtures in the main chamber is reduced by 70 %, suggesting an improved overall ignition performance. Regarding the equivalence ratio, more pronounced effects on jet ignition and flame propagation are identified under lean-burning conditions. For mixtures at the hydrogen/ammonia ratio of 20 %, as the equivalence ratio increases from 0.6 to 0.8 and from 0.8 to 1.0, the decay rates for 10 %–90 % combustion duration are 82.5 % and 20.6 %, respectively. Besides, with the increase of equivalence ratio, flame initiation is shifted from the jet head to the side surface. The role of hydrogen/ammonia ratio and equivalence ratio is different, with the former modifying mixture reactivity while the latter affecting ignition energy.

Two-Photon Planar Laser-Induced Fluorescence Of Near-Wall CO During The Interaction Between A Flame And A Cooling Air Film

Reducing the anthropogenic pollutant emissions is a major concern in the aeronautical community. Implementing high-power density core engines made of lighter materials would undoubtedly increase the thermal efficiency and reduce CO 2 emissions. Nevertheless, near-wall combustion processes will become significant, raising concerns regarding thermal management. As a result, walls in aero-engine combustors are commonly cooled, but it also introduces more complex physical phenomena associated to pollutant formation during flame-cooling air interaction (FCAI). This experimental study aims at elucidating the role of the cooling air film on CO emissions during FCAI. The experiments are conducted in an atmospheric pressure and optically-accessible lab-scale combustion test rig dedicated to near-wall combustion. It generates a lean premixed turbulent methane/air V-shaped flame, with one branch of the flame that interacts with an oil-cooled stainless-steel wall. A splash-cooling plate system enables to generate a momentum-controlled parietal cooling air film. The novelty of this study is to develop and implement planar laser-induced fluorescence of the CO molecule (CO-PLIF), complemented by planar laser-induced fluorescence of the OH radical (OH-PLIF) as well as global CO emissions in the exhaust gases. Results show that the excitation of the Hopfield-Birge system enables to detect the AN ngström bands and the third positive system. However, significant interferences with C 2 and CN emissions are highlighted, being more pronounced in rich combustion conditions, and originating from the incomplete fuel oxidation as well as the high energy density of the two-photon excitation process. A broadband collection strategy of the AN ngström bands is selected, since these interferences are limited in lean combustion regimes. The analysis of the flame dynamics indicates that the increase of the cooling air film momentum shifts the reactive flow away from the wall, but also controls the flame wrinkling through modified aerodynamics. As a result, the flame is not influenced anymore by thermal quenching processes, and can effectively burn further away from the wall. Global emission measurements indicate an increase in CO when the cooling air film momentum increases. While thermal quenching favors a faster oxidation downstream of the flame, the establishment of the cooling air film leads to a longer flame with more production areas. CO-PLIF imaging concurs this phenomenon, with a CO distribution all along the wall when the cooling air film is well established.

Evaluating the feasibility of machine learning algorithms for combustion regime classification in biodiesel-fueled homogeneous charge compression ignition engines

The present study explores the feasibility of machine-learning algorithms in classifying the combustion regimes in a homogenous charge compression ignition (HCCI) engine fueled with biodiesels from diverse sources. A distinctive feature of this research is the extensive database of HCCI combustion regimesgenerated for neat biodiesel fuels of different compositions. The engine experiments reveal significant variability in the load range of operation of HCCI engines across different biodiesel fuels. Strategies including compression ratio variation and charge dilution were employed to achieve higher engine loads, and the results confirm that the interplay of fuel reactivity and engine load, along with compression ratio and charge dilution, dictates the combustion stability. This complex phenomenon necessitates ascertaining the prevailing combustion type under a given engine load for biodiesel fuels derived from diverse sources. Thus, the present study aims to investigate the applicability of machine learning algorithms for developing models that classify combustion regimes in a neat biodiesel-fueled light-duty HCCI diesel engine. Models are developed to categorize biodiesel HCCI combustion into four groups: engine misfire, stable combustion without dilution, stable combustion with dilution, and knocking combustion. Biodiesel composition (13 different methyl esters), compression ratio and engine load are considered the input variables for the machine learning (ML) models. The results show that Naive Bayes and logistic regression models exhibit poor applicability among the investigated ML models due to a low F1 score (<0.8). The k-NN and SVM models demonstrate practical suitability with reasonably good F1 scores. However, the decision tree model outperforms all, precisely classifying 94 % of calibration and validation samples. It could precisely classify all the validation samples corresponding to misfire combustion, 21 of 22 samples to stable combustion, 4 of 6 samples to stable combustion with dilution, and 30 of 31 to knocking combustion. Thus, it emerges as the best approach to classify the combustion regimes of HCCI engines operated with neat biodiesel fuels.

1-Hexene Ozonolysis across Atmospheric and Combustion Temperatures via Synchrotron-Based Photoelectron Spectroscopy and Chemical Ionization Mass Spectrometry.

This study investigates the complex interaction between ozone and the autoxidation of 1-hexene over a wide temperature range (300-800 K), overlapping atmospheric and combustion regimes. It is found that atmospheric molecular mechanisms initiate the oxidation of 1-hexene from room temperature up to combustion temperatures, leading to the formation of highly oxygenated organic molecules. As temperature rises, the highly oxygenated organic molecules contribute to radical-branching decomposition pathways inducing a high reactivity in the low-temperature combustion region, i.e., from 550 K. Above 650 K, the thermal decomposition of ozone into oxygen atoms becomes the dominant process, and a remarkable enhancement of the conversion is observed due to their diradical nature, counteracting the significant negative temperature coefficient behavior usually observed for 1-hexene. In order to better characterize the formation of heavy oxygenated organic molecules at the lowest temperatures, two analytical performance methods have been combined for the first time: synchrotron-based mass-selected photoelectron spectroscopy and orbitrap chemical ionization mass spectrometry. At the lowest studied temperatures (below 400 K), this analytical work has demonstrated the formation of the ketohydroperoxides usually found during the LTC oxidation of 1-hexene, as well as of molecules containing up to nine O atoms.

Biodiversity monitoring for the jarrah (Eucalyptus marginata) forest in south-west Western Australia: An extension to ten-year findings of Forestcheck

Long-term research and monitoring programs can provide important insights into biodiversity patterns to inform sustainable management of multi-use forest ecosystems. This study investigated biodiversity patterns in two forest ecosystems in the southwest region of Australia, as part of a long-term monitoring program, Forestcheck, established to track changes and trends in forest biodiversity associated with management activities. We assessed the effects of forest ecosystem, silvicultural treatment types (gap release and shelterwood/selective harvesting) and time since fire, on taxonomic richness and assemblage patterns across six taxonomic groups (terrestrial vertebrates, birds, macro-invertebrates, vascular plants, macro-fungi and cryptogams). Further, temporal comparisons in taxonomic richness and assemblage were completed for vascular plants and terrestrial vertebrates using data collected in a second round of monitoring. There was a low-level effect of harvesting and prescribed burning disturbance on combined community assemblage, but forest ecosystem had a greater influence on biodiversity patterns. Plant assemblages changed significantly according to ecosystem, silvicultural treatment type and monitoring round when two rounds of monitoring data were considered. These findings support previous assessments of Forestcheck sites, in that the strongest determinant of biodiversity patterns was forest ecosystem, and the greatest effects of disturbance were on assemblage patterns. These results provide some insight into post disturbance patterns and may assist with decision-making in relation to silviculture and prescribed burning regimes, and post-silviculture management, to provide increased opportunity for restoration of diversity.

Discovery of a Long Thermonuclear X-Ray Burst from the Ultracompact Binary 4U 1850–087

We report the detection of a long X-ray burst from the ultracompact binary 4U 1850–087. The type I X-ray burst was observed on MJD 60171.65 by the Monitor of All-sky X-ray Image and Neutron Star Interior Composition Explorer (NICER). We analyze the NICER data between MJD 60095.19 and 60177.43, which includes the observation covering parts of the long X-ray burst decay phase, i.e., 0.15–3.8 hr after the trigger. The persistent spectra are well described by a multicolor disk blackbody, with an inner temperature of ∼0.5 keV, a thermally Comptonized continuum, an asymptotic power-law photon index of Γ ∼ 2.2, and an electron temperature of kT e ∼ 20–30 keV. The mean persistent flux was around 3.8 × 10−10 erg cm−2 s−1, corresponding to a local mass accretion rate of ∼1%ṁEdd . Part of the time-resolved burst spectra show a clear deviation from the blackbody model, which can be improved by considering the enhanced persistent emission due to the Poynting–Robertson drag or the reflected disk emission illuminated by the burst. From the burst flux during the cooling phase, we estimate the burst duration, τ ≈ 0.78 hr, the burst fluence, E b ≈ 4.1 × 1041 erg, and the ignition column depth, y ign ≈ 3.5 × 1010 g cm−2. These long X-ray burst parameters from 4U 1850–087 suggest a regime of unstable burning of a thick, pure helium layer slowly accreted from a helium donor star. Moreover, we identify 7σ significant ∼1 keV emission lines in the burst spectra, which may originate from the surrounding accretion disk.

Analysis of the Efficiency of Burning Briquettes from Agricultural and Industrial Residues in a Layer

The combustion of briquettes made from organic and industrial residues in small boilers requires researchers to consider the characteristics of this type of fuel and methods of its combustion. For the efficient combustion of fuel briquettes, a layered combustion method with the ability to regulate the supply of combustion air is better suited. The purpose of this research is to study the thermal technical conditions of briquetted fuel combustion. In order to carry this out, a stand was created, which made it possible to determine the combustion efficiency of this type of fuel. Two types of briquettes were studied: one with 30% sunflower husks and 70% leaves, and one with and 70% sunflower husks and 30% coke breeze. The combustion results of the briquettes show that heat loss from chemical under-burning was no more than 6.25%. To determine the temperature distribution in the fuel layer, a model of unsteady heat transfer in a fixed layer was used. A calculation of the temperature fields in the layer of burned fuel briquettes was carried out, which showed that the most favorable conditions for burning briquettes were created with a layer about 15–20 cm thick for both burned briquette options. The temperature was in the range of 450–750 °C, which on the one hand corresponds to experimental data and on the other hand provides a combustion regime that occurs with a relatively low loss to the environment. This installation and mathematical model will help future studies based on the processes of other types of organic waste combustion with a grate system.

Preparation and properties of porous rice husk ash for internal curing of high performance cement pastes

To address the challenges posed by significant autogenous shrinkage in high-performance concrete and mitigate the adverse effects of conventional internal curing materials on its mechanical properties, this study explored the potential use of rice husk ash (RHA) as internal curing materials. The research investigated the impact of combustion regime (combustion temperature and dwelling time) and the milling process on the microstructure and physical-chemical properties of RHA. Moreover, the effect of RHA on autogenous shrinkage, compressive strength, and microstructure in cement pastes were also reported. Experimental results demonstrated that the combustion regime and milling time could significantly affect the pore structure, particle size and pozzolanic activity of RHA. RHA had good potential for internal curing when it was burned at 600°C for 1 h and milled for 15–30 min. RHA released the entrained water after 12 h to slow down the depletion of free water, and thus inhibited the development of self-drying and autogenous shrinkage. The 168 h shrinkage was reduced by 56.8 %, 62 %, and 51.8 %, respectively when the milling time increased to 30 min. Moreover, the addition of RHA with a smaller particle size could refine the microstructure of matrix and enhance the compressive strength due to its higher pozzolanic activity. These findings suggest promising applications for RHA as an effective internal curing material in high-performance cementitious materials.