| 1. |
- Zhong, Shenghui, et al.
(författare)
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Assessment of grid/filter size dependence in large eddy simulation of high-pressure spray flames
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 329
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Tidskriftsartikel (refereegranskat)abstract
- With the increase of computing power, large eddy simulation (LES) within the Lagrangian–Eulerian two-phase flow framework has evolved as an useful numerical tool to gain insights to the spray combustion processes in advanced internal combustion engines. However, the inherent effect of grid/filter size (G/FS) on the physics revealed by LES is not fully understood. In this paper, we present several new viewpoints on this by analyzing the results from a Lagrangian–Eulerian LES study of Engine Combustion Networks Spray A with a newly developed multi-region adaptive mesh refinement method in OpenFOAM. It is found that G/FS affects the predicted spray characteristics, the modes of spray combustion, the ignition delay time and the liftoff length in different ways. First, owing to the nature of Lagrangian particle-in-cell approach, the spray liquid penetration length converges as the G/FS approaches to the nozzle diameter. The convergence behavior is rather independent of the spray model parameters. Second, it is critical to have a well resolved spray liquid region; the main spray combustion characteristics, e.g., the mean pressure rise profile, the onsets of the cool flame and hot flame, and the main flame structure, are shown to be similar and independent of G/FS once the liquid region is properly resolved. However, the detailed flame structures at the liftoff position are sensitive to G/FS; the upstream auto-ignition events, the low temperature ignition assisted flame propagation and the hot flame propagation, are shown to partially rely on the adopted G/FS. Finally, it is found that a finer G/FS predicts a slower fuel/oxidizer mixing and a shorter flame liftoff length, yielding a higher soot mass.
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| 2. |
- Li, Shenghui, 1994-, et al.
(författare)
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Blades : A Unified Benchmark Suite for Byzantine Attacks and Defenses in Federated Learning
- 2024
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Konferensbidrag (refereegranskat)abstract
- Federated learning (FL) facilitates distributed training across different IoT and edge devices, safeguarding the privacy of their data. The inherent distributed structure of FL introduces vulnerabilities, especially from adversarial devices aiming to skew local updates to their advantage. Despite the plethora of research focusing on Byzantine-resilient FL, the academic community has yet to establish a comprehensive benchmark suite, pivotal for impartial assessment and comparison of different techniques. This paper presents Blades, a scalable, extensible, and easily configurable benchmark suite that supports researchers and developers in efficiently implementing and validating novel strategies against baseline algorithms in Byzantine-resilient FL. Blades contains built-in implementations of representative attack and defense strategies and offers a user-friendly interface that seamlessly integrates new ideas. Using Blades, we re-evaluate representative attacks and defenses on wide-ranging experimental configurations (approximately 1,500 trials in total). Through our extensive experiments, we gained new insights into FL robustness and highlighted previously overlooked limitations due to the absence of thorough evaluations and comparisons of baselines under various attack settings. We maintain the source code and documents at https://github.com/lishenghui/blades.
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| 3. |
- Li, Xinyu, et al.
(författare)
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Biodegradable MOFilters for Effective Air Filtration and Sterilization by Coupling MOF Functionalization and Mechanical Polarization of Fibrous Poly(lactic acid)
- 2023
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 15:22, s. 26812-26823
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Tidskriftsartikel (refereegranskat)abstract
- High-performance air filtration materials are importantfor addressingthe airborne pollutants. Herein, we propose an unprecedented accessto biodegradable poly-(lactic acid) (PLA)-based MOFilters with excellentfiltering performance and antibacterial activity. The fabricationinvolved a stepwise in situ growth of zeolitic imidazolate framework-8(ZIF-8) crystals at the surface of microfibrous PLA membranes, followedby mechanical polarization under high pressure and low temperature(5 MPa, 40 degrees C) to trigger the ordered alignment of dipoles inPLA chains and ZIF-8. The unique structural features allowed thesePLA-based MOFilters to achieve an exceptional combination of excellenttensile properties, high dielectric constant (up to 2.4 F/m), andenhanced surface potential as high as 4 kV. Arising from the remarkablesurface activity and electrostatic adsorption effect, a significantincrease (from over 12% to nearly 20%) in PM0.3 filtrationefficiency was observed for the PLA-based MOFilters compared to thatof pure PLA counterparts, with weak relation to the airflow velocities(10-85 L/min). Moreover, the air resistance was controlledat a considerably low level for all the MOFilters, that is, below183 Pa even at 85 L/min. It is worth noting that distinct antibacterialproperties were achieved for the MOFilters, as illustrated by theinhibitive rates of 87 and 100% against Escherichiacoli and Staphylococcus aureus, respectively. The proposed concept of PLA-based MOFilters offersunprecedented multifunction integration, which may fuel the developmentof biodegradable versatile filters with high capturing and antibacterialperformances yet desirable manufacturing feasibility.
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| 4. |
- Sha, Jingeng, et al.
(författare)
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Stratigraphy of the Triassic-Jurassic Boundary Successions of the Southern Margin of the Junggar Basin, Northwestern China
- 2011
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Ingår i: Acta Geologica Sinica. - : Wiley. - 1000-9515. ; 85:2, s. 421-436
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Tidskriftsartikel (refereegranskat)abstract
- The Triassic-Jurassic (Tr-J) boundary marks a major extinction event, which (similar to 200 Ma) resulted in global extinctions of fauna and flora both in the marine and terrestrial realms. There prevail great challenges in determining the exact location of the terrestrial Tr-J boundary, because of endemism of. : and the scarcity of fossils in terrestrial settings leading to difficulties in linking marine and terrestrial sedimentary successions. Investigation based on palynology and bivalves has been carried out over a 1113 m thick section, which is subdivided into 132 beds, along the Haojiagou valley on the southern margin of the Junggar Basin of the northern Xinjiang, northwestern China. The terrestrial Lower Jurassic is conformably resting on the Upper Triassic strata. The Upper Triassic covers the Huangshanjie Formation overlaid by the Haojiagou Formation, while the Lower Jurassic comprises the Badaowan Formation followed by the Sangonghe Formation. Fifty six pollen and spore and one algal on were identified from the sediments. Based on the key-species and abundance of spores and pollen, three zones were erected: the Late Triassic (Rhaetian) Aratrisporites-Alisporites Assemblage, the Early Jurassic (Hettangian) Perinopollenites-Pinuspollenites Assemblage, and the Sinemurian Perinopollenites-Cycadopites Assemblage. The Tr-J boundary is placed between bed 44 and 45 coincident with the boundary between the Haojiagou and Badaowan formations. Beds with Ferganoconcha (?), Unio-Ferganoconcha id Waagenoperna-Yananoconcha bivalve assemblages are recognized. The Ferganoconcha (?) bed is limited to the upper Haojiagou Formation, Unio-Ferganoconcha and Waagenoperna-Yananoconcha assemblages are present in the middle and upper members of the Badaowan Formation. The sedimentary succession is interpreted as terrestrial with two mainly lake deposit intervals within Haojiagou and Badaowan formations, yielding fresh water algae and bivalves. However, the presence of brackish water algae Tasmanites and the marine-littoral facies bivalve Waagenoperna from the Badaowan Formation indicate that the Junggar Basin was influenced by sea water caused by transgressions from the northern Tethys, during the Sinemurian.
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| 5. |
- Xu, Huan, et al.
(författare)
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Osteoconductive and Antibacterial Poly(lactic acid) Fibrous Membranes Impregnated with Biobased Nanocarbons for Biodegradable Bone Regenerative Scaffolds
- 2021
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 60:32, s. 12021-12031
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Tidskriftsartikel (refereegranskat)abstract
- Carbonaceous nanostructures featuring unique structural characteristics and high cytocompatibility offer a wealth of possibilities to impart enhancements of mechanical properties and biological activities for osteogenic tissue scaffolds. Here, we unveil the fabrication of osteoconductive and antibacterial porous poly(lactic acid) (PLA) membranes by direct electrospinning of microfibers impregnated with coffee-ground-derived quantum dots (QDs). It enabled a straightforward pathway to regulate the diameter and its distribution for the electrospun PLA microfibers, as well as the improved hydrophilicity. The QDs exhibited high affinity to the PLA matrix, permitting remarkable promotion of tensile strength and elastic modulus for the QD-modified PLA membranes while maintaining comparable extensibility. More importantly, osteoblast adhesion and stretching on the electrospun membranes were significantly enhanced with the existence of QDs, as exemplified by the nearly 1.8-fold increase in cell viability cultured onto the composite membrane loaded with 1.5% QDs compared to that of pure PLA. This was accompanied by rapid biomimetic mineralization and uniform apatite formation in an osteofriendly manner. Unexpectedly, immediate and broad-spectrum antibacterial performance was achieved for the composite membranes, likely arising from the synergistic effects of QD-imparted membrane stress and oxidative stress. The unusual combination of mechanical, biomineralization, biological, and antibacterial performances makes the biodegradable membrane scaffolds promising for guided bone/tissue regeneration therapy.
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| 6. |
- Xu, Shijie, et al.
(författare)
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Large-eddy simulation of the injection timing effects on the dual-fuel spray flame
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 310
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations (LES) coupled with a partially-stirred reactor model and a finite-rate chemistry are carried out to investigate the effects of n-heptane injection timing on the methanol fueled dual-fuel (DF) combustion. Methanol is premixed with air in a constant volume chamber (T=1000 K, ρ=14.8 kg/m3) to form a homogeneous mixture (equivalence ratio ϕm of 0.3). Liquid fuel n-heptane is provided from a high pressure injector to mimic the pilot fuel injection in DF engines. First, mesh sensitivity analysis and LES model validation are conducted. The experimental data of Spray-H (n-heptane fueled) from the Engine Combustion Network is used for model validation. It is shown that the present mesh and LES model are capable of replicating the liquid and vapor penetration length, mixture fraction, temperature distribution, pressure rise profile and ignition delay time (IDT). Second, the effects of n-heptane injection timing are investigated, by varying the start of injection (SOI) time. The LES results reveal that there are three stage heat releases in the DF combustion. With the delay of SOI, the mass fraction of hydrogen peroxide in the ambient mixture increases, leading to an early formation of hydroxyl. Therefore, the two-stage IDTs of n-heptane decrease, while the ambient methanol IDT increases. Results also show the cool flame and high-temperature flame evolution after methanol auto-ignition. The cool flame disappears while the high-temperature flame is found near the injector nozzle, which leads to a relatively high heat release rate.
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| 7. |
- Xu, Shijie, et al.
(författare)
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On element mass conservation in Eulerian stochastic fields modeling of turbulent combustion
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 239
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Tidskriftsartikel (refereegranskat)abstract
- Eulerian stochastic fields (ESF) based transported PDF method has evolved as one of the general purpose methods for the numerical simulation of complex chemically reactive turbulent flows in which the effect of turbulence on the chemical reaction rates can be computed directly without the need of modeling. It is found that ESF method may suffer from violation of the element mass conservation law due to the stochastic Wiener process and the highly non-linear dependence of the chemical reaction rates on the stochastic fields. This paper presents a modified ESF method to remove the error in the element mass conservation. The method is evaluated in numerical simulation of two turbulent flames, (i) Reynolds averaged Navier-Stokes (RANS) simulation of a turbulent non-premixed methane/air counterflow flame under stationary burning and transient flame extinction conditions, (ii) large eddy simulation of swirling turbulent methane/air non-premixed flames under local extinction and re-ignition conditions. The original ESF method violates the element mass conservation in both flames, and the element mass error would not disappear even if a large number of stochastic fields were used. The new method yields a satisfactory prediction of the element mass conservation even with a small number of stochastic fields. The new and original methods predict similar stationary flame structures but the results under flame extinction and re-ignition conditions are rather different.
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| 8. |
- Yang, Miao, et al.
(författare)
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CFD modeling of biomass combustion and gasification in fluidized bed reactors using a distribution kernel method
- 2022
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Ingår i: Combustion and Flame. - : Elsevier Inc.. - 0010-2180 .- 1556-2921. ; 236
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Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional reactive multi-phase particle-in-cell (MP-PIC) model is employed to investigate biomass combustion and gasification in fluidized bed furnaces. The MP-PIC model considered here is based on a coarse grain method (CGM) which clusters fuel and sand particles into parcels. CGM is computationally efficient, however, it can cause numerical instability if the clustered parcels are passing through small computational cells, resulting in over-loading of solid particles in the cells. To overcome this problem, in this study, a distribution kernel method (DKM) is proposed and implemented in an open-source CFD code, OpenFOAM. In DKM, a redistribution procedure is employed to spread the solid volume and source terms of the particles in the parcel to the domain in which the particles are clustered. The numerical stiffness problem caused by the CGM clustering can be remedied by this method. Validation of the model was performed using data from different lab-scale reactors. The model was shown to be able to capture the transient heat transfer process in a lab-scale bubbling fluidized bed reactor under varying fluidization velocities and loads of sand. Then, the model was used to study the combustion/gasification process in a bubbling fluidized bed reactor under varying ambient temperatures, equivalent air ratios, and steam-to-biomass ratios. The performance of DKM was shown to improve the accuracy and the robustness of the model. © 2021 The Author(s)
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| 9. |
- Ye, Fanghua, et al.
(författare)
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Slot Self-Attentive Dialogue State Tracking
- 2021
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Ingår i: Proceedings of the World Wide Web Conference 2021 (WWW 2021). - New York, NY, USA : Association for Computing Machinery (ACM). - 9781450383127 ; , s. 1598-1608
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Konferensbidrag (refereegranskat)abstract
- An indispensable component in task-oriented dialogue systems is the dialogue state tracker, which keeps track of users' intentions in the course of conversation. The typical approach towards this goal is to fill in multiple pre-defined slots that are essential to complete the task. Although various dialogue state tracking methods have been proposed in recent years, most of them predict the value of each slot separately and fail to consider the correlations among slots. In this paper, we propose a slot self-attention mechanism that can learn the slot correlations automatically. Specifically, a slot-token attention is first utilized to obtain slot-specific features from the dialogue context. Then a stacked slot self-attention is applied on these features to learn the correlations among slots. We conduct comprehensive experiments on two multi-domain task-oriented dialogue datasets, including MultiWOZ 2.0 and MultiWOZ 2.1. The experimental results demonstrate that our approach achieves state-of-the-art performance on both datasets, verifying the necessity and effectiveness of taking slot correlations into consideration.
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| 10. |
- Zhang, Cunzhi, et al.
(författare)
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Eco-Friendly Bioinspired Interface Design for High-Performance Cellulose Nanofibril/Carbon Nanotube Nanocomposites
- 2020
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:49, s. 55527-55535
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Tidskriftsartikel (refereegranskat)abstract
- Inspired by a wood-like multicomponent structure, an interface-reinforced method was developed to fabricate high-performance cellulose nanofibril (CNF)/carbon nanotube (CNT) nanocomposites. Holocellulose nanofibrils (HCNFs) with core-shell structure were first obtained from bagasse via mild delignification and mechanical defibration process. The well-preserved native hemicellulose as the amphiphilic shell of HCNFs could act as a binding agent, sizing agent, and even dispersing agent between HCNFs and CNTs. Remarkably, both the tensile strength at high relative humidity (83% RH) and electrical conductivity of the HCNF/CNT nanocomposites were significantly improved up to 121 MPa and 321 S/m, respectively, demonstrating great superiority compared to normal CNF/CNT composite films. Furthermore, these HCNF/CNT composites with outstanding integrated performances exhibited great potential in the field of flexible liquid sensing.
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| 11. |
- Zhong, Shenghui, et al.
(författare)
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Combustion characteristics of n-heptane spray combustion in a low temperature reform gas/air environment
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 293
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case.
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| 12. |
- Zhong, Shenghui, et al.
(författare)
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Cool flame wave propagation in high-pressure spray flames
- 2023
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 39:2, s. 2513-2522
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Tidskriftsartikel (refereegranskat)abstract
- Cool flame has an important impact on the performance and emission of advanced low-temperature combustion (LTC) internal combustion engines (ICEs) in which the liquid fuel is injected earlier at a lower ambient temperature (Tam) than that in the conventional diesel combustion engines. However, the cool flame characteristics of spray combustion under ICEs conditions are not fully understood, e.g., the effect of cool flame on the spray ignition and flame stabilization is not well studied. In this paper, the so-called cool flame wave propagation (CFWP) in the Engine Combustion Networks Spray A flames at three ambient temperatures Tam (800 K, 900 K and 1000 K) is analyzed using the data from large eddy simulation with an improved Eulerian based transported probability density function sub-grid combustion model. A good agreement between the LES results and experimental data is obtained for the spray liquid penetration length, vapor fuel penetration length, mean pressure rise profile, and flame liftoff length. It is shown that CFWP in the spray ignition process promotes the ignition of the fuel-rich and cold reactant mixture, leading to the most reactive mixture shifting toward fuel-richer locations due to the spatial stratification of temperature and concentration, and turbulent mixing. As a result, the high temperature ignition (HTI) can be shortened compared to the ignition in the homogeneous mixture. At Tam = 800 K, the HTI kernels are consistently formed following the CFWP propagating toward the spray head region. However, as Tam increases, the spatial correlation between HTI kernels and CFWP fronts is weakened. On the other hand, the turbulent mixing (quantified using local scalar dissipation rate) contributes more to the formation of HTI kernels at higher Tam. The present results indicate that CFWP is more profound at lower Tam in the spray ignition process. Finally, it is found that cool flame propagates mainly into pre-reacted fuel-rich mixture in an ignition wave propagation mode from the spray upstream region toward downstream region, whereas the ignition assisted flame mode is found in the spray upstream region where the combustion heat release is negligible.
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| 13. |
- Zhong, Shenghui, et al.
(författare)
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Large eddy simulation of n-heptane/syngas pilot ignition spray combustion : Ignition process, liftoff evolution and pollutant emissions
- 2021
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Ingår i: Energy. - : Elsevier BV. - 0360-5442. ; 233
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Tidskriftsartikel (refereegranskat)abstract
- The utilization of syngas in the internal combustion engine is one way to reach low carbon emission engine. The n-heptane/syngas pilot ignition spray combustion is simulated here by high fidelity model with different syngas compositions. It is found that the ambient syngas suppresses the ignition by diluting the ambient oxidizer (non-chemical effect) and by affecting the chemical reactions (chemical effect). The consumption of OH radicals through H2+OH = H + H2O and CO + OH = CO2+H in low temperature combustion (LTC) stage is shown to be the main reason for the suppression of ignition. Cool flame propagation into the rich mixture in the mixing layer of n-heptane jet is observed during the transition process from LTC to high temperature combustion (HTC). CO is found to assist the transition to HTC through CO + HO2 = CO2+OH. On the contrary, H2 slows down the cool flame propagation and narrows down the cool flame flammability range, which retards the onset of HTC. The effects of syngas compositions on the flame structure and emission formation are discussed in detail. Due to the upstream auto-ignition, a cool/diffusion/hot flame structure is identified at the liftoff position in dual-fuel case, which drastically changes the flame structure. And more soot formation in early stage is found in dual-fuel cases due to the naturally richer mixture.
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| 14. |
- Zhong, Shenghui, et al.
(författare)
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Structure and propagation of n-heptane/air premixed flame in low temperature ignition regime
- 2020
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 275
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a large eddy simulation of n-heptane/air turbulent premixed combustion in a reactor assisted turbulent slot (RATS) burner under different preheating conditions. N-heptane/air mixture at an equivalence ratio of 0.6, pressure of 1 atm and temperature of 600, 650 and 700 K is considered to investigate the effect of low temperature chemistry on turbulent burning velocities and flame regimes, including chemically frozen (CF) regime where the fuel/air mixture inside the burner is chemically frozen, low temperature ignition (LTI) regime where the fuel/air mixture inside the burner undergoes LTI reactions, and transition regime from CF to LTI. The results show that the flame in the LTI regime exhibits the highest turbulent burning velocity. Differential diffusion is found to play an important role in the LTI regime whereas it is less important in the CF regime. To investigate the effect of LTI reactions on the flame, a series of two-dimensional laminar flames are simulated, in which the effect of turbulence on the flames is eliminated. The results show that in the LTI regime, the laminar burning velocity is drastically enhanced and the heat release zone is broadened. Budget term analysis shows that the enhanced rate of production and diffusion towards the preheat zone of the flames and the smaller gradient of reactant mass fraction are the main reasons behind the increased laminar burning velocity in the LTI regime.
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