| 24331. |
- Li, Songyang, et al.
(författare)
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Theoretical and experimental analysis of ceiling-jet flow in corridor fires
- 2011
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Ingår i: Tunnelling and Underground Space Technology. - : Elsevier BV. - 1878-4364 .- 0886-7798. ; 26:6, s. 651-658
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Tidskriftsartikel (refereegranskat)abstract
- In tunnels or long corridors, the combustion products of the fire are confined to spread in one or two directions, forming a ceiling-jet flow. For safety assessment and emergency treatment, it is important to investigate and understand the behavior of the ceiling-jet flow. In this paper, a simple model has been presented, in terms of Richardson number and non-dimensional ceiling-jet thickness, to predict the temperature and the velocity of fire-induced ceiling-jet in a rectangular corridor. Besides, the location of hydraulic jump, occurring in ceiling-jet flow, has been estimated theoretically. In order to validate the theoretical predictions, a series of reduced-scale fire experiments were conducted in a 5 m long corridor. The predicted results, concerning non-dimensional excess temperature, agree favorably with experimental data in different fuels and heat release rates of the fire tests. Finally, the scaling issue has also been discussed and validated. (C) 2011 Elsevier Ltd. All rights reserved.
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| 24332. |
- Li, Shian, et al.
(författare)
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Three-dimensional modeling and investigation of high temperature proton exchange membrane fuel cells with metal foams as flow distributor
- 2017
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 42:44, s. 27323-27333
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Tidskriftsartikel (refereegranskat)abstract
- Flow field design of fuel cells has a significant impact on the distribution of the reactants over the active surface area. In this work, the cell performance and transport characteristics of high temperature proton exchange membrane fuel cells with metal foams as flow distributor were numerically investigated by a three-dimensional and non-isothermal model. Compared to the fuel cell with conventional straight flow channels, the cell performance is considerably improved by the application of a metal foam as flow distributor. The current density improvements at the operating cell voltages 0.6 V and 0.3 V are 4.96% and 6.45%, respectively. Moreover, the species, temperature and current density are uniformly distributed because the reactants can also flow through the area covered by the ribs in the conventional design. It is concluded that a metal foam can be used as the flow distributor due to its high electrical conductivities and low weight advantages compared to the conventional graphite and metallic bipolar plates. In addition, the effects of permeability, operating pressure, operating temperature, and stoichiometry ratio on the cell performance were investigated. The present results provided and improved the fundamental understanding of the application of metal foams as flow distributor and its effects on transport characteristics and cell performance.
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| 24333. |
- Li, Songyu, 1994-
(författare)
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Vision based perception systems for unmanned forestry machines
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- After years of continuous development in the past decades, the field of forest production has achieved a notable degree of mechanization. Nonetheless, the unceasing demand for increased productivity in forestry operations continues to drive the rapid expansion of forestry automation. Critical aspects of forest production, including activities such as harvesting, forwarding, seedling planting, and soil preparation, hold significant potential for improved automation. These advancements promise numerous advantages, such as heightened production efficiency, reduced environmental impact, reduced operational costs, and the creation of a more favourable work environment. Beyond fostering industrial progress, such innovations could also make a positive contribution to the sustainable development of forestry.Forestry operations take place in intricate natural forest environments, rendering the automation of these processes complex and challenging. To realize full automation of forest machines, it is imperative to equip them with environmental awareness and cognitive capabilities. This can be achieved by integrating advanced imaging sensors with sophisticated algorithms, thereby enabling forest machinery to possess a vision system. By utilizing both novel and existing solutions for object detection, positioning, categorization, and status analysis within the machine's surroundings, combined with intelligent decision-making and control mechanisms, forest machinery can attain a higher degree of operational automation.This thesis primarily focuses on the development, deployment, and validation of vision systems on an unmanned forestry machine platform, with a special emphasis on the prototype development of essential features that are still manually executed in contemporary forestry operations. These features include forest terrain obstacle detection, roundwood pose estimation, automated selection of seedling planting locations, and autonomous obstacle avoidance for mounders.In this thesis, vision systems rooted in color and stereo camera sensing are designed and deployed on an unmanned forest machinery platform. These systems encompass the following key functionalities:• A precise positioning system for detecting forest terrain obstacles, such as stones and stumps, using stereo camera data in conjunction with deep learning techniques.• Localization and pose estimation capabilities for ground logs, leveraging stereo cameras and deep learning.• An analysis of the planning area for obstacle detection, along with the extraction of feasible planting zones for establishing seedling planting locations.• The development of an automated obstacle avoidance system for forestry mounders, powered by visual solutions.Through rigorous testing in real-world scenarios spanning logging, loading, planting, and site preparation, this paper demonstrates the feasibility and practicality of enhancing the level of automation in forest machinery operations through the integration of vision systems. It culminates in the creation of tangible and efficient functional prototypes, leading towards a new era of automation in the field of forestry.
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| 24334. |
- Li, Shian, et al.
(författare)
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Wavy Surface Cathode Gas Flow Channel Effects on Transport Processes in a Proton Exchange Membrane Fuel Cell
- 2017
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Ingår i: Journal of Electrochemical Energy Conversion and Storage. - : ASME International. - 2381-6872 .- 2381-6910. ; 14:3
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Tidskriftsartikel (refereegranskat)abstract
- The flow field design of current collectors is a significant issue, which greatly affects the mass transport processes of reactants/products inside fuel cells. Especially for proton exchange membrane (PEM) fuel cells, an appropriate flow field design is very important due to the water balance problem. In this paper, a wavy surface is employed at the cathode flow channel to improve the oxygen mass transport process. The effects of wavy surface on transport processes are numerically investigated by using a three-dimensional anisotropic model including a water phase change model and a spherical agglomerate model. It is found that the wavy configurations enhance the oxygen transport and decrease the water saturation level. It is concluded that the predicted results and findings provide the guideline for the design and manufacture of fuel cells.
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| 24335. |
- Li, Tian, et al.
(författare)
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A fast-solving particle model for thermochemical conversion of biomass
- 2020
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 213, s. 117-131
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics (CFD) simulations of large-scale furnaces or reactors for thermal conversion of solid fuels remains challenging partially due to the high computational cost related to the particle sub-models. Owing to the thermally thick nature, it is particularly expensive to simulate the conversion of large fuel particles such as biomass particles. To address this issue, a fast-solving particle model was developed in this work with special attention to the computational efficiency. The model spatially discretizes a fuel particle in one homogenized dimension. The conversion process of the fuel particle is treated as a reactive variable-volume one-dimensional transient heat conduction problem. The model also utilizes several features that are typically found in sharp interphase-based models to reduce the computational cost. Validation of the model was carried out by comparing with experimental results under both pyrolysis and combustion conditions. The accuracy and computational efficiency of the model was thoroughly examined by varying the degrees of temporal and spatial discretization. It was found that the model well predicted pyrolysis and combustion of a single biomass particle within a broad range of temporal and spatial discretization. The time used to simulate the conversion of a biomass particle using the developed model can be more than one order of magnitude smaller than the conversion process itself. It was also revealed that a well-predicted conductive heat transfer inside the particle is essential for a precise simulation of the drying and devolatilization process. The char conversion process, however, is less sensitive to the external heat transfer as it is mainly controlled by the mass diffusion process. Further studies showed that a time step of 1×10−3 s and a spatial discretization of 20 cells were sufficient for simulating the conversion of typical fuel particles in grate-fired and fluidized-bed furnaces. We also demonstrated that when the particle model was implemented in a CFD solver, only 2.2% of computational overhead was introduced by the model. As the model can efficiently employ fixed time stepping, optimal load balancing during parallel computing of many simultaneous conversion processes becomes trivial. This performance opens up new possibilities for treating fuel polydispersity in Eulerian CFD simulations of biomass conversion.
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| 24336. |
- Li, Tianyou, 1997, et al.
(författare)
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Modeling collision avoidance maneuvers for micromobility vehicles
- 2023
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Ingår i: Journal of Safety Research. - 0022-4375. ; 87
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: In recent years, as novel micromobility vehicles (MMVs) have hit the market and rapidly gained popularity, new challenges in road safety have arisen, too. There is an urgent need for validated models that comprehensively describe the behaviour of such novel MMVs. This study aims to compare the longitudinal and lateral control of bicycles and e-scooters in a collision- avoidance scenario from a top-down perspective, and to propose appropriate quantitative models for parameterizing and predicting the trajectories of the avoidance—braking and steering— maneuvers. Method: We compared a large e-scooter and a light e-scooter with a bicycle (in assisted and non-assisted modes) in field trials to determine whether these new vehicles have different maneuverability constraints when avoiding a rear-end collision by braking and/or steering. Results: Braking performance in terms of deceleration and jerk varies among the different types of vehicles; specifically, e-scooters are not as effective at braking as bicycles, but the large e-scooter demonstrated better braking performance than the light one. No statistically significant difference was observed in the steering performance of the vehicles. Bicycles were perceived as more stable, maneuverable, and safe than e-scooters. The study also presents arctangent kinematic models for braking and steering, which demonstrate better accuracy and informativeness than linear models. Conclusions: This study demonstrates that the new micromobility solutions have some maneuverability characteristics which differ significantly from those of bicycles, and even within their own kind. Steering could be a more efficient collision- avoidance strategy for MMVs than braking under certain circumstances, such as in a rear-end collision. More complicated modelling for MMV kinematics can be beneficial but needs validation. Practical Applications: The proposed arctangent models could be used in new advanced driving assistance systems to prevent crashes between cars and MMV users. Micromobility safety could be improved by educating MMV riders to adapt their behavior accordingly. Further, knowledge about the differences in maneuverability between e-scooters and bicycles could inform infrastructure design, and traffic regulations.
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| 24337. |
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| 24338. |
- Li, Tianyou, 1997, et al.
(författare)
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Modelling Braking and Steering Avoidance Maneuvers for Micromobility
- 2023
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Ingår i: The Evolving scholar. - 2667-2812. ; 2023:5th edition
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Recent advancements in technology make it possible for advanced driving assistance systems (ADAS) to recognize micromobility vehicles (MMV) and include them in their threat assessment. However, today we lack the rider-vehicle models which are of great importance in understanding the interconnection between the MMV and its rider. These models may help ADAS predict micromobility kinematics and provide accurate threat assessments, especially when avoidance maneuvers from micromobility must be considered. In this study, we modelled avoidance maneuvers from micromobility vehicles to support ADAS threat assessment. We compared traditional bicycles (with and without assistance) with e-scooters (a small personal scooter and a large scooter) in a field test, where 36 participants avoided a stationary obstacle by either braking or steering. Kinematic data such as longitudinal and latera speed, acceleration, jerk and steering angle and rate were collected and analyzed.
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| 24339. |
- Li, Tianyou, 1997
(författare)
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Modelling E-scooterist Braking and Steering for Collision Avoidance
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Introduction: In the last few years, new road safety issues have emerged with the growing popularity of novel micromobility vehicles (MMVs). The term ‘novel MMV’ includes, in addition to electrically assisted bicycles, electric kick scooters (e-scooters) and Segway balance scooters (Segways)—which are usually distinct in appearance and operation from traditional MMVs (conventional bicycles). Developing validated models that offer a comprehensive understanding of the behavior of these new types of MMVs is crucial to inform the design of new, safer MMVs, active safety systems, infrastructure, and the making of regulations. This licentiate thesis includes two studies that shed light on how MMV riders perform longitudinal and lateral control of both traditional and novel MMVs at an operational and tactical level. Methods: Study 1 analyzed field test data and compared the longitudinal control (i.e., accelerating and braking) of bicycles, an electrically-assisted bicycles (e-bicycle/e-bike), a light personal electric kick scooters, and a Segway balance scooters. Study 2 conducted field tests and compared both the longitudinal and lateral (i.e., steering) maneuvers in a rear-end collision avoidance scenario. A larger e-scooter from the public sharing systems replaced the Segway in the test. Comparisons were made among different types of vehicles, maneuvers, and urgency levels (i.e., maneuver comfortably as daily riding, or harshly as avoiding close danger), to determine the extent to which these novel MMVs, compared to traditional bicycles, demonstrate constraints in maneuverability, safety, and comfort. Results: The results showed differences in longitudinal performance across vehicles, while no statistically significant differences were observed in lateral performance. Novel MMVs demonstrated poorer braking capability than bicycles and were perceived as less safe and maneuverable. Among the two e-scooters, the larger one could achieve shorter braking distances. Additionally, riders were able to improve their collision-avoidance performance by compromising comfort when urgency increased. Two kinematic models, linear and arctangent, were derived to predict MMV trajectories. Conclusion: Compared to bicycles, novel MMVs demonstrate statistically significantly poorer in braking performance, which usually constrains the rider’s collision avoidance capability. No statistically significant difference was observed in steering. The findings of this thesis have implications for improving road safety for MMVs by informing MMV design, infrastructure planning, policy-making, and consumer assessment programs. For example, EuroNCAP can incorporate the findings to design test scenarios for this new group of vulnerable road users (VRUs). They can also support the development of active safety systems and automated driving features for automobiles, enabling more accurate and acceptable system activations with respect to timing and magnitude in itneractions with MMVs.
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| 24340. |
- Li, Wei, et al.
(författare)
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Generalized stacking fault energies of alloys
- 2014
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 26:26, s. 265005-
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Tidskriftsartikel (refereegranskat)abstract
- The generalized stacking fault energy (gamma surface) provides fundamental physics for understanding the plastic deformation mechanisms. Using the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation, we calculate the. surface for the disordered Cu-Al, Cu-Zn, Cu-Ga, Cu-Ni, Pd-Ag and Pd-Au alloys. Studying the effect of segregation of the solute to the stacking fault planes shows that only the local chemical composition affects the. surface. The calculated alloying trends are discussed using the electronic band structure of the base and distorted alloys. Based on our. surface results, we demonstrate that the previous revealed 'universal scaling law' between the intrinsic energy barriers (IEBs) is well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning measure parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, Al, Zn and Ga increase the twinnability, while Ni decreases it. Aluminum and gallium yield similar effects on the twinnability.
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