HIGH INJECTION PRESSURE DME IGNITION AND COMBUSTION PROCESSES: EXPERIMENT AND SIMULATION

With nearly smokeless combustion, Dimethyl Ether (DME) can be pressurized and used as a liquid fuel for compression-ignition (CI) combustion. However, due to its lower heating value and liquid density compared with diesel fuel, DME has a smaller energy content per unit volume. To obtain an equivalent energy content of diesel, approximately 1.86 times more quantity of DME is required. This can be addressed by a larger nozzle size or higher injection pressure. However, the effect of high injection pressure on DME spray combustion characteristics have not yet been well understood. In order to fill this gap, spray and combustion processes of DME were studied extensively via a series of experiments in a constant-volume and optically accessible combustion vessel. In the current study, a hydraulic electric unit injector (HEUI) with a 180 µm single-hole nozzle was driven by an oil-pressurized fuel injection (FI) system to achieve injection pressure of 1500 bar. The liquid and vapor regions of DME jet were visualized using a hybrid Schlieren/Mie scattering at non-reacting conditions. At reacting conditions, high-speed natural flame luminosity of DME combustion was used to capture the flame intensity, and planar laser-induced fluorescence (PLIF) imaging was used to characterize CH2O evolution. Spray and combustion characteristics of DME were compared with diesel in terms of rate of injection (ROI), liquid/vapor penetration and, ignition delay. Flame lift-off length (LOL), flame structure, and formaldehyde (CH2O) formation of DME were also studied through high-speed imaging. The RANS Converge CFD simulation was validated against the experimental and used as a powerful tool to explore the DME spray characteristics under various conditions. Further insights into DME spray and flame structure were obtained through experimentally validated Large Eddy Simulations (LES) simulations

Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

Objective of this work is the incorporation of the flame stretch effects in an Eulerian-Lagrangian model for premixed SI combustion in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. To this end, effects of energy transfer from electrical circuit and turbulent flame propagation were fully decoupled. The first ones are taken into account by Lagrangian particles whose main purpose is to generate an initial burned field in the computational domain. Turbulent flame development is instead considered only in the Eulerian gas phase for a better description of the local flow effects. To improve the model predictive capabilities, flame stretch effects were introduced in the turbulent combustion model by using formulations coming from the asymptotic theory and recently verified by means of DNS studies. Experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel were used to validate the proposed approach. In the vessel, a shrouded fan blows fresh mixture directly at the spark-gap generating highly inhomogeneous flow and turbulence conditions close to the ignition zone. Experimental and computed data of gas flow velocity profiles and flame radius were compared under different turbulence, air/fuel ratio and pressure conditions

Detection and characterization of coastal tidal wetland change in the northeastern US using Landsat time series

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yang, X., Zhu, Z., Qiu, S., Kroeger, K. D., Zhu, Z., & Covington, S. Detection and characterization of coastal tidal wetland change in the northeastern US using Landsat time series. Remote Sensing of Environment, 276, (2022): 113047, https://doi.org/10.1016/j.rse.2022.113047.Coastal tidal wetlands are highly altered ecosystems exposed to substantial risk due to widespread and frequent land-use change coupled with sea-level rise, leading to disrupted hydrologic and ecologic functions and ultimately, significant reduction in climate resiliency. Knowing where and when the changes have occurred, and the nature of those changes, is important for coastal communities and natural resource management. Large-scale mapping of coastal tidal wetland changes is extremely difficult due to their inherent dynamic nature. To bridge this gap, we developed an automated algorithm for DEtection and Characterization of cOastal tiDal wEtlands change (DECODE) using dense Landsat time series. DECODE consists of three elements, including spectral break detection, land cover classification and change characterization. DECODE assembles all available Landsat observations and introduces a water level regressor for each pixel to flag the spectral breaks and estimate harmonic time-series models for the divided temporal segments. Each temporal segment is classified (e.g., vegetated wetlands, open water, and others – including unvegetated areas and uplands) based on the phenological characteristics and the synthetic surface reflectance values calculated from the harmonic model coefficients, as well as a generic rule-based classification system. This harmonic model-based approach has the advantage of not needing the acquisition of satellite images at optimal conditions (i.e., low tide status) to avoid underestimating coastal vegetation caused by the tidal fluctuation. At the same time, DECODE can also characterize different kinds of changes including land cover change and condition change (i.e., land cover modification without conversion). We used DECODE to track status of coastal tidal wetlands in the northeastern United States from 1986 to 2020. The overall accuracy of land cover classification and change detection is approximately 95.8% and 99.8%, respectively. The vegetated wetlands and open water were mapped with user's accuracy of 94.6% and 99.0%, and producer's accuracy of 98.1% and 93.5%, respectively. The cover change and condition change were mapped with user's accuracy of 68.0% and 80.0%, and producer's accuracy of 80.5% and 97.1%, respectively. Approximately 3283 km2 of the coastal landscape within our study area in the northeastern United States changed at least once (12% of the study area), and condition changes were the dominant change type (84.3%). Vegetated coastal tidal wetland decreased consistently (~2.6 km2 per year) in the past 35 years, largely due to conversion to open water in the context of sea-level rise.This study was supported by USGS North Atlantic Coast Cooperative Ecosystem Studies Unit (CESU) Program for Detection and Characterization of Coastal Tidal Wetland Change (G19AC00354)

Ignition Process and Flame Lift-Off Characteristics of dimethyl ether (DME) Reacting Spray

Advanced combustion systems that utilize different combustion modes and alternative fuels have significantly improved combustion performance and emissions compared to conventional diesel or spark-ignited combustions. As an alternative fuel, dimethyl ether (DME) has been receiving much attention as it runs effectively under low-temperature combustion (LTC) modes such as homogeneous charge compression ignition (HCCI) and reactivity control combustion ignition (RCCI). Under compression-ignition (CI), DME can be injected as liquid fuel into a hot chamber, resulting in a diesel-like spray/combustion characteristic. With its high fuel reactivity and unique chemical formula, DME ignites easily but produces almost smokeless combustion. In the current study, DME spray combustion under several different conditions of ambient temperature (Tamb = 750–1100 K), ambient density (ρamb = 14.8–30 kg/m3), oxygen concentration (O2 = 15–21%), and injection pressure (Pinj = 75–150 MPa) were studied. The results from both experiments (constant-volume combustion vessel) and numerical simulations were used to develop empirical correlations for ignition and lift-off length. Compared to diesel, the established correlation of DME shows a similar Arrhenius-type expression. Sensitivity studies show that Tamb and Pinj have a stronger effect on DME\u27s ignition and combustion than other parameters. Finally, this study provides a simplified conceptual mechanism of DME reacting spray under high reactivity ambient (high Tamb, high O2) and LTC conditions. Finally, this paper discusses engine operating strategies using a non-conventional fuel such as DME with different reactivity and chemical properties

An Experimental Study of Diesel Spray Impingement on a Flat Plate: Effects of Injection Conditions

[EN] Advanced injection strategies for internal combustion engines have been extensively studied although there still exists a significant fundamental knowledge gap on the mechanism for high-pressure spray interaction with the piston surface and chamber wall in the internal combustion engine. The current study focuses on providing qualitative and quantitative information on spray-wall impingement and its characteristics by expanding the range of operating parameters under engine-like conditions. Parameters considered in the experiment are ambient gas and fuel injection conditions. The test included the non-vaporizing spray at the different ambient density (14.8, 22.8 and 30 kg/m3 ) and injection pressure (1200, 1500 and 1800 bar) with the isothermal condition (ambient, and plate temperatures of 423 K). The test was conducted in the constant-volume vessel with the 60-degree impinging spray angle relative to the plate. The free spray and impinged spray properties were qualitatively analysed based on Mie and schlieren images. The results showed that the lower ambient density and higher injection pressure tended to result in relatively higher impinged spray height. The expanding shape of the impinged spray on the wall showed the oval shape.This material is based upon work supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) and the Department of Defense, Tank and Automotive Research, Development, and Engineering Center (TARDEC), under Award Number DE‐EE0007292.Zhu, X.; Zhao, L.; Zhao, Z.; Ahuja, N.; Naber, J.; Lee, S. (2017). An Experimental Study of Diesel Spray Impingement on a Flat Plate: Effects of Injection Conditions. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 208-215. https://doi.org/10.4995/ILASS2017.2017.4733OCS20821

Route Design Model of Feeder Bus Service for Urban Rail Transit Stations

As an important part of urban public transportation systems, the feeder bus fills a service gap left by rail transit, effectively extending the range of rail transit’s service and solving the problem of short-distance travel and interchanges. By defining the potential demand of feeder bus services and considering its relationship with the traffic demands of corresponding staging areas, the distance between road and rail transit, and the repetition factor of road bus lines, this paper established a potential demand model of roads by opening feeder bus services and applying a logit model for passenger flow distribution. Based on a circular route model, a route starting and ending at urban rail transit stations was generated, and a genetic algorithm was then applied to solve it. The Wei-Fang community of Shanghai was selected as the test area. Per the model and algorithm, the feeder route length was conformed to a functional orientation of short-distance travel and the feeder service of a feeder bus; the route mostly covered where conventional bus lines were fewer, which is a finding that is in agreement with the actual situation; the feasibility of the model and algorithm was verified

Optimization of urban mini-bus stop spacing: a case study of Shanghai (China)

U posljednjih se nekoliko godina služba mini-autobusa brzo razvija, a odgovarajući razmak između stajališta značajno poboljšava učinkovitost putovanja stanovnika. Kako bi mini-bus stajališta bila praktično locirana, u ovom se radu najprije predlaže matematički model temeljen na Voronoi dijagramima u svrhu smanjenja ukupnog vremena putovanja putnika. Za rješenje tog modela primijenjena je poboljšana tehnika Wilson-Han-Powell sekvencionalno kvadratno programiranje (SQP). Na temelju geografskih informacijskih sustava (GIS) korišten je Kartezijanski koordinatni sustav za izračunavanje pješačke udaljenosti između mini-bus stanica i početnih ili odredišta, koja se ne mogu izravno izmjeriti. Drugo, u skladu sa stvarnim stanjem, u radu se predlažu metode modifikacije za podešavanje stajališta mini-autobusa. Valjanost i korisnost metodologije ispitana je njenom primjenom na stvarnom slučaju u Šangaju (Kina). Rezultati proračuna pokazuje da se modelom dobro rješava problem određivanja razmaka između stajališta mini autobusa.In recent years, the mini-bus service has gained a rapid development, while the reasonable stop spacing is significant to improve residents’ travel efficiency. In order to locate the mini-bus stops practically, this paper firstly proposes a mathematical model based on Voronoi Diagrams to minimize residents’ total travel time. Improved Wilson-Han-Powell Sequential Quadratic Programming (SQP) technique is employed to solve the model. Based on Geographic Information Systems (GIS), the Cartesian coordinate system is used to calculate the walking distance between mini-bus stops and origins or destinations, which cannot be directly measured. Secondly, based on the actual situation, this paper proposes modification methods for adjusting mini-bus stops. The validity and usefulness of the methodology is tested through applying it to a real case in Shanghai (China). The calculation results suggest that the model deals well with the mini-bus stop spacing problem

Sustainable Layout Planning Methods for Taxi Service Centres: A Case Study of Jiangning District, Nanjing

Taxi fleet size has undergone a continuous rapid growth in Chinese cities, while the service facility layout planning has relatively lagged behind, causing a decrease in the service quality. Therefore, it is crucial to carry out taxi service centre layout planning. This study analyses the functional orientation and hierarchical layout mode of taxi service centres, and then proposes stepwise layout planning methods combining existing transportation facilities. Considering the demand for integrated service centres is relatively low and convenient service centres are highly flexible, their layout can be determined by qualitative analysis. The constrained layout model of functional service centres is established by optimizing the layout scheme among various alternative taxi service centres. The study finds the model is a mixed 0–1 integer programming problem, genetic algorithms are applied to solve it. Finally, the proposed methods are applied to a real case of Jiangning district, Nanjing. The results show that one integrated service centre, four functional service centres, and twenty-three convenient service centres could satisfy taxi drivers’ demand, which suggests that the proposed methods should effectively address the layout planning problem of taxi service centres, thus have important implications for other cities’ downtown areas in China and beyond

Operating Characteristics of Dockless Bike-Sharing Systems near Metro Stations: Case Study in Nanjing City, China

With the growth of dockless bike-sharing (DLBS) systems, the first-and-last mile connection to public transport, such as metro and light railway stations, could be improved. DLBS systems complete the trip chain by connecting metro stations with points of interest and enhance the sustainability of urban transportation. Therefore, it is necessary to understand the trans-shipment characteristics of DLBS systems for metro stations. In this study, we collected data from the Mobike DLBS system in Nanjing City, China and applied K-means clustering to analyse the activity patterns of DLBS systems near local metro stations. Metro stations were categorised into five types on workdays and three types on weekends. An analysis of the relationships between activity patterns and spatial distribution characteristics demonstrated that the distribution of clusters possesses a strong connection with the surrounding environment. Low land development rates and a sparse distribution of metro stations cause a large range of influences. This research has direct implications for understanding the operating state of DLBS systems near metro stations and promoting the proper management of DLBS systems

Evaluation Method for Node Importance of Urban Rail Network Considering Traffic Characteristics

As a sustainable means of public transport, the safety of the urban rail transit is a significant section of public safety and is highly important in urban sustainable development. Research on the importance of urban rail stations plays an important role in improving the reliability of urban rail networks. This paper proposed an improved method for evaluating the importance of urban rail stations in a topology network, which was used to identify the key stations that affect the urban rail network performance. This method was based on complex network theory, considering the traffic characteristics of the urban rail network that runs on specific lines and integrating the structural characteristics and interrelationship of the lines where the stations are located. Hereafter, this method will be abbreviated as CLI. In order to verify that the high importance stations evaluated by this method were the key stations that had a great impact on the urban rail network performance, this paper designed a comparative attack experiment of betweenness centrality and CLI. The experiment was carried out by taking the Suzhou Rail Transit (SZRT) network as an example and the largest connected subgraph as well as the network efficiency as indicators to measure the network performance. The results showed that CLI had a greater impact on network performance and could better evaluate the key stations in the urban rail network than node degree and betweenness centrality