13 research outputs found
Characterization and Chemical Kinetic Analysis of the Ignition of Representative Conventional and Bio-Derived Fuels
Fossil fuels are the main energy source in the world. However, they are responsible for negative environmental impacts, such as global climate change and rising sea levels. Biofuels are an environmentally friendly alternative which can substitute fossil fuels without major engine modications, especially in the transportation sector. Furans, a class of biofuels, are considered as possible alternative fuels for SI engines. They can be produced from sugars, derived from non-food biomass sources. This thesis is a contribution to fundamental characterization of their combustion properties.
Reactivity trends in furan combustion are established through ignition delay measurements of selected furans; 2,5-dimethyl furan (DMF), 2-methyl furan (2-MF), and furan. The isomer effect on the ignition of alkylated furans is also investigated to understand the general trends between dimethyl and ethyl isomers of cyclic fuel components. Since near term use of biofuels involves blends with fossil fuels, the relative ignition behavior of the least reactive furan, DMF, the gasoline surrogate, iso-octane, and their blends, is investigated. Experiments are carried out in a shock tube, a reactor that can generate instantaneous high temperature and pressure conditions by means of reflected shock wave, leading to chemical reactions and subsequent ignition of a test mixture of fuel and oxidizer.
Experimental results are compared with chemical kinetic model simulations and the models are analyzed to gain insight on leading chemical pathways. The experimental results for furans and iso-octane are compared to the most recent chemical kinetic models of each fuel and a combined DMF/iso-octane model is developed for the analysis of fuel blend combustion. The new blend model is used to clarify the chemical interactions during ignition of fuel blends.
The thesis also considers the ignition of saturated furans. In this respect the ignition behavior of tetrahydrofuran (THF) and methyl tetrahydrofuran (MTHF) is investigated to establish relative reactivity trends. The results are put into context by comparing with the unsaturated furan, 2-MF.
Cyclic fuel components of non-biofuel nature are considered. The high-temperature autoignition delay times of dimethyl and ethyl isomers of cyclohexane are carried out behind reflected shock waves to establish reactivity dierences between these dimethyl and ethyl isomers, which could further be explored in chemical kinetic modeling. The study is designed to test whether the observed trend is indicative of general reactivity differences between dimethyl and ethyl isomers of cyclic hydrocarbons, oxygenated or non-oxygenated. The ignition delay times of ECH are compared to model predictions to test the model performance. The pronounced dierences in the high-temperature ignition delay times of these isomers are clearly established using the shock tube technique and motivate further mechanistic explorations of distinguishing reaction pathways, without necessarily invoking the more
complex low-temperature chemistry.
With regards to model reduction, the existing Alternate Species Elimination (ASE) model reduction method is employed for the reduction of recently reported iso-octane and n-heptane models. The ASE approach is expanded into a stochastic species sampling approach, referred to as the Stochastic Species Elimination (SSE) method. The SSE method allows for a linear reduction process, and involves new features leading to reduced computational resource requirements, compared to the standard ASE method. Larger systems, such as the recent literature model of n-octanol, are approached with the SSE method with multiple species sampling, which allows for a less time consuming model reduction process. Resulting skeletal models are shown to adequately predict ignition delay times as well as flame propagation, compared to the predictions of the detailed models.
The work advances understanding of biofuel combustion. The established reactivity trends between the various fuels investigated in this work is of great importance to transportation fuel technology. The resulting experimental data sets are expected to fill the gap in the understanding of furans and gasoline combustion. The combined DMF/iso-octane model is a main contribution that allows for better insight into the combustion chemistry of furans, iso-octane, and their blends. Further, the proposed SSE model reduction method contributes to the use of combustion chemistry in practical combustion analysis in the form of cost-effective reduced models
Development of Reduced Chemical Kinetic Models for the Numerical Simulation of Combustion and Emissions Behavior of Representative Conventional and Bio-derived Fuels
The study addresses two of the main challenges facing combustion modeling for transportation fuels: simultaneous simulation of non-related combustion problems and reducing the computational cost of the modeling process itself. To address the first challenge, researchers determine a characteristic flame time from thermal diffusivity and laminar burning velocity. Researchers examine parametric dependence of flame time and ignition delay time on pressure, temperature and equivalence ratio for methane, based on validated chemical kinetic mechanisms. The study reveals flame time and ignition delay time show similar temperature dependence, flame time has stronger dependence on equivalence ratio and weaker dependence on pressure than ignition delay time. The study also establishes a correlation to predict flame time, and subsequently, burning velocity, based on knowledge of the relevant auto ignition time. Differences between methane, propane and ethanol are also explored.
Researchers address the second challenge in a chemical kinetic modeling study of the high-temperature ignition behavior of Tetrahydrofuran (THF), a promising second-generation transportation biofuel. The study implements a Stochastic Species Elimination (SSE) reduction approach to develop multiple skeletal versions of a detailed chemical kinetic model of THF from the literature based on ignition delay time simulations at various pressures and temperatures. The developed skeletal versions are combined into a global skeletal model. The study uses ignition delay time simulations using detailed and skeletal models, with good agreement observed at higher temperatures. Next, researchers use sensitivity analysis to identify the most important reactions responsible for the performance of the skeletal model. Finally, they perform reaction rate parameter modification for such reactions in order to improve the agreement of detailed and reduced model predictions with literature experimental ignition data.
This work contributes toward improved understanding and modeling of the oxidation kinetics of conventional and bio-derived transportation biofuels, as well as the estimation of laminar burning velocity that can be encountered in turbulent combustion simulations. This would ultimately contribute into the design of cleaner and more efficient transportation systems, and support the testing and adoption of novel fuels as additives and/or replacement to conventional non-renewable fossil fuel
Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis
BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London
Surgical site infection after gastrointestinal surgery in high-income, middle-income, and low-income countries: a prospective, international, multicentre cohort study
Background: Surgical site infection (SSI) is one of the most common infections associated with health care, but its importance as a global health priority is not fully understood. We quantified the burden of SSI after gastrointestinal surgery in countries in all parts of the world.
Methods: This international, prospective, multicentre cohort study included consecutive patients undergoing elective or emergency gastrointestinal resection within 2-week time periods at any health-care facility in any country. Countries with participating centres were stratified into high-income, middle-income, and low-income groups according to the UN's Human Development Index (HDI). Data variables from the GlobalSurg 1 study and other studies that have been found to affect the likelihood of SSI were entered into risk adjustment models. The primary outcome measure was the 30-day SSI incidence (defined by US Centers for Disease Control and Prevention criteria for superficial and deep incisional SSI). Relationships with explanatory variables were examined using Bayesian multilevel logistic regression models. This trial is registered with ClinicalTrials.gov, number NCT02662231.
Findings: Between Jan 4, 2016, and July 31, 2016, 13 265 records were submitted for analysis. 12 539 patients from 343 hospitals in 66 countries were included. 7339 (58·5%) patient were from high-HDI countries (193 hospitals in 30 countries), 3918 (31·2%) patients were from middle-HDI countries (82 hospitals in 18 countries), and 1282 (10·2%) patients were from low-HDI countries (68 hospitals in 18 countries). In total, 1538 (12·3%) patients had SSI within 30 days of surgery. The incidence of SSI varied between countries with high (691 [9·4%] of 7339 patients), middle (549 [14·0%] of 3918 patients), and low (298 [23·2%] of 1282) HDI (p < 0·001). The highest SSI incidence in each HDI group was after dirty surgery (102 [17·8%] of 574 patients in high-HDI countries; 74 [31·4%] of 236 patients in middle-HDI countries; 72 [39·8%] of 181 patients in low-HDI countries). Following risk factor adjustment, patients in low-HDI countries were at greatest risk of SSI (adjusted odds ratio 1·60, 95% credible interval 1·05–2·37; p=0·030). 132 (21·6%) of 610 patients with an SSI and a microbiology culture result had an infection that was resistant to the prophylactic antibiotic used. Resistant infections were detected in 49 (16·6%) of 295 patients in high-HDI countries, in 37 (19·8%) of 187 patients in middle-HDI countries, and in 46 (35·9%) of 128 patients in low-HDI countries (p < 0·001).
Interpretation: Countries with a low HDI carry a disproportionately greater burden of SSI than countries with a middle or high HDI and might have higher rates of antibiotic resistance. In view of WHO recommendations on SSI prevention that highlight the absence of high-quality interventional research, urgent, pragmatic, randomised trials based in LMICs are needed to assess measures aiming to reduce this preventable complication
Recent Trends in the Production, Combustion and Modeling of Furan-Based Fuels
There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion
Reactivity Trends in Furan and Alkyl Furan Combustion
A systematic
study of the ignition behavior of furan and the substituted
furans 2-methyl furan (2-MF) and 2,5-dimethyl furan (DMF) is presented.
Ignition delay times are measured over a temperature range from 977
to 1570 K and pressures up to 12 atm for lean, stoichiometric, and
rich mixtures of fuel, oxygen, and argon. It is found that when the
equivalence ratio ϕ, the argon-to-oxygen ratio <i>D</i>, and pressure <i>p</i> are kept constant over a range
of temperatures <i>T</i>, DMF generally has the longest,
while 2-MF has the shortest, ignition delay times, and furan shows
intermediate reactivity. Ignition delay times decrease with increasing
equivalence ratios, except for DMF, which does not show a conclusive
trend over the temperature range investigated. The experimental data
are also found to agree with published ignition data, showing differences
in some cases partly related to disparities in endwall and sidewall
ignition measurements. The ignition delay times of 2-MF and DMF are
compared to predictions using furan chemical kinetic models by Sirjean
et al. and Somers et al. The models show qualitatively that DMF has longer ignition
delay times than 2-MF under similar conditions of ϕ, <i>D</i>, <i>p</i>, and <i>T</i>, as revealed
by the experiments. Quantitatively, the model predictions agree with
experimental data at conditions similar to those used in their development,
and deviations from experiment at other conditions are mostly related
to unmatched temperature sensitivities over a wider temperature range,
revealed by varying pressure and reduced dilution. The reported experimental
data set contributes toward further understanding and improved modeling
of the combustion of furans, a promising class of alternative fuels
Experimental and Chemical Kinetic Modeling Study of Dimethylcyclohexane Oxidation and Pyrolysis
A combined
experimental and chemical kinetic modeling study of
the high-temperature ignition and pyrolysis of 1,3-dimethylcyclohexane
(13DMCH) is presented. Ignition delay times are measured behind reflected
shock waves over a temperature range of 1049–1544 K and pressures
of 3.0–12 atm. Pyrolysis is investigated at average pressures
of 4.0 atm at temperatures of 1238, 1302, and 1406 K. By means of
mid-infrared direct laser absorption at 3.39 μm, fuel concentration
time histories are measured under ignition and pyrolytic conditions.
A detailed chemical kinetic model for 13DMCH combustion is developed.
Ignition measurements show that the ignition delay times of 13DMCH
are longer than those of its isomer, ethylcyclohexane. The proposed
chemical kinetic model predicts reasonably well the effects of equivalence
ratio and pressure, with overall good agreement between predicted
and measured ignition delay times, except at low dilution levels and
high pressures. Simulated fuel concentration profiles agree reasonably
well with the measured profiles, and both highlight the influence
of pyrolysis on the overall ignition kinetics at high temperatures.
Sensitivity and reaction pathway analyses provide further insight
into the kinetic processes controlling ignition and pyrolysis. The
work contributes toward improved understanding and modeling of the
oxidation and pyrolysis kinetics of cycloalkanes
Surgical site infection after gastrointestinal surgery in children : an international, multicentre, prospective cohort study
Introduction Surgical site infection (SSI) is one of the most common healthcare-associated infections (HAIs). However, there is a lack of data available about SSI in children worldwide, especially from low-income and middle-income countries. This study aimed to estimate the incidence of SSI in children and associations between SSI and morbidity across human development settings. Methods A multicentre, international, prospective, validated cohort study of children aged under 16 years undergoing clean-contaminated, contaminated or dirty gastrointestinal surgery. Any hospital in the world providing paediatric surgery was eligible to contribute data between January and July 2016. The primary outcome was the incidence of SSI by 30 days. Relationships between explanatory variables and SSI were examined using multilevel logistic regression. Countries were stratified into high development, middle development and low development groups using the United Nations Human Development Index (HDI). Results Of 1159 children across 181 hospitals in 51 countries, 523 (45 center dot 1%) children were from high HDI, 397 (34 center dot 2%) from middle HDI and 239 (20 center dot 6%) from low HDI countries. The 30-day SSI rate was 6.3% (33/523) in high HDI, 12 center dot 8% (51/397) in middle HDI and 24 center dot 7% (59/239) in low HDI countries. SSI was associated with higher incidence of 30-day mortality, intervention, organ-space infection and other HAIs, with the highest rates seen in low HDI countries. Median length of stay in patients who had an SSI was longer (7.0 days), compared with 3.0 days in patients who did not have an SSI. Use of laparoscopy was associated with significantly lower SSI rates, even after accounting for HDI. Conclusion The odds of SSI in children is nearly four times greater in low HDI compared with high HDI countries. Policies to reduce SSI should be prioritised as part of the wider global agenda.Peer reviewe
Exploring the cost-effectiveness of high versus low perioperative fraction of inspired oxygen in the prevention of surgical site infections among abdominal surgery patients in three low- and middle-income countries
Background: This study assessed the potential cost-effectiveness of high (80–100%) vs low (21–35%) fraction of inspired oxygen (FiO2) at preventing surgical site infections (SSIs) after abdominal surgery in Nigeria, India, and South Africa. Methods: Decision-analytic models were constructed using best available evidence sourced from unbundled data of an ongoing pilot trial assessing the effectiveness of high FiO2, published literature, and a cost survey in Nigeria, India, and South Africa. Effectiveness was measured as percentage of SSIs at 30 days after surgery, a healthcare perspective was adopted, and costs were reported in US dollars (216 compared with 6 (95% confidence interval [CI]: −1) difference in costs. In India, the average cost for high FiO2 was 195 for low FiO2 leading to a −15 to −1164 compared with 93 (95% CI: −65) difference in costs. The high FiO2 arm had few SSIs, 7.33% compared with 8.38% for low FiO2, leading to a −1.05 (95% CI: −1.14 to −0.90) percentage point reduction in SSIs. Conclusion: High FiO2 could be cost-effective at preventing SSIs in the three countries but further data from large clinical trials are required to confirm this