| 1. |
- Xu, Zhen, et al.
(författare)
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Computational optimization of fuel supply, syngas composition, and intake conditions for a syngas/diesel RCCI engine
- 2018
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 234, s. 120-134
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Tidskriftsartikel (refereegranskat)abstract
- By utilizing the promising alternative fuel of syngas, and the syngas/diesel dual-fuel reactivity controlled compression ignition (RCCI) is a potential combustion strategy for internal combustion engines. However, the optimal operating parameters for syngas/diesel RCCI engines under wide operating conditions have not been investigated. In this study, the operating parameters include fuel supply, syngas composition, and intake conditions of a syngas/diesel RCCI engine were optimized under wide load by integrating the KIVA-3V code and the non-dominated sort genetic algorithm II (NSGA-II). The results indicated that nitrogen oxides (NOx) emissions can be controlled in considerably low levels, and the efficient combustion of the premixed syngas in the squish region can be realized with high premix ratio and early pilot injection of diesel. Equivalent indicated specific fuel consumption (EISFC) and ringing intensity (RI) are the major issues for the optimization at low and mid load, respectively. The double diesel injection strategy with the relatively late main injection timing is an effective way to both improve combustion efficiency at the low load and reduce RI at the mid load. For the double diesel injection, the ratio of pilot injection is controlled in a narrow range to provide sufficient high reactivity fuel in the piston bowl and to avoid the local high-temperature combustion region simultaneously. With the restrictions of EISFC and RI, the optimal H2 fraction in the syngas is 60–80%. Based on the optimal fuel supply and intake conditions, a syngas with 75% H2 and the diluent factor C of 0.8 is capable of realizing the high efficiency, moderate combustion, and low emissions for the RCCI engine at full load range.
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| 2. |
- Chen, Yuqian, et al.
(författare)
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Experimental and numerical study of flow and ignition and lean blowout characteristics of jet-cooled wall flameholder in a dual-mode combustor
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 122
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Tidskriftsartikel (refereegranskat)abstract
- The wall flameholder is one of the credible alternatives to realize pilot ignition in augmented/ramjet combustors. To overcome the ablation for a long-term operation, two types of jet cooling, external-inhaled air and pressure-driven jet cooling, are proposed for the wall flameholder. In this work, the flow and combustion process in a laboratory scale rig is studied for different cooling schemes and cooling conditions using experimental and numerical methods. Flow analysis in pressure-driven jet cooling scheme shows that the flow field of flameholder is influenced significantly by the cooling hole angle on the oblique plate α and on the rear plate β. In particular, the cooling jet angle combinations (α=30o, β=30o) and (α=90o, β=150o) are the two schemes with the most different characteristics. To investigate the effects of jet cooling type and cooling jet angle on the flow, ignition, and lean blowout (LBO) characteristics, the two distinctively different angle combinations are applied to form two kinds of jet cooling schemes mentioned above. Results suggest that the jet cooling type has less impact on the flow field but more influence on the flow loss than the cooling jet angle. The ignition performance of cooling schemes with α=30o and β=30o is better than that of those with α=90o and β=150o, but it has a more significant flow loss. The LBO limits of external-inhaled air cooling are lower than that of pressure-driven jet cooling. Moreover, the ignition and LBO limits decrease gradually with the increased mainstream temperature and they are only slightly affected by the mainstream velocity. Notably, the pressure-driven jet cooling scheme can slightly reduce the flow loss but it leads to a deteriorated ignition and LBO performance. The external-inhaled air cooling scheme with α=30o and β=30o has an excellent ignition and LBO performance, and the ignition and LBO limits increase with the increasing cooling air flow rate and the decreasing cooling air temperature.
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| 3. |
- Chen, Yuqian, et al.
(författare)
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Experimental study on combustion and flow resistance characteristics of an afterburner with air-cooled bluff-body flameholder
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- In the afterburner assembled with an air-cooled bluff-body flameholder, cooling air is directly injected into the recirculation zone behind the bluff-body, which can reduce the local temperature and increase the oxygen concentration of the gas mixture in the wake of the bluff-body, thereby affecting the total pressure loss and combustion characteristics. To better understand the flow and combustion process of the system, the exhaust gas temperature, cold and hot total pressure losses in a rectangular premixed combustor are investigated under different cooling air jet conditions. Experimental results show that the added cooling air could improve the combustion efficiency and widen the blowout limit, whereas it could also give rise to an extra total pressure loss. However, when the cooling air flow rate was higher than a critical value, i.e., after the blowing ratio reached 2.5, the recirculation zone could be blown away, resulting in a failed ignition in the afterburner. Notably, the decreased temperature difference between the mainstream and the cooling air could improve the combustion efficiency and reduce the thermal resistance loss but enlarge the cold flow loss and hot total pressure loss. Moreover, since the oxygen content declined and autoignition appeared after the mainstream temperature reached 1100 K, the exhaust gas temperature and combustion efficiency declined rapidly, and the hot total pressure loss also decreased. In addition, with the fuel-gas ratio increasing, the combustion efficiency significantly dropped, the exhaust gas temperature and thermal resistance loss firstly increased to a peak value (at the equivalence ratio of 1.14) and then decreased for excessively fuel-rich combustion.
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| 4. |
- Huang, Jianqing, et al.
(författare)
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A detailed study on the micro-explosion of burning iron particles in hot oxidizing environments
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 238
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Tidskriftsartikel (refereegranskat)abstract
- As a promising carbon-free fuel, iron powder can directly combust with air and has great potential to provide clean and high-grad heat for various applications. The combustion characteristics of iron particles are of great significance for developing iron combustion model, designing efficient combustor, and optimizing combustion technologies. In this work, the micro-explosion behavior of burning iron particles was experimentally investigated based on optical diagnostics. With two high-speed cameras operating at 10,000 frames per second, the three-dimensional (3D) motion and mean surface temperature of burning iron particles during the micro-explosion process were measured using the stereo imaging technique and two-color pyrometry, respectively. The probability of micro-explosions in different oxidizing environments were statistically studied. Three distinct micro-explosion modes have been observed. The results showed that the micro-explosion of burning iron particles heavily depended on oxygen concentration. The micro-explosion would slightly reduce the particle surface temperature by 30–70 K within 0.5 ms, since a lot of smaller fragments were produced. In addition, the 3D velocity of most fragments would sharply increase to 2–6 times within 0.2 ms after the micro-explosion occurred. Regarding the mechanism of the micro-explosion, three types of potential gas sources inside the particle were discussed. The sharp gradients of gas temperature and oxygen concentration may facilitate the rapid increase of the internal pressure in the particle, which eventually causes the micro-explosion.
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| 5. |
- Lozano, Rafael, et al.
(författare)
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Measuring progress from 1990 to 2017 and projecting attainment to 2030 of the health-related Sustainable Development Goals for 195 countries and territories: a systematic analysis for the Global Burden of Disease Study 2017
- 2018
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Ingår i: The Lancet. - : Elsevier. - 1474-547X .- 0140-6736. ; 392:10159, s. 2091-2138
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Tidskriftsartikel (refereegranskat)abstract
- Background: Efforts to establish the 2015 baseline and monitor early implementation of the UN Sustainable Development Goals (SDGs) highlight both great potential for and threats to improving health by 2030. To fully deliver on the SDG aim of “leaving no one behind”, it is increasingly important to examine the health-related SDGs beyond national-level estimates. As part of the Global Burden of Diseases, Injuries, and Risk Factors Study 2017 (GBD 2017), we measured progress on 41 of 52 health-related SDG indicators and estimated the health-related SDG index for 195 countries and territories for the period 1990–2017, projected indicators to 2030, and analysed global attainment. Methods: We measured progress on 41 health-related SDG indicators from 1990 to 2017, an increase of four indicators since GBD 2016 (new indicators were health worker density, sexual violence by non-intimate partners, population census status, and prevalence of physical and sexual violence [reported separately]). We also improved the measurement of several previously reported indicators. We constructed national-level estimates and, for a subset of health-related SDGs, examined indicator-level differences by sex and Socio-demographic Index (SDI) quintile. We also did subnational assessments of performance for selected countries. To construct the health-related SDG index, we transformed the value for each indicator on a scale of 0–100, with 0 as the 2·5th percentile and 100 as the 97·5th percentile of 1000 draws calculated from 1990 to 2030, and took the geometric mean of the scaled indicators by target. To generate projections through 2030, we used a forecasting framework that drew estimates from the broader GBD study and used weighted averages of indicator-specific and country-specific annualised rates of change from 1990 to 2017 to inform future estimates. We assessed attainment of indicators with defined targets in two ways: first, using mean values projected for 2030, and then using the probability of attainment in 2030 calculated from 1000 draws. We also did a global attainment analysis of the feasibility of attaining SDG targets on the basis of past trends. Using 2015 global averages of indicators with defined SDG targets, we calculated the global annualised rates of change required from 2015 to 2030 to meet these targets, and then identified in what percentiles the required global annualised rates of change fell in the distribution of country-level rates of change from 1990 to 2015. We took the mean of these global percentile values across indicators and applied the past rate of change at this mean global percentile to all health-related SDG indicators, irrespective of target definition, to estimate the equivalent 2030 global average value and percentage change from 2015 to 2030 for each indicator. Findings: The global median health-related SDG index in 2017 was 59·4 (IQR 35·4–67·3), ranging from a low of 11·6 (95% uncertainty interval 9·6–14·0) to a high of 84·9 (83·1–86·7). SDG index values in countries assessed at the subnational level varied substantially, particularly in China and India, although scores in Japan and the UK were more homogeneous. Indicators also varied by SDI quintile and sex, with males having worse outcomes than females for non-communicable disease (NCD) mortality, alcohol use, and smoking, among others. Most countries were projected to have a higher health-related SDG index in 2030 than in 2017, while country-level probabilities of attainment by 2030 varied widely by indicator. Under-5 mortality, neonatal mortality, maternal mortality ratio, and malaria indicators had the most countries with at least 95% probability of target attainment. Other indicators, including NCD mortality and suicide mortality, had no countries projected to meet corresponding SDG targets on the basis of projected mean values for 2030 but showed some probability of attainment by 2030. For some indicators, including child malnutrition, several infectious diseases, and most violence measures, the annualised rates of change required to meet SDG targets far exceeded the pace of progress achieved by any country in the recent past. We found that applying the mean global annualised rate of change to indicators without defined targets would equate to about 19% and 22% reductions in global smoking and alcohol consumption, respectively; a 47% decline in adolescent birth rates; and a more than 85% increase in health worker density per 1000 population by 2030. Interpretation: The GBD study offers a unique, robust platform for monitoring the health-related SDGs across demographic and geographic dimensions. Our findings underscore the importance of increased collection and analysis of disaggregated data and highlight where more deliberate design or targeting of interventions could accelerate progress in attaining the SDGs. Current projections show that many health-related SDG indicators, NCDs, NCD-related risks, and violence-related indicators will require a concerted shift away from what might have driven past gains—curative interventions in the case of NCDs—towards multisectoral, prevention-oriented policy action and investments to achieve SDG aims. Notably, several targets, if they are to be met by 2030, demand a pace of progress that no country has achieved in the recent past. The future is fundamentally uncertain, and no model can fully predict what breakthroughs or events might alter the course of the SDGs. What is clear is that our actions—or inaction—today will ultimately dictate how close the world, collectively, can get to leaving no one behind by 2030.
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| 6. |
- Xu, Leilei, et al.
(författare)
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Large eddy simulation of spray and combustion characteristics of biodiesel and biodiesel/butanol blend fuels in internal combustion engines
- 2023
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Ingår i: Applications in Energy and Combustion Science. - 2666-352X. ; 16
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Tidskriftsartikel (refereegranskat)abstract
- Biofuel is a crucial renewable and environmentally friendly energy source for addressing greenhouse gas emissions and other energy-related issues. Biodiesel and butanol, among alternative biofuels, possess complementary physical and chemical properties, offering multiple possibilities for their use in existing internal combustion engines. However, biodiesel's distinctly different physical and combustion properties from conventional diesel fuels make its combustion process substantially different. The complex composition of biodiesel presents significant challenges in accurately simulating its spray combustion characteristics. This paper presents a systematic evaluation of six single-component surrogate fuel models and a five-component model for the prediction of biodiesel spray characteristics under various conditions using large-eddy simulation (LES). The results show that single-component surrogate fuel models can only predict the gaseous penetration of biodiesel but not the liquid-phase penetration. A five-component fatty acid methyl ester surrogate fuel model is proposed, demonstrating an accurate simulation of biodiesel spray evaporation characteristics under different conditions. Based on the five-component evaporation model, LES is utilized to examine three strategies of biodiesel/butanol-fueled internal combustion engines: direct injection of pure biodiesel in conventional diffusion-controlled combustion (CDC) engines, direct injection of biodiesel–butanol blend in CDC engines, and biodiesel/butanol reactivity-controlled compression ignition (RCCI) engines. The simulation results are validated against engine experiment results, showing that the five-component model can successfully predict spray and combustion characteristics in internal combustion engines. The RCCI concept can significantly reduce NOx emissions; however, CO and UHC emissions are higher than in the CDC engines due to incomplete combustion in the fuel-lean butanol/air mixture.
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| 7. |
- Xu, Leilei, et al.
(författare)
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Performance and emission characteristics of an ammonia/diesel dual-fuel marine engine
- 2023
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Ingår i: Renewable and Sustainable Energy Reviews. - 1364-0321. ; 185
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Tidskriftsartikel (refereegranskat)abstract
- This study explores the challenges and possibilities of using ammonia (NH3) as a carbon-free fuel for marine propulsion in a Wärtsilä ammonia/diesel dual-fuel engine using a reactivity-controlled compression ignition (RCCI) concept. The main issues in ammonia RCCI engines are high ammonia slip and high emissions of nitrogen oxides and nitrous oxide. A joint experimental and computational investigation was conducted to understand the combustion process, pollutant and greenhouse gas (GHG) formation mechanisms, and the effects of injector configuration and injection timing on engine performance. A comparative assessment between the ammonia/diesel RCCI engine and a baseline natural gas/diesel RCCI engine showed that, upon replacing premixed natural gas with ammonia and maintaining the same energy share of diesel (8.5% of the total energy from diesel), the engine yielded considerably poor combustion efficiency. Increasing diesel usage to 24% share of the total energy allowed a successful engine operation, cutting GHG by 70%. However, higher diesel usage increased CO and CO2 emissions. N2O emission was attributed to the slow premixed ammonia/air flame propagation and near-wall flame quenching. The primary sources of NO emissions in ammonia/diesel RCCI engines were identified as fuel-NOx from premixed ammonia oxidation and thermal NOx in the diesel flame region. A recommendation was put forth for enhancing the operating conditions and injection strategies of ammonia RCCI engines. The proposed operation and injection strategy results in a 45% reduction in CO emission, a 60% reduction in total GHG emissions, and, notably, an 89% decrease in CO2 emissions compared to the LNG/diesel engine.
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| 8. |
- Xu, Shijie, et al.
(författare)
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Phase change and combustion of iron particles in premixed CH4/O2/N2 flames
- 2024
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Ingår i: Combustion and Flame. - 0010-2180. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- Metal powder is a promising carbon-free and recyclable energy carrier. Direct combustion of the micron-sized iron particles involves complex physical and chemical processes, such as heat transfer, surface reaction, and phase change. In this work, computational modelling of these processes is investigated and validated against experiments. A single iron particle combustion and phase change model is proposed in an Eulerian–Lagrangian framework. The new phenomenological model considers five stages, i.e., solid phase oxidation, melting of iron oxides and raw iron, liquid phase oxidation, cooling of liquid iron oxides, and solidification of super-cooled liquid iron oxides. The proposed model is first validated and then adopted in simulations of micron-sized iron particle combustion in premixed CH4/O2/N2 flames to study the effects of ambient temperature and oxygen concentration on single iron combustion. Results show that the new model is capable of replicating the melting, heterogeneous surface reaction, cooling, and solidification processes. Two-stage solidification is observed in experiments and modelled in simulations. This two-stage solidification includes a fast solidification with a significant temperature rise (∼150–200 K) and a thermal equilibrium solidification featuring a constant temperature and a slight particle radiant intensity decrease. In addition, a diffusion-controlled mechanism is identified during the melting process, in which the oxygen concentration dominates the melting time and the subsequent burning time. Furthermore, it is found that the reaction between iron and CH4/O2/N2 flame products, such as CO2 and H2O, plays a non-negligible role in the iron combustion process.
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| 9. |
- Yang, Miao, et al.
(författare)
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CFD Simulation of Biomass Combustion in an Industrial Circulating Fluidized Bed Furnace
- 2023
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Ingår i: Combustion Science and Technology. - 0010-2202. ; 195:14, s. 3310-3340
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Tidskriftsartikel (refereegranskat)abstract
- In this study, a three-dimensional computational fluid dynamics (CFD) model is employed to investigate the hydrodynamic and combustion characteristics of biomass particles in an industrial-scale circulating fluidized bed (CFB) furnace. The CFD model considered here is based on the Eulerian-Lagrangian framework, the multi-phase particle-in-cell (MP-PIC) collision model, the coarse grain method (CGM), and a recently developed distribution kernel method (DKM). The challenge of simulating industrial-scale CFB furnaces using CFD lies in the large number of particles in the system. MP-PIC and CGM showed that local particle overloading could occur, causing the numerical simulation to diverge. The combination of MP-PIC with CGM and DKM was shown to overcome this problem. The CFD predictions werecompared with onsite temperature experiments in the furnace, and the predicted furnace temperature agreed fairly well with the measured data. Using the CFD results, the study analyzed the transient solids mixing and fluidization characteristics, as well as the thermochemical process in biomass combustion. The simulated individual particle provided insight into the physical and chemical processes of the granular flow in the dilute/dense regions of the CFB furnace. The simulated results revealed the CO and NOx emission processes in the furnace.
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| 10. |
- Zhang, Yanzhi, et al.
(författare)
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Numerical study on liquid ammonia direct injection spray characteristics under engine-relevant conditions
- 2023
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 334
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on CFD modeling of the fuel/air mixing characteristics of liquid ammonia under direct injection engine-relevant conditions. Several questions are addressed including, whether the widely used Lagrange particle tracking (LPT) spray models developed for traditional fossil fuels are suitable for ammonia spray, whether certain improvement of the current models should be introduced, and under what conditions the improvement of models needs to be considered. It is found that liquid ammonia spray characteristics can be well reproduced by the current Lagrange-based spray models under non-flash boiling conditions. However, there are obvious gaps between measurements and predictions under strong flash boiling conditions. A strong flash boiling region is found near the nozzle while its intensity decreases downstream due to the significant cooling effect of ammonia spray, and initial superheat degree defined as the ratio of ambient pressure to the saturation vapor pressure at initial fuel temperature, can be adopted to determine the boundary of flash boiling model that needs to be employed in the modeling of liquid ammonia spray. Spray included angle is a crucial parameter within the LPT simulation framework to reproduce the collapse effect for multi-plume sprays. The flash model considering only the evaporation promotion effect cannot replicate the ammonia spray characteristics and it does not result in obvious differences compared with the results from the normal evaporation model. Liquid ammonia is a thermal sensitive fuel and has a strong tendency of flash boiling, and a more accurate flash boiling model that considers the thermal breakup effect should be proposed to accurately predict ammonia fuel/air mixing characteristics under wide engine-relevant conditions.
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