Numerical investigation of the ignition delay time of a translucent solid at high radiant heat fluxes

A one-dimensional numerical model describing the physical and chemical phenomena occurring in a translucent solid fuel up to ignition is used to investigate the failure of the classical ignition theory at radiant heat fluxes above 70 kW/m². Comparison with a very large dataset of experimental measurements of time to piloted ignition for black PMMA (PolyMethylMethAcrylate) samples shows that model predictions agree well for heat fluxes from 20 to 200 kW/m². The only two available sets of experimental data for ignition at high heat fluxes for black-carbon coated and uncoated samples are used. Predictions of the transient temperature profiles inside the solid at different heat fluxes also agree well with measurements. Among all the mechanisms investigated, agreement with measurements at heat fluxes above 70 kW/m² is only possible when in-depth radiation absorption is included in the model. Observed behaviour at high heat fluxes cannot be explained by the reaction scheme, ignition criterion, temperature dependency of material properties, surface heat losses or radiation attenuation by pyrolyzates. The model is also used to show that the traditional coating of black carbon added on the sample does not cancel in-depth radiation absorption but its effect is to absorb at the surface around 35% of the incoming radiation. The work explains the failure of the classical ignition theory at high heat fluxes and it is the first time that the effect of black-carbon coating is explained and quantified.     

An experimental study on ignition of single coal particles at low oxygen concentrations

An experimental study on the ignition of single coal particles at low oxygen concentrations (X_{{\mathrm{O}}_2}<21%) was conducted using a tube furnace. The surface temperature (T_s) and the center temperature (T_c) of the coal particles were obtained from the images taken by an infrared camera and thermocouples respectively. The ignition processes were recorded by a high-speed camera at different X_{{\mathrm{O}}_2} values and furnace temperatures T_w. Compared with literature experimental data obtained at a high X_{{\mathrm{O}}_2} value, the ignition delay time t_i decreases more rapidly as X_{{\mathrm{O}}_2} increases at the low X_{{\mathrm{O}}_2} region. The responses of T_s and T_c to the variation of X_{{\mathrm{O}}_2} are different: T_s decreases while T_c remains nearly constant with increasing X_{{\mathrm{O}}_2} at a low X_{{\mathrm{O}}_2} value. In addition, t_i is less sensitive to T_w while the ignition temperature T_i is more sensitive to T_w at a low X_{{\mathrm{O}}_2} value than in air. Observations of the position of flame front evolution illustrate that the ignition of a coal particle may change from a homogeneous mode to a heterogeneous or combined ignition mode as X_{{\mathrm{O}}_2} decreases. At a low X_{{\mathrm{O}}_2} value, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process.     

Effect of ignition,initial pressure and temperature on the lower flammability limit of hydrogen/air mixture

In this research, the effect of ignition, initial pressure (50–250 kPa) and temperature (20–100 °C) on the lower flammability limit (LFL) of hydrogen/air mixture are investigated experimentally and numerically. The results show that with the ignition energy increases, the LFL of hydrogen decreases. When high voltage direct current power supply (HVDC) is used, the time for the flame to propagate to the edge of the window is much shorter than that of 15 kV high voltage transformer (15 kV HVT) ignition. As the initial pressure increases, the LFL of hydrogen increases. When HVDC is used, the time to reach the peak deflagration overpressure increases with the increase of initial pressure. However, when 15 kV HVT is used, the time to reach the peak deflagration overpressure is almost the same. As the initial temperature increases, the LFL of hydrogen decreases. The change of the LFL of hydrogen with 15 kV HVT ignition is greater than that of HVDC. Through the analysis of chemical kinetic factors, the effect of OH radical generation when the LFL of hydrogen increases with the increase of initial pressure is revealed.     

Three-dimensional modeling of pressure effect on operating characteristics and performance of solid oxide fuel cell

A three-dimensional (3-D) model for planar, anode-supported, solid oxide fuel cell (SOFC) is developed to investigate the effect of operating pressure on cell characteristics. The results show that the elevated operating pressure can improve cell performance by increasing open circuit voltage and reducing activation overpotential, and enhance the electrochemical reaction in the vicinity of electrolyte. Besides, the high pressure can also change the distributions of species and internal reforming reactions. Compared to the case using syngas as fuel, the operating pressure has more significant effects on temperature gradient along flow direction when partly pre-reformed gas is supplied. In addition, efficient control of cell temperature could be achieved by decreasing fuel utilization in the case of partly pre-reformed gas, but this is achieved at the expense of cell efficiency, especially under high pressure condition. Another way to reduce the temperature gradient is to adopt higher air ratio. Moreover, when partly pre-reformed gas is used, the counter-flow configuration has a better performance due to the higher overall temperature.     

Effect of distributions of fuel concentration and temperature on ignition processes in a diesel PCCI combustion

The distributions of fuel concentration and temperature have significant effect on the ignition processes of diesel premixed charge compression ignition (PCCI) combustion. It was found in this study that the ignition process of PCCI combustion organized by multi-pulse injection was strongly influenced by conditions of fuel stratification. The start of low temperature reactions occurred in the leaner area of the combustion chamber in the test engine because the temperature here first reached the point of low temperature reactions. Ignition always occurred in the position where the mixture featured with equivalence ratios close to the mean equivalence ratio of the overall mixture, while the neighboring area of the initial ignition area accumulate heat with a finite speed until finally autoigniting. Moreover, the appearance of highest combustion temperature occurred in the same area at the combustion chamber. For more homogeneous mixture, a higher amount of mixture reached ignition simultaneously, resulting in a larger initial ignition area and a higher temperature at the ignition area. Furthermore, V-type distribution of equivalence ratio was found to be beneficial to retarding high temperature reaction.     

The effect of tube internal geometry on the propensity to spontaneous ignition in pressurized hydrogen release

Spontaneous ignition of compressed hydrogen release through a length of tube with different internal geometries is numerically investigated using our previously developed model. Four types of internal geometries are considered: local contraction, local enlargement, abrupt contraction and abrupt enlargement. The presence of internal geometries was found to significantly increase the propensity to spontaneous ignition. Shock reflections from the surfaces of the internal geometries and the subsequent shock interactions further increase the temperature of the combustible mixture at the contact region. The presence of the internal geometry stimulates turbulence enhanced mixing between the shock-heated air and the escaping hydrogen, resulting in the formation of more flammable mixture. It was also found that forward-facing vertical planes are more likely to cause spontaneous ignition by producing the highest heating to the flammable mixture than backward-facing vertical planes.     

Theoretical analysis of the characteristics of the solid oxide fuel cells with a bi-layer electrolyte

From the analytical model derived earlier [17], analytical expressions for the relative thickness ratio r_s of a bi-layer electrolyte and the maximum power density of a fuel cell are developed. Using these expressions, together with the other relationships from the analytical model, the characteristics of solid oxide fuel cells (SOFCs) with a bi-layer electrolyte are analyzed and theoretical analysis of the effect of the configuration of a bi-layer electrolyte on the SOFC performance is performed. The results show that the effectiveness of the bi-layer electrolyte depends strongly on its configuration. In the analyses, the variations of open circuit voltage and the maximum power density with the thickness ratio at different total electrolyte thicknesses and different operating temperatures are obtained. Furthermore, by taking into considerations of the oxygen partial pressure at the interface between the two layers of electrolytes, an analytical expression for the critical relative thickness ratio, above which, the electrolyte is stable, is obtained.     

The effect of coal syngas containing HCl on the performance of solid oxide fuel cells: Investigations into the effect of operational temperature and HCl concentration

The performance of solid oxide fuel cells (SOFCs) using simulated coal-derived syngas, with and without hydrogen chloride (HCl), was studied. Electrolyte-supported SOFCs were tested potentiostatically at 0.7 V at 800 and 900 °C with simulated coal syngas containing 0, 20, and 160 ppm HCl. The results from the tests without HCl show good performance with little degradation over 100 h of operation. Both 20 and 160 ppm HCl were shown to cause performance losses in the SOFCs after injection into the system. Although the tests presented in this paper show that HCl does cause degradation to SOFC performance, the cell performance was recoverable upon the removal of HCl from the fuel. Also recent results from anticipated Integrated Gasification Combined Cycle IGCC warm/hot-gas-cleanup technologies suggest that HCl will be removed to levels that will not cause any significant performance losses in SOFCs.     

The effect of an obstacle plate on the spontaneous ignition in pressurized hydrogen release: A numerical study

Spontaneous ignition of a pressurized hydrogen release has important implications in the risk assessment of hydrogen installations and design of safety measures. In real accident scenarios, an obstacle may be present close to the release point. Relatively little is known about the effect of such an obstacle on the salient features of highly under-expanded hydrogen jets and its spontaneous ignition.In the present study, the effect of a thin flat obstacle on the spontaneous ignition of a direct pressurized hydrogen release is investigated using a 5th-order WENO scheme and detailed chemistry. The numerical study has revealed that, for the conditions studied, the presence of the obstacle plays an important role in quenching the flame following spontaneous ignition for the release conditions considered.     

Effects of a YSZ porous layer between electrolyte and oxygen electrode in solid oxide electrolysis cells on the electrochemical performance and stability

The delamination of (La,Sr)MnO₃ (LSM) oxygen electrode is considered as a key reason for the degradation of solid oxide electrolysis cells (SOEC). In this study, a YSZ porous layer prepared by spinning coating has been introduced to inhibit significant degradation of LSM oxygen electrode during anodic polarization for 100 h under constant 500 mA cm⁻². Impedance spectra of LSM oxygen electrode are recorded before and after anodic polarization. By performing distribution of relaxation time (DRT) processing on the impedance spectra, it is indicated that the introduction of YSZ porous layer provides more active sites or three phase boundary (TPB) for oxygen oxidation reaction. The potential relaxation process of LSM oxygen electrode is measured by three-sequence chronopotentiometry. The result proves that the sample with a YSZ porous layer has lower the oxygen activity and faster the oxygen ion diffusion at the solid-solid two-phase (oxygen electrode and electrolyte) interface (SSTPI) due to more TPB and shorter oxygen ion diffusion paths.     

Investigation of the effect of molecular structure on sooting tendency in laminar diffusion flames at elevated pressure

A specially designed High Pressure Vessel and Burner and fueling system (called “doped flame”) are presented in this paper. This setup allows for soot measurements in laminar diffusion flames of liquid fuel blends at elevated pressure. Fuels with two typical molecular structures, namely linear and saturated cyclic hydrocarbons, are examined in both non-oxygenated (n-hexane (C₆H₁₄) and cyclohexane (C₆H₁₂)) and oxygenated form (1-hexanol (C₆H₁₄O) and cyclohexanol (C₆H₁₂O)). All compounds are blended into n-heptane. Focus of the research is on soot volume fraction at elevated pressure in the range of 1.5–2.0 bar. Sooting tendency is evaluated by means of Laser Induced Incandescence (LII) with Line of Sight Attenuation calibration (LOSA), and the data suggests that soot is more prevalent for cyclic structures relative to their linear counterparts.     

Pre-chamber turbulent jet ignition of methane/air mixtures with multiple orifices in a large bore constant volume chamber: effect of air-fuel equivalence ratio and pre-mixed pressure

Liquefied natural gas (LNG), mainly composed of methane, is in progress to substitute diesel fuel in heavy-duty marine engine for practical, economic, and environmental considerations. However, natural gas is relatively difficult to be ignited in a large bore combustion chamber. A combustion enhancement technique called pre-chamber turbulent jet ignition (TJI) can permit combustion and flame propagation in a large-bore volume. To investigate the effect of air-fuel equivalence ratio and pre-mixed pressure on pre-chamber TJI of methane/air mixtures with multiple orifices in a large bore volume, experimental tests and computational simulations were implemented to study the discharge of hot turbulent jets from six orifices of the pre-chamber. Different initial pressures and air-fuel equivalence ratios were considered to analyze the characteristics of TJI. The asymmetry of the turbulent jet actuated from six different orifices were found due to the asymmetric orientation of the spark plug, resulting in the inhomogeneous distribution of combustion in the constant volume chamber, which should be considered seriously in the marine engine design. Besides, as the premixed pressure increases, it has more effect on the flame propagation and plays a more important role, as it further increases.     

The effect of ambient pressure on the evaporation of a single droplet and a spray

The effect of ambient pressure on the evaporation of a droplet and a spray of n-heptane was investigated using a model for evaporation at high pressure. This model considered phase equilibrium using the fugacities of the liquid and gas phases for the behavior of a gas being real, and its importance in the calculation of the evaporation of a droplet or spray at high pressures was demonstrated. For the evaporation of a single droplet, the fact that the droplet's lifetime increased with pressure at a low ambient temperature, but decreased at high temperatures, was explained with pressure and the droplet's temperature determining phase equilibrium. In this study, it was also found that the evaporation of a spray can be explained in terms of multiplex dependencies of the atomization and evaporation of a single droplet. The evaporation of a spray was enhanced by increasing the ambient pressure and this effect was more dominant at higher ambient temperatures.     

Ethanol electrooxidation on a carbon-supported Pt catalyst at elevated temperature and pressure: A high-temperature/high-pressure DEMS study

The electrooxidation of ethanol on a Pt/Vulcan catalyst was investigated in model studies by on-line differential electrochemical mass spectrometry (DEMS) over a wide range of reaction temperatures (23–100 °C). Potentiodynamic and potentiostatic measurements of the Faradaic current and the CO₂ formation rate, performed at 3 bar overpressure under well-defined transport and diffusion conditions reveal significant effects of temperature, potential and ethanol concentration on the total reaction activity and on the selectivity for the pathway toward complete oxidation to CO₂. The latter pathway increasingly prevails at higher temperature, lower concentration and lower potentials (∼90% current efficiency for CO₂ formation at 100 °C, 0.01 M, 0.48 V), while at higher ethanol concentrations (0.1 M), higher potentials or lower temperatures the current efficiency for CO₂ formation drops, reaching values of a few percent at room temperature. These trends result in a significantly higher apparent activation barrier for complete oxidation to CO₂ (68 ± 2 kJ mol⁻¹ at 0.48 V, 0.1 M) compared to that of the overall ethanol oxidation reaction determined from the Faradaic current (42 ± 2 kJ mol⁻¹ at 0.48 V, 0.1 M). The mechanistic implications of these results and the importance of relevant reaction and mass transport conditions in model studies for reaction predictions in fuel cell applications are discussed.     

The numerical investigation of a planar single chamber solid oxide fuel cell performance with a focus on the support types

Single chamber solid oxide fuel cells (SC-SOFCs) could be an alternative to the conventional dual chamber types since they do not need any sealant and electrolyte crack growth does not lead to failure in performance. However, the reduced reactant activity due to spectator species present at anode and cathode results in a significantly decreased performance. The focus of this paper is to present a comparative study on the performance of single-chamber anode-, cathode, and electrolyte-supported cells. Our results show that anode-supported cells offer significantly better performance compared to electrolyte-supported cells. The cathode-supported cells show a similar performance to anode-supported cell close to open circuit voltages, i.e. voltages above 0.92 V, after which the cell current density decreases due to lack of oxygen at the cathode catalyst layer. Finally, a time-dependent performance study of the cathode-supported cell concept is presented and discussed.     

汽油醇发动机点火时刻及空燃比对输出扭矩影响的仿真分析

采用汽油醇后的汽油发动机的最佳点火时刻及最佳空燃比将发生变化,为合理地分析汽油醇发动机点火时刻及空燃比对发动机输出扭矩的影响,首先,通过构建汽油醇发动机数学模型,利用Matlab/Simulink设计汽油醇发动机仿真系统,并通过仿真系统分析汽油醇发动机点火提前角分别为13°,14°,16°时及空燃比A/F分别为14.5,14.7,14.8时对发动机输出扭矩的影响。经过对不同曲线的对比分析,得到了汽油醇发动机最佳点火提前角为16°,最佳A/F为14.5。所设计的汽油醇发动机仿真系统能真实地反映出汽油醇发动机的工作情况,对于分析汽油醇发动机的性能有较大的帮助。     

Flammability limits,limiting oxygen concentration and minimum inert gas/combustible ratio of H2/CO/N2/air mixtures

The flammability limits, the limiting oxygen concentration (LOC) and the inert gas/combustible ratio (ICR) of hydrogen/carbon monoxide/nitrogen/air mixtures are determined for hydrogen fuel molar fractions of 0.44, 0.62 and 0.71, at atmospheric pressure and initial temperatures up to 200 °C. The experiments are performed in a glass cylindrical tube with an internal diameter of 80 mm. The mixtures are ignited by a spark discharge between two electrodes placed at the bottom of the tube. Flame propagation is said to have occurred if the flame propagates a distance of at least 100 mm. The experimental procedure is based upon EN 1839 and EN 14756. Le Chatelier's law is used to estimate the flammability limits of the hydrogen/carbon monoxide mixtures, while the LOC and ICR are estimated based upon the lower flammability limit. The estimates are compared with the experimental data.     

The effect of gas compositions on the performance and durability of solid oxide electrolysis cells

High-temperature electrolysis with various gas compositions has been performed to investigate the effects of the hydrogen partial pressure and the humidity generated by the steam electrode on the performance and durability of solid oxide electrolysis cells. The power density of the button cell used in this research is 0.48 W cm⁻² at 750 °C, and the flow rates of the air and humidified hydrogen are 100 cc min⁻¹. By changing the flow ratio of H₂:Ar:H₂O(g) from 10:0:4 to 1:9:4, the cell's OCV decreases from 0.973 V to 0.877 V, and the charge transfer resistance increases from 1.126 Ω cm² to 1.645 Ω cm². The close relationship between the conversion efficiency of high-temperature electrolysis and steam composition is evident in the increase in the cell's charge transfer resistance from 0.381 Ω cm² to 1.056 Ω cm² as the steam content changed from 40 vol% to 3 vol%. Although the electrochemical splitting of water is stimulated in the short term by excessive steam flow, the Ni-YSZ electrodes have been damaged by the steam electrode's low H₂ partial pressure. Consequently, the steam electrode's gas composition must be optimized in the long-term because of the trade-off between performance and durability, which depends on the water concentration of the steam electrodes.     

The effects of ethanol–unleaded gasoline blends and ignition timing on engine performance and exhaust emissions

In this study, the effects of using unleaded gasoline (E0) and unleaded gasoline–ethanol blends (E10, E20 E40 and E60) on engine performance and exhaust emissions have been experimentally investigated. The investigation was conducted on a Hydra single-cylinder, four-stroke, spark ignition engine. The experiments were performed by varying the compression ratio (8:1, 9:1 and 10:1) and ignition timing at a constant speed of 2000 rpm at wide open throttle (WOT). The experimental results showed that blending unleaded gasoline with ethanol slightly increased the brake torque and decreased carbon monoxide (CO) and hydrocarbon (HC) emissions. It was also found that blending with ethanol allows increasing the compression ratio without knock occurrence.     

Energy and exergy analyses of a combined ammonia-fed solid oxide fuel cell system for vehicular applications

In this study, both energetic and exergetic performances of a combined heat and power (CHP) system for vehicular applications are evaluated. This system proposes ammonia-fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-H⁺) with a heat recovery option. Fuel consumption of combined fuel cell and energy storage system is investigated for several cases. The performance of the portable SOFC system is studied in a wide range of the cell’s average current densities and fuel utilization ratios. Considering a heat recovery option, the system exergy efficiency is calculated to be 60-90% as a function of current density, whereas energy efficiency varies between 60 and 40%, respectively. The largest exergy destructions take place in the SOFC stack, micro-turbine, and first heat exchanger. The entropy generation rate in the CHP system shows a 25% decrease for every 100 °C increase in average operating temperature.