Effects of different combustion modes on the thermal efficiency and emissions of a diesel pilot-ignited natural gas engine under low-medium loads

Research on dual-fuel(DF)engines has become increasingly important as engine manufacturers seek to reduce carbon dioxide emissions.There are significant advantages of using diesel pilot-ignited natural gas engines as DF engines.However,different combustion modes exist due to variations in the formation of the mixture.This research used a simulation model and numerical simulations to explore the combustion characteristics of high-pressure direct injection(HPDI),partially premixed compression ignition(PPCI),and double pilot injection premixed compression ignition(DPPCI)combustion modes under a low-medium load.The results revealed that the DPPCI combustion mode provides higher gross indicated thermal efficiency and more acceptable total hydrocarbon(THC)emission levels than the other modes.Due to its relatively good performance,an experimental study was conducted on the DPPCI mode engine to evaluate the impact of the diesel dual-injection strategy on the combustion process.In the DPPCI mode,a delay in the second pilot ignition injection time increased THC emissions(a maximum value of 4.27g/(kW·h)),decreased the emission of nitrogen oxides(a maximum value of 7.64 g/(kW·h)),increased and then subsequently decreased the gross indicated thermal efficiency values,which reached 50.4%under low-medium loads.

On the Design and Optimization of a Clean and Efficient Combustion Mode for Internal Combustion Engines through a Computer NSGA-Ⅱ Algorithm

In order to address typical problems due to the huge demand of oil for consumption in traditional internal combustion engines,a new more efficient combustion mode is proposed and studied in the framework of Computational Fluid Dynamics(CFD).Moreover,a Non-dominated Sorting Genetic Algorithm(NSGA-Ⅱ)is applied to optimize the related parameters,namely,the engine methanol ratio,the fuel injection time,the initial temperature,the Exhaust Gas Re-Circulation(EGR)rate,and the initial pressure.The so-called Conventional Diesel Combustion(CDC),Homogeneous Charge Compression Ignition(HCCI)and the Reactivity Controlled Compression Ignition(RCCI)combustion modes are compared.The results show that RCCI has a higher methanol ratio and an earlier injection timing with moderate EGR rate and higher initial pressure.The initial temperature increases as the methanol ratio increases.In comparison,CDC has the lowest hydrocarbon and CO emissions and the highest combustion efficiency.At different crankshaft rotation angles corresponding to 50%of the combustion amount(CA50),the combustion temperature and boundary layer temperature of HCCI change significantly,while those of RCCI undergo limited variations.At the same CA50,the exergy losses of HCCI and RCCI are lower than that of the CDC.On the basis of these findings,it can be concluded that the methanol/diesel RCCI engine can be used to obtain a clean and efficient combustion process,which should be regarded as a promising combustion mode.

Scaling effects on combustion modes in a single-side expansion kerosene-fueled scramjet combustor

The combustion modes in two different scramjet combustors with the mass flow rates of 1.8 kg/s and 3.6 kg/s are experimentally investigated to explore the scaling effects on supersonic combustion with a Mach number 2.0 inflow.It is found that the scramjet combustor with a larger scale can broaden the flame rich blowout limit.As the Equivalence Ratio(ER)increases,the combustion in the small-scale combustor maintains in the cavity-stabilized mode,and the flamebase moves downstream along the cavity shear layer;however,the combustion in the large-scale combustor gradually transfers from the cavity-stabilized mode to the jet-wake-stabilized mode.The differences in the cavity residence time,the ignition delay time and the Damkohler number caused by different scales of the scramjet combustor are likely to account for the scaling effects on the combustion modes.

Experimental study on combustion characteristics of supersonic combustor based on alternating-wedge strut

Acetone Planar Lase-Induced Fluorescence(PLIF)and OH-PLIF were employed to capture the fuel distribution and OH distribution downstream for the supersonic combustor based on the alternating-wedge strut.The combustion establishment process and combustion mode in the combustor under different fuel injection methods and different equivalence ratios were analyzed.Combined with the kerosene-PLIF and OH-PLIF results in the cavity combustor,a comparative analysis was conducted to understand the combustion characteristics and combustion modes between the alternating-wedge strut-based combustor and the cavity-based combustor.The results show that the combustor is in weak combustion mode in the case of low equivalence ratio,and the combustor is in intensive combustion mode in the case of high equivalence ratio.The lower limit of the equivalence ratio of the combustor to maintain the intensive combustion mode varies based on different fuel injection methods.The OH distribution under reacting condition has a strong correlation with the fuel distribution under non-reacting condition.The OH fluorescence signal near the injector is weaker when the fuel distribution is more concentrated.The injector position located at the base of the strut rear has better mixing performance,enabling the combustor to be in intensive combustion mode at a lower equivalence ratio.The combustion reaction in the alternating-wedge strut-based combustor is not necessarily dominated by mass transfer due to the mixing enhancement and premixed zone downstream of strut,while the combustion reaction process in the cavity-based combustor is mainly influenced by mass transfer.

Numerical simulation for explosion wave propagation of combustible mixture gas

A two-dimensional multi-material code was indigenously developed to investigate the effects of duct boundary conditions and ignition positions on the propagation law of explosion wave for hydrogen and methane-based combustible mixture gas. In the code,Young's technique was employed to track the interface between the explosion products and air,and combustible function model was adopted to simulate ignition process. The code was employed to study explosion flow field inside and outside the duct and to obtain peak pressures in different boundary conditions and ignition positions. Numerical results suggest that during the propagation in a duct,for point initiation,the curvature of spherical wave front gradually decreases and evolves into plane wave. Due to the multiple reflections on the duct wall,multi-peak values appear on pressure-time curve,and peak pressure strongly relies on the duct boundary conditions and ignition position. When explosive wave reaches the exit of the duct,explosion products expand outward and forms shock wave in air. Multiple rarefaction waves also occur and propagate upstream along the duct to decrease the pressure in the duct. The results are in agreement with one-dimensional isentropic gas flow theory of the explosion products,and indicate that the ignition model and multi-material interface treatment method are feasible.

Thermal behavior of an isolator with mode transition inducing back-pressure of a dual-mode scramjet

Combustion mode transition is a valuable and challenging research area in dual-mode scramjet engines.The thermal behavior of an isolator with mode transition inducing backpressure is investigated by direct-connect dual-mode scramjet experiments and theoretical analysis.Combustion experiments are conducted under the incoming airflow conditions of total temperature1270 K and Mach 2.A small increment of the fuel equivalence ratio is scheduled to trigger mode transition.Correspondingly,the variation of the coolant flow rate is very small.Based on the measured wall pressures,the heat-transfer model can quantify the thermal state variation of the engine with active cooling.Compared with the combustor,mode transition has a greater effect on the isolator thermal behavior,and it significantly changes the isolator heat-flux and wall temperature.To further study the isolator thermal behavior from flight Mach 4 to Mach 7,a theoretical analysis is carried out.Around the critical point of combustion mode transition,sudden changes of the isolator flowfield and thermal state are discussed.

Anisotropic Combustion of Aluminum Nanoparticles in Carbon Dioxide and Water Flows

Shock-induced combustion of aluminum nanoparticles was examined in the CO_(2)and H_(2)O flows up to 8 km/s using reactive molecular dynamics.The morphological evolutions and heat/mass transfer of ANPs were discussed to reveal the nature of anisotropic combustion.The breakage of triatomic gas molecule and the formation of key intermediates were identified to illustrate the reaction mechanisms at the atomic level.It was found that surface reactions prevail for cases in lower flow velocity(≤6 km/s),and gas-phase reactions govern the oxidation process under the intense impact(8 km/s).In particular,we converted the flow velocity to the initial kinetic energy of flow molecules to highlight the impact of oxidizing ability on the shock-induced combustion.In the regime of low initial kinetic energy(<122.2 kJ/mol),the oxidation follows the diffusion mechanism,and the ignition delay is mainly affected by the reaction rate and heat release of oxidizers.Further increasing the initial kinetic energy(<458.1 kJ/mol),the impact of oxidizers weakens and the heat transfer becomes dominant.In the extreme scenarios(>458.1 k J/mol),the overall oxidation is governed by the microexplosion mechanism,and different oxidizers share almost the same ignition delay.

Numerical study on the combustion process in a gas turbine combustor with different reference velocities

The mixing and combustion processes under different reference velocities in a gas turbine combustor were numerically investigated using the Flamelet Generated Manifold(FGM)model based on the Reynolds Averaged Navier-Stokes(RANS)method.The flow and combustion fields show strong self-similarity except on the slow auto-ignition in the mixing layer between fuel-rich product and fresh air upstream of the flame stabilization position.The time-scale analysis was carried out to understand the combustion modes inside the combustor.In general,the residence time of the fuel-mixture is much longer than both the chemical time scale and the mixing time scale.Thus,the combustion properties in each sub-zone were dominated by the mean flow structures.Furthermore,the combustion process exhibits a mixing-controlled feature in total.However,partially premixed combustion still appears on the flame base.Most of the fuel was found to be oxidized in the primary zone and the intermediate zone;however,the slow oxidization reactions also play a non-negligible role on the whole combustion process.Finally,a sketch map on the space of mixture fraction and combustion efficiency was proposed to understand the mixing and oxidization experiences of the fuel mixture.

Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.

Numerical simulations of turbulent flows in aeroramp injector/gas-pilot flame scramjet

To uncover the internal flow characteristics in an ethylene-fueled aeroramp injector/gaspilot（ARI/G-P）flame scramjet,a Reynolds-averaged Navier-Stokes（RANS）solver is constructed under a hybrid polyhedral cell finite volume frame.The shear stress transport（SST）k-x model is used to predict the turbulence,while the Overmann’s compressibility corrected laminar flamelet model is adopted to simulate the turbulent combustion.Nonreactive computations for Case 1（G-P jet on）,Case 2（ARI jets on）,and Case 3（both ARI and G-P jets on）were conducted to analyze the mixing mechanism,while reactive Cases 4–7 at equivalent ratios of 0.380,0.278,0.199 and0.167 respectively were calculated to investigate the flame structure and combustion modes.The numerical results are compared well to those of the experiments.It is shown that the G-P jet plays significant role in both the fuel/air mixing and flame holding processes;the combustion for the four reactive cases takes place intensively in the regions downstream of the ARI/G-P unit;Cases 4 and 5are under subsonic combustion mode,whereas Cases 6 and 7 are mode transition critical and supersonic combustion cases,respectively;the mode transition equivalent ratio is approximately 0.20.

Ignition Characteristics of Lean Coal Used a Novel Alternating-Current Plasma Arc Approach

In order to achieve the target of reducing oil consumption to zero for pulverized coal(PC)boiler in power plant,the paper developed a novel coal pulverized ignition approach,called as Alternating-Current plasma(AC plasma)ignition,with the advantages of excellent PC combustion behavior and longer electrode life-span.The scientific principle of how to generate the AC plasma arc was elaborated in detail.First,the experiments on life-span of electrodes inside AC plasma generator had been conducted,finding a workable way to extend its life-span beyond 530 hours.Second,a new AC plasma burner specifically designed for lean coal according to the principle of PC staged combustion had been illustrated with diagrams and then used to ignite the PC-air stream under four kinds of conditions with a varying AC plasma power from 150 kW to 300 kW,focusing on analyses of the influence of AC plasma power on combustion behaver,such as combustion temperature,carbon burnout rate as well as PC combustion regime.The following results showed that in the case of the power of the AC plasma was P=300 kW,a satisfied PC combustion process could achieved,with the average PC combustion temperature of about 940°C,combustion flame length of 6.3 m,and the total carbon burnout rate of up to 52.2%.In addition,about 80%of the nozzle outlet section was filled with bright flame,while 81%of the PC was in zone of the cylindrical flame regime.The PC combustion modes were changed repeatedly during the process of combustion,which went from homogeneous combustion mode at initial ignition stage to combined combustion mode and heterogeneous combustion mode at middle stage,finally to combined combustion mode at later stage.The research conclusion in this paper has proved that the AC plasma ignition approach is feasible and effective to ignite low-rank coal without the present of fuel oil.