Fuel-air Mixing Characteristics Research Articles

In-depth comparison of methanol port and direct injection strategies in a methanol/diesel dual fuel engine

For methanol/diesel dual-fuel combustion, there are two different methanol injection implementations: port injection into the intake manifold and direct injection into cylinder. In this work, an experimental comparison was conducted for the two methanol injection strategies to study the effects of the methanol-energy-substation ratio (ESR) and diesel injection timing on the fuel-air mixing and combustion characteristics. The results showed that the ignition delay was increased with ESR due to the cooling effect of methanol evaporation in the cylinder. The combustion duration under methanol direct injection condition was much shorter than that of the methanol port injection strategy. The methanol direct injection strategy had a more controllable and stable heat release than that of methanol port injection strategy. The maximum ESR could reach up to 96.0% in the methanol direct injection strategy. The methanol direct injection strategy had a better fuel economy, and its maximum indicated thermal efficiency could reach up to 41.55% at 50% ESR. The methanol direct injection strategy yielded higher NOx emissions than that with the port injection strategy, accompanied by a mild increase of soot emissions. Compared with methanol port injection, the methanol direct injection strategy exhibits lower CO emissions, however, it suffers from higher HC emissions.

Numerical investigation into fuel–air mixing characteristics and cold flow field of single hydrogen-rich Micromix nozzle

As a promising gas turbine combustor for hydrogen-rich syngas, Micromix combustor may produce new design criteria due to its novel structure. Therefore, the mixing characteristics and flow field of single Micromix nozzle with multiple micro-mixing tubes were studied numerically. The results showed that, the Large-Eddy simulation model was more accurate in cold-state prediction of Micromix combustor than the Reynolds averaged equations model. For mixing characteristics, mass diffusion had greater effect on mixing process of hydrogen-rich syngas than momentum-driven diffusion. With the increase of premixing distance, the effect of fuel–air momentum ratio gradually disappeared. And the difference in mixing characteristics was mainly caused by the shear-layer instability at the initial stage of jet. By reducing the jet-direction diameter of micro-mixing tubes, the mixing uniformity improved, indicating that the jet-direction mixing ability was weak, but there was an upper limit of the mixing uniformity. In addition, fuel hole shapes had obvious effect on vortex structure and vorticity, but little impact on mixing characteristics. For the cold flow field, there was a recirculation region between jets near the nozzle outlet, which was helpful for flame stabilization. Compared with the multiple circular jets, the axial scale of recirculation region formed by multiple elliptical jets was larger, the jet merging point and the combined point moved to the burner outlet.

Effect of Inlet-Mixture Stratification and Preheating on a C3H8 Premixer and Bluff-Body Combustor

AbstractThis work presents the computational investigation of the nonreacting flow field and of the fuel–air mixing characteristics in a stratified, premixer/bluff-body combustor configuration unde...

Numerical investigation into the fuel evaporation and mixture formation characteristics of a free-piston diesel engine

The free-piston engine generator becomes a new-type potential substitute for the conventional crankshaft combustion engine. This article presents a simulation to study the fuel spray and mixing characteristics of a diesel free-piston engine generator by comparing a corresponding crankshaft combustion engine. A full-cycle model which couples with piston dynamics, combustion, and gas exchange is developed to simulate the fuel spray, atomization, and mixing in the free-piston engine generator. The result indicates that compared with the crankshaft combustion engine, the free-piston engine generator provides a higher temperature and pressure for fuel spray and mixing during the ignition delay, but its ignition delay lasts shorter. The free-piston engine generator shows a shorter spray penetration and more fuel impingement due to its smaller combustion chamber volume during the injection process. The free-piston engine generator exhibits a lower level of air utilization and worse uniformity of fuel–air mixture in combustion chamber. In addition, the shorter ignition delay of free-piston engine generator makes the time of atomization, evaporation, and mixing of fuel shorter, and the mixing effect of free-piston engine generator is worse, resulting in less combustible mixture formed during the ignition delay. In addition, some guiding suggestions have been proposed to improve the fuel spray and fuel–air mixing characteristics of free-piston engine generator.

Evaporation and atomization characteristics of dual-fuel system under flash boiling conditions

Flash boiling sprays are formed when injecting the fuel into an environment with the local pressure below the saturation pressure of the fuel. The formation of flash-boiling bubbles can enhance the atomization and evaporation of the fuel spray, thus enhancing fuel-air mixing characteristics. A significant factor impacting the performance of flash-boiling sprays is the co-evaporation of fuels with various volatilities, especially for combusting practical fuels. In the present study, two single component hydrocarbon fuels (i-pentane and n-undecane) with remarkably different boiling points were mixed with each other to examine the multi-components impact on the flash boiling properties of the resultant fuel spray. Experimental studies on spray structure with various blending ratio, fuel temperature, and ambient pressure were conducted in a constant volume chamber. The difference in spray penetration and collapse rate among various blending ratios obtained from high-speed Mie scattering measurements were compared. Results showed that fuel with a higher volume fraction of i-pentane had a higher tendency to achieve flash boiling as fuel temperature increase or ambient pressure decrease. Superheat index, i.e. the ratio of ambient pressure and saturated pressure was examined for predicting the flash-boiling phenomenon of the dual fuel system, was found no longer completely applicable for dual-fuel system. An equivalent saturation pressure considering both dewing and bubble point pressures, and molar fraction of light fuel component is proposed to calculate superheat index which can be used to predict flash-boiling behavior of a dual-fuel system.

Experimental and numerical study on the influence of intake swirl on fuel spray and in-cylinder combustion characteristics on large bore diesel engine

In the present study, the numerical simulation of the performance of fuel injection and spray atomization on diesel engine combustion was conducted, which was validated against fuel injection test, spray test and single cylinder diesel engine test. Thus, the effect of the intake swirl on the fuel spray and diesel engine combustion performance was explored. Quantitative analysis of the characteristics of fuel-air mixing and combustion characteristics under different swirl ratio was given. It was found that the spray atomization and fuel-air mixing begins from the front of fuel beam which has deflection due to the presence of the intake swirl. The function of swirl is to accelerate the mixing and diffusion speed of fuel, thus promoting the combustion process in cylinder. Meanwhile, the NO and Soot emissions were quite sensitive to the swirl ratio, which leads higher NO but lower Soot as the swirl ratio is increased. In addition, the performance of varied swirl ratio on spray penetration and spray angle was well captured by numerical studies in the paper. Consequently, this study reveals the influence of different swirl ratio on the fuel spray performance, output power and fuel consumption, thus exploring the influence of inlet swirl of the controllable swirl diesel engine on the fuel atomization behavior as well as dynamic performance, and verifying the accuracy of the simulation results.

Investigation of Combustion Process of Biodiesel Derived from Waste Cooking Oil, Jatropha, Palm Oil and Algae using Rapid Compression Machine

Biodiesel essentially comes from plants and animal fats which is safe and can be used in diesel engines with few or no modification needed toward existing engine. Higher of the biodiesel blending ratio reflects to the higher viscosities that influences to the fuel-air premixing, ignition process and combustion characteristics. The objective of this research is to investigate the effect of different blending ratio on ignition delay, ignition process, combustion characteristics and emission production using Rapid Compression Machine (RCM). Rapid Compression Machine is used to simulate combustion behaviour similar with the real combustion for internal combustion engines. The standard petroleum diesel was blend with biodiesel derived from waste cooking oil, algae and jatropha with blending ratio from 2vol% to 40vol%. The biodiesel were tested using 130 MPa of injection pressure with constant ambient temperature of 950 K. Increased of the blending ratio of biodiesel significantly influences the longer of ignition delay. Changes of biodiesel properties thus influence to the fuel-air mixing, ignition process and combustion characteristics. In addition, the emission of NOx and CO2 increased when increasing biodiesel blending ratio.

Multi-Component Model of Diesel Sprays Under High Injection Pressure

The combustion efficiency of a diesel engine depends not only on spray characteristics but also on fuel-air mixing characteristics. Based on the original spray model, a new spray model is established in this paper to accurately predict the diesel spray, and then a multi-component evaporation model is added into it. The model takes the influence of component concentration gradient and species on its evaporation rate in the liquid phase into account. This paper studies the spray characteristics (spray penetration, spray angle and spray morphology) and fuel-air mixing characteristics (spray area, spray volume and air entrainment mass) using the spray model, and the results are compared with the experimental results. The comparison shows that the simulated spray penetration and spray angle are close to the experimental results with the average deviations less than 3%. Moreover, this paper studies the spray area, spray volume and air entrainment using empirical formula under different conditions. And the maximum deviations of the spray volume, spray area and air entrainment mass are less than 5% as compared with the test values. Overall, this spray model can predict the diesel spray characteristics and fuel-air mixing characteristics under high injection pressure accurately.

Flow, Mixing, and Combustion Characteristics of High Velocity Ratio Plane Coaxial and Convoluted Trailing Edge Nozzles

AbstractThe work presents a comparative study of the fuel-air mixing and flame stabilization characteristics of two, high velocity ratio, coaxial jet configurations comprising either a plain circular arrangement or a convoluted trailing edge nozzle. The high velocity ratio, coaxial stream interaction results in a stable central recirculation zone at a distance downstream of the nozzle exit where a range of flame configurations can be successfully stabilized under suitable flow and mixing conditions. In this configuration, the reacting front region is supplied with both a radial and an axial propane-air mixture gradient which are formulated through a central upstream premixer duct and the coannular jet interaction downstream of the exit plane. Depending on the nozzle shape, plain circular or convoluted, a variety of partially premixed turbulent flame configurations can be obtained at a position detached farther from the nozzle exit. Measurements of mixture concentrations, mean temperatures, and flame chemi...

Effects of Temperature and Ambient Pressure on Spray Characteristics of Biodiesel Combustion

s: Diesel fuel injection is the most dominant in ignition process of the diesel engines combustion. Diesel engines have been widely used in heavy-duty and light-duty vehicles due to their higher fuel economy, efficient and powerful than spark ignition (SI) engines. The principal objective of this research is to seek the effect of temperature and pressure on the spray characteristics, as well as fuel-air mixing characteristics. Experiments were performed in a constant volume chamber at specified ambient gas temperature and pressure. This research was continued with injecting diesel fuel into the chamber using a Bosch common rail system. Direct photography technique with a digital camera was used to clarify the real images of mixture formation such as spray penetration, fuel evaporation and spray interference. The liquid phase of the spray reaches a maximum penetration distance soon after the start of injection, while the vapour phase of the spray continues to penetrate downstream. The condition to which the fuel is affected was estimated by combining information on the wall chamber temperature, ambient temperature and photographs of the spray. The increases in ambient pressure inside the chamber resulting in gain of spray area and wider spray angle. Thus predominantly promotes for a better fuel-air premixing.

A Review Paper on Simulation and Modeling of Combustion Characteristics under High Ambient and High Injection of Biodiesel Combustion

This paper describes simulation of combustion characteristics under high ambient and high injection of biodiesel combustion by using CFD simulation. Diesel engine performance and emissions is strongly couple with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. The principal objective of this research is to seek the effect of temperature and pressure on the spray characteristics, as well as fuel-air mixing characteristics. Experiments were performed in a constant volume chamber at specified ambient gas temperature and pressure. This research was continued with injecting diesel fuel into the chamber using a Bosch common rail system. Direct photography technique with a digital camera was used to clarify the real images of spray pattern, liquid length and vapor penetration. The method of the simulation of real phenomenon of diesel combustion with optical access rapid compression machine is also reviewed and experimental results are presented. The liquid phase of the spray reaches a maximum penetration distance soon after the start of injection, while the vapor phase of the spray continues to penetrate downstream. The condition to which the fuel is affected was estimated by combining information on the block temperature, ambient temperature and photographs of the spray. The increases in ambient pressure inside the chamber resulting in gain of spray area and wider spray angle. Thus predominantly promotes for a better fuel-air mixing. All of the experiments will be conducted and run by using CFD. The simulation will show in the form of images.

An Analysis of the Ambient Condition Effect on Biodiesel Spray Using Constant Volume Chamber

Diesel engines are high compression ignition engine which are now very vastly used for heavy vehicles and machineries. Diesel fuel is compressed under the right condition to ignite inside the constant volume chamber. Researchers have been studying for many years on ways to increase the efficiency of diesel engine as well as reduce the emission. The main idea of this research is to understand the effect of temperature on the spray characteristics, as well as fuel-air mixing characteristics. These are the characteristics responsible for ignition of diesel sprays. This research is first conducted by investigating the influence of biodiesel properties and ambient condition on the mixture formation especially at early stage of fuel-air premixing. This research was continued with injecting diesel fuel into the chamber using a Bosch common rail system. Direct photography technique with a digital camera was used to capture the real images of spray evaporation, spray length, and mixture formation with the time changes. The values of the temperature were recorded at ambient temperature, 55°C, 70°C, 85°C as well as 100°C. Injection pressure of 0.1 MPa up to 0.7 MPa was induced into the chamber with an increment of 0.1 MPa. The condition to which the fuel is affected was estimated by combining information on the block temperature, ambient temperature and photographs of the spray. The increase in block temperature increases the ambient temperature inside the chamber resulting in gain of spray area and wider spray angle. Thus predominantly promotes for a better fuel-air mixing.

Study of the characteristic of diesel spray combustion and soot formation using laser-induced incandescence (LII)

In this paper, the planar images of diesel spray combustion flame and soot formation were measured and analyzed by using LII, in a constant volume combustion vessel. The effects of combustion flame and fuel–air mixing characteristics on soot formation and distribution of soot concentration were studied at different conditions. The result indicates that, with increase in ambient temperature and pressure, the ignition delay of diesel fuel is shorter. The increase of ambient temperature and pressure and the reduction of injection pressure shorten the diesel flame lift-off length. The lower the ambient temperature and pressure, the weaker LII signal intensity. At the same ambient temperature and pressure condition, the higher the diesel injection pressure, the smaller the soot production in diesel jet spray, and soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel jet.

Effect of Preheated Fuel on Mixture Formation of Biodiesel Spray

The key issue in using vegetable oil-based fuels is oxidation stability, stoichiometric point, bio-fuel composition, antioxidants on the degradation and viscosity thus influences to the different spray atomization and fuel air mixing characteristics. Purpose of this study is to investigate the effect of preheated biodiesel on fuel properties, spray characteristics and mixture formation. The detail behavior of mixture formation was investigated using the direct photography system with a digital color camera. This method can capture spray evaporation, spray length and mixture formation clearly with real images. Increased preheated fuel is found to enhance the spray penetration, resulting in increased the spray area and enhanced fuel-air premixing.

Effects of the throttle opening ratio and the injection timing of CNG on the combustion characteristics of a DI engine

We investigated the effects of the fuel injection timing — both for early and late injection — in conjunction with the throttle opening ratio on the fuel-air mixing characteristics, engine power, combustion stability and emission characteristics of a DI CNG spark engine and control system that had been modified and designed according to the author’s original idea. We verified that the combustion characteristics were affected by the fuel injection timing and that the engine conditions were affected by the throttle opening ratios and the rpm. The combustion characteristics were greatly improved for a complete open throttle ratio with an early injection timing and for a partial throttle ratio with a late injection timing. The combustion duration was governed by the duration of flame propagation in late injection timing scenarios and by the duration of early flame development in cases of early injection timing. As the result, the combustion duration is shortened, the lean limit is improved, the air-fuel mixing conditions are controlled, and the emissions are reduced through control of the fuel injection timing and vary according to ratio of the throttle opening.

Fuel-Air Mixing Characteristics of DI Hydrogen Jets

Fuel-Air Mixing Characteristics of DI Hydrogen Jets

Confined Vortex Shedding Past a Square Cylinder with a Planar Jet.

In this investigation, confined vortex shedding past a square cylinder with a planar jet is numerically studied. Flow and scalar-transport simulations are presented for various cases including both laminar and turbulent flow situations. It is shown that the ratio of jet velocity to uniform inlet velocity significantly affects the overall flow structures and thus scalar transport downstream of the cylinder. Especially, when the ratio is large enough, the jet penetrates the main vortices shed from the cylinder, resulting in significant changes in the flow and scalar fields. In the case of laminar flow, regions of intense scalar are formed along the streamlines from the jet exit, and the oscillation of the force on the cylinder eventually disappears as the jet velocity is close to the inlet velocity. Large Eddy Simulation of turbulent flow also reveals complex flow structures and intense mixing depending on the velocity ratio; regions of intense scalar coincide with those of high turbulence intensity. The results obtained exhibit fuel-air mixing characteristics observed in a planar combustor where the square cylinder plays the role of a flame-holder.

PSR-based microstructural modeling for turbulent combustion processes and pollutant formation in double swirler combustors

The present study numerically investigates the fuel-air mixing characteristics, flame structure, and pollutant emission inside a double-swirler combustor. A PSR (Perfectly Stirred Reactor) based microstructural model is employed to account for the effects of finite rate chemistry on the flame structure and NO formation. The turbulent combustion model is extended to nonadiabatic flame condition with radiation by introducing an enthalpy variable, and the radiative heat loss is calculated by a local, geometry-independent model. The effects of turbulent fluctuation are taken into account by the joint assumed PDFs. Numerical model is based on the non-orthogonal body-fitted coordinate system and the pressure/velocity coupling is handled by PISO algorithm in context with the finite volume formulation. The present PSR-based turbulent combustion model has been applied to analyze the highly intense turbulent nonpremixed flame field in the double swirler combustor. The detailed discussions were made for the flow structure, combustion effects on flow structure, flame structure, and emmission characteristics in the highly intense turbulent swirling flame of the double swirler burner.

A Comprehensive Simulation Model for Mixing and Combustion Characteristics of Small Direct Injection Diesel Engines

An analytical model has been developed to predict the fuel-air mixing and burning characteristics of small direct injection (DI) diesel engines. Four mass and momentum conservation equations are written along the tangential and normal directions to the spray for both free and wall regions based on the continuum integral approach. The spray structure is multi-zonal and the simulation of atomization, droplet-size distribution, evaporation, turbulent mixing and combustion processes is included. The model predictions show good correlation with the experimental data and respond well to the variations in input parameters. The level of predictions from the model is satisfactory and the model is quite suitable to optimize the injection characteristics bowl geometry and air motion in small DI diesel engines.

Numerical study of mixing in supersonic combustors with hypermixing injectors

A numerical study was conducted to evaluate the performance of wall mounted fuel-injectors designed for potential Supersonic Combustion Ramjet (SCRAM-jet) engine applications. The focus of this investigation was to numerically simulate existing combustor designs for the purpose of validating the numerical technique and the physical models developed. Three different injector designs of varying complexity were studied to fully understand the computational implications involved in accurate predictions. A dual transverse injection system and two streamwise injector designs were studied. The streamwise injectors were designed with swept ramps to enhance fuel-air mixing and combustion characteristics at supersonic speeds without the large flow blockage and drag contribution of the transverse injection system. For this study, the Mass-Averaged Navier-Stokes equations and the chemical species continuity equations were solved. The computations were performed using a finite-volume implicit numerical technique and multiple block structured grid system. The interfaces of the multiple block structured grid systems were numerically resolved using the flux-conservative technique. Detailed comparisons between the computations and existing experimental data are presented. These comparisons show that numerical predictions are in agreement with the experimental data. These comparisons also show that a number of turbulence model improvements are needed for accurate combustor flowfield predictions.