Fuel-air Mixing Research Articles

Experimental and numerical investigation on fuel/air mixing and macroscopic spray characteristics of free spray, convergent-divergent, and straight duct sprays under non-vaporizing conditions

In this study, the convergent-divergent (CD) and straight (ST) duct sprays were conducted for comparison with free spray to investigate the diesel macroscopic spray characteristics and mixing process. The schlieren visualization system with a high-speed camera was performed with a 0.22 mm single-hole heavy-duty diesel injector. The injection pressure varied from 120 via 140 to 160 MPa, with the ambient pressure being set at 40 bar. The experimental result showed that CD and ST duct spray had much higher spray tip velocity compared to free spray during the early spray stage. CD duct spray presented a more dispersed and more distributed spray morphology along with larger-scale flow vortexes and gas entrainments, indicating the promoted fuel-air mixing process. Large-eddy simulations (LES) were conducted to analyze the pumping effect, the local separation of the boundary layer, and the air entrainment of different sprays. Compared with free spray, a much stronger pumping effect was generated for ST duct spray followed by CD duct spray; Less flow interaction with the spray body inside the ST duct, retaining more energy for spray development; A much stronger flow interaction was noted as the external gas was sucked into the expanding section of the CD duct, produce the local air vortexes in the narrow inner wall space of the CD duct, leading to the promoted spray diffusion and development with a wider spray cone angle. These phenomena from CD duct spray were beneficial for achieving a more sufficient fuel-air mixing process.

Experimental Study of Dual-Fuel Diesel/Natural Gas High-Pressure Injection.

Dual-fuel diesel/natural gas direct-injection engine is promising and highly attractive due to its low-carbon emission and high thermal efficiency, and both high-pressure diesel and natural gas injections are critical for air-fuel mixing. This study presents an optical experimental investigation on the high-pressure dual-fuel diesel/methane injection process based on a constant-volume vessel test rig. The results show that the diesel penetration process of the dual-fuel injection experiences two stages: Stage I, the diesel tip penetration S diesel, the diesel spray area A diesel, and the diesel spray perimeter C diesel of the dual-fuel injection are smaller than those of the single diesel injection. Stage II, both the diesel and methane continue to penetrate forward, and S diesel, A diesel, and C diesel of the dual-fuel injection become larger than those of the single diesel injection do. The diesel injection pressure causes effect on the dual-fuel spray penetration. The diesel injection pressure directly causes linear influence on the two-stage dual-fuel injection characteristic. As the diesel injection pressure increases, the diesel spray meets the methane jet advancer and the cross point occurs linearly earlier. Furthermore, the dual-fuel injection is asymmetric and the methane gas jet enhances this asymmetry so that the spray cone shifts to the side of the methane gas jet.

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.

Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), high-temperature pre-reaction time (Δτ2), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.

Reduced-order modeling of supersonic fuel–air mixing in a multi-strut injection scramjet engine using machine learning techniques

Dual-mode ramjet/scramjet engines promise extended flight speed range and are the commonly preferred air-breathing propulsion system from within the family of hypersonic aircraft concepts. One of the main challenges that should be hurdled in their design is modeling the fuel–air mixing process to provide optimal fuel distribution and yield the best engine performance. Injecting fuel into high-speed air stream along multiple parallel struts can augment the fuel penetration and improve mixing efficiency. Mixing intensity is increased with turbulence by the shock-expansion waves in post-strut regions. However, this enhancement in mixing can bring about detrimental effects on the aerodynamic performance by increasing losses on total pressure. Designing the optimal working configuration requires testing the interaction between many design variables. This can be a tedious and computationally costly task. Machine learning models thus appear well-suited for multi-objective optimization of design variables that can be elusive to designers. In particular, non-linear regression models can be built over the available sparse simulation data to predict unseen mixing conditions. In the present work, we carry out a detailed investigation of the effect of multi-strut configuration parameters on three objective functions: mixing efficiency, mixing length, and the total pressure recovery (TPR) factor. These objective functions are linked with the most relevant physical phenomena in the supersonic fuel–air mixing flow field. We first generate a CFD database by solving compressible, non-reactive, Reynolds-averaged Navier–Stokes (RANS) filtered flow equations in a 2D scramjet engine domain with three varying design variables: struts location, strut wedge angle and strut V-settlement angle. We then apply various regression models – artificial neural network (ANN), Gaussian process regression (GPR), and kernel regression – to this database and formulate a surrogate model relevant for fuel injection that can be utilized in reduced-order modeling studies that estimate the hypersonic engine performance. We find that regression is generally more difficult in the vicinity of fuel struts (due to turbulence/shock effects) and easier further downstream from the struts, but ANN performs generally better than other regression models. Thus, our reduced-order tool incorporates a mixing efficiency model predicted by the ANN. It computes the thrust of a hypersonic engine with less than 10% error. We also present a detailed discussion of the physical insights gained from our CFD database; we link this discussion with the earlier findings from the machine learning tools. In our sensitivity study, we find the strut wedge angle to be the most influencing parameter on the mixing properties and aerodynamic losses.

A comparative study on operating range and combustion characteristics of methanol/diesel dual direct injection engine with different methanol injection timings

Improved combustion and reduction in emissions levels could be achieved with optimization of in-cylinder fuel–air mixing. According to the engine operating condition, the spatial and temporal features of in-cylinder mixture can be flexibly and accurately adjusted by methanol/diesel dual direct injection in real time. Present research investigated the effect of dual direct injection strategy on the operating range, combustion and emissions characteristics of a methanol/diesel dual direct injection engine. The results showed that the limits of the engine operating range could be obtained by observing the unstable and roar combustion in the cylinder when the methanol was injected directly during the intake stroke. The onset of roar combustion provided the upper bound of operating range at the methanol injection timing (SOIM) of − 60 °CA aTDC. High indicated thermal efficiency (ITE) could be achieved accompanied by widening the operating range of the engine. Systematic analysis results of combustion characteristics indicated that the increments of engine load and methanol energy substation ratio (ESR) were conducive to improving the combustion stability of the engine. Because of the reduced diffusion-controlled combustion, the methanol/diesel dual fuel direct injection strategy could realize a relatively low NOx emissions, which were insensitive to the SOIM at low ESR conditions. With the increase of ESR, injecting methanol close to TDC resulted in lower CO and HC emissions, especially at higher engine load conditions. This study confirms that the implementation of dual direct injection strategy has the potential to extend the operating range of a methanol/diesel engine, accompanied with high fuel economy and low emissions.

Numerical and Experimental Study on Combustion Characteristics of Micro-Gas Turbine Biogas Combustor

The use of biogas in land-based gas turbines for power generation is a promising approach to reducing greenhouse gases and our dependence on fossil fuels. The focus of this research was to investigate the fuel/air mixing and combustion performance in an DLE (dry low emission) type can combustor designed for a micro-gas turbine. The fuel and air mixing uniformity was studied considering the air flow characteristic and fuel injection performance through the numerical simulation. The influence of the fuel/air mixing characteristics on the combustion characteristics was studied by numerical simulation and experimental tests. The combustion characteristics studied included the temperature field in the combustor, the pattern factor at the combustor outlet, combustion efficiency, and pollutant emission characteristics. The results show the position of the fuel nozzle has little effect on the mixing uniformity due to the limited mixing space for the micro-gas turbine combustor, while there are optimal fuel nozzle diameters to generate the suitable fuel jet momentum for the mixing process. The fuel/air mixing characteristics had an obvious influence on the combustion performance for the studied DLE combustor. The increase in the fuel air mixing uniformity can decrease the NOx emissions and generate a better temperature distribution at the combustor outlet. The increased mixing uniformity may decrease the combustion efficiency and increase the CO emissions of the micro-gas turbine combustor.

An experimental study on penetration and mixing characteristics of liquid fuel in preheated supersonic airflows

The effect of air temperature on the penetration and mixing of a liquid hydrocarbon fuel injected into supersonic airflows was investigated. The supersonic air of Mach 1.8 with a total temperature in the range of 373–673 K was supplied to the test section. The liquid fuel was injected into the supersonic airflow by means of an alternating-wedge pylon with a fuel-to-air momentum flux ratio of 15. A laser sheet imaging system using Mie scattering has been used to investigate the instantaneous and ensemble-averaged characteristics of the fuel plume, the fuel/air mixing process, and fuel evaporation. As the air temperature increased, the Mie scattering signal gradually weakened due to fuel-air mixing enhancement and fuel evaporation downstream of the pylon. At an air temperature of 673 K, most of the fuel evaporated before reaching the outlet of the test section (X/d = 703). As the air temperature increased from 373 to 673 K, the total pressure loss without fuel injection decreased from 20.6 to 15.4%. In addition, the total pressure losses due to fuel injection was about 3.5% greater than that without fuel injection. As the air temperature increased, the fuel penetration height decreased. However, at the air temperatures higher than the fuel boiling temperature, the fuel jet/spray penetration heights were almost similar, showing little effect of temperature any further.

Evaluation of impingement effects on high-power diesel engine mixing process with an optimized stochastic combustion model

• The mixing-controlled combustion can be modelled by a stochastic combustion model. • The impingement is evaluated by a volume ratio between squish part and total spray. • The modeling is classified into different types according to the impinging location. • The spray impingement significantly enhances the mixing rate. Spray impingement has been proved to have remarkable influence on the mixing process, and particularly for high-power diesel engines, where the dominant diffusion combustion is mixing-controlled. Therefore, in this paper, the stochastic combustion model, which has the capability to indicate the heterogeneity of the fuel–air mixing, was optimized to account for the influence of spray impingement. The combustion chamber was divided into squish region and non-squish region, and by judging the location of the spray/wall impingement, the spray was correspondingly divided into squish part and bowl part. After that, a parameter that was defined as the volume ratio between squish part and the total spray was introduced to evaluate the influence of impingement on spray mixing rate. Finally, the optimized stochastic combustion model was validated by a single-cylinder high-power-density diesel engine to have the ability to estimate the influence of spray impingement on the mixing rate of mixture and the performance of diesel combustion with a pretty good accuracy, and the results showed that, spray impingement significantly enhanced the fuel–air mixing rate, contributed to leaner and more homogenous mixture and hence optimizing the combustion.

Combined effect of intake pipe deflection and injection timing on In-cylinder flow and combustion characteristics of a gasoline direct injection Wankel rotary engine

To improve the fuel–air mixing and burning processes in a rotary gasoline engine to achieve efficient combustion, this paper used a computational fluid dynamics method to establish numerical simulation models for the in-cylinder flow and combustion of the rotary engine with a peripheral intake system. The effects of the intake pipe deflection angle and fuel injection timing on the in-cylinder flow, fuel distribution, and combustion characteristics were studied. The results showed that deflecting the intake pipe to the left generated a stronger tumble flow in the cylinder to effectively increase the turbulence intensity. A greater deflection angle provided a more obvious improvement. Moreover, deflecting the intake pipe to the left made the fuel distribution more towards the rear part of the combustion chamber and a more uniform fuel distribution in the cylinder. The spray impingement effect on the rotor surface was also greatly improved. Fuel injection timing also has a great impact on the mixing process of the combustible mixture in the cylinder. More advanced fuel injection timing resulted in longer mixing time of the fuel and air to make the mixture more uniform. Finally, deflecting the intake pipe to the left significantly increased the in-cylinder peak pressure and improved accumulated heat release. However, these effects became weakened when the fuel injection timing was retarded. Particularly, when the intake pipe was deflected to the left by 10°, the in-cylinder peak pressure increased most with a 27.9% increase compared with the original design. When the intake pipe was deflected to the left by 30°, the turbulent kinetic energy increased most with a 93% increase compared with the original design. The intake pipe was deflected by 10° to the left and the fuel injection timing was 250° CA BTDC, which is the most helpful for improving engine performance. This study can provide guidance for design of the injection and intake systems of the rotary engine.

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.

Influence of isolator section on the shock augmented mixing in SCRAMJET engine

The paper aims to numerically study the effect of isolator design on shock-augmented mixing in a generic supersonic Ramjet (SCRAMJET) engine and reduce what would be a very complicated thermodynamics+aerodynamic optimization in the design to a simpler aerodynamic one. Changing the length of the isolator section effectively varies the thickness of the boundary layer that is formed in the isolator, which then enters into the SCRAMJET combustor. The study is conducted on a three-dimensional geometry using the Menter's SST model for turbulence on an in-house unstructured grid RANS solver. In this paper, the authors quantify the effect of the incoming boundary layer thickness on the shock-augment air-fuel mixing in the SCRAMJET combustor. It is observed that for the maximum variation in the isolator length, the normalized mixing volume only improves by 7%, which is only a marginally better mixing of air and fuel obtained as with an increase in the isolator length. The authors conclude that the Prandtl-Meyer expansion fan (PMEF) formed at the flame-holder effectively isolates the combustor from the isolator section to a degree that the isolator dimensions can therefore be chosen entirely on considerations of flow homogenization of inlet air before it enters the combustor section of the SCRAMJET engine. Although, the paper analyses a generic SCRAMJET with specific dimensions, the results are more generally applicable than for the specific design actually considered in the paper. Thus this study is also relevant to SCRAMJET engines that may differ radically from our considered design in form and dimensions.

A coupling study of ignition position on fuel-air mixing and diffusion of a linear hydrogen engine

Linear hydrogen engine (LHE) is a new technology of hydrogen energy utilization due to its flexible compression ratio coupling with ignition, fuel-air mixing, and combustion to optimize thermal efficiency. Fuel-air mixing in LHE is expected to be promoted by using ignition, which differs with conventional engine. This paper develops a full-cycle model which couples with dynamics, hydrogen-air mixing and combustion to describe the effect of ignition position, meanwhile a loop iterative calculation method is proposed to solve the coupling model for hydrogen-air mixing predication. The results show that ignition position variation can cause the piston trajectory to change significantly, and the higher equivalent speed is obtained in the medium ignition position. Besides, the higher equivalent speed in the injection stage is conducive to the diffusion of hydrogen, but the higher equivalent speed is not conducive to diffusion and mixing in the diffusion stage. More importantly, the equivalence ratio distribution at the ignition position is more uniform for the later ignition position due to the longer mixing stroke, and the mixture uniformity index at the ignition position is inversely proportional to the advance of the ignition position. Therefore, the late ignition is recommended to obtain a uniform hydrogen mixture.

Pulverized biomass flame under imposed acoustic oscillations: Flame morphology and emission characteristics

Forced intermittent combustion with periodical variations of pressure, velocity, and air-fuel ratios is a promising method to increase efficiency and reduce emissions from combustion and gasification applications. In this work, flame characteristics and emissions from a pulverized biomass burner are investigated under oscillations induced by an acoustically-driven synthetic jet. Instantaneous images of incandescent light emitted from flame were captured using high-speed cameras. The images were analyzed to identify the liftoff distance, flame length, and shape. The flame liftoff distance decreased under excited conditions, notably at high forcing amplitude applied to small particle size distribution (63-112 μm). In such conditions, acoustic forcing increases particle dispersion as presented in the previous work, providing conditions for earlier ignition due to enhanced fuel-air mixing besides reducing CO emissions. Flue gas emissions were influenced mainly by the particle size distribution, from which the 63-112 μm particle size presented the lowest values of CO and highest levels of NO emissions. The results presented stable flame edge positions for the particle size of 63-112 μm, while wide range particle distributions (0–600, 0-400 μm) had strong fluctuations, indicating high flame instability. The experimental work adds new insights regarding acoustic excitation in swirl burners, which could be used to optimize pulverized fuel combustion.

Co-flow jet diffusion flames in a multi-slot burner: Flow field and emissions

Motivated by the application of internally air-assisted flares and recent anomalous data that black carbon (BC) emissions changes are not monotonic with the amount of air added inside the fuel stream, a burner was designed to study the flow, emissions, and stability aspects of the existence of both normal and inverse jet diffusion flames in close proximity. Since the radius of curvature of a burner affects all aspects of combustion, such as dynamics, stability, flame structure, and emissions, a slot burner configuration, inspired by the Wolfhard-Parker burner, was adopted. This multi-slot burner consists of five parallel rectangular slots (i.e., the central slot for the inner air, sandwiched between two fuel slots, and surrounded by two outer air slots). When operated with laminar fuel and air flows, this burner produced flame sheets at each fuel-air mixing layer with open optical access to all the flows. Flow fields were characterized using two-dimensional two-component particle image velocimetry, while simultaneous single-lens reflex photography was used to establish the overall height and the location of the flames. Emission measurements specifically targeted BC, a precursor of soot, and oxides of nitrogen (NOx). The experimental test conditions involved constant flows of outer air and propane, and variable inner air flow. At one extreme, i.e., zero inner air flow, two flame bases were produced at the interfaces between the fuel and outer air streams. These flames merged to produce a normal diffusion flame. With the addition of inner air, two more flame bases appeared (referred to as the inner flame) and formed an inverse diffusion flame. With increasing inner air flow, the inner flame transitioned from a closed-tip flame, to an open-tip flame, and eventually to a lifted flame. With increasing inner air flow, the NOx emissions remained constant while the BC emissions, which were 3 g/kg-fuel with no inner air, rose by an order of magnitude as it became an open-tip flame. Only when the inner flame finally lifted off due to the inner air flow, did the BC emissions collapse to near zero. Phenomenological models associated with the importance of partial premixing were proposed to explain this collapse, thereby generalizing this finding to other combustion systems when attempting to reduce BC emissions through secondary internal air addition.

Mixing Enhancement of Inverse Jet Flame with Circumferential Fuel Ports

ABSTRACT Despite more than a decade-long effort to understand the flow structure in Inverse Jet flame (IJF) configuration, the performance enhancements on such configurations are sparsely investigated. A passive technique to improve the air-fuel entrainment in IJF configuration is implemented in this study by altering the central-air port geometry with an elliptical port. The flow and mixing characteristics are studied using a three-dimensional Favre-averaged method for the elliptical central air-port IJF compared to its circular counterpart. The numerical model is validated using the standard k-ε turbulence model to capture the complex flow physics and comprehend the mixing characteristics of IJF in this burner configuration. The elliptical central port brought the differences in the spreading and entrainment rates. The present work emphasizes flow dynamics and mixing characteristics, including the features of the recirculation zone, turbulence kinetic energy, and velocity distribution. The (omega) Ω vortex identification method and turbulent kinetic energy distribution provided more profound insights into the role of vortex structures on air-fuel mixing. The vortex structures are prominent in the novel elliptical port IJF compared to the circular counterpart, thus affecting the flame structure. The vortex strength Ω was found to be 0.94 in elliptical IJF and 0.88 for the circular central port IJF. This work further quantifies the enhanced air-fuel mixing as observed with the elliptical inverse jet diffusion flame based on mixture fraction mixing index, which can be applied to any IJF configurations. The mixing index at the axis switching location was greater than 80% for the elliptical IJF compared to 45% in the circular IJF configuration.

Effects of a CFD-improved dimple stepped-lip piston on thermal efficiency and emissions in a medium-duty diesel engine

Diesel piston-bowl shape is a key design parameter that affects spray-wall interactions and turbulent flow development, and in turn affects the engine’s thermal efficiency and emissions. It is hypothesized that thermal efficiency can be improved by enhancing squish-region vortices as they are hypothesized to promote fuel-air mixing, leading to faster heat-release rates. However, the strength and longevity of these vortices decrease with advanced injection timings for typical stepped-lip (SL) piston geometries. Dimple stepped-lip (DSL) pistons enhance vortex formation at early injection timings. Previous engine experiments with such a bowl show 1.4% thermal efficiency gains over an SL piston. However, soot was increased dramatically [SAE 2022-01-0400]. In a previous study, a new DSL bowl was designed using non-combusting computational fluid dynamic simulations. This improved DSL bowl is predicted to promote stronger, more rotationally energetic vortices than the baseline DSL piston: it employs shallower, narrower, and steeper-curved dimples that are placed further out into the squish region. In the current experimental study, this improved bowl is tested in a medium-duty diesel engine and compared against the SL piston over an injection timing sweep at low-load and part-load operating conditions. No substantial thermal efficiency gains are achieved at the early injection timing with the improved DSL design, but soot emissions are lowered by 45% relative to the production SL piston, likely due to improved air utilization and soot oxidation. However, these benefits are lost at late injection timings, where the DSL piston renders a lower thermal efficiency than that of the SL piston. Energy balance analyses show higher wall heat transfer with the DSL piston than with the SL piston despite a 1.3% reduction in the piston surface area. Vortex enhancement may not necessarily lead to improved efficiency as more energetic squish-region vortices can lead to higher convective heat transfer losses.

The evolution and structure of ignited high-pressure cryogenic hydrogen jets

The anticipated upscaling of hydrogen energy applications will involve the storage and transport of hydrogen at cryogenic conditions. Understanding the potential hazard arising from leaks in high-pressure cryogenic storage is needed to improve hydrogen safety. The manuscript reports a series of numerical simulations with detailed chemistry for the transient evolution of ignited high-pressure cryogenic hydrogen jets. The study aims to gain insight of the ignition processes, flame structures and dynamics associated with the transient flame evolution. Numerical simulations were firstly conducted for an unignited jet released under the same cryogenic temperature of 80 K and pressure of 200 bar as the considered ignited jets. The predicted hydrogen concentrations were found to be in good agreement with the experimental measurements. The results informed the subsequent simulations of the ignited jets involving four different ignition locations. The predicted time series snapshots of temperature, hydrogen mass fraction and the flame index are analyzed to study the transient evolution and structure of the flame. The results show that a diffusion combustion layer is developed along the outer boundary of the jet and a side diffusion flame is formed for the near-field ignition. For the far-field ignition, an envelope flame is observed. The flame structure contains a diffusion flame on the outer edge and a premixed flame inside the jet. Due to the complex interactions between turbulence, fuel-air mixing at cryogenic temperature and chemical reactions, localized spontaneous ignition and transient flame extinguishment are observed. The predictions also captured the experimentally observed deflagration waves in the far-field ignited jets.

Optical and computational investigations of the effect of Spray-Swirl interactions on autoignition and soot formation in a compression ignition engine fuelled by Diesel, dieseline and diesohol

Diesel engine injection strategy plays a crucial role in fuel–air mixing and consequent soot formation, requiring optimisation. Fuel properties affect the combustion characteristics of the engine significantly. In this study, the fundamental mechanisms of pilot-main injection interactions and the effects of fuel properties have been investigated using a combined approach of optical diagnostics in an optical engine and computational fluid dynamics (CFD) tools. A single-cylinder optical research engine (SCORE) with a cylindrical piston bowl was used for the optical engine investigations. Simulations were carried out using a re-entrant piston geometry model. Reynolds-Averaged Navier-Stokes (RANS) based Re-Normalization Group (RNG) k-ε model was adopted and solved using CONVERGE CFD solver. SAGE detailed chemistry solver was used for the calculations of combustion kinetics. The adaptive mesh refinement (AMR) strategy was used for improved grid resolution. The investigation results highlighted a striking difference between the combustion mechanisms of the pilot injection and the main injection. The pilot-main injection interactions became crucial for igniting the main fuel spray. The swirl intensity was dominant in the pilot-main injection interactions and the combustion initiation. The impinging spray jet and swirl interactions improved the radial mixing and reduced the soot in the up-swirl direction. Dieseline (a blend of diesel and gasoline) and diesohol (a blend of diesel and alcohol) resulted in lower soot formation than baseline diesel. This study demonstrated that in-cylinder swirl and oxygen concentration could be optimised to simultaneously reduce oxides of nitrogen (NOx) and soot emissions using oxygenated fuels.

A numerical study on the effects of bowl and nozzle geometry on performances of an engine fueled with diesel or bio-diesel fuels

Abstract One of the most important trends for advanced diesel engines is downsizing, i.e., higher power density, which means more fuel is burned in a shorter period. In order to achieve rapid combustion for a high-power density diesel engine, the effects of bowl shape, diameter-to-depth ratio of bowl, and arrangement of nozzle holes on combustion and performance were investigated by CFD simulation. The effects of four bowl shapes, two of which were double-layer split bowls (DLSBs), as well as four diameter-to-depth ratios and three arrangements of nozzle holes were numerically assessed. The results show that the DLSB with a shallow dish-like structure yielded a remarkable effect of swirling flow by fuel splitting into upper- and lower-layer zones, which improved fuel–air mixing, shortened combustion duration, thus, resulting in high combustion efficiency and power density. Moreover, with the increase in diameter-to-depth ratio of the B type DLSB, the turbulent kinetic energy and the peak of pressure and heat release rate increased, further increasing power density. Finally, when the DLSB with a diameter-to-depth ratio of 2.0 is coupled with the staggered double-layer arrangement of nozzle holes, the in-cylinder mixtures became more uniform at both the circumferential and radial directions, and the combustion was considerably accelerated, achieving an optimum specific power of 122.6 kW·L−1. Meanwhile, there was a slight decrement for peak pressure and NOx emission, and smoke decreased by 49.1%, which revealed substantial improvement in reduction in mechanical load and emissions.