Unburned Zone Research Articles

A numerical investigation of flame acceleration and deflagration-to-detonation transition: Effects of the position and the delayed injection time of fluidic obstacles

Fluidic obstacles, employed as a form of turbulence generator, are often utilized to facilitate flame acceleration and deflagration-to-detonation transition (DDT). This paper conducts a detailed numerical study focusing on the position (L1) and the delayed injection time. The results indicate that reducing L1 leads to an earlier interaction between the flame and vortex structures, thus enhancing the initial flame acceleration effect. However, this also results in a reduction in the movement of vortex structures, which prevents the enhancement of turbulence intensity within the channel. Conversely, notwithstanding increasing L1 can improve the turbulence intensity within the channel, vortex structures fail to interact with the flame, which is unfavorable for the DDT process. Therefore, an optimal L1 exists which not only improves initial flame acceleration but also accommodates DDT. Furthermore, from a comprehensive perspective, the effectiveness of the delayed injection strategy is constrained by the range of L1. When L1 is small, the delayed injection strategy can enhance the time window for the flame–jet interaction, thereby improving turbulence and finally enhancing the DDT performance. However, as L1 increases, this improvement gradually diminishes and ultimately disappears. Regarding the DDT process, this study reveals that the distribution and strength of the wave structure in the channel, the size of the recirculation zone, the motion effect of the vortex structure, and various flow instability are the internal causes of DDT. The intensity of the pressure and velocity fields in the unburned zone ahead of the flame plays a crucial role in the DDT process.

Numerical investigation on the effect of ignition timing on a low-temperature hydrogen-fueled Wankel rotary engine with passive pre-chamber ignition

Numerical investigation on the effect of ignition timing on a low-temperature hydrogen-fueled Wankel rotary engine with passive pre-chamber ignition

Post-fire vegetation dynamic patterns and drivers in Greater Hinggan Mountains: Insights from long-term remote sensing data analysis

Post-fire vegetation dynamic patterns and drivers in Greater Hinggan Mountains: Insights from long-term remote sensing data analysis

Numerical investigation of premixed syngas/air flame evolution in a closed duct with tulip flame formation

Numerical investigation of premixed syngas/air flame evolution in a closed duct with tulip flame formation

Numerical analysis of thermal mechanism in downward flame spread over inverted surfaces of discrete inclined solid fuels

Numerical analysis of thermal mechanism in downward flame spread over inverted surfaces of discrete inclined solid fuels

Exergy Analysis in Highly Hydrogen-Enriched Methane Fueled Spark-Ignition Engine at Diverse Equivalence Ratios via Two-Zone Quasi-Dimensional Modeling

In the endeavor to accomplish a fully de-carbonized globe, sparkling interest is growing towards using natural gas (NG) having as vastly major component methane (CH4). This has the lowest carbon/hydrogen atom ratio compared to other conventional fossil fuels used in engines and power-plants hence mitigating carbon dioxide (CO2) emissions. Given that using neat hydrogen (H2) containing nil carbon still possesses several issues, blending CH4 with H2 constitutes a stepping-stone towards the ultimate goal of zero producing CO2. In this context, the current work investigates the exergy terms development in high-speed spark-ignition engine (SI) fueled with various hydrogen/methane blends from neat CH4 to 50% vol. fraction H2, at equivalence ratios (EQR) from stoichiometric into the lean region. Experimental data available for that engine were used for validation from the first-law (energy) perspective plus emissions and cycle-by-cycle variations (CCV), using in-house, comprehensive, two-zone (unburned and burned), quasi-dimensional turbulent combustion model tracking tightly the flame-front pathway, developed and reported recently by authors. The latter is expanded to comprise exergy terms accompanying the energy outcomes, affording extra valuable information on judicious energy usage. The development in each zone, over the engine cycle, of various exergy terms accounting too for the reactive and diffusion components making up the chemical exergy is calculated and assessed. The correct calculation of species and temperature histories inside the burned zone subsequent to entrainment of fresh mixture from the unburned zone contributes to more exact computation, especially considering the H2 percentage in the fuel blend modifying temperature-levels, which is key factor when the irreversibility is calculated from a balance comprising all rest exergy terms. Illustrative diagrams of the exergy terms in every zone and whole charge reveal the influence of H2 and EQR values on exergy terms, furnishing thorough information. Concerning the joint content of both zones normalized exergy values over the engine cycle, the heat loss transfer exergy curves acquire higher values the higher the H2 or EQR, the work transfer exergy curves acquire slightly higher values the higher the H2 and slightly higher values the lower the EQR, and the irreversibility curves acquire lower values the higher the H2 or EQR. This exergy approach can offer new reflection for the prospective research to advancing engines performance along judicious use of fully friendly ecological fuel as H2. This extended and in-depth exergy analysis on the use of hydrogen in engines has not appeared in the literature. It can lead to undertaking corrective actions for the irreversibility, exergy losses, and chemical exergy, eventually increasing the knowledge of the SI engines science and technology for building smarter control devices when fueling the IC engines with H2 fuel, which can prove to be game changer to attaining a clean energy environment transition.

Explosion overpressure behavior and flame propagation characteristics in hybrid explosions of hydrogen and magnesium dust

• The explosion safety of magnesium-based hydrogen storage materials was concerned. • Explosion characteristics of hybrid hydrogen/magnesium dust mixture were clarified. • Using reaction kinetics to analyze the hybrid explosion mechanisms. The explosion risk of a hybrid mixture of hydrogen and magnesium dust is relatively high during the production of magnesium-based hydrogen storage materials and industrial magnesium products. To explore the explosion characteristics of a hybrid hydrogen/magnesium dust mixture, the overpressure variation and flame propagation characteristics of the hybrid hydrogen/magnesium dust mixture in a 5-L semi-closed explosion duct were experimentally studied. The results show that the explosion pressure ( P ex ) and the explosion pressure rise rate ((d P /d t ) ex ) of the hybrid hydrogen/magnesium dust mixture significantly increase only when the hydrogen concentration is greater than 10 %. The flame of a pure magnesium dust explosion mainly consists of the unburned zone, preheating zone, and combustion reaction zone. Compared with pure magnesium dust, the flame structure of the hybrid hydrogen/magnesium dust mixture explosion does not change significantly, but a red emission zone appears at the front of the preheating zone. Moreover, the range of the preheating zone is significantly larger compared to pure magnesium dust, and it increases with the hydrogen concentration. When the hydrogen concentration is low, the flame propagation speed of the hybrid hydrogen/magnesium dust mixture exhibits strong pulsation. The pulsation frequency of the flame propagation speed of the hybrid mixture explosion gradually decreases with the hydrogen concentration, but the average flame propagation speed continually increases. Based on the experimental results, the explosion mechanism of the hybrid hydrogen/magnesium dust mixture was analyzed from the perspective of reaction kinetics by numerical simulation.

On the propagation stability of droplet-laden two-phase rotating detonation waves

The propagation characteristics and stability of droplet-laden two-phase rotating detonation waves are studied by theoretical analysis and numerical simulations. The stability criterion of rotating detonation waves is proposed for an annular combustor fueled by liquid kerosene, which combines the pre-evaporation equivalence ratio φ pre and a dimensionless parameter Δ. The latter one is defined as the ratio of the droplet evaporation distance L E to the detonation wave front height L D . The maximum droplet diameter as well as the stability boundary for the rotating detonation wave without pre-evaporation is found theoretically. Furthermore, numerical simulations are conducted on the two-phase rotating detonation waves produced by kerosene droplets and high-temperature air by means of the Eulerian-Lagrangian method. The effects of initial droplet diameter d 0 and φ pre on the propagation characteristics of rotating detonation waves are analyzed. The mechanism of detonation instability and wave-quenching as d 0 and φ pre exceed the stability boundary is explored. Results show that for φ pre = 0, the droplet evaporation becomes longer and the rotating detonation wave tends to be less stable as Δ gradually increases. The unburned reactant pockets are generated at the detonation front and are consumed by the transverse detonation waves. When Δ is approximately equal to 1.0, the insufficient evaporation leads to the formation of local larger unburned reactant pockets. If the unburned reactant zones gradually enlarge, the flame and shock wave will be decoupled, and the detonation quenched soon. Increasing φ pre can significantly improve the propagation stability of detonation wave for Δ ∼ O(1.0). The stability regime of droplet-laden two-phase rotating detonation waves is obtained and the above stability criterion is verified based on the simulation cases. The conclusion will inspire the optimization design of the liquid-fuel rotating detonation engine.

Flame Propagation Characteristics of Hybrid Explosion of Ethylene and Polyethylene Mixture under Pressure Accumulation

In order to study the flame propagation characteristics of a ethylene/polyethylene hybrid explosion under pressure accumulation, a visual pressure-bearing gas/power hybrid-explosion experimental platform was built. The flame propagation characteristics of polyethylene and ethylene/polyethylene hybrid explosions in the closed vessel were analyzed. The results show that the flame brightness, flame front continuity and average flame propagation velocity of polyethylene dust explosion in the closed vessel increased first and then decreased when the polyethylene dust concentration increased. The curve of the flame propagation velocity with time had obvious pulsation characteristics. Adding 1% and 3% ethylene to different concentrations of polyethylene dust significantly improved its explosion flame brightness, flame front continuity and average flame propagation velocity. Moreover, it also improved the fluctuation amplitude of the explosion flame propagation velocity with time curve. The explosion flame of the polyethylene dust and ethylene/polyethylene hybrid mixture included four zones during the propagation process, which were denoted as the unburned zone, preheated zone, premixed flame zone and dust flame zone. The addition of ethylene to polyethylene dust can significantly increase its thickness of premixed flame zone and preheated zone, and the thickness increased when the ethylene concentration increased.

Explosion behavior of methane-air mixtures and Rayleigh-Taylor instability in the explosion process near the flammability limits

• After generating a stable flame, the flame speed is a fixed value within a certain flame radius. • The upward movement of the spherical flame is due to the density gradient produced by the combustion. • Rayleigh-Taylor instability in the explosion process near the flammability limits is analyzed. Methane is the main component of natural gas and biogas, and it has been applied in various fields in daily life and industrial production. A better understanding of the explosion behavior of methane mixture can contribute to the safe usage of this fuel and developing advanced explosion protection technology and devices. The main purpose of this study is to investigate the explosion behavior and the Rayleigh-Taylor (RT) instability in the explosion process of methane-air mixtures using a 20 L bomb. Overpressure trajectories and flame images using high-speed schlieren technique are recorded for each experiment. Explosion behavior of methane-air mixtures is investigated at 280 K covering wide equivalence ratios (0.61–1.42) and initial pressures (50 kPa ∼ 250 kPa). By analyzing the explosion image data, flame speed and laminar burning velocity are then calculated. Results indicate that as initial pressure rises, the maximum explosion pressure increases, while the flame speed and the laminar burning velocity decrease. The effect of initial pressure and equivalence ratio ( Ф ) on the spherical flame is also presented and analyzed through the combination of image data and pressure data. It is suggested that mass of reactants per unit volume and fuel composition have a strong relationship to ignition feasibility and ignition delay time. When Ф is 1.42 and the initial pressure is less than 150 kPa, the dynamic ignition process is recorded. Fores (pressure gradient, density gradient) are generated at the position of the flame front due to combustion. The density of spherical flame zone (burned zone) is lower than the unburned zone, therefore the spherical flame moves upward. The moving density stratification of the flame surface changes the structure of the flame front, which induces the RT instability. Near the flammability limits ( Ф = 0.61 or 1.42), the explosion pressure is weak and flame speed is slow (∼0.4 m/s). Due to the longer duration of forces, the effect of forces on the spherical flame becomes apparent. It is found that the RT instability leads to structural instability of the flame.

Numerical investigation on combustion and knock formation mechanism of hydrogen direct injection engine

Hydrogen is an ideal alternative fuel of internal combustion engine because of its high thermal efficiency and no carbon emissions, but knock has become one of the main obstacles restricting its application in engines. The characteristic of flame propagation and the interaction of flame and pressure wave during knocking development was studied numerically in a hydrogen fueled engine. The results showed that the knock is caused by the spontaneous combustion of the end mixture and the interaction between flame and pressure wave. HO2 and H2O2 play a significant role in the flame development by affecting the formation of OH in low-temperature reaction zone. Because the low-temperature reaction zone is preheated by reaction heat of the chain reaction, H2+OH=H2O+H, to prepare for the subsequent high-temperature reactions. Therefore, the faster pressure wave accelerates the flame propagation by promoting the generation of HO2 and H2O2 in the unburned zone at the flame front. The accelerated flame in turn further enhances the pressure wave. Like this, the pressure wave and flame promote and strengthen each other, and finally develop into detonation.

Exergy evaluation of equivalence ratio, compression ratio and residual gas effects in variable compression ratio spark-ignition engine using quasi-dimensional combustion modeling

This study is founded on previously validated, in-house, comprehensive, two-zone, quasi-dimensional combustion model, predicting performance and emissions in spark-ignition engines. The model is extended to include exergy terms complementing the first-law analysis results, which provide further useful information by implementation on test results acquired from an experimental, Ricardo E6, variable compression ratio (VCR) gasoline engine at the authors’ laboratory, operated under various CR and equivalence ratio (EQR) values at wide-open throttle. The model consists of unburned and burned zones simulating the combustion process, following strictly the flame-front movement in combustion chamber. The eventual combustion of the unburned mixture turbulent entrainment into burned zone through the flame-front area is followed closely. The exergy terms of each simulated zone are identified and computed, considering also chemical exergy components (diffusion plus reactive). The accurate account of temperature and species histories in burned zone after entrainment from the unburned one, should lead to more precise evaluation. This is important if irreversibility is computed from balance of all other exergy terms. The investigation examines and compares exergy-wise various EQR, CR and residual gas fractions. Presented diagrams of exergy terms for each zone and cylinder charge provide detailed information for chemical exergy, irreversibility and exergy losses.

Study of Displacement Characteristics of Fire Flooding in Different Viscosity Heavy Oil Reservoirs

A field test in the Xinjiang oilfield in China shows that the viscosity of heavy oil has a certain influence on the combustion dynamics and injection-production performance of fire flooding. The experiment in this study uses a one-dimensional combustion tube to study the temperature, gas composition, and air injection pressure and the production performance of the fire flooding of heavy oil with different viscosities. The results show that the oil viscosities of 1180–22500 mPa·s can achieve stable combustion, and the O2 content of the gas produced during the stable combustion stage is <0.5%. The higher the viscosity of the heavy oil, the higher the temperature in the burned zone and the smaller the range of the temperature increase in the unburned zone. The air injection pressure will increase rapidly until a stable seepage channel is formed, and then, it will drop to a level close to the formation pressure. High-viscosity heavy oil requires a higher air injection pressure and will remain in the high-pressure stage for a longer period of time. Low-viscosity heavy oil has a low water cut in the early stage of fire flooding, a large oil production rate, and a low and stable air–oil ratio. The water cut of high-viscosity heavy oil increases rapidly in the early stage of fire flooding and then decreases gradually, so a good air–oil ratio can only be obtained in the middle and late stages of fire flooding. Thus, fire flooding may be more suitable for application in common heavy oil and some extra heavy oil reservoirs with lower viscosities.

On the evolution of fuel droplet evaporation zone and its interaction with flame front in ignition of spray flames

Evolution of fuel droplet evaporation zone and its interaction with propagating flame front are studied in this work. A general theory is developed to describe the evolutions of flame propagation speed, flame temperature, droplet evaporation onset and completion locations in ignition and propagation of spherical flames. The influences of liquid droplet mass loading, heat exchange coefficient (or evaporation rate) and Lewis number on spherical spray flame ignition are studied. Two flame regimes are considered, that is, heterogeneous and homogeneous flames, based on the mixture condition near the flame front. The results indicate that the spray flame trajectories are considerably affected by the ignition energy addition. The critical condition for successful ignition for the fuel-rich mixture is a coincidence of inner and outer flame balls from igniting kernel and propagating flame. The flame balls always exist in homogeneous mixtures, indicating that ignition failure and critical successful events occur only in purely gaseous mixture. The fuel droplets have limited effects on minimum ignition energy, which, however, increases monotonically with the Lewis number. Moreover, flame kernel originates from heterogeneous mixtures due to the initially dispersed droplets near the spark. The evaporative heat loss in the burned and unburned zones of homogeneous and heterogeneous spray flames is also evaluated, and the results show that for the failed flame kernels, evaporative heat loss behind and before the flame front first increases and then decreases. The evaporative heat loss before the flame front generally increases, although non-monotonicity exists, when the flame is successfully ignited and propagate outwardly. For heterogeneous flames, the ratio of the heat loss from the burned zone to the total one decreases as the flame expands. Moreover, droplet mass loading and heat exchange coefficient considerably affect the evaporating heat loss from burned and unburned zones.

Modelling and simulation of virtual NOx sensor for diesel engine using thermodynamic model

This paper presents a thermodynamic model for virtual NOx sensor which is capable of predicting nitrogen oxides (NOx) emissions in diesel engines. In-cylinder volume and pressure are the inputs from estimator. The proposed method is based on a two-zone thermodynamic model which divides the combustion chamber into burned and unburned gas zone. The contribution of proposed method arises from: In-cylinder volume and pressure are the inputs from estimator using slider crank mechanism. And the two-zone thermodynamic modeling framework is applied in a way that the thermodynamic equations can be solved in a closed form, which provides the basis for achieving high level of predictiveness using MATLAB which uses ADVISOR as prerequisite for the project. The model is validated using real data values. The model is suitable for two different direct injection diesel engines, i.e. a light and a heavy-duty engine. Also, it maintains a low pipe emission which minimizes fuel consumption.

Two-Zone Simulation of an Axial Vane Rotary Engine Cycle

An axial vane rotary engine (AVRE) is a novel type of rotary engines. The engine is a positive displacement mechanism that permits the four “stroke” action to occur in one revolution of the shaft with a minimum number of moving components in comparison to reciprocating engines. In this paper, a two-zone combustion model is developed for a spark ignition AVRE. The combustion chamber is divided into burned and unburned zones and differential equations are developed for the change in pressure and change in temperature in each zone. The modelling is based on equations for energy and mass conservation, equation of state, and burned mass fraction. The assumption is made that both zones are at the same pressure <i>P</i>, and the ignition temperature is the adiabatic flame temperature based on the mixture enthalpy at the onset of combustion. The developed code for engine simulation in MATLAB is applied to another engine and there is a good agreement between results of this code and results related to the engine chosen for validation, so the modelling is independent of configuration.

Kinetic modelling of combustion in a spark ignition engine with water injection

Abstract This work models the impact of direct water injection on the combustion process in a spark ignition engine. It uses a two-zone kinetic model coupled with detailed combustion chemistry to highlight the thermodynamic and chemical-kinetic interactions between gasoline combustion and water injection. The modelling results agree closely with measurements from a highly boosted, direct injection gasoline engine. This study first proposes an approach to model the mass fraction burned (MFB) profile using a representative in-cylinder pressure trace. The derived MFB profile is then used as the input for a two-zone kinetic model. Within this model, predictive kinetic modelling is used to estimate the knock limited spark advance (KLSA) for a baseline engine operating condition without water injection and subsequently, for several conditions with water injection. Predicted KLSA values obtained using this method agree closely with measured results. Utilising the approach developed in this study, the modelled MFB profile at the baseline operating condition was found to be similar to that obtained at the condition with a water/fuel ratio (WFR) of 60%. This result is likely due to the competing and contrasting effects of reduced in-cylinder temperature versus more advanced combustion phasing at conditions with water injection. Further thermodynamic analysis shows that the charge cooling effect afforded by direct water injection is much greater than the dilution effect in terms of advancing the knock limited combustion phasing. Water injection also affects the kinetic processes that take place in the unburned gas zone, but mainly by altering the in-cylinder thermodynamic conditions – the injected water is not directly involved in the low temperature chemistry in the unburned gas zone, it simply acts as a collision partner.

Exploring the Influences of Conductive Graphite on Hydroxylammonium Nitrate (HAN)‐Based Electrically Controlled Solid Propellant

AbstractThe electrically controlled solid propellant (ECSP) has many advantages that are not available with traditional propellants, such as multiple start/stop operations and controllable thrust. In this research, conductive graphite (C) was employed as an additive to hydroxylammonium nitrate (HAN)‐based ECSPs to enhance electrical conductivity and thermal conductivity, and the influences of C on the burning characteristics of ECSPs were investigated by some characterization methods. The addition of 5 % C increases the electrical conductivity and thermal conductivity by 13.2 % and 18.9 %, while reduces the theoretical specific impulse (Isp) and adiabatic flame temperature (Tf) by 7.9 % and 18.4 %. C acting as electric and heat energy transfer media was demonstrated by TG‐DSC. ECSPs containing 1 %, 3 %, and 5 % C reinforce the burning rate by 11.4 %, 25.6 %, 35.9 % and mass loss by 6.9 %, 22.3 %, 47.8 % at 200 V; and enhance the burning rate by 39.2 %, 62.4 %, 104.9 %, and mass loss by 46.7 %, 84.7 %, 200.1 % at 0.6 MPa. This can be explained by the fact that the increase in electrical conductivity promotes the electrochemical decomposition rate of the ionic oxidizer in ECSPs at atmospheric pressure (0.1 MPa) and the rise in thermal conductivity promotes the heat transfer to the unburned zone of the propellant under high pressure (0.2 MPa∼0.6 MPa).

A Novel Two-Zone Thermodynamic Model for Spark-Ignition Engines Based on an Idealized Thermodynamic Process

The thermodynamic model is a valuable simulation tool for developing combustion engines. The most widely applied thermodynamic models of spark-ignition engines are the single-zone model and the two-zone model. Compared to the single-zone model, the two-zone model offers more detailed in-cylinder thermodynamic conditions, but its governing equations are numerically stiffer, therefore it is restricted when applied in computationally intensive scenarios. To reduce the two-zone model’s stiffness, this paper isolates an idealized thermodynamic process in the unburned zone and describes this idealized thermodynamic process by an algebraic equation. Assisted with this idealized thermodynamic process, this paper builds a novel two-zone model for spark-ignition engines, whose governing equations are simplified to a set of two ordinary differential equations accompanied by a set of three algebraic equations. Benchmarked against the single-zone model and conventional two-zone model, the novel two-zone model is formed and validated by experimental results, and its stiffness is quantitatively evaluated by linearizing its governing equations at simulation steps. The results show that the novel two-zone model inherits the conventional two-zone model’s ability to estimate both zones’ state variables highly accurately while its simplified structure reduces its stiffness down to the level of the single-zone model, accelerating the computation speed.

Characterization of the turbulent flame front surface in spark ignition engines during spark ignition operation to identify controlled auto-ignition and abnormal combustion

The combustion diagnostics and subsequent analysis are standardized tools based on the estimation of the heat release law (HRL). From this estimation, the different combustion parameters can be obtained: combustion phasing and duration, heat release rate, and so on. This analysis might be usually enough to study traditional spark ignition (SI) engines. However, with the new upcoming SI engines, this is probably not the case anymore, since different combustion modes can be operated in the same engine, as for instance a combination of SI and controlled auto-ignition (CAI) combustion modes. When different combustion modes are combined, it seems interesting to study in more depth the HRL, trying to get more data and to study the differences among the diverse combustion modes. Toward this end, a methodology to go deeper in the study of the HRL is proposed in this work, consisting of, mainly quantifying and taking into account the most relevant influencing parameters: the fuel properties (mainly its lower heating value), the in-cylinder oxygen content, the density of the burned and unburned zones, the laminar combustion speed, and the turbulence effect. With the proposed methodology, a standard SI combustion, developed by a flame front, can be characterized at any given operating point. This would allow to predict which the combustion developement would be, at this operating point, assuming it to be developed by a flame front. Subsequently, this SI combustion prediction can be compared to the one obtained experimentally, making it possible to identify and analyze abnormal combustion phenomena, as well as to study the differences between a combustion developed by a flame front (SI) and by auto-ignition (CAI). Derived from this work, an alternative equation to experimentally characterize the laminar combustion velocity has also been proposed, in order to improve its applicability in a wider range of fuel/air ratios and dilution degrees.