Constant Volume Combustion Chamber Research Articles

Effects of energy-share and ambient oxygen concentration on hydrogen-diesel dual-fuel direct-injection (H2DDI) combustion in compression-ignition conditions

This study investigates the ignition and combustion characteristics of interacting hydrogen (H2) and diesel surrogate jets under simulated compression-ignition engine conditions. The experimental setup includes two converging single-hole injectors in an optically accessible constant-volume combustion chamber (CVCC). The parameters varied in the study are fuel injection durations and ambient O2 concentrations (10 to 21 vol.%). The results show that a longer interaction between the diesel products and the H2 jet is required to achieve ignition of the H2 jet at lower O2 concentrations. Once ignited, the flame stabilises near or at the nozzle, except under the lowest ambient O2 condition of 10 vol.% where a lifted flame is observed. The lift-off response, however, is influenced by the relative injection duration of the fuels, with the interaction between the incoming H2 jet and the diesel combustion recession products possibly playing a role. The interaction between the jets also affects the recorded intensity and the distribution of the diesel fuel jet soot zone.

Spray-evaporation characteristics of n-pentanol/n-dodecane binary fuel at ultra-high injection pressure

The spray and evaporation characteristics of the fuel directly affect the combustion and emission performance of the engine. In this work, the atomization and evaporation characteristics of the n-pentanol/n-dodecane binary fuel were investigated by varying the injection pressure (100–300 MPa). Process of breaking, atomization, evaporation and oil-gas mixing were measured by schlieren technique. The liquid phase length of the spray was measured using the Mie scattering method. Numerical simulations were performed in a constant volume combustion chamber to further investigate the effects of injection pressure. The findings demonstrate that n-pentanol/n-dodecane binary fuel has slightly more liquid penetration than pure n-dodecane fuel, which may be attributed to n-pentanol's higher density. The results also show that the ultra-high injection pressure notably improved the spray penetration and reduced the droplet size that led to fragmentation of the fuel droplets and effectively improved the atomization characteristics. Moreover, a substantial reduction of over 20% in droplet size was observed with the increase in injection pressure from 100 to 200 MPa. However, it has been observed that the reduction in droplet size due to an increase in injection pressure diminishes when the pressure exceeds 200 MPa. In such cases, the size reduction is less than 10%, which implies that the impact of ultra-high injection pressure on size reduction is restricted. Although the impact of ultra-high injection pressure on fuel evaporation under evaporative conditions is limited, it can enhance fuel air mixing performance through intensified turbulence activity within the spray, thereby promoting greater air roll-up.

Experimental study of the effect of structural differences in the composition of kerosene-like blended fuels on low-temperature autoignition in a constant volume combustion chamber

The cold start performance of compression ignition engines at high altitudes is expected to be improved by means of fuel design involving kerosene-like blended fuels. In this study, the autoignition characteristics of ten blended fuels with different chemical compositions and physical properties were investigated in a constant volume combustion chamber. Steady-state conditions in a low thermal atmosphere are considered at varying temperatures (723, 773, 823 K), pressures (2.2, 2.8, 4 MPa), injection pressures (40, 60, 80 MPa), and injection pulse widths (1.5, 2.5, 5 ms). The relative importance and relevance of the physical and chemical properties of the fuel to the ignition delay were evaluated based on characteristic parameters e.g. total ignition delay, physical delay, chemical delay, combustion delay, pressure and heat release rate evolution. It was found that adding ether-oxygenated component content to kerosene-like significantly shortened the ignition delay, followed by chain alkanes, naphthenes, and aromatics, in that order. The effect of oxidation chemistry changes on the ignition performance of fuels in low-temperature-pressure environments is more significant than physical properties. As the thermal atmosphere or fuel activity intensifies, the percentage of physical delay in the total ignition delay gradually exceeds that of chemical delay as the main part, and the variability in the ignition qualities of fuels with different physicochemical properties diminishes. There is a good linear correlation between ignition delay and cetane number (ranging from 44 to 60), density, viscosity, and recovery temperature, whilst the role of cetane number and density is more prominent.

Inhibition effect and mechanism of CF3CHCl2 on methane flame in oxygen-enriched environment

In oxygen-enriched environment (oxygen concentration above 21%), flammable materials become even more flammable with higher net heat release rate, burning velocity and combustion intensity. Unfortunately, most investigations concentrated on the characteristics and mechanisms of oxygen-enriched combustion, the methods to control or weaken their perniciousness of oxygen-enriched fire have been rarely reported. CF3CHCl2 (HCFC-123, R123) has been demonstrated excellent inhibition performance for oxygen-normal combustion in the fire suppression experiment, was considered as the potential fire extinguishing agents of aircraft cargo. This study performed the comparison between the inhibition performance and kinetic mechanism of CF3CHCl2 for the CH4-N2-O2 (21% of O2) flame and the flames enriched up to 23.1%, 25.2%, 27.3% of oxygen with constant-volume combustion chamber and numerical prediction. Even for the oxygen-enriched flames, CF3CHCl2 showed substantial inhibition effect on the burning velocities and adiabatic flame temperature. The chain-radicals production/consumption rate and normalized sensitivity coefficients were investigated to analyze the in-depth inhibition mechanisms of CF3CHCl2 for oxygen-enriched flames. The mechanism analysis found that the inhibition effect of CF3CHCl2 and relevant halogenated species on oxygen-enriched flame are smaller than that in normal flame.

Investigation of ammonia cracking combined with lean-burn operation for zero-carbon combustion and NO/N2O/NO2 improvements

Ammonia has received significant attention as a carbon-free fuel and a promising carrier of hydrogen. Ammonia cracking presents an effective method for on-line hydrogen production, eliminating the need for hydrogen transportation and storage. This paper aims to study the comprehensive impacts of ammonia cracking ratio (CR) and equivalence ratio (ER) on combustion performance and NO/N2O/NO2 behaviors under lean-burn operation mode. The ammonia/hydrogen mechanism was updated and simplified to enhance the calculation efficiency in 3D simulation. Visualization measurements for flame propagation and NO and 3D simulations were simultaneously performed in a constant volume combustion chamber (CVCC). The results showed that ammonia blending with the cracked gas of H2/N2 for CR can significantly enhance the combustion rate under lean-burn operation at an ER of 0.5. Nevertheless, this enhancement leads to an increase in NOx due to elevated OH and HNO formation, regardless of N2 separation. In the context of varying ER, ranging from 0.5 to 0.3, the study revealed that increasing CR combined by decreasing ER at a certain point can sustain a higher combustion rate while reducing NOx emissions. For instance, CR45ER0.4 yields a much shorter combustion duration and lower NOx compared to pure ammonia at an ER of 1.0. However, a further increase in CR, such as CR80ER0.3, results in higher NOx emissions again. Moreover, complete cracking or separation from the non-cracked ammonia proves favorable for NOx elimination due to the absence of fuel-NO. The current study provides insights on high-efficiency and clean zero-carbon combustion technology.

Fundamental Experiment of Ignition and Flame Development in Turbulent Jet Ignition Ignited Methane Jet Flame

ABSTRACT In heavy-duty natural gas engines, diesel is usually injected before natural gas to initiate methane combustion. However, such an approach requires additional fuel and precise injection strategies. In the present work, an alternative approach of pre-chamber turbulent jet ignition (TJI) ignited methane jet flame is performed experimentally. The underlying mechanism of TJI-HPDI jet combustion is investigated in an optically visualized constant-volume combustion chamber with an active pre-chamber. It is found that with the increase of the injection-ignition delay (ti), the combustion process undergoes several transitions. In addition, the success ignition region and ignition failure region separated by ti and T MI are discerned on the ti-T MI diagram. As ti increases, there is an initial rise in both the mean flame front velocity and peak pressure, followed by a subsequent decrease This behavior highlights the TJI-HPDI system’s notable advantages in expanding the lean combustion limit and enhancing overall combustion characteristics compared to premixed combustion. These advantages can be attributed to the mixture stratification and turbulence. Furthermore, pre-chambers fueled with hydrogen can further improve the intensity and energy of the turbulent hot jet, which further decreases the critical injection-ignition delay to initiate global flame propagation in the TJI-HPDI system. The present work deals with real engineering technologies, and it will provide new insights into the design and control of TJI-HPDI engines, especially for heavy-duty natural gas engines.

Impact of n-butanol addition to hydrogenated catalytic biodiesel fueled a constant volume combustion chamber; a computational study

ABSTRACT The continuous quest for renewable, efficient, and clean energy sources has led to significant advances in the field of biodiesel production. Hydrogenated catalytic biodiesel (HCB) seems to be a promising option among various renewable biodiesel fuels which can be obtained from waste cooking oil. However, its extreme ignitability limits its application in existing engines. Therefore, a blend of n-butanol and HCB has been proposed in the present study, aiming to explore the spray combustion and soot formation behavior of these blends as a step toward the best possible utilization of this blend in engine. The objective was to examine the impact of adding n-butanol to HCB fuel on spray combustion and soot particles. A new skeletal chemical kinetic model of HCB/n-butanol blend was introduced and validated through measured data reported in the literature. Numerical studies have been performed in a constant volume combustion chamber under reactive spray environments. The simulation results concluded that, blending 20%, 40%, and 60% n-butanol with HCB fuel increased the heat release rates by 31.6%, 47.8%, and 87%, respectively. Furthermore, introducing a 20% of n-butanol into HCB fuel resulted in a 35.7% increase in ignition delay time and a 35.3% increase in flame liftoff length. These results demonstrated the reactivity of n-butanol/HCB mixture experienced a decrease when increasing the n-butanol fraction in the blend, while the fuel atomization was improved. The simulation results also demonstrated a notable reduction in the equivalence ratio when the n-butanol addition ratio was increased, which was a result of the entrainment effect that enhanced the mixing of fuel spray and surrounding fresh air. The simulation results also demonstrated that, adding 20%, 40%, and 60% n-butanol to HCB fuel resulted in a 25%, 43%, and 50% reduction in soot mass of HCB, respectively. This was mainly attributed to the less amount of C2H2, A1, and A4 species with n-butanol addition. Consequently, this sequential reduction leads to suppression of PAHs growth, nucleation, surface growth, and soot coagulation. Besides, a longer flame length resulting from the increased blending ratio of n-butanol could also have an additional soot suppression effect by improving air-fuel mixing and thus suppressing the overall soot mass.

Effect of the β-hydroxy group on ester reactivity: Combustion kinetics of methyl hexanoate and methyl 3-hydroxyhexanoate

One of the major biofuels used today is biodiesel, composed of fatty acid methyl esters. The fats and oils feedstocks used to make biodiesel are in relatively short supply. To meet increasing global demand, engineered microorganisms have been developed to catalyze conversion of sugar to fatty acid esters that contain unique β‑hydroxy-esters. This study investigated the effect of a hydroxyl group on the combustion characteristics of methyl esters by comparing the chemical behavior of methyl hexanoate (MHx) and methyl 3-hydroxyhexanoate (M3OHHx)—used as surrogates for diesel-boiling-range esters with longer fatty acid chains. The oxidation of these esters was studied experimentally in a flow reactor at 0.84 and 10 bar, 600 to 1,100 K, and stoichiometric conditions; and in a constant volume combustion chamber at 5 and 10 bar, 600 to 900 K, and equivalence ratios of 0.3 and 0.6. MHx was more reactive in the constant volume chamber at temperatures below 800 K at 10 bar and equivalence ratio of 0.6. MHx also exhibited a higher indicated cetane number (16.4) than M3OHHx (8.1). We investigated this reactivity trend using an updated MHx kinetic model and M3OHHx model developed as part of this work. The kinetic models predicted that radicals formed from MHx were consumed by low-temperature chemistry reactions, whereas the M3OHHx reactivity was significantly governed by the β-radical (on the same carbon as the OH group) chemistry which mainly terminated via the less reactive chain propagation pathway to methyl-3-oxohexanoate + HO2. This study extends our understanding of structural effects on ester reactivity, which will allow for accurate surrogate formulation and performance simulations.

Development of Physical-Chemical Surrogate Models and Skeletal Mechanisms for the Combustion Simulation of Several Jet Fuels

Abstract The physical–chemical surrogate models for S-8, Jet-A, and RP-3 fuels to capture their physical and kinetics properties have been developed in this study. n-dodecane (nC12H26), 2,5-dimethylhexane (C8H18-25), and toluene (C6H5CH3) were chosen as candidate surrogate components and formulated by the function group based surrogate fuel methodology. Some important physical properties and spray characteristics for S-8, Jet-A, and RP-3 surrogate models were validated. The results indicate that present surrogate models can well emulate various physical properties to accurately reproduce the spray characteristics. Then, a minimal and high-precision surrogate skeletal mechanism that can be suitable for computational fluid dynamics (CFD) simulations was developed and validated against some fundamental combustion experiments for each surrogate component. Furthermore, the performances of surrogate models that contain the surrogate formulation and associated skeletal mechanisms were validated against the experimental data on ignition delay times (IDTs), species concentration profiles, and laminar flame speeds (Su0) in a wide range of conditions. Finally, the surrogate fuels were used to combustion CFD simulations to model the spray combustion process in a constant volume combustion chamber. It can be seen that the agreements between the simulation and experiment in fundamental and spray combustion characteristics are reasonably good, which proves that present surrogate models are accurate and robust to be applied in CFD simulations.

Experimental Investigation on Cross-Impingement Characteristics Under Various Biodiesel-Butanol Blended Proportions and Ambient Conditions

Abstract The cross-impingement phenomenon always appears in several diesel engines with two or more injectors. Meanwhile, the application of biofuels has a great potential in realizing clean and efficient combustion. Therefore, the investigation aims to explore the cross-impingement characteristics at small (10%), middle (30%), and large (50%) biodiesel-butanol blended proportions. Experiments are conducted in a constant-volume combustion chamber with twin injectors. Spray images are captured by optical diagnosis techniques. Several macroscopic parameters are obtained, including diffusion length, collision width, and spray area. Results show that the cross-impingement accelerates the droplet interaction, and the spray presents a “fan-shaped” behavior after the collision, which promotes a more uniform mixing between the fuel and ambient gas. As the twin sprays collide at 120 deg, the vapor-phase vertical diffusion rate is close to the vertical component of the single spray, and the horizontal diffusion rate is about 1.2 times the vertical diffusion rate. The cross-impingement is likely to decrease the spray-wall impingement owing to a change in the diffusion direction. At various blended fuels, the biodiesel blended with 30% n-butanol displays the smallest liquid-phase diffusion length, width, and area. The further increase in the n-butanol mixing ratio leads to larger liquid-phase parameters. Contrary to the biodiesel blended with 10% n-butanol, the biodiesel blended with a higher proportion of n-butanol presents faster vapor-phase diffusion, which promotes fuel-gas mixing.

Manipulating hydrogen oxy-combustion through carbon dioxide addition

Hydrogen (H2) is increasingly viewed as an attractive carbon-free fuel due to its potential compatibility with the existing transportation and conversion infrastructure. However, one of the major challenges facing large-scale deployment is the fundamentally different combustion properties it has in comparison to commonplace hydrocarbon fuels such as natural gas. For example, the laminar burning velocity (LBV) of a combustible mixture has a direct impact on how it can be used in internal combustion engine or gas turbines; the LBV of hydrogen is over 7 times greater than that of natural gas when combusted in air. Carbon dioxide (CO2) can be used as a working fluid, as opposed to nitrogen (in air), to reduce the flame speed of hydrogen combustion. A mixture of hydrogen, oxygen, and carbon dioxide as a working fluid can provide LBVs comparable to natural gas in air, which potentially enables existing conversion architectures. Furthermore, by replacing nitrogen (N2) with CO2 in the mixture, NOx emissions are avoided and opportunities for carbon sequestration or closed-cycle processes are possible. This study experimentally explores fundamental premixed oxy-combustion properties of H2-CO2 mixtures in a constant volume combustion chamber (CVCC) across a range of initial pressures (1, 1.5, and 2bar) and equivalence ratios (0.4, 0.6, 0.8, and 1). The spherically expanding flames are examined to determine the flame speed, LBV, and lower flammability limits (LFL) with respect to different CO2 concentrations (40%, 60%, 65%). Furthermore, at an initial pressure of 1bar and stoichiometric conditions, it was found that the flame speed of the 65% CO2 case was lower than that of CH4-air combustion.

Autoignition Characterization of Hydrogen Directly Injected into a Constant-Volume Combustion Chamber through a Heavy-Duty Injector

One factor limiting the exploitation of hydrogen as a fuel in internal combustion engines is their tendency to autoignition. In fact, on one hand, its low activation energy facilitates autoignition even with low compression ratios; on the other hand, this can become uncontrollable, due, for instance, to the presence of hot spots in the combustion chamber or to the collision of hydrogen on close surfaces. This represents a limit to the use of hydrogen at medium–high loads, therefore limiting the power density of the engine. In this work, hydrogen was injected at a pressure ranging between 15 and 25 bars into a constant-volume combustion chamber in which the temperature and pressure were increased by means of a previous combustion event. The phenomena taking place after hydrogen injection were observed through fast image acquisition and characterized by measuring the chamber pressure and temperature. In particular, ignition sites were established. The physical system was also modeled in Ansys Fluent environment, and the injection and mixture formation were simulated in order to evaluate the thermo-fluid dynamic field inside the combustion chamber just before autoignition.

Assessment of Self-Ignition Properties of Canola Oil–n-Hexane Blends in a Constant Volume Combustion Chamber and Compression Ignition Engine

This article presents the results of an assessment of the combustion process of blends of n-hexane and canola oil. Tests were conducted for pure canola oil and its blends with n-hexane, with a max. n-hexane content of 20% by volume. The tests were carried out using the constant volume combustion chamber (CVCC) method as well as a diesel engine. For comparison purposes, the results for typical diesel fuel are also presented. Tests on the self-ignition properties of the n-hexane–canola oil blend, conducted in a CVCC according to the normative method for diesel fuel, showed little effect on the combustion process. However, previous tests conducted on a diesel engine of a passenger car showed a favorable effect of the n-hexane addition to canola oil on the combustion process in the engine, the performance and environmental parameters obtained. This shows that for some fuels, the evaluation of self-ignition and combustion properties in a constant volume combustion chamber, under conditions corresponding to diesel fuel tests, is not sufficient. The findings of this research may be beneficial in optimizing the diesel engine combustion systems fueled by renewable fuels. As the results of the tests have shown, the standardized method for determining the combustion process of the CVCC method does not always fully reflect the results obtained in engine tests. The results obtained by the CVCC method can be successfully used when comparing fuels, but when considering the selection of fuel for the engine, the results obtained in engine tests should be relied on first and foremost.

Development of a Skeletal Mechanism of a Four-Component Diesel Surrogate Fuel Using the Decoupling Method.

Alkylcyclohexanes with a long alkyl chain account for more than 30% of diesel fuel but seldom used in the oxidation mechanism of diesel surrogate fuel due to the lack of a reduced skeletal mechanism. Hence, a four-component diesel surrogate fuel was developed with n-butylcyclohexane (NBCH) as the representative of alkylcyclohexanes with a long alkyl chain in real diesel. The surrogate fuel can reproduce the physicochemical characteristics of real diesel, especially the distillation range. The reduced mechanism of NBCH was developed, and the skeletal mechanism of the surrogate fuel was formulated including 80 species and 251 reactions based on the decoupling method. The mechanism was validated under a wide range of conditions with the experimental results of ignition delay time (IDT), laminar flame speed, and species concentrations of both pure components and diesel. The accuracy of the mechanism on the spray and ignition performance was further validated against the experimental data obtained in a constant volume combustion chamber system. The calculated results showed a satisfactory agreement, in which the maximum error of flame lift-off length is 7.82 mm and that of IDTs is 0.16 ms. It was proven that the mechanism is suitable to reproduce the physicochemical properties of diesel and further predict the diesel spray and ignition performance.

On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pecr2/Pecr1≈2. The temporal acceleration exponents (αR) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as SF/SL,b∼Pedt and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (dt) of turbulent flames continuously increase with u’/SL in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the dt of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H2/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.

Visualization and simulation study of ammonia blending with hydrogen as combustion application in lean-burn condition

Ammonia (NH3), as one of the two zero-carbon fuels, has attracted lots of attention. However, its poor properties, including slow laminar burning velocity (LBV), high ignition energy, and low flammability, make it unfavorable for use in engines. On the other hand, hydrogen (H2), another zero-carbon fuel, has a much higher LBV and flammability. Adding a certain amount of hydrogen to ammonia mixture has the potential to improve the combustion process and extend the lean-burn limit of ammonia. This paper investigates the combustion and emission improvements of ammonia blended with a small amount of hydrogen under lean-burn conditions, both experimentally and numerically. A new NH3/H2 chemical reaction mechanism was developed to accurately predict ignition delay time (IDT), LBV and NO generation under a wide range of initial conditions. The flame propagation schlieren and NO chemiluminescence measurements with a homogenous NH3/H2 mixture ignited by a spark plug in constant volume combustion chamber (CVCC) were conducted. The 3D numerical simulations in CVCC, coupled with the new mechanism, were validated with the experimental results. The hydrogen energy fraction up to 30 % (X30) and ER down to 0.4 (ER0.4) were studied in experiments and simulations. The results showed that, in comparison to pure ammonia (X0) at ER1.0, lean-burn conditions can be extended to ER0.6 and ER0.4 for X10 and X30, respectively, while still achieving a shorter combustion duration. NO is initially increased and then decreased with decreasing ER, regardless of hydrogen addition. Comparatively, X30ER0.4 can achieve relatively low NOx emissions, and maintaining an equivalent combustion rate with a low combustion temperature.

The properties of sustainable aviation fuel I: Spray characteristics

This study measures the spray penetration, spray angle and spray volume of sustainable aviation fuel (SAF) experimentally using an optical constant volume combustion chamber under different ambient temperatures, injection pressures, and ambient pressures. The results show that when the ambient temperature increases from 200 °C to 300 °C, the spray penetration decreases by ∽10%–20%, while it further decreases by ∽40%–50% when the temperature increases from 300 °C to 400 °C. Furthermore, the spray penetration of SAF will reach its stable stage fastest at 400 °C. As the ambient temperature increases, the spray angle and spray volume of SAF decreased by 10%–20% and 50%, respectively. In addition, the spray penetration and spray volume of SAF increased by 10% and 20%, respectively, with an increase in injection pressure at 200 °C. An increase in injection pressure at 400 °C causes the decrease in spray penetration and spray volume of SAF by 20%, and both of them showed increasing and decreasing trends at the stable stage. Thus, increase in injection pressure has little effect on the spray angle. Moreover, the spray penetration and spray volume of SAF showed a decreasing trend on increasing ambient pressure, but significant effect was observed at a high temperature of 400 °C than at a low temperature of 200 °C. Conversely, the spray angle of SAF showed an increasing trend with an increase in ambient pressure, but this influence was larger at a low temperature of 200 °C than that at a high temperature of 400 °C.

DESIGN OF A CONSTANT VOLUME COMBUSTION CHAMBER WITH OPTICAL APPROACH TECHNIQUE

The combustion process in the internal combustion engine is realized as the core of the engine's power and exhaust emission formation. This paper describes the design of a constant volume combustion chamber (CVCC) to simulate the combustion process of the diesel engine with the optical approach technique. The technical design standards of the chamber were based on the actual conditions of diesel engines with a compression ratio of 16 to 28 and an injection profile with a fuel pressure varying between 400 and 1600 bar. The system included 2 quartz crystal windows that allow observing the entire process of fuel injection and combustion taking place inside the chamber. The structural strength analysis of the design under simulating pressure was carried out by ANSYS software. The results revealed that the chamber met durable standards with each type of material based on design criteria with a pressure of 200 bar selected as the ambient pressure inside the chamber.

Experimental observation of the TJI-initiated HPDI gas combustion: Vertically crossed flame jet and methane jet

In the field of natural gas marine engines, the high-pressure direct injection (HPDI) technology is widely used to achieve emission reduction while maintaining equivalent thermal efficiency and power output compared to diesel engines. In these engines, the in-cylinder combustion is primarily initiated by the diesel spray flame near the top dead center (TDC). In the present work, pre-chamber turbulent jet ignition (TJI) is employed to substitute diesel injection to initiate the combustion of HPDI methane jets. The optical experiments of ignition and flame development in the TJI-HPDI system with various injector configurations and injection/ignition control parameters are conducted in a constant-volume combustion chamber (CVCC). It is revealed that with the increase of injection-ignition delay(ti), three ignition modes are observed sequentially: methane jet suppresses ignition, methane jet first suppresses ignition then promotes flame propagation, and direct ignition and promoted flame propagation. The effect of various injection-ignition delays and injection pulse width were explored. The injection-ignition delay significantly influences the combustion characteristics such as heat release rate, while the injection pulse width influences the duration of the lifted jet flame. The flame lift-off length first decreases and then increases due to variations in thermodynamic conditions and oxygen concentration. With a larger-nozzle injector, the critical injection-ignition delay to initiate the main chamber combustion is significantly reduced. Moreover, the use of different nozzle diameters leads to varying levels of turbulent intensity in the methane jet, which in turn affects the combustion behavior.

Laser ignition and flame propagation of methanol-air mixture in a constant volume combustion chamber

ABSTRACT The combustion performance of laser-ignited methanol-air mixture in a constant volume combustion chamber was investigated using the Schlieren photography technique. The experimentation was conducted at 5, 7, 9, and 11 MPa fuel injection pressure and 363 K initial chamber temperature using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser of 1064 nm wavelength. The gasoline direct injection (GDI) system was used for injecting the fuel at different pressures inside the chamber. The laser beam is focused into the chamber using a 150 mm focal length lens, and laser energy of 200 mJ was used for all experimentation. The Schlieren system was used for capturing flame propagation of methanol-air mixture for different equivalence ratios. The pressure-time history for equivalence ratio (ϕ) of 0.8, 0.9, and 1.0 was plotted and analyzed at different chamber filling pressures. Flame propagation was found faster for ϕ of 1.0 due to higher laminar velocity than 0.8 and 0.9. Flame visualization showed that the flame attains the ellipsoid shape during the early stage of ignition. It generates two lobes in the vertical direction and then again expands in a horizontal direction toward the ignition source. For all equivalence ratios, the flame propagation toward the incoming laser source (X-) was observed faster compared to the flame propagation in the direction and perpendicular to the laser beam direction. For all equivalence ratios, higher laminar flame speed was observed at 5 MPa injection pressure; however, it decreases as the injection pressure increases. At ϕ of 1.0 and 11 MPa injection pressure, the maximum peak pressure of 0.67 MPa and combustion duration of 19.3 ms were noted. At 9 MPa injection pressure and ϕ of 1.0, the peak pressure and combustion duration was found as 0.61 MPa and 19.4 ms, respectively. At ϕ of 0.8, the peak pressure and combustion duration were observed 0.27 MPa with 34.6 ms duration and 0.47 MPa with 36.9 ms duration at 5 MPa and 11 MPa injection pressures, respectively.