Constant Volume Combustion Chamber Research Articles

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.

Experimental investigation combined with steady-state and transient-state tests on soot characteristics of hydrogenated catalytic biodiesel/n-butanol blends

The utilization of biodiesel and biobutanol offers a promising approach to achieve energy conservation and emission reduction in combustion engines. This study investigates the soot characteristics of hydrogenated catalytic biodiesel mixed with n-butanol (20% by volume) under steady-state and transient-state conditions using diffused background-illumination extinction imaging and two-color method, respectively. Both conditions in constant volume combustion chamber (steady-state) and optical engine (transient-state) exhibit similar soot stages, namely Formation Dominating and Oxidation Dominating. Between the above Dominating stages, the soot development under steady-state conditions is further segmented into Initial Oxidation and Secondary Oxidation, while a Stalemate stage manifests under transient-state conditions owing to wall impingement and strong turbulence. Under steady-state conditions, formation and oxidation processes are mainly controlled by the soot peak of spray head. By decreasing ambient temperature and increasing injection pressure, soot accumulation is notably diminished by 30.27% and 45.72% during the Formation Dominating stage, achieved through the deceleration of precursor reactions and enhancement of air-fuel mixing characteristics within the flame lift-off zone, respectively. Additionally, higher injection pressure notably amplifies the efficacy of the Initial Oxidation, primarily through the reduction of soot residence time. Under transient-state conditions, the Formation Dominating stage for SOI-25 and SOI-20 occur away from the early combustion, resulting in a reduction of 10.26% and 68.36% in soot compared to SOI-30. The Stalemate stage is characterized by a competition between the oxidation of high-temperature flames above 2000 K and the formation of low-temperature flames below 1800 K. A portion of soot originating from the high-temperature flames becomes integrated into the low-temperature flames after cooling, subsequently emerging as the primary hindrance to the ensuing oxidation process.

Investigation of the injection pressure impact on non-monotonic two-stage ignition delay of diesel engines under cold-start

Poor startability under cold-start conditions is a long-term obstacle of the compression ignition engine and adjustment of the injection pressure is an effective way to improve the startability. Therefore, a detailed investigation of the effect of injection pressure (Pi) on the ignition characteristics of the fuel spray under cold-start conditions is needed. Though numerous studies have been conducted on the effect of Pi on the ignition characteristics of the fuel spray, the two-stage ignition process of the fuel spray under cold-start conditions is still an unknown field. In this paper, the effect of Pi on the two-stage ignition process of the fuel spray under cold-start conditions is investigated in a constant volume combustion chamber through simultaneous Shadowgraphy and high-speed natural luminosity method an analytical model for two-stage ignition delay is established accordingly for further investigation. Based on the experiment results, a monotonic decreasing trend on low-temperature ignition delay time LTI (i.e., form 4.5 ms to 2.5 ms under the ambient temperature of 760 K) and an increasing trend on the cool flame propagation time τ (i.e., form 0.4 ms to 1.6 ms under the ambient temperature of 760 K) are found and finally results in a non-monotonic trend of high-temperature ignition delay (HTI) as the injection pressure raises from 40 MPa to 160 MPa. According to the established model, increased Pi leads to promoted atomization and intensified dilution effect of the fuel spray, which is concluded as the reason for decreased LTI and increased τ, respectively. In this way, the non-monotonic trend of HTI under engine cold-start as Pi increases is quantitatively explained through the promoted model.

Optical characterization of ethanol spray flame on a constant volume combustion chamber

Studying pure ethanol spray flame has the potential to achieve the carbon neutrality vision. This paper studies the effects of fuel injection masses (12, 24, 36 mg) and fuel injection pressures (30, 40, 50 MPa) on ethanol spray flame on an optically visualized constant volume combustion chamber. Further compared with the spray flame of methanol and n-butanol. The combustion characteristics and flame development process were revealed by flame self-illumination high-speed imaging method, and the soot distribution was revealed by wavelength integration two-color method. Results show that ethanol spray flame presents an unstable yellow flame with many wrinkles. Small injection masses exist a partial flame-quenching phenomenon. As injection mass increases, the soot lift-off length decreases, and the flame brightness, soot concentration, and ignition delay increase. The high soot concentration areas locate upstream of the flame, and there is almost no soot downstream. Increased injection pressure increases the soot lift-off length and decreases the flame brightness. The ignition delay is shortened from 8.388 ms to 6.955 ms when injection pressure increases from 30 MPa to 40 MPa. But higher injection pressure has a negligible effect on reducing ignition delay. Finally, an ethanol spray combustion conceptual model is proposed. This paper gives particular guiding significance to the future use of carbon-neutral ethanol in diesel engines.

Effect of initial pressure on the critical characteristics and overpressure of hydrogen-air premixed gas combustion and explosion

In order to ensure the safe utilization of hydrogen, the flame characteristics and explosion overpressure at lean-burn, stoichiometric and rich-burn conditions are investigated systematically by varying initial pressures. Experiments were conducted in a constant-volume combustion chamber using the Schlieren system to investigate the critical characteristics of hydrogen-air premixed flames at different equivalence ratios (from 0.2 to 6.5) and initial pressures (from 50 kPa to 150 kPa). The cellular instabilities of hydrogen-air premixed flames were interpreted and evaluated from the viewpoint of thermal-diffusive and hydrodynamic instabilities. The influence of initial pressure on the cellular instability and combustion and explosion characteristics were analyzed. The results show that the variation of thermal-diffusive instability dominantly governs the cellular instability at lean-burn conditions (φ = 0.3–1.0). In contrast, the cellular instability at rich-burn conditions (φ > 1.5) is governed by the variation of hydrodynamic instability. The increase of initial pressure promotes the cellular instabilities noticeably caused by the rise of the hydrodynamic instability. With the increase of P0, the Lewis number Le and thermal expansion ratio σ almost remain the same while flame thickness δ noticeably decreases. Increasing initial pressure at lean-burn and stoichiometric conditions (φ < 2.0) facilitates cellular instability, making the explosion more intense and resulting in higher explosion overpressure. In contrast, the variation of initial pressure suppresses the laminar combustion at rich-burn conditions (φ > 3.0).

The effect of 1-octanol blending on the multi-stage autoignition of conventional diesel and HVO fuels

Alternative fuels can reduce the carbon intensity of diesel engine use, and a detailed understanding of the combustion process for these fuels can minimize resulting fuel consumption and emissions. 1-Octanol has been considered as an alternative diesel fuel, but little work evaluating its combustion process in detail exists. In this work, ignition of 1-octanol, both pure and in blends with conventional and renewable diesel base fuels, is analyzed in a constant volume combustion chamber and a modified Cooperative Fuels Research Octane Rating engine operating in homogeneous charge compression ignition combustion mode. 1-Octanol blending reduced the derived cetane number of either base fuel, but modest increases in chamber temperature at injection could compensate for even dramatic reductions in derived cetane number. It was further found that greater amounts of low-temperature heat release were required to reach main ignition as the 1-octanol blending volume fraction was increased in the constant volume combustion chamber. This finding is attributed to the changing local equivalence ratio, as the reacting spray diffuses further into the chamber and approaches the (lean) global equivalence ratio as the ignition delay grows. Finally, it was found that, despite lowering the derived cetane number, 1-octanol blending enhanced the reactivity of the conventional diesel fuel observed in the variable compression ratio engine. This finding is attributed to the higher rates of hydrogen abstraction from the weaker average C–H bonds in 1-octanol, as hydrogen abstraction governs intermediate temperature heat release. These findings clarify the effect of 1-octanol on physical processes and the various chemical regimes of heat release in compression ignition. The dependence of the effects of 1-octanol blending on the base fuel are emphasized, as these can inform the deployment of 1-octanol as a diesel blendstock. Application of these results can support the optimization of conventional combustion strategies for use with 1-octanol, as well as the development of 1-octanol-fueled low-temperature combustion strategies with the associated promise of very low criteria pollutant emissions.

Hydrogen addition effect on cellularization and intrinsic instability of ethyl acetate spherical expanding flame

The cellularization and intrinsic instability of ethyl acetate (EA) spherical expanding flame with different proportions of hydrogen (4%, 8%, 12%) were studied through experiments and theoretical analysis. The high-speed schlieren imaging technique combined with the image processing method was used in the constant volume combustion chamber to quantify the development of flame surface cracks and saturation time, crack length, number of cellular units, and average cellular area. The results show that the evolution of flame cracks can be mainly classified into two periods. In the first period, several initial cracks are generated on the flame surface. In the second period, cracks rapidly expand while forming a large number of small cellular units, and the surface areas also increase rapidly. The increase in the proportion of hydrogen led to an increase in minor cellular cracks and a more moderate structural evolution of flame cells during the second period. Afterwards, the involved thermodynamic parameters, including the thermal expansion ratio, flame thickness, effective Lewis number, critical Peclet number, and perturbation logarithmic growth rate of unstable flames, were quantitatively analyzed. The results show that an increase in initial pressure significantly reduces the flame thickness, leading to more unstable flame. In contrast, the proportion of hydrogen has less impact on the involved thermodynamic parameters, while the equivalence ratio is the primary factor influencing instability parameters. Additionally, the study examined the flame's self-acceleration characteristics and found that increasing the proportion of hydrogen reduces the tendency for ethyl acetate self-acceleration.

Combustion comparative analysis of pyrolysis oil and diesel fuel under constant-volume conditions

The article discusses the research results on the combustion of pyrolysis oil derived from the pyrolysis of HDPE plastics after its distillation. The tests were carried out in a constant-volume combustion chamber in conditions similar to those in a compression-ignition engine with a compression ratio of 17.5:1. The phases of premixed and diffusion combustion and the ignition lag were determined. Diesel fuel combustion tests were performed under similar pressure-temperature conditions. Comparative analysis was used to draw conclusions as follows: the duration of the premixed combustion phase for pyrolysis oil is approximately 5% shorter than that for diesel oil; with an increase in the initial pressure in the chamber, the premixed combustion time in both cases was shortened, while the diffusion combustion time slightly increased, practically without any significant change; the maximum combustion rate for the premixed combustion phase for pyrolysis oil was nearly 20% higher than that for diesel fuel; the heat released from the premixed combustion phase for pyrolysis oil was found to be 20% higher compared to that one for diesel fuel; the ignition lag for pyrolysis oil was slightly longer compared to that for diesel fuel. The presented parameters significantly impact both the development of combustion and the thermal efficiency of the internal combustion engine. Summing up, pyrolysis oil can be applied as a substitute for diesel fuel as a single or blended component with it.

Effect of CO2 and its concentration variation on the ignition and combustion of diesel surrogate fuel: Optical experiments and numerical simulations

To investigate the influence of CO2 on diesel ignition and combustion, an experimental setup comprising a constant volume combustion chamber with optical channels was constructed. High-speed direct photography was employed to capture flame propagation images during fuel ignition and combustion under different CO2/O2 atmospheres. Additionally, large eddy simulation was conducted to model diesel combustion characterized by CO2/O2 environment. The theoretical calculations were then compared with experimental results, focusing on flame natural luminosity, flame structure, flame field, and heat release analysis. The results indicate that the flame structure in a CO2/O2 atmosphere consists of a diffusion flame surrounding a premixed flame, characterized by elevated levels of CO and a smaller amount of C2 radicals at the flame front. The O2 concentration ranged from 21% to 57%, resulting in a flame morphology with thin strips, intensified flame edge instability, and increased natural luminosity. The inhibitory effect of CO2 concentration within the range of 35%–50% counteracted the promoting effect of O2 concentration within the range of 29%–44%. Consequently, the chemical impact of CO2 plays a crucial role in combustion.

Validation of octane hyperboosting phenomenon in prenol and structurally related olefinic alcohols

Hyperboosting is a recently discovered phenomenon in which the research octane number (RON) of a blend is higher than both the neat blendstock and the neat fuel it was blended into. RON is a measure of a fuel’s resistance to knock, and knock is a cause of engine inefficiency. Blends which exhibit hyperboosting are blends in which an overall improvement in engine efficiency may be expected. The first discovery of hyperboosting came from blending experiments in which prenol was blended into several different base fuels. Here, ignition delay time (IDT) measurements taken using a commercially available constant volume combustion chamber called the Advanced Fuel Ignition Delay Analyzer (AFIDA) are presented. The data show that some prenol blends have longer IDTs (lower reactivity) than either neat prenol or the base fuel, providing further evidence of hyperboosting. Additionally, more blending data is presented in which the base fuel is varied, which allows for a better understanding of hyperboosting sensitivity to chemical classes. The data indicate that aromatics may inhibit, and branched alkanes may enhance the magnitude of hyperboosting observed. Enthalpy of vaporization estimates are also given for several molecules which are blended into a 4-component surrogate. These estimates are derived from Equation of State simulations and reveal that there is no observable correlation between hyperboosting and enthalpy of vaporization. Blending data for molecules which share structural similarities with prenol are also presented. Structure property relationships are suggested, in which the double bond motif of prenol seems to play an important role in hyperboosting. This may help to understand hyperboosting and its underlying mechanism. Lastly, blending curves of surrogate blends with prenol experienced hyperboosting under lean (Homogeneous Charged Compression Ignition-HCCI) operating conditions, which validates that hyperboosting is not an artifact of the octane test methods, but inherent to the properties of prenol.

Combustion characteristics of waste styrene–butadiene rubber oil derived from thermal pyrolysis process: experiment and numerical simulation

Owing to the similar carbon range in components, the pyrolytic oil derived from waste styrene–butadiene rubber (SBR) is preferred in aviation, for achieving waste-to-energy and fly net zero missions. The present study investigated the combustion and emission characteristics of SBR using a constant volume combustion chamber (CVCC) spray at 673–873 K initial temperature (Tini), 10 and 15 bar initial pressure (Pini), and 0.5 equivalence ratio (φ). SBR combustion properties were benchmarked with conventional jet propellant 5 (JP-5), and JP-5 and SBR were blended at a 50% volume ratio (JP-5-SBR-50). SBR has a shorter ignition delay (ID) and combustion delay (CD) at 15 bar Pini, compared to 10 bar. SBR exhibits negative temperature coefficient (NTC) behavior starting from Tini = 773–823 K, which was more significant at 10 bar. In the NTC region, at Tini = 723 and 823 K, the ID nearly has a 3-fold variation between 10 and 15 bar, and CD has 4- and 1.7-fold variations at Tini = 723 and 823 K, respectively. Finally, τ has a 3- and 1.5-fold variation at Tini = 723 and 823 K. JP-5 has a shorter ID and CD, followed by JP-5-SBR-50 and SBR. At Tini = 673 K, the addition of JP-5 to SBR significantly shortened ID and CD by nearly 24% and 17.5%. JP-5 has higher CO2, thermal NOx, and unburned HC emissions, whereas those were reduced with SBR addition to JP-5. Moreover, CO emissions at the SBR NTC region (773–823 K) gradually increased more than at any other tested Tini. Meanwhile, the measured ID periods and NTC characteristics for SBR were replicated using a reduced mechanism of 106 species and 2,162 reactions. The reduced mechanism also predicted the measured SBR inhibition effect on the ID periods for JP-5.

Effect of equivalence ratio and mixing time on combustion of ammonia/oxygen/argon mixture using a constant volume combustion chamber with sub-chamber

Effect of equivalence ratio and mixing time on combustion of ammonia/oxygen/argon mixture using a constant volume combustion chamber with sub-chamber

Experimental study on the explosion characteristics of NH3/DME/air mixtures

Co-firing ammonia (NH3) fuel with dimethyl ether (DME) is a promising approach of achieving clean combustion. However, the fundamental explosion characteristics of NH3/DME blends have not been well understood. This paper presents an experimental study of the explosion characteristics of NH3/DME blends using a spherical constant-volume combustion chamber. The effects of equivalence ratio (ϕ = 0.7–1.5), initial pressure (Pu = 0.05–0.5 MPa), and DME fraction (XDME = 0–100%) on the explosions were examined. The results show that peak explosion pressure Pmax, maximum pressure rise rate (dP/dt)max, and deflagration index KG, vary non-monotonically with equivalence ratio, and reach peak values at around ϕ = 1.1–1.2. DME addition has a positive effect on explosion intensity. Both (dP/dt)max and KG show a linear increase trend with increasing DME, while the growth rate of Pmax decreases with DME fraction. The relatively higher increase in Pmax between 0 %DME and 20 %DME is mainly due to greater heat release. Pmax increases with increasing initial pressure, and this is because of the increase in mixture mass promoting the heat release, and the enhanced possibility of effective collision between molecules of gas mixtures. The influence of initial pressure on (dP/dt)max and KG depends on the equivalence ratio. Both (dP/dt)max and KG show a linear increase with increasing initial pressure at ϕ = 1.0 and 1.5, whereas they are insensitive to initial pressure at ϕ = 0.7. This is because the decrease in laminar burning velocity leads to a significant increase in explosion time at ϕ = 0.7 compared to ϕ = 1.0 and 1.5. In addition, the actual explosion pressure is closely associated with non-negligible heat loss during flame propagation, and the heat loss decreases with the addition of DME, but increases with increasing initial pressure.

Role of Altitude in Influencing the Spray Combustion Characteristics of a Heavy-Duty Diesel Engine in a Constant Volume Combustion Chamber. Part I: Free Diesel Jet

Heavy-duty diesel engines operating in plateau regions experience deteriorated combustion. However, the lack of up-to-date information on the spray-combustion process limits the fundamental understanding of the role of altitude. In this work, the in-cylinder thermodynamic conditions of a real diesel engine operating under different altitudes were reproduced in a constant-volume combustion chamber (CVCC). The liquid spray, ignition, and combustion processes were visualized in detail using different optical diagnostics. Apart from predictable results, some interesting new findings were obtained to improve the understanding of free spray-combustion processes with different altitudes. The spatial distributions of ignition kernels provided direct evidence of higher peak pressure rise rates for high-altitude diesel engines. The percent of stoichiometric air was calculated to confirm that the net effect of altitude was an increase in the amount of air-entrained upstream of the lifted flame; therefore, the soot levels deduced from flame images were inconsistent with those from real engines, revealing that accelerating the soot oxidation process could effectively reduce engine soot emissions in plateau regions. Finally, a novel schematic diagram of the spray flame structure was proposed to phenomenologically describe the role of altitude in influencing the spray-combustion process of a free jet.

Investigation of cellularization characteristics of hydrogen-methane-ethanol expanding spherical flame at elevated pressures

The incorporation of ethanol into hydrocarbon fuels for use is attracting increasing interest and it is necessary to investigate its inherent flame instability for better application in combustion units. The instability of hydrogen-methane-ethanol spherically expanding flame has been investigated at an initial temperature of 400 K, initial pressures of (2–4 bar), ethanol fraction of (20%, 50%, 80%), and equivalence ratios (Ф) of (0.7–1.4) using the constant volume combustion chamber (CVCC). High-speed schlieren technology was used to record flame propagation images. The effects of hydrodynamic and thermal-diffusion effect on the inherent instability of the flame were investigated. As the ethanol ratio increased, the hydrodynamic effect was enhanced. The thermal-diffusion effect was discovered to stabilize the flame surface under all conditions, as judged jointly by the effective Lewis number and the critical Lewis number. The critical conditions (critical radius and Peclet number) at the onset of unstability were evaluated, and it was found that the flames were more prone to flame instability at higher pressures. The critical Peclet number increased with the increase in the equivalence ratio when the ethanol ratio was 20%, and showed the opposite trend when the ethanol ratio was 50% and 80%. In addition, as the ethanol ratio increases, the stability of the lean mixtures flame increases, while the rich mixtures flame suffers from early onset of instability. The theoretical and experimental results were consistent, with some differences at Ф = 1.4. An empirical correlation formula for the critical Peclet number (Pec) and Markstein number (Mb) was further proposed (Pec= 18.03Mb+214.78). Finally, the Karlovitz number was used to study the instability behavior of the flame. The critical Karlovitz number (Kac) decreased with increasing Mb and the tendency of the flame to suffer from instability diminished, and the following correlation was obtained Kac=0.05635×e−0.13852Mb. Furthermore, the flame was more unstable in rich mixtures, this was consistent with the conclusion of instability derived from the critical radius.

Study on chemical kinetics and NO behaviors in pre-chamber jet-induced ignition mode with ammonia

Ammonia has been supposed to be an important carbon-neutral fuel for the applications in future engines. Its unique physical and chemical properties have great effect on engine combustion. The pre-chamber jet-induced ignition mode can yield very high turbulent jet ignition energy, which should be favorable for the ammonia combustion. To study the effects of key initial parameters in this combustion mode, the ammonia combustion and NOx formation processes were analyzed deeply in constant volume combustion chamber. A 3D CFD model was developed and validated with the visualization experiments, and the initial pressures (Pin) and temperatures (Tin) were numerically studied. The comparisons of different Pin for constant fuel mass (CFM) and constant equivalence ratio (CER) showed that Pin with CFM affects the combustion phase and burning rate more obviously. A higher Pin reduces the NOx emission slightly for CER but leads to higher NOx obviously for CFM, and a higher Pin for CFM also results in a slightly higher N2O emission due to the lower combustion temperature. The comparisons of different Tin showed that a higher Tin can enhance the combustion process greatly, while it also results in higher NOx due to high activity of OH and HNO for HNO + OH → NO+H2O.