Low-temperature Ignition Research Articles

An experimental and modeling study on the ozone-assisted low-temperature ignition of methylcyclohexane in an RCM

Methylcyclohexane (MCH) is a significant component of practical fuels and has garnered significant research interest in recent years. However, due to the low reactivity of MCH, ignition data below the temperature of 680 K are still lacking. To extend the experimental condition of MCH ignition to a lower temperature range, ozone (O3) was used as an active additive to assist in the kinetic investigation of the low-temperature ignition chemistry of MCH. Ignition delay times (IDTs) and the first-stage IDTs for MCH containing varying concentrations of O3 (0, 100, and 500 ppm) were determined in a rapid compression machine (RCM) under the pressure of 20 bar, equivalence ratios of 1.0 and 1.5, and temperatures in the range of 616–864 K. Species profiles during the auto-ignition of MCH at 615 K and 795 K were recorded using a fast sampling system combined with the gas chromatography (GC) technique. A kinetic model for MCH ignition with O3 addition was developed with some key reaction rates updated based on our newly obtained data. The promotional effects of O3 on the ignition of MCH were observed through both the IDTs and species evolutions. The negative temperature coefficient (NTC) behavior of MCH ignition disappeared in the presence of 500 ppm O3. The production of intermediate species displayed varying tendencies, with the formation of acetaldehyde (CH3CHO) being favored over other small alkenes. Moreover, the production of ethane (C2H6) was detected under 615 K with 500 ppm O3 addition, which was not reported in previous works concerning MCH combustion. Model analysis revealed that the oxidation pathways for ROO radicals of MCH altered because the addition of O3 shifted the onset of the oxidation temperature to a lower region, making the generation of RO radicals significantly favored. Therefore, the experimental species profiles can provide additional kinetic information for the low-temperature oxidation chemistry of MCH, and the corresponding data effectively highlighted and constrained the reaction rates of ROO radicals yielding RO radicals. Further model simulation indicated that the intramolecular hydrogen immigration of ROO radicals and the following secondary O2 addition oxidation pathways of MCH can be nearly frozen if the onset oxidation temperature is shifted as low as 550 K with 1000 ppm O3.

Numerical study of combustion mode and diffusive transport underlying fuel ignitions in lean stratified dimethyl ether-air premixture at various turbulence intensities

This paper presents a systematic numerical investigation of the auto-ignition phenomenon in stratified dimethyl ether (DME)/air turbulent mixtures. The study was conducted at an initial pressure of 30 atm, mean temperature of 650 K, and equivalence ratio of 0.45, with varying Damköhler numbers (Da) ranging from 0.01 to 1.1. The simulations indicated that the transition point between two combustion modes, deflagration wave and spontaneous ignition, occurred at a Da value of 0.06. The chemical explosive mode analysis identified the leading role of diffusive transport in both modes. A diffusion index (DI) was defined to quantify contributions of each diffusive sub-process. The DI results revealed that heat conduction and oxygen diffusion are the most important processes for the low-temperature ignition, followed by DME, CH2O, and HOCH2OCO with decreasing importance. Further DI analysis of the detailed local flame structure demonstrated that heat, DME, O2, and H2O2 diffusive transports have moderate importance for depressing turbulent propagation at local extinction modes. In conclusion, the proposed DI method has the potential to be a more efficient and rigorous substitution for sensitivity analysis and transport budget analysis.

Effect of composition and process control agents on the microstructure and ignition properties of ball-milled Al-Zr powders

We demonstrate the effect of varying Al-Zr chemistry on the morphology, microstructure, intermetallic reactions and ignition temperature of powders prepared using arrested reactive ball milling in hexane. Upon slow heating in Ar, 3Al:Zr, Al:Zr, and Al:3Zr (at.%) produces 19 ± 2.6 kJ/mol 22.0 ± 1.6 kJ/mol, and 14.0 ± 2.6 kJ/mol of heat respectively. When rapidly heated in air 3Al:Zr powders did not ignite, while Al:Zr and Al:3Zr had ignition temperatures of 461 ± 25 °C and 475 ± 25 °C, suggesting Zr oxidation is aiding ignition.To reduce cold welding of 3Al:Zr during milling, acetonitrile replaced a fraction of hexane. 3Al:Zr milled with 6.25 vol% and 12.5 vol% acetonitrile had smaller particles, higher heats and lower ignition temperatures compared to 0 vol%. 3Al:Zr with 6.25 vol% demonstrated the lowest ignition temperatures (433 ± 34 °C). 3Al:Zr with 12.5 vol% had the smallest mean diameter, largest surface area, smallest Zr inclusion areas, and highest heats, but had higher ignition temperatures (492 ± 27 °C), likely due to ZrC inhibiting the reaction of Zr with N and O.

Investigation of the ignition temperature of ethane-air mixtures at elevated pressure

Determining the ignition temperature of combustible substances is crucial in enhancing energy utilization efficiency and preventing fire and explosion accidents. This study investigates the ignition temperature of ethane-air mixtures with concentrations of 29.6%, 31.8%, and 34.4% at pressures up to 1 MPa in a 20 L spherical closed vessel, using a “heating” method. The high-pressure gas mixture was heated until spontaneous rapid reaction occurred in the reaction vessel. The study found that the ignition temperature of ethane in air decreases with an increase in initial pressure and ethane concentration. For example, at an ethane concentration of 31.8%, the ignition temperature decreased from 374℃ to 296℃ when the pressure increased from 0.3 MPa to 1 MPa. Additionally, larger container sizes resulted in lower ignition temperatures for ethane. Furthermore, the study simulated the self-oxidation reaction of ethane using the San Diego Mechanism kinetic reaction mechanism. The results revealed that the initial pressure's effect on important free radical reaction rates was the fundamental cause of autoignition. Under high-pressure conditions, the reaction between C2H5 and oxygen, C2H5 + O2<=>C2H4 + HO2, was the most critical reaction pathway. Overall, this study provides valuable insights into the ignition temperature of ethane-air mixtures and identifies key factors that influence autoignition. The findings can be utilized in improving safety measures and optimizing energy utilization in various industrial processes.

Effect of hexafluorophosphate and conjugated fluoride on the combustion of aluminum powder

In this study, the alumina layer on the surface of aluminum powder was removed with hydrofluoric acid. 2-Azido-1,3-dimethylimidazolinium Hexafluorophosphate (ADMHFP) and 4-azido-2,3,5,6-tetrafluorobenzoic acid (ATFBA) were coated on the surface of aluminum to form Al@ADMHFP and Al@ATFBA. Scanning electron microscope and FTIR test results proved that the ADMHFP and ATFBA were coated on aluminum powder. The Al@ADMHFP and Al@ATFBA showed better thermal reaction and combustion performance. The exothermic enthalpy of Al@ADMHFP and Al@ATFBA were increased by about 1.6 times and 1.0 time, with lower ignition temperature and longer burning duration, compared with raw aluminum powder.

Preparation of perfluorobutane glycidyl azide polymer and its coating on aluminum powder

In this study, perfluorobutyl iodide (PI) and glycidyl azide polymer (GAP) were used to synthesize perfluorobutane glycidyl azide polymer (PGAP) and coated on aluminum powder to form Al@PGAP. PGAP, an energetic fluoride, was used to explore the combustion promotion of aluminum powder. The Al@PGAP showed better thermal reaction and combustion performance. The exothermic enthalpy of Al@PGAP were increased by about 1.5 times, with higher combustion heat, lower ignition temperature and longer burning duration, compared with raw Al.

Valorization potential of pine needle waste biomass: recent trends and future perspectives.

Pines play a significant role in forest biodiversity globally and generate huge forest litter. Dry pine needles due to low ignition temperature and high frictional force with the ground catch fire quickly. Annual forest fires in the northern states of India greatly impact the Indian economy besides causing huge loss to biodiversity, livelihood, and environment. Pine needles are also considered unfit for fodder consumption due to presence of tannins. Although the presence of softwood lignin in pine needles makes it difficult to degrade easily, the presence of holocellulose (68.5%) containing 45-51% cellulose makes this biomass a potential substrate to be used in pulp-making industries for low-grade paper sheets. The good fiber length of pine needles (1.3-1.4mm) with a diameter of 30-32μm, maybe considered important property for paper making. The use of pine needles in the pharmaceutical and food industries are due to the presence of secondary metabolites (α-pinene, β-pinene, caryophyllene etc.). The various other potential applications of pine needles are for producing bio-ethanol (yield, 3.98%; purity, 94%), biogas (yield, 23.1 L kg-1), smokeless briquettes (calorific value, 18.77MJkg-1), biochar (calorific value, 25.6MJkg-1), bio-composites (tensile strength, 21-60MPa), and bio-pesticides. This paper comprehensively reviews the current applications of pine needles along with its future prospective applications that can have the dual advantage of providing employment opportunities to the people along with environmental protection.

Enhancing the combustion and propulsion performance of Al/CuO via introducing AlH3 as fuel substitute

The research on nanothermites mainly focuses on formulation and structural modification, performance optimization and application research. Whereas, the few gaseous products of nanothermites limit the practical applications, especially in microthrusters. Here, various contents of AlH3 was introduced in typical nanothermites (Al/CuO) to replace Al as fuel. The characteristics of Al/AlH3/CuO were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ignition and combustion performance such as burning rate, unconstrained combustion, ignition temperature and delay time, pressurization and propulsion properties was investigated. Results revealed that when AlH3 content was 20%, Al/AlH3/CuO had the fastest burning rate (189 m/s), the lowest ignition temperature (441.4 °C), the shortest ignition delay time (5.32 ms), the maximum Pmax (1141.3 kPa), the highest pressurization rate (75.06 kPa/μs) and the maximum microthrust (1.61 N). In short, replacing Al with AlH3 in the Al/CuO provides an innovation for creating nanothermites with completely new metal fuels, and also enhances the combustion and propulsion performance.

A Brief Review of the Impregnation Process with Dielectric Fluids of Cellulosic Materials Used in Electric Power Transformers

In the manufacturing of power transformers, the impregnation of the solid electric insulation systems (cellulosic materials) with a dielectric liquid is a key issue for increasing the breakdown voltage of the insulation, and this prevents the apparition of partial discharges that deteriorate the insulation system. After introducing the problem, this article presents the theory of impregnation and later carries out a bibliographical review. Traditionally, mineral oils have been used as the dielectric liquid in electrical transformers, but for environmental (low biodegradability) and safety (low ignition temperature) reasons, since the mid-1980s, their substitution with other ester-type fluids has been studied. However, these liquids have some drawbacks, including their higher viscosity (especially at low temperatures). This property, among other aspects, makes the impregnation of cellulosic materials, which is part of the transformer manufacturing process, difficult, and therefore this tends to lengthen the manufacturing times of these machines.

High energy density entrainment-based catalytic micro-combustor for portable devices

The increasing demand for low-cost, high energy density heat sources has motivated the development of compact and lightweight combustion-based devices. In this work, we developed an energy-dense (≈236 MW/m3) entrainment-based catalytic micro-combustor for heating portable systems. The multichannel micro-combustor (coated with Pt/Al2O3 catalyst) leverages a copper-nichrome wire to enable quick and localized ohmic preheating durations (2–3 mins). Furthermore, we demonstrated low ignition temperature (108–125 °C), which facilitates low energy consumption (∼1948 J). In addition, an optimal fuel flow rate (3.09 × 10-8 m3/s) was determined via FEM simulations and experiments to enable fuel savings (high fuel conversion) while achieving high heat fluxes by analyzing the reaction kinetics and species transport behavior in the microchannels. Through independent testing, we established the micro-combustor’s ability to maintain long-term autothermal combustion at a high saturation wall temperature (585 °C), which was attained at short timescales to enable fast heating/cooling cyclability. The successful cyclic heating demonstration of large thermal mass additions (at least 41 times the micro-combustor’s mass), coupled with the combustor’s high energy density, shows promise for device-level implementation for a range of commercial, defense, and energy conversion applications.

한옥의 내화 및 방충 성능 개선에 관한 연구

This paper is a study on the fire resistance of wood and securing the performance of insects, which are still emerging as problems in order to convert the various advantages and excellence of hanok into modern architectural methods. To verify the research results, the test was conducted by applying it to the building materials of hanok, and the results of the research were summarized as follows.&#x0D; First, wood with fire resistance regulations is relatively safe from fire. Wood has a very low heat transfer rate and a relatively low ignition temperature on the surface is a relatively safe material compared to other materials because when a fire breaks out, the carbonized part of the surface additionally blocks heat transfer and prevents the inside from being carbonized.&#x0D; Second, in the case of hanok, the standard of the member that can be used as a fireproof material is 460mm×600mm long in the case of beams, 360mm in diameter, 3,600mm high in the case of cylinders, and 300mm×300mm high in each column.&#x0D; Third, in order to prevent insects such as termites, insect repellent paint is directly applied to wood, a building material, to show the performance of the room. In comparison to existing termite damage prevention methods such as fumigation, soil treatment, and termite colony removal systems, insect repellent paints show excellent performance in difficulty, cost, use of eco-friendly products, and room effects.

Laser-Induced Ignition and Combustion of Single Micron-Sized Al-Li Alloy Particles in High Pressure Air/N2

To solve the problems associated with micron-sized aluminum (Al), including sintering, agglomeration, and slag deposition during the combustion of aluminized propellants, aluminum–lithium (Al-Li) alloy, prepared by introducing a small amount of Li (1.0 wt.%) into Al, was used in place of Al. Then, the ignition and combustion characteristics of single micron-sized Al-Li alloy particles were investigated in detail using a self-built experimental apparatus and multiple characterization methods. The ignition probability, ignition delay time, flame propagation rate, burn time, combustion temperature, flame radiation spectra, and microexplosion characteristics were obtained. The TG-DSC results demonstrated that, as compared to the counterpart Al, the Al-Li alloy had a lower ignition temperature. The emission lines of AlO revealed the gas-phase combustion of the Al-Li alloy, and thus the Al-Li alloy exhibited a mixed combustion mode, including surface combustion and gas-phase combustion. Moreover, during combustion, a microexplosion occurred, which increased the combustion rate and reduced the burn lifetime. The ambient pressure had a significant effect on the ignition and combustion characteristics of the Al-Li alloy, and the ignition delay time and burn time exponentially decreased as the ambient pressure enhanced. The combustion temperature of the Al-Li alloy at atmospheric pressure was slightly higher than those at elevated pressures. The Al-Li alloy burned in N2, but no microexplosion was observed. Finally, the ignition and combustion mechanism of the Al-Li alloy in air was demonstrated by combining SEM, EDS, and XRD analyses of the material and residues. The results suggest that the addition of Li promoted the combustion performance of Al by changing the surface structure of the oxide film and the combustion mode.

Pelletization of nitrogen-containing biowaste hydrochar for solid biofuel preparation

In this study, typical N-containing biowaste including food waste (F), sludge (S), and chlorella (C) was modeled using pine (P) as a carbohydrate component and protein/glycine as a nitrogen-containing component, and hydrothermal carbonization and pelletization were performed to investigate the fuel properties of hydrochar pellet including proximate analysis, elemental composition, tensile strength, and combustion characteristics. Results suggested that the introduction of N can obviously enhance the compressive strength of hydrochar pellet, of which chlorella model derived hydrochar pellet possessed higher tensile strength (1–3.75 MPa) compared to others. Protein/P model hydrochar had a higher water resistance index, but glycine/P model hydrochar had higher hydrophily than that of protein/P model. Most importantly, H/C in all protein/P model hydrochar was lower than that of glycine/P, and bio-oil derived “liquid bridge” may account for the high tensile strength. Moreover, combustion analysis showed that glycine in model enhanced volatile matter combustion of hydrochar with a low ignition temperature. This work suggested that N in biowaste had significant effect on the fuel properties of hydrochar pellet, and proper strategy of N-containing biowaste for HTC can be expected for the value-added utilization as solid biofuel.

Experimental study on the ignition and combustion of Mg particles enhanced by metal oxides in CO2

Due to the high cost and long lifecycle of manned Mars exploration missions, in-situ resource utilization technology, including Mg/CO 2 processes, has attracted increasing attention. At present, Mg/CO 2 engines face the challenges of long ignition delay, serious carbon deposition, and low combustion efficiency. The root of these challenges lies in the Mg/CO 2 combustion reaction. Therefore, based on an Mg-based powder prepared by mechanical mixing combined with ultrasonic dispersion, the effects of various nano-metal oxides (CuO, Fe 2 O 3 , MnO 2 , MoO 3 , and Bi 2 O 3 ) on the ignition and combustion of Mg particles in CO 2 were studied through constant volume combustion experiments. It was found that MoO 3 undergoes the optimal covered-type of binding with Mg particles. The other oxides achieved embedded-type binding, of which MnO 2 binding was the worst. The enhanced ignition and combustion effects of Bi 2 O 3 and CuO were best. Thermogravimetric analysis found that CuO and Bi 2 O 3 improve combustion efficiency by lower ignition temperatures, inhibiting low-temperature oxidation reaction and increasing reaction ratio. • The enhancement effects of metal oxides on ignition and combustion of Mg particles in CO 2 are firstly studied. • The binding abilities of different nano-metal oxides to micro-Mg particles are analyzed. • The enhancement effects of CuO and Bi 2 O 3 on ignition and combustion of Mg particles in CO 2 are analyzed. • The influences of metal oxide addition ratio on enhancement are analyzed.

Coupling effect of thermal conducting and low-temperature reaction process on ignition characteristics under diesel-like conditions

The typical behavior of misfiring after low-temperature ignition (LTI) in the cold-start of a diesel engine is mainly caused by thermo-kinetic imbalance and is related to the coupling effect of thermal conducting and low-temperature chemistry. This paper abstractly constructed a two-zone model to characterize such a diesel-like operating condition and investigated the coupling effect of thermal conducting and low-temperature chemistry on ignition characteristics mechanistically. The results show that at relatively low temperature/equivalence ratio environments, thermal conducting has a stronger effect on the ignition process and acts mainly on the chemical reaction rate rather than the reaction pathway. At lower temperature environments (600 K–700 K), the two zones' temperature trajectories almost overlap before LTI. At a higher temperature (700 K–800 K), thermal conducting and low-temperature reactions together affect the ignition process, causing the lower-temperature zone to earlier translate from LTI to high-temperature ignition (HTI), in contrast to the situation without thermal conducting. Thermal conducting induced by fuel-concentration gradient has an effect after the LTI of richer zones, which causes the HTI to delay, but for the leaner zone, it increases the low-temperature reaction rates sharply and advances the ignition. This study provides theoretical support to overcome the unstable ignition problem of cold-start.

Macroscopic and mesoscopic properties of HTPB propellant under low temperature dynamic biaxial compression loading

In order to study the macroscopic mechanical properties and meso damage mechanism of solid fuel hydroxy-terminated polybutadiene (HTPB) propellant under low temperature and dynamic biaxial compression loading, the INSTRON high strain rate hydraulic testing machine was used to carry out dynamic biaxial compression test of HTPB propellant in the temperature range of −50 °C ∼ -30 °C and the strain rate range of 1s−1–85s−1. Meanwhile, the control test at room temperature (25 °C) was set up, and microcomputed tomography (Micro-CT) was used to observe and quantitatively characterize the meso damage of the loaded specimens. The test results show that temperature and strain rate significantly affect the mechanical properties of HTPB propellant in biaxial compression, and the mechanical properties under biaxial compression loading are more susceptible to temperature than strain rate. In addition, there is a significant difference between uniaxial and biaxial strength under compression loading, and finally, the effect of temperature and strain rate on mechanical properties is closely related to the variation in properties and mesoscopic damage mechanisms of HTPB propellants. This paper can deepen researchers' understanding of the performance of composite solid propellant and other energetic materials at the macro and micro level, and provide more effective theoretical and technical support for accurate analysis of the grain structure integrity of airborne missile solid rocket motor (SRM) under low temperature ignition conditions.

ФРИКЦІЙНІ ЯВИЩА У ВУГІЛЬНИХ ШАХТАХ: ПРИЧИНИ, НАСЛІДКИ ТА ВПЛИВ НА ГАЗОДИНАМІЧНІ ЯВИЩА

Coal mines are objects of increased danger, since dynamic, gas-dynamic and thermal phenomena occur in them. These phenomena lead to violations of the operating regime, the failure of mining equipment, the death of miners, an increase in the cost of coal mining and other consequences. The purpose of the article is to present the results of studies of the conditions for the formation of thermal effects in coal mines and the reasons for their occurrence, which has both scientific and applied relevance. The conducted studies made it clear that sparking can occur under the influence of both man-made and natural factors. Friction phenomena leading to thermodynamic effects have been recorded in many mines and countries of the world. The main part of explosions and fires of methane occurs during frictional sparking of rocks containing sulfides. The latter produce sparks of a sufficiently high temperature, which can lead to the ignition of combustible gases, as well as the release of hydrogen sulfide, which has a low ignition temperature. In world practice, cases of ignition of combustible materials during the landing of the roof in the absence of coal have been described. British researchers concluded that the methane-air mixture can be ignited by a spark generated by the friction of sandstone against sandstone or mudstone against sandstone. According to the literature data, the ignition of combustible materials and coals can occur as a result of: emissions and explosions, rock collapse during landing, when a spark discharge occurs in quartz during a tribo- or piezoelectric effect; the presence of sulfides in coals or rocks, pyroelectric effect; electrification of coals. At the micro level, in quartz grains of sandstones, the formation of static charges by dislocations, which are decorated with dust, has been established. A particularly strong effect can occur if, in addition to methane, carbon monoxide, hydrogen, hydrogen sulfide, and acetylene are present in the air. An increase in sparking during frictional friction of quartz in the presence of coal dust has been established. The presence of gas inclusions in sandstone minerals during shear deformations leads to the appearance of additional concentrations of hydrocarbons. As a result of contact of such a mixture with a heated area, a short-term small flash occurs, which is a spark.

Plasma-assisted deflagration to detonation transition in a microchannel with fast-frame imaging and hybrid fs/ps coherent anti-Stokes Raman scattering measurements

This study examines kinetic enhancement by nanosecond dielectric barrier discharge (ns-DBD) plasma on fuel-lean, Φ = 0.7, dimethyl ether (DME), oxygen (O2), and argon (Ar) premixture during deflagration to detonation transition (DDT) experiments in a microchannel. Nonequilibrium plasma produces active species and radicals as well as fast and slow heating of a mixture to promote ignition due to energetic electrons, ions, and electronic and vibrational excitations. Experiments are conducted to examine the influence of the plasma discharge on the premixture and on the resultant deflagration to detonation transition (DDT) onset time and distance through the use of high-speed imaging and one-dimensional, two-beam, femtosecond/picosecond, coherent anti-Stokes Raman scattering (CARS). A high-speed camera is used to trace the time histories of flame front position and velocity and to identify the dynamics and onset of DDT. The results show that plasma discharge can nonlinearly affect the onset time and distance of DDT. It is shown that a small number of plasma discharge pulses prior to ignition result in reduced DDT onset time and distance by 60% and 40%, respectively, when compared to the results without pre-excitation by ns discharges. The results also show that an increase of number of the plasma discharge pulses results in an extended DDT onset time and distance of 224% and 94%, respectively. Time history of the deflagration wave speed of DME and the analysis of ignition timescale under the choking condition of the burned gas of the deflagration wave suggest low temperature ignition may play a role for DME near the isobaric choking condition of the burned gas and the DDT. Plasma-induced low temperature oxidation of the reactive mixture is assessed via the CO2 to O2 ratio as measured through fs/ps CARS during the gas excitation in discharges. CARS measurements also confirm negligible vibrational and rotational heating of the gas by discharge. The present experiments demonstrate the ability of nonequilibrium plasma to alter the chemistry of DME/O2/Ar premixtures in order to control DDT for applications in advanced propulsion engines.

Computational Fluid Dynamics Modeling of Low Temperature Ignition Processes From a Nanosecond Pulsed Discharge at Quiescent Conditions

Abstract Recent interest in nonequilibrium plasma discharges as sources of ignition for the automotive industry has not yet been accompanied by the availability of dedicated models to perform this task in computational fluid dynamics (CFD) engine simulations. The need for a low-temperature plasma (LTP) ignition model has motivated much work in simulating these discharges from first principles. Most ignition models assume that an equilibrium plasma comprises the bulk of discharge kernels. LTP discharges, however, exhibit highly nonequilibrium behavior. In this work, a method to determine a consistent initialization of LTP discharge kernels for use in engine CFD codes like converge is proposed. The method utilizes first principles discharge simulations. Such an LTP kernel is introduced in a flammable mixture of air and fuel, and the subsequent plasma expansion and ignition simulation is carried out using a reacting flow solver with detailed chemistry. The proposed numerical approach is shown to produce results that agree with experimental observations regarding the ignitability of methane-air and ethylene-air mixtures by LTP discharges.

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

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