High-temperature Ignition Research Articles

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.

The role of cool-flame fluctuations in high-pressure spray flames, studied using high-speed optical diagnostics and Large-Eddy Simulations

This work investigates the low- and high-temperature ignition and combustion processes, applied to the Engine Combustion Network Spray A flame, combining advanced optical diagnostics and large-eddy simulations (LES). Simultaneous high-speed (50 kHz) formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) and line-of-sight OH* chemiluminescence imaging were used to measure the low- and high-temperature flame, during ignition as well as during quasi-steady combustion. While tracking the cool flame at the laser sheet plane, the present experimental setup allows detection of distinct ignition spots and dynamic fluctuations of the lift-off length over time, which overcomes limitations for flame tracking when using schlieren imaging [Sim et al.Proc. Combust. Inst. 38 (4) (2021) 5713–5721]. After significant development to improve LES prediction of the low-and high-temperature flame position, both during the ignition processes and quasi-steady combustion, the simulations were analyzed to gain understanding of the mixture variance and how this variance affects formation/consumption of CH2O. Analysis of the high-temperature ignition period shows that a key improvement in the LES is the ability to predict heterogeneous ignition sites, not only in the head of the jet, but in shear layers at the jet edge close to the position where flame lift-off eventually stabilizes. The LES analysis also shows concentrated pockets of CH2O, in the center of jet and at 20 mm downstream of the injector (in regions where the equivalence ratio is greater than 6), that are of similar length scale and frequency as the experiment (approximately 5–6 kHz). The periodic oscillation of CH2O match the frequency of pressure waves generated during auto-ignition and reflected within the constant-volume vessel throughout injection. The ability of LES to capture the periodic appearance and destruction of CH2O is particularly important because these structures travel downstream and become rich premixed flames that affect soot production.

Shock-tube study of high-temperature ignition of propane-air mixtures at elevated pressures

Ignition delay time measurements in high-temperature propane-air mixtures have been performed behind reflected shock wave at temperatures T = 1043 ÷ 1778 K, pressures p = 17.7 ÷ 42.8 atm, and equivalence ratios φ = 0.5, 1.0, and 2.0. Simultaneously, the emission characteristics of the flammable mixture associated with the emission of electronically excited OH* radical were recorded. The available detailed kinetic mechanisms of propane ignition were used to describe the obtained experimental data. Sensitivity analyses of the propane-air mixtures with various equivalence ratios was carried out to single out the key reactions proceeded in these mixtures. The data obtained make it possible to specify the available information on the ignition of combustible mixtures and thereby to improve the understanding of the processes occurring in the combustion chambers.

Cool flame wave propagation in high-pressure spray flames

Cool flame has an important impact on the performance and emission of advanced low-temperature combustion (LTC) internal combustion engines (ICEs) in which the liquid fuel is injected earlier at a lower ambient temperature (Tam) than that in the conventional diesel combustion engines. However, the cool flame characteristics of spray combustion under ICEs conditions are not fully understood, e.g., the effect of cool flame on the spray ignition and flame stabilization is not well studied. In this paper, the so-called cool flame wave propagation (CFWP) in the Engine Combustion Networks Spray A flames at three ambient temperatures Tam (800 K, 900 K and 1000 K) is analyzed using the data from large eddy simulation with an improved Eulerian based transported probability density function sub-grid combustion model. A good agreement between the LES results and experimental data is obtained for the spray liquid penetration length, vapor fuel penetration length, mean pressure rise profile, and flame liftoff length. It is shown that CFWP in the spray ignition process promotes the ignition of the fuel-rich and cold reactant mixture, leading to the most reactive mixture shifting toward fuel-richer locations due to the spatial stratification of temperature and concentration, and turbulent mixing. As a result, the high temperature ignition (HTI) can be shortened compared to the ignition in the homogeneous mixture. At Tam = 800 K, the HTI kernels are consistently formed following the CFWP propagating toward the spray head region. However, as Tam increases, the spatial correlation between HTI kernels and CFWP fronts is weakened. On the other hand, the turbulent mixing (quantified using local scalar dissipation rate) contributes more to the formation of HTI kernels at higher Tam. The present results indicate that CFWP is more profound at lower Tam in the spray ignition process. Finally, it is found that cool flame propagates mainly into pre-reacted fuel-rich mixture in an ignition wave propagation mode from the spray upstream region toward downstream region, whereas the ignition assisted flame mode is found in the spray upstream region where the combustion heat release is negligible.

DNS Of the ignition process of n-heptane/air premixed combustion with low-temperature chemistry in turbulent boundary layer

In the present work, three-dimensional direct numerical simulation (DNS) of n-heptane/air premixed combustion in turbulent boundary layer was performed to explore the near-wall ignition process with low-temperature chemistry. A reduced chemical mechanism with 58 species and 387 elementary reactions for n-heptane combustion was used in the DNS. The general characteristics of the ignition process near the wall were examined. It was found that low-temperature ignition (LTI) dominates the upstream region, and high-temperature ignition (HTI) appears in the downstream region. The ignition process and the low-temperature chemistry pathways of the DNS are compared with those of a corresponding laminar case. It was found that the ignition process was affected by turbulence, which results in thickened reaction zones. However, the carbon flow analysis of low-temperature chemistry showed that turbulence rarely affects the low-temperature chemistry pathway. The combustion modes of various regions were scrutinized based on the budget terms of species transport equations and the chemical explosion mode analysis (CEMA). It was shown that the reaction term of RO2 is significant during the LTI process of the upstream region, and the reaction terms of CH2O and CO2 are evident in the downstream region, indicating the occurrence of HTI. It was also shown that auto-ignition is dominant in the upstream region. With increasing streamwise distance, the contribution of flame propagation increases, which takes over that of auto-ignition in the near-wall region.

Large eddy simulation of n-dodecane spray flame: Effects of injection pressure on spray combustion characteristics at low ambient temperature

The present study uses large-eddy simulations (LES) to identify the underlying mechanism that governs the ignition and stabilization mechanisms of ECN Spray A flame for different injection pressures (Pinj) and ambient temperatures (Tam). Two Pinj of 50 MPa and 150 MPa, as well as two Tam of 750 K and 900 K are considered. The numerical model is validated against the experimental fuel penetration, radial mixture fraction distribution, ignition delay time, and lift-off length. The combustion characteristics of all four spray flames are well predicted, with a maximum relative difference of 15% to the measurements. At 900 K, high-temperature ignition (HTI) occurs in the fuel-rich mixture at the spray head of the high Pinj spray flame, but at the spray periphery of the low Pinj spray flame. This is due to the low Pinj case having fuel-richer mixture in the inner spray region. Nonetheless, the spray flames at both Pinj exhibit double-flame structure. At 750 K, HTI occurs at the fuel-rich and fuel-lean regions for spray flames with Pinj=50 MPa and 150 MPa, respectively. Reducing the Pinj leads to a lower injection velocity, less turbulent fluctuation, slower mixing, and hence the occurrence of HTI at the fuel-rich mixtures. The spray flame in the low Pinj case at 750 K exhibits a triple-flame structure at the lift-off position, while the high Pinj case exhibits a lean premixed reaction zone. This difference is attributed to the distribution of fuel-rich mixtures. Despite differences in the flame structures, auto-ignition process plays a key role to stabilize the lift-off position for all four spray flames. The auto-ignition process is also found to be dependent on the cool-flame products upstream of the lift-off position. In particular for the low Tam cases, the heat transfer effect from the main flame to the fuel-rich regions is suggested to also contribute to the flame stabilization mechanisms.

Effect of Methane on the Ignition Process of Spray Flame Using Large Eddy Simulation

Abstract The ignition process of spray flame in a methane environment was studied using large eddy simulation. By comparing the combustion process of n-dodecane spray (single fuel, SF) and n-dodecane/methane dual fuels (DF), the effect of methane addition on the low- and high-temperature ignition was investigated. Results showed that although the ignition delay times (IDTs) for the SF and DF cases are very similar, methane in the ambient gas mainly prolongs the time interval between the low- and high-temperature combustion for the DF case. A high gas temperature of 900 K can increase the reactivity and promote the early oxidization of n-dodecane. Thus, the formation of formaldehyde appears very early at 900 K. Then, the interactions between the early oxidation process of methane and the multistage ignition process of spray are investigated. By delaying the n-dodecane injection timing, different active environments including intermediate radicals are created, which can delay or even shorten the ignition process of n-dodecane spray depending on the gas temperature. However, the formation of formaldehyde is less dependent on the injection timing at low temperatures, indicating that methane has a negligible influence on the onset of low-temperature reactions. But at high temperatures, the early oxidation process in methane increases the gas temperature, which plays the dominant role in shortening the IDT.

Reliable determination of SiO2 concentrations in sediments via sequential leaching and ICP-OES/MS analysis

Although one of the most abundant elements in bulk Earth, silicon has not received enough attention during routine elemental analysis by ICP-OES and ICP-MS as the standard methods cannot readily obtain Si due to its volatile SiF4 removal during standard HF-HNO3 digestion. This study developed a detailed calculation method for SiO2 concentrations in a sequential analysis of sediment samples based on the mass balance principle. The prerequisite of our calculation is that (in)organic carbon is effectively removed through acid leaching and high-temperature ignition. In this study, powdered geologic reference materials were treated with a two-step sequential leaching method of i) 1 mol/L HCl to digest authigenic minerals such as carbonates followed by ii) ignition in a muffle furnace to remove remaining organic matter and a concentrated HF-HNO3 high pressure-temperature digestion to attack the detrital silicate residues. Five reference materials, GSR4, GSR5, GSR6, GSD9, and GSD10, were selected to verify the trueness of our procedure. The SiO2 contents of geostandards showed good agreement with reference values. The precisions of calculated SiO2 values are better than 0.46 %; 1σ, n ≥ 3, and the accuracies are better than 2 %; 1 RSD, n ≥ 3, compared to the reference values. For other elements, the accuracy and precision of calculated results in bulk samples (i.e., the sum of leached and residual phases) are generally better than 15 %; 1 RSD, n ≥ 3, except for a few elements with extremely low concentrations. This strategy has the advantages of low sample consumption, high efficiency, and helps saving time and labour costs compared to separate sample preparation and analysis for SiO2 and the other major/trace elements.

Unsteady Flamelet modeling study on OMEx-type fuels under Engine Combustion Network Spray A conditions

Poly-Oxymethylene Dimethyl Ethers OMEx are synthetic and potentially-renewable fuels that lead to a notable reduction of the lifecycle CO2 emissions while promoting lower soot emissions than conventional Diesel fuel. In the present contribution, a computational study with a single component OME1 and a multicomponent OMEx fuel has been carried out under reference Spray A conditions from the Engine Combustion Network (ECN), which mimic in-cylinder conditions representative of Diesel engines. For both fuels, three ambient temperature conditions have been swept at constant ambient density. Calculations have been carried out using an Unsteady Flamelet Progress Variable (UFPV) combustion model and detailed chemical mechanisms. For both OMEx-type fuels, low temperature ignition in flamelet configurations start in lean mixtures, which shifts towards the fuel-rich zone and eventually leads to high temperature ignition, similar to typical hydrocarbons. In agreement with corresponding fuel cetane numbers, ignition of OMEx occurs at timings similar to those of n-dodecane, which is the reference fuel for ECN studies, while a delayed ignition is obtained for OME1. However, the actual difference in ignition timing between OMEx and n-dodecane depends on diffusion in the mixture fraction space. Moreover, ignition in spray calculations seems to occur fully on the lean side, especially for OME1, as well as for the low temperature cases. This difference in ignitable mixture range between the canonical flamelet configuration and the spray calculations results from the finite residence time for relevant mixtures in the latter case, compared to an infinite residence time in flamelets. Comparison with experiments show that the modeling approach predicts most combustion metrics for both fuels and temperature values. The combination of ambient temperature and fuel-related reactivity has enabled a transition from a short ignition lifted diffusion flame structure (OMEx at 1000–900 K) towards long ignition cases, where lift-off length may eventually be longer than the maximum length of the stoichiometric surface. This results in a reaction front stabilization at very lean conditions, i.e. a type of lean mixing-controlled flame.

Methanol oxidation up to 100 atm in a supercritical pressure jet-stirred reactor

Methanol (CH3OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. The experimental results show that the onset temperature of CH3OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH3OH oxidation from the literature and added some new low-temperature reaction pathways such as CH2O + HO2 = HOCH2O2 (RO2), RO2 + RO2 = HOCH2O (RO) + HOCH2O (RO) + O2, and CH3OH + RO2 = CH2OH + HOCH2O2H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO2 chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO2 chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO2 formation.

Study of mechanism of ammonia decomposition and oxidation: From NOx reduction to ammonia auto-ignition problem

A chemical kinetic mechanism for ammonia decomposition and combustion from the CRECK modeling group is analyzed in this study with respect to the fast and slow chemical processes and the results are used to generate skeletal mechanisms for different applications. As examples two regimes of system parameters are chosen: values typical for engine exhaust gas systems (low temperature ammonia decomposition and NOX reduction) and a high temperature ignition problem studied by flow reactors and shock tube experiments. The mechanism is first tested and compared to existing experimental results. A time scale analysis is implemented to study the mechanism. A combination of local and global analysis based on the Global Quasi-Linearisation (GQL) concept is suggested and employed to estimate the number of relatively fast and slow processes governing the system dynamics. A GQL decomposition into invariant sub-spaces is used to determine the complimentary sub-sets of relatively fast and slow reactions. These are used to devise specific skeletal mechanisms for the chosen applications, which are verified in the considered parametric ranges. The suggested approach can be used to identify rate limiting processes, study how the coupling of relatively fast and slow (slaved) reactions proceed. The latter is extremely important for mechanism development and model reduction. The results show a very good agreement not only with respect to the ignition delay times, but also with the results from flow reactor experiments.

Experimental and Kinetic Modeling Study on High-Temperature Autoignition of Cyclohexene.

Cyclohexene is an important intermediate during the oxidation of cycloalkanes, which comprise a significant portion of real fuels. Thus, experimental data sets and kinetic models of cyclohexene play an important role in the understanding of the combustion of cycloalkanes and real fuels. In this work, an experimental and kinetic modeling study of the high-temperature ignition of cyclohexene is performed. Ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST). The studied pressures are 5, 10, and 20 bar; the equivalence ratios are 0.5, 1.0, and 2.0; and the temperatures range from 980 to 1400 K for IDT in HPST. It is shown that the IDTs of cyclohexene exhibit Arrhenius behaviors as a function of temperature, and the IDTs decrease as the equivalence ratio and pressure increase. The experimental results are simulated using three previous detailed kinetic mechanisms and an updated detailed mechanism in this work. The updated detailed kinetic mechanism exhibits good agreement with experimental results. Reaction path analysis and sensitivity analysis are performed to provide insights into the chemical kinetics controlling the ignition of cyclohexene. The results demonstrate that different detailed kinetic mechanisms are significantly different, and there are still no unified conclusions about the major reaction path for cyclohexene oxidation. However, it is worth noting that the abstraction reaction by oxygen at the allylic site and the submechanism of cyclopentene are of significant importance for the accurate prediction of IDTs of cyclohexene. The present experimental data set and kinetic model should be valuable to improve our understanding of the combustion chemistry of cycloalkanes.

Effect of injection timing on knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures

The knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures are still unclear, especially under different injection timings. Therefore, this study illustrates the above issues through CONVERGE simulation. The results show that with the start of injection (SOI) sweeps from −7°CA to −32°CA, a large amount of liquid-phase fuel adheres to the wall, and the wet-wall ratio of fuel at SOI = −32°CA is as high as nearly 30%. The fuel film evaporates slowly, coupled with the effect of low temperature on chemical reactions, the high-temperature ignition (HTI) is delayed seriously until the end of injection. The amount of premixed mixture formed during long ignition delay is significantly increased, but its uniformity is better and the concentration is more suitable for ignition. Once HTI is triggered, high-frequency and strong pressure oscillation occurs in the cylinder, and the maximum oscillation amplitude is as high as nearly 10 MPa, far exceeding the threshold of destructive knock combustion. Delayed fuel injection can effectively alleviate the above problems, such as the best when the SOI in this study is −17°CA. In addition, HC emissions are positively correlated with the amount of fuel film, but the trend of CO quantity with injection timing shows the opposite result. NOx emission increases as the injection timing advances, while soot is the opposite, because the mixture concentration is leaner at the earlier SOI and the expanded high-temperature region leads to an accelerated oxidation rate of soot.

Comparative study on ignition characteristics of n-propylbenzene, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene behind reflected shock waves

Alkyl aromatics comprise a significant portion of real fuels. Among various alkyl aromatics, the C9H12 aromatic fuels including n-propylbenzene and trimethylbenzenes are representative alkyl aromatics, which are widely detected in real fuels, and they are widely employed as surrogate compounds in modeling real fuels. Thus, the combustion kinetic study of C9H12 fuels is necessary and urgent for fuel combustion. In this work, comparative experimental and kinetic modeling study of the high-temperature ignition of three C9H12 fuels is performed. New ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST) for 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene. The studied pressure is 2, 5 and 10 bar, the equivalence ratios are 0.5, 1.0 and 2.0, and the temperature range is from 1090 K to 1600 K for IDT in HPST. The experimental results are simulated using an updated detailed kinetic mechanism. Reaction path analysis and sensitivity analysis are performed to provide insight into the chemical kinetics controlling the ignition of the three C9H12 fuels. The present experimental data set and kinetic model results should be valuable to improve our understanding of the combustion chemistry of alkyl aromatics and to offer practical guidance for the surrogate model development of real fuels.

Insight into High-temperature Ignition Processes of Methane–T425 Mixtures at Various Methane Contents: Experimental and Chemical Kinetic Analysis

ABSTRACT A surrogate fuel model named as T425 was constructed for diesel with 57.5% n-heptane and 42.5% toluene by mole fraction, and then ignition characteristics of stoichiometric methane–T425 mixtures with various methane contents at elevated initial temperatures were investigated at initial pressures up to 30 atm via a shock tube facility. The result shows that behaviors of two reaction mechanisms studied in accurate prediction of ignition of methane–T425 mixture depend on initial pressure and temperature. At higher methane content, more significant change will occur in ignition delay time of the mixture if continually increasing the methane content. In the ignition process, methane is consumed through reactions with radicals in a short time near the ignition time point. Attacks from radicals to n-heptane dominate its consumption, but at initial stage of ignition period the decomposition reaction plays the major role for the consumption of n-heptane. Radicals including H and OH show the control over the consumption of toluene, and such consumption lasts for a relatively longer time compared to methane. When elevating the methane content, decomposition reactions for consumption of n-heptane are promoted, and reactions involving CH3 are enhanced in oxidation processes of methane and toluene.

Ignition Sensitivity Analysis for Energy-Assisted Compression-Ignition Operation on Jet Fuels with Varying Cetane Number

<div class="section abstract"><div class="htmlview paragraph">Local deposition of thermal energy can be used to assist the combustion process of low cetane number (CN) fuels in compression-ignition engines, here termed energy-assisted compression ignition (EACI). In the current work, a commercial ceramic glow plug, operated beyond its conventional operation range, was used as the ignition assistant (IA) and sensitivity of fuel jet ignition to operation parameters was studied for two fuels using EACI in an optical engine. A design-of-experiments (DoE) study was devised to determine which engine parameters influenced the energy-assisted pilot injection ignition process the most. The DoE was constructed with four parameters: injection pressure, injected mass, injection timing, and ignition assistant temperature. The fuels used were F24 (Jet-A with military additives) with a cetane number of 48 and a cetane number 35 fuel mixture consisting of 60% F24 and 40% of an alcohol-to-jet fuel (ATJ), blended on a volumetric basis. Simultaneous OH chemiluminescence and schlieren imaging were employed along with in-cylinder pressure measurements to assess ignition delay, jet development, and the combustion process. A Gaussian Kriging emulator was utilized to perform the sensitivity analysis. The results indicate that ignition of the fuel jet directed toward the ignition assistant is mainly driven by the ignition assistant temperature, with around 90% of the sensitivity for both fuels ascribed to ignition assistant temperature. However, for ignition assistant surface temperatures greater than ~1350 K ignition delay is approximately constant with increasing temperature. Ignition delay from the time the fuel reaches the ignition assistant to start of combustion, here termed hot surface ignition delay, was analyzed to gain a better understanding of the EACI ignition process. The hot surface ignition delay for ignition assistant temperatures <i>></i>1350 K ranged from approximately 200 to 500 <i>μ</i>s, independent of fuel cetane number. This indicates that ignition for ignition assistant surface temperatures <i>></i>1350 K is likely the result of high-temperature ignition chemistry.</div></div>

Practical Application of Oxygen Augmentation Observation in the Process of Unsealing Fire Zone: Case study of Yu Wu coal mine in China

It is very important to unseal the closed coal mine fire zone safely. Although the fire source disappears in the closed fire zone, it is uncertain whether there are still have high-temperature ignition points, and there is a risk of fire or even explosion again after unsealing. To ensure that the closed fire zone can be unsealed successfully at one time, the study puts forward the method of oxygen augmentation observation to understand the combustion state of the closed fire zone in underground coal mine from the perspective of gas analysis technology. When the mine fire is detected to be extinguished, the method is to inject 8–12% oxygen into the fire area, and the fire area shall be monitored for 2 weeks to determine whether the fire area is completely extinguished. The accuracy and practicality of the method of oxygen augmentation observation are verified by comparing and analyzing the difference results generated by two oxygen augmentation observations in closed zone of Yuwu Coal Mine. In this paper, the use conditions of unsealing in fire zone, detailed methods and steps in the whole process of unsealing and emergency rescue plan are given. The research results can provide valuable reference for one-time safe unsealing of fire zone during the process of disaster relief.

Numerical study on the wall-impinging diesel spray mixture formation, ignition, and combustion characteristics in the cylinder under cold-start conditions of a diesel engine

The combustion of wall-impinging diesel spray is a typical phenomenon in a diesel engine due to the limited in-cylinder space, especially for small and medium-sized diesel engines under the cold-start condition. To investigate the wall-impinging diesel spray combustion process at low-temperature and low-speed conditions, the mixture formation, ignition, and combustion characteristics of the fuel were quantitatively analyzed using a computational fluid dynamics model. It was discovered that the diesel spray was guided by the piston surface and the limited in-cylinder space, resulting in more vapor-phase fuel flowing into the region between the two spray paths, Thus, high-temperature ignition (HTI) occurred in this region. During a cold-start, in-cylinder HTI also occurred in the region between the two spay paths with different injection timings. The flame then propagated first to the region with the higher equivalence ratio near the piston surface and finally to the region with the lower equivalence ratio in the center of the combustion chamber. Because an earlier injection timing resulted in both a leaner mixture in the cylinder at top dead center and the transition period from the low-temperature reactions to the high-temperature reactions, there was a limited ability to adjust the HTI and thermal power conversion by controlling the injection timing. The optimal control of in-cylinder combustion during the cold start of a diesel engine depends on the HTI, the center of combustion, and the deposited fuel mass. The optimal of injection timing during a cold start of the diesel engine in this study was −13 °CA ATDC.

Fire Risk Assessment and Experimental Study of Transformer Insulating Oil

Most previous works concentrated on burning characteristics of pool fire using common fuels such as heptane, propane, biodiesel, and diesel, whereas burning characteristics for transformer oil are barely involved, although transformer oil is arguable of more practical importance in power system. This study performed a series of pool fire experiments using transformer oil to study the burning characteristics in open and confined spaces. Three fuel thicknesses and five initial temperatures are changed in open space. The essential parameters of mass loss rate, flame height, and fire plume temperature are obtained and analyzed. Moreover, three oil pool diameters are varied in a chamber. The main conclusions are summarized as follows: the variations of mass loss rate, flame height, and fire plume temperature not only obviously increase as the fuel thickness increases from 0.5 cm to 1.0 cm, but also insignificantly increase as the fuel thickness changes from 1.0 cm to 2.0 cm. The mass loss rate is less sensitive to the initial temperature of transformer oil, but the flame height and fire plume temperature significantly rise with the initial temperature. Moreover, the modified models to predict the flame height and fire plume temperature for 25°C initial temperature conditions are proposed, but the fitting coefficients are obviously different from that for common liquid fuels. The flame height in confined space is higher and will rapidly increase to the maximum, then decreases, and tends to be stable, which is obviously different from the oil pool fire burning in open space. In addition, the phenomenon of burning blast and the ignition of the adjacent oil pool will be observed with a high‐temperature ignition source and a certain high temperature in a chamber under 30 cm oil pool diameter, which will not be recorded in 15 cm and 20 cm.

Effects of speed extension on PCCI combustion and emissions in a heavy-duty diesel engine at medium load

Effects of boost pressure, engine speed and multi-pulse injection timing on premixed charge compression ignition (PCCI) combustion and emissions in a heavy-duty diesel engine at medium load were studied experimentally and numerically. Results show that, intake boost makes the high temperature ignition timing advance, but the mass fraction of lean mixture increases dramatically and the mass fractions of rich and over-rich mixtures decrease significantly, indicating that intake boost helps the formation of lean and homogeneous mixture; the indicated thermal efficiency increases and the combustion loss decreases. When the engine speed is increased with other variables constant, the high temperature ignition timing is retarded, the mass fraction of over-rich mixture decreases; the NOx, soot, CO and UHC emissions are all decreased; the indicated thermal efficiency increases, the combustion loss decreases, but the exhaust loss increases. The delay of multi-pulse fuel injection timing with other variables constant shortens the mixing time, increases the mass fraction of over-rich mixture, and increases the initial soot generation. However, due to the combined effect of mixing space and combustion temperature, the final soot, CO and UHC emissions are all decreased, and the NOx emissions have little change; the indicated thermal efficiency increases, the combustion loss decreases, but the exhaust loss increases.