Ignition Delay Times Of Fuels Research Articles

Reactions of Hydroperoxyl Radicals (HO2) with Oxygenated Aromatic Bio-oil Model Compounds

During thermal utilization of biomass, a complex array of reactions ensues in the oxidative environment between fragments of the biomass and reactive species. In the low-temperature window, hydroperoxyl (HO2) radicals assume a central role prior to the establishment of the O/H radical pool. Anisole, cresol, guaiacol and vanillin are often utilized as representative model compounds to comprehend the combustion chemistry of the phenolic entities in biomass and oxygen-containing derived bio-oil. This paper reports thermo-kinetic parameters for the reactions of HO2 radicals with anisole, cresol, guaiacol and vanillin. Computed activation enthalpies entail facile values for H abstraction from the side-chain functional groups in the investigated compounds. H abstraction from the hydroxyl groups incurs lower activation energies in reference to methyl sites in cresol and guaiacol. Hence, H abstraction from the OH sites in these two molecules proceeds at a faster reaction rate constant when compared to abstraction from the methyl site. In the vanillin molecule, abstraction from the aldehydic and hydroxyl sites portrays comparable importance. The effect of scaling vibrational frequencies and the type of applied quantum tunneling on the computed reaction rate constants were thoroughly explored. Analysis of entropies of activation indicates that the transition states exhibit a product-like character. While ignition delay times of fuels often exhibit a profound sensitivity towards initial H abstraction reactions by HO2, we found that ignition delay times of the four title molecules remain almost unchanged when updating literature kinetics models with new reaction rate constants calculated herein. It is anticipated that reported outcomes from this study aid in formulating a better understanding of the complex chemical reactions that dictate oxidative decomposition of oxygenated bio-oil model compounds.

Co-combustion of methane hydrate and conventional fuels

This paper presents the experimental data on the co-combustion of methane hydrate powder with conventional fuels: gasoline, kerosene, Diesel fuel, coal, and coal slime. For the experiments we used a laboratory-scale combustion chamber based on an induction system on the walls and a spark ignition system. Fuel ignition delay times were measured during separate and simultaneous supply of fuel components. It was found that the combustion of slow-burning fuels can be intensified by injecting methane hydrate granules. Liquid fuels with high concentrations of light fractions can be used to intensify the combustion of methane hydrate at relatively low temperatures in the combustion chamber. After recording the concentrations of the main components of combustion products, we rationalized the environmental benefits of gas hydrates used as the primary and secondary fuel components. The relative efficiency coefficients were calculated for fuels in terms of their environmental, economic, and energy performance indicators. We also formulated recommendations on the co-combustion of gas hydrates with conventional fuels in combustion chambers. Finally, we proposed engineering solutions for the effective use of gas hydrates in power plants.

An experimental investigation into the fuel oil-free start-up of a coal-fired boiler by the main solid fossil fuel with additives of brown coal, biomass and charcoal for ignition enhancement

The research is focused on the ignition of solid fuels in a heated air flow. The experimental conditions correspond closely to solid fuel combustion in coal-fired boiler furnaces. Charcoal, wood, brown and bituminous coals were used as independent solid fuels and as fuel mixture components. The particle sizes of these fuels were less than 40 μm, 40–140 μm, 140–250 μm, 250–375 μm and 375–800 μm. They were ignited under the conditions of radiant-convective heating as they were moving in a heated air flow with a temperature of 500–800 °C and at a velocity of 5 m/s. The ignition delay time was the main investigated characteristic, recorded by a high-speed video camera. The induction period of bituminous coal shortens by 25–35% when 25–50% brown coal or wood in an air-dry state is added. The lowest ignition delay times were established for the fuel mixtures of bituminous coal with dry charcoal. The experimental curves of the ignition delay times of fuels and their mixtures versus the heated air temperature were presented as approximation equations. Based on the conducted research findings, a muffle burner design with a vortex structure of the fuel-air flow was proposed for the fuel oil-free start-up of coal-fired boilers.

Experimental research and numerical simulation of gel fuel ignition by a hot particle

The research investigates interdependent physical and chemical processes, occurring when a typical gel fuel is ignited by a hot particle. In the conducted experiments, high-speed video recording made it possible to establish the consistent patterns and characteristics of the ignition of gel fuel, based on an organic polymer thickener, by a disc-shaped steel particle 5 mm in radius and height. The fuel ignition delay times are 0.5–2.5 s when the initial temperature of the steel particle is varied from 1230 to 1500 K. A mathematical model of the investigated process was developed using experimental data obtained. It describes the interdependent processes of conductive heating and melting of gel fuel, melt evaporation, cooldown of the particle and its gradual immersion in the near-surface layer of the fuel, formation of a combustible gas–vapor mixture around the local heating source and its ignition under the conditions of diffusion-convective heat and mass transfer. The numerical simulation results are in good agreement with the experimental data. The developed mathematical model can be used in practice to predict the characteristics of gel fuel ignition by hot particles, when a group of factors in the particle – fuel – air system is varied in wide ranges.

Applicability of high dimensional model representation correlations for ignition delay times of n-heptane/air mixtures

It is difficult to predict the ignition delay times for fuels with the two-stage ignition tendency because of the existence of the nonlinear negative temperature coefficient (NTC) phenomenon at low temperature regimes. In this paper, the random sampling-high dimensional model representation (RS-HDMR) methods were employed to predict the ignition delay times of n-heptane/air mixtures, which exhibits the NTC phenomenon, over a range of initial conditions. A detailed n-heptane chemical mechanism was used to calculate the fuel ignition delay times in the adiabatic constant-pressure system, and two HDMR correlations, the global correlation and the stepwise correlations, were then constructed. Besides, the ignition delay times predicted by both types of correlations were validated against those calculated using the detailed chemical mechanism. The results showed that both correlations had a satisfactory prediction accuracy in general for the ignition delay times of the n-heptane/air mixtures and the stepwise correlations exhibited a better performance than the global correlation in each subdomain. Therefore, it is concluded that HDMR correlations are capable of predicting the ignition delay times for fuels with two-stage ignition behaviors at low-to-intermediate temperature conditions.

Autoignition of varied cetane number fuels at low temperatures

The reactivity of six kerosene based control fuels, specifically formulated for cetane number variation, are investigated by measuring ignition delay time in a heated rapid compression machine. Cetane numbers vary from 30 to 55 (increment of 5) while holding other properties relatively constant by adjusting chemical group composition. Main cetane variation was controlled through the distribution of normal alkanes and isoalkanes, which was fine-tuned using additives. Other fuel properties such as density, viscosity, H/C ratio, etc. were balanced using cyclic compounds and aromatics. Fuels were tested in the RCM at compressed pressures of Pc = 10 and 20 bar, equivalence ratios of ϕ = 0.25, 0.5 and 1.0, in the low to intermediate temperature range (620 K ≤ Tc ≤ 730 K). Relations between cetane number and ignition delay time have been evaluated at multiple test conditions, and further analysis on multistage ignition has been conducted. Ignition delay times of fuels with higher cetane numbers are shorter at these temperatures for most conditions. First stage ignition delay time measurements have been observed to be relatively insensitive to Pc, ϕ, and fuel type, while deviations in overall ignition delay times are mainly attributed to second stage ignition delay time, impacted by variations in the first stage temperature. Control fuels of this type offer an opportunity to be used in practical experiments to determine the impact of cetane number on combustion dynamics.

The impact of physicochemical property interactions of iso-octane/ethanol blends on ignition timescales

This work presents new measurements of liquid fuel ignition delay times of iso-octane and ethanol fuel blends obtained from an ignition quality tester at the National Renewable Energy Laboratory (NREL IQT), which are compared to previous ignition delay data from the University of Michigan rapid compression facility (UM RCF), at the same experimental conditions. Pressure-time histories were used to determine liquid fuel ignition delays at global stoichiometric non-premixed conditions for iso-octane, ethanol and iso-octane/ethanol blends of 25, 50, 75% by volume in mixtures of 10% oxygen diluted in nitrogen. Temperatures ranging from 880 to 970 K were studied at a pressure of 10 atm. By comparing total ignition delay times from the NREL IQT with chemical ignition delay times from the UM RCF, the contributions of physical phenomena were quantified as representative time scales for spray injection, breakup and evaporation processes, and for gas-phase turbulent mixing. Regression analyses were developed for ignition time scales as function of blend level and charge temperature. Non-dimensional analyses were also carried out to determine the relative effects of physical time scales with respect to chemical ignition delay times.

Shock Tube Ignition Delay Data Affected by Localized Ignition Phenomena

ABSTRACTShock tubes have conventionally been used for measuring high-temperature ignition delay times of approximately O(1 ms). In the last decade or so, the operating regime of shock tubes has been extended to lower temperatures by accessing longer observation times. Such measurements may potentially be affected by some non-ideal phenomena. The purpose of this work is to measure long ignition delay times for fuels exhibiting negative temperature coefficient and to assess the impact of shock tube non-idealities on ignition delay data. Ignition delay times of n-heptane and n-hexane were measured over the temperature range of 650–1250 K and pressures near 1.5 atm. Driver gas tailoring and long length of shock tube driver section were utilized to measure ignition delay times as long as 32 ms. Measured ignition delay times agree with chemical kinetic models at high (>1100 K) and low (<700 K) temperatures. In the intermediate temperature range (700–1100 K), however, significant discrepancies are observed between the measurements and homogeneous ignition delay simulations. It is postulated, based on experimental observations, that localized ignition kernels could affect the ignition delay times at the intermediate temperatures, which lead to compression (and heating) of the bulk gas and result in expediting the overall ignition event. The postulate is validated through simple representative computational fluid dynamic simulations of post-shock gas mixtures, which exhibit ignition advancement via a hot spot. The results of the current work show that ignition delay times measured by shock tubes may be affected by non-ideal phenomena for certain conditions of temperature, pressure, and fuel reactivity. Care must, therefore, be exercised in using such data for chemical kinetic model development and validation.

Experimental research of heat transfer conditions influence on the distillate fuels ignition characteristics

The experimental studies results of diesel fuel ignition regularities by hot single metal particles and fuel droplets on a heated metal plate surface are presented. The ignition delay time dependences on the initial heat source temperature were determined. The results are a basis for the development of typical liquid fuels ignition models in terms of heat and mass transfer processes in a small neighborhood. Liquid fuels are widely used in road transport and under certain conditions (storage, transport, congestion) are sources of increased danger. They can cause a fire during thermal exposure of high intensity. Heat sources can be different, as well as the conditions of exposure (1-3). To date, mathematical models and solving methods for the ignition of solid (4, 5) and liquid (2, 3) fuels are worked out. Experimental conditions and the ignition characteristics of solid monolithic (9) and dispersed (7) fuels with a local heating source are established. Ignition regularities for films and drops of distillate fuels are studied much worse. The purpose of this work is an experimental study of diesel fuel ignition conditions and characteristics. Two of the most important options for practice are considered: a single fuel film heated up to high temperatures and particle droplets on the heated metal plate surface. Film fuel ignition process research was conducted on the plant (9), the concept of which is shown in Fig. 1, and drop method (1) (the experimental scheme is shown in Fig. 2). In all experiments a video registration of the investigated processes was conducted. The time moments of heat source (particle or plate) contact with a liquid (film or drop, respectively) and the ignition delay time were registered. As the heat sources the steel particles (diameter of dp = 6 · 10 −3 m and a height of hp = 3 · 10 −3 ÷7 · 10 −3 m) and the steel plate (size 5 · 10 −2 × 5 · 10 −2 m and a height hw = 5 · 10 −3 m) were used. Particles fall from a small height (0.01m) onto the fuel film surface. A possible change in its temperature is due to heat sink in the air. It is established that during the fall of the particles temperature is reduced by not more than 5K, which is about ±0.5% depending on the absolute value of the initial particle temperature. Experiments at constant heat source temperature T0 is repeated at least 7 times under identical conditions. Random error in ign determining is no more than ±10% in the experiments with the particle heat source and up to ±16% in the experiments with hot plate. The dependences of the diesel fuel ignition delay times from the initial heat source temperature obtained from the experiments results are shown in Fig. 3. Measurements have shown that the temperature of the massive plate during the experiment remained virtually unchanged (droplet sizes This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Chemical kinetics modeling of n-nonane oxidation in oxygen/argon using excited-state species time histories

Chemical reactions of ground-state species strongly govern the formation of excited-state species, including OH* and CH*, which are commonly used to determine ignition delay times of fuels. With well-characterized chemiluminescence rates embedded in chemical kinetics mechanisms, time histories of excited-state species can aid in identifying influential ground-state reactions which are important to processes such as ignition delay time. Placing emphasis on the high-temperature regime, improvements were made to a detailed chemical kinetics mechanism of n-nonane oxidation developed previously by the authors. Using characteristic features of OH* time histories measured in shock-tube experiments as a metric, detailed model analyses were performed over a broad range of conditions: T>1100K, 1.5<P (atm) < 10.5, ϕ=0.5, 1.0, 2.0. OH* time history measurements, particularly under fuel-rich conditions (ϕ=2.0), displayed a two-peak behavior, with the first peak occurring within the first 5–10μs after reflected-shock passage, and the second, wider peak corresponding to main oxidation and ignition. In the initial version of the kinetics mechanism, the two peaks at rich conditions were predicted to merge, blurring the main ignition process prior to the second peak. The work herein presents modifications to the initial chemical kinetics mechanism which led to improved agreement between measurements and model-based predictions, with emphasis on the fuel-rich condition. To this end, the predicted shapes of the OH* time histories were crucial to matching the two-peak behavior detected in the experiments. A first-order resistance–capacitance (RC) model of the experimental time response of the optical setup was developed and shown to reproduce the measured time dependence and peak behavior that are vital for matching the OH* behavior near time-zero. The RC model processes the kinetics predictions in a way that allows the kinetics model predictions to directly correspond to the true conditions in the experiment. In moving towards improved agreement in OH*-profile predictions for all conditions, improvements in the kinetics mechanism were also realized at the two leaner equivalence ratios (ϕ=1.0 and 0.5), both in terms of OH* profile shape and ignition delay times. Model calculations of oxidation processes indicate that reactions leading to the first OH* peak originate from fuel homolysis. The resulting (alkyl) radicals lead to the formation of methyl which then, through a series of H-abstraction reactions, leads to the production of the methylidyne radical (CH) that reacts with molecular oxygen to form OH*. The oxidation processes near time-zero terminate, in part, due to methyl depletion by methylene forming C2H4+H2. In addition to the insight gained on n-nonane ignition and oxidation chemistry, the present study highlights the utility of correctly interpreted OH* measurements for inference of kinetic information other than ignition delay times.

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O2/argon mixtures at pressures from 2.3 to 8.0atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.

Ignition and Emission Characteristics of Surrogate and Practical Jet Fuels

Aircrafts have been identified as a significant source of particulate matter (PM) emissions, which have been associated with adverse health effects and decreased ambient air quality. Recent published studies have shown that synthetic fuels, such as iso-paraffinic Fischer−Tropsch (F−T) fuels, can significantly reduce PM emissions compared to operation with petroleum-derived fuels. It is believed that the absence of aromatics in synthetic fuels slows the molecular growth to higher ringed polycyclic aromatic hydrocarbons (PAHs), which ultimately leads to lower soot emissions. However, it is not known if differences in chemical composition between synthetic and conventional (JP-8) fuel also change the ignition and combustion characteristics, which likely impact high altitude relight and combustor lean blowout (LBO) limits. This study was conducted to investigate how differences in fuel composition can impact the chemical ignition and emissions characteristics of a fuel and to help deconvolute the effect of physical and chemical processes on combustion efficiency. Only the chemical ignition delay was considered as the fuels were prevaporized and premixed prior to ignition. The fuels investigated in this study were a F−T processed synthetic jet fuel (synjet), a conventional JP-8, 2-methylheptane (surrogate synjet fuel), and a mixture of 80% heptane and 20% toluene (surrogate JP-8). The experiments were conducted using a modified single-pulse reflected shock tube instrumented to measure fuel ignition delay times and PAH and soot yields. The shock tube results show that both the actual and surrogate synjet fuels produced lower soot and PAH emissions than the actual and surrogate JP-8 fuels at temperatures of interest. These results are consistent with previous turbine engine test results with the actual fuels. It was observed, however, that, for the conditions tested, both surrogate and actual synjet and JP-8 fuels evaluated had similar ignition delays, which may imply that there is no correlation between soot emissions and chemical ignition delay. However, these results may be controlled by the test conditions, and accordingly, future tests at different conditions (e.g., higher equivalence ratios, pressures, lower pre-ignition temperatures) are warranted.

Jet fuel ignition delay times: Shock tube experiments over wide conditions and surrogate model predictions

Ignition delay times were measured for gas-phase jet fuel (Jet-A and JP-8) in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: temperatures of 715–1229 K, pressures of 17–51 atm, equivalence ratios of 0.5 and 1, and oxygen concentrations of 10 and 21% in synthetic air. Ignition delay times were measured using sidewall pressure and OH* emission at 306 nm. Longer ignition delay times at low temperatures (715–850 K) were accessed by utilizing driver-gas tailoring methods. Also presented is a review of previous ignition delay time measurements of kerosene-based fuels and recent work on surrogate fuel and kinetic mechanism development. To our knowledge, we report the first gas-phase shock tube ignition delay time data for JP-8, and our measurements for Jet-A are for a broader range of conditions than previously available. Our results have very low scatter and are in excellent agreement with the limited previous shock tube data for Jet-A. Although JP-8 and Jet-A have slightly different compositions, their ignition delay times are very similar. A simple 1 / P dependence was found for ignition delay times from 874 to 1220 K for the pressure range studied for both fuels. Ignition delay time variations with equivalence ratio and oxygen concentration were also investigated. The new experimental results were compared with predictions of several kinetic mechanisms, using different jet fuel surrogate mixtures.

A study of ignition delay of diesel fuel sprays

Diesel fuel ignition delay times were characterized in a rapid compression machine (RCM) using cylinder ambient gas temperature and pressure measurements as diagnostics. The objective of the study was to investigate the dependency of ignition delay time on: (a) cylinder ambient gas temperature, (b) cylinder ambient gas pressure, (c) injection pressure, (d) injector nozzle orifice diameter, (e) base fuel cetane number and (f) 2-ethylhexyl nitrate (2-EHN) cetane improver additive. The results presented here show that diesel ignition delay times can be shortened by increasing cylinder gas ambient temperatures and pressures, injection pressures and base fuel cetane number, either through blend components or by addition of cetane improver. Decreasing the injector nozzle orifice diameter also decreases the ignition delay time. It was also found that ignition chemistry is rate controlled by the molar concentration of the cylinder gas oxygen.