Fundamental Combustion Characteristics Research Articles

Extinction limits and structure of counterflow nonpremixed methane-ammonia/air flames

An experimental and computational investigation on the fundamental combustion characteristics of methane (CH4)-ammonia (NH3) blends is conducted to confirm their potential as a clean fuel with low carbon dioxide (CO2) emissions and determine their reasonable burning conditions, considering counterflow nonpremixed CH4NH3/air flames. Extinction limits and structure of the nonpremixed CH4NH3/air flames are measured and predicted. Results show that flames gradually become orange and the flame thickness increases with NH3 addition, compared with the pure CH4/air flames. Also, flames can sustain less NH3 at high strain rates. Compared with the pure CH4/air flames, CH4NH3/air flames exhibit remarkable reduction of CO2 emissions with moderate reduction of combustion stability limits and no remarkable temperature drop in the flame, supporting the potential of CH4NH3 blends as a clean fuel with low CO2 emissions. However, additional investigations for reducing the enhanced NOx emissions mainly via the fuel NOx mechanism with NH3 addition are needed. Finally, the quantitative discrepancy among the present measurements and predictions merits the development of a new reaction mechanism which is optimized for the reaction of CH4NH3 fuel blends and air.

Effects of diluent on laminar burning speed and flame structure of gas to liquid fuel air mixtures at high temperatures and moderate pressures

Gas to liquid (GTL) fuel has gained attention recently because of its clean combustion behavior. Experimental studies have been performed to investigate fundamental combustion characteristics such as laminar burning speed and flame structure of GTL/air/diluent premixed flames. In the present study, the GTL fuel was designated by Syntroleum S-8, supplied by US Air Force Research Laboratory (AFRL), which was synthesized from natural gas using the Fisher–Tropsch (F–T) process. A mixture of 32% iso-octane, 25% n-decane and 43% n-dodecane by volume was used as a surrogate for GTL fuel. In this work, two diluent concentrations of 5% and 10% were used. The diluent is a blend of 86% N2 and 14% CO2 having the same specific heat as the burned gases. Experiments were conducted using a spherical vessel for laminar burning speeds measurement and a cylindrical vessel to investigate the flame structures. The cylindrical vessel was set up in a Z-shape Schlieren system coupled with a high-speed CMOS camera that was used to capture evolutionary behavior of flames at up to 40,000 frames per second. A multi-shell thermodynamic model was used to calculate the laminar burning speed for the smooth and low stretch flames. During the flame expansion, measured pressure rise as a function of time was the input into the thermodynamic model. Power law correlations for laminar burning speeds of GTL/air/diluent premixed flames over a wide range of temperatures (from 490 K to 610 K), pressures (from 0.5 atm to 3.2 atm), equivalence ratios (from 0.7 to 1.2), and two different diluent concentrations of 5% and 10% have been reported. Experimental burning speed results were compared with simulation values calculated by the solution of one dimensional steady premixed flame code from CANTERA using Ranzi’s chemical kinetics mechanisms. Results of simulations are close to the measured values.

Development of a diesel/biodiesel/alcohol (up to n-pentanol) combined mechanism based on reaction pathways analysis methodology

Recently, diesel/biodiesel/alcohol blend fuels have attracted tremendous attention as a potential alternative fuel in internal combustion engines, because of their beneficial effects on pollution emission and engine performance. However, there are few chemical mechanisms to forecast combustion characteristics of the ternary blends, especially for those blending with long-chain alcohols, such as butanol and pentanol. The objective of this paper is to develop a combined reduced combustion mechanism for the combustion simulation of diesel/biodiesel/alcohol (up to n-pentanol). This new combined mechanism, consisting of 229 species and 902 elementary reactions, was constructed by integrating two separated reduced mechanisms; one is the PRF-alcohols’ mechanism (Liu et al., 2016), and another is the biodiesel mechanism (Luo et al., 2012). The new combined mechanism reproduced well predictions of fuel ignition and flame speed compared with those obtained using the original ones. Extensive validations were performed against various experimental data for 0-D homogenous systems, 1-D freely propagating premixed flames and 3-D spray combustion flames. The proposed combined mechanism was shown to be versatile and robust, successfully integrating the biodiesel reaction mechanism and the long-chain alcohol reaction mechanisms into a single reaction scheme. Coupling with advanced simulation technology, it could be a powerful numerical tool in the design and optimization of new generation engines which are fueled with clean alternative fuels, especially when concerning long-chain alcohols. In addition, the effects of adding biodiesel and different kinds of alcohols on fundamental combustion characteristics of diesel/biodiesel/alcohol blend fuels were investigated using the proposed combined mechanism.

Investigation of Methane Oxy-Fuel Combustion in a Swirl-Stabilised Gas Turbine Model Combustor

CO2 has a strong impact on both operability and emission behaviours in gas turbine combustors. In the present study, an atmospheric, preheated, swirl-stabilised optical gas turbine model combustor rig was employed. The primary objectives were to analyse the influence of CO2 on the fundamental characteristics of combustion, lean blowout (LBO) limits, CO emission and flame structures. CO2 dilution effects were examined with three preheating temperatures (396.15, 431.15, and 466.15 K). The fundamental combustion characteristics were studied utilising chemical kinetic simulations. To study the influence of CO2 on the operational range of the combustor, equivalence ratio (Ф) was varied from stoichiometric conditions to the LBO limits. CO emissions were measured at the exit of the combustor using a water-cooled probe over the entire operational range. The flame structures and locations were characterised by performing CH chemiluminescence imaging. The inverse Abel transformation was used to analyse the CH distribution on the axisymmetric plane of the combustor. Chemical kinetic modelling indicated that the CO2 resulted in a lower reaction rate compared with the CH4/air flame. Fundamental combustion properties such as laminar flame speed, ignition delay time and blowout residence time were found to be affected by CO2. The experimental results revealed that CO2 dilution resulted in a narrower operational range for the equivalence ratio. It was also found that CO2 had a strong inhibiting effect on CO burnout, which led to a higher concentration of CO in the combustion exhaust. CH chemiluminescence showed that the CO2 dilution did not have a significant impact on the flame structure.

Combustion behaviors of wasted plastics coated around electric cables in electrically heated drop tube furnaces

For thermal recycle by wasted plastics coated around electric cables, ignition and combustion experiments of the pulverized samples were carried out, using two types of electrically heated drop tube furnace, in which the particles are pulsatively fed and continuously fed, respectively. The ignition phenomenon was defined as occurrence of luminous emission from a cloud of the particles fed in the former furnace. The wasted plastics coated around the electric cables mainly make of polyethylene with a little amount of carbon particles. The ignition characteristics of the pulverized plastic samples as well as graphite particles as a reference sample were studied, changing the particle diameters and the furnace wall temperatures. In the electrically heated drop tube furnace with the continuous feeding system, the wall temperature and stoichiometric combustion air ratio were varied to elucidate the fundamental combustion characteristics. As a result, the ignition temperature depended on the particle diameter. The plastics with the particle diameter ranged from 150 to 300 µm could ignite at the lowest furnace temperature. From the continuous combustion tests, CO and lower molecule hydrocarbons like CH4, C2H2 and C2H4 were produced in the upper region of furnace. Finally, all of them gradually burned out.

Investigation of C8-C12 Paraffins’ Fundamental Combustion Characteristics and Surrogate Fuel Model

In order to investigate fundamental combustion characteristics of high carbon number paraffins and their role in surrogate fuel model of aviation fuel, ignition delay time and laminar premixed flame speed of iso-octane, n-decane and n-dodecane were measured under different initial conditions. Ignition delay time and laminar premixed flame speed were measured in shock tube and constant volume combustion chamber respectively. The experimental results show that ignition delay times of iso-octane and n-dodecane are relatively sensitive to the variation of equivalence ratio in low temperature and high temperature range, while n-decane’s influence by equivalence ratio is comparatively uniform in the entire temperature range; laminar premixed flame speed of iso-octane is sensitive to the variation of initial pressure under lean and rich fuel conditions, while n-dodecane couldn’t be ignited successfully in combustion chamber and gave flame speed data. Under overall consideration, n-dodecane is recommended as constitute of surrogate fuel model of aviation fuel which has high ignition point and is hard to combust, while high proportion iso-octane can be employed in surrogate fuel model which is sensitive to equivalence ratio, and high proportion n-decane can be employed in model which changes mildly with equivalence ratio.

Experimental study on stoichiometric laminar flame velocities and Markstein lengths of methane and PRF95 dual fuels

Natural gas is one of the most promising alternative fuels. The main constituent of natural gas is methane. The slow burning velocity of methane poses significant challenges for its utilization in future energy efficient combustion applications. Methane-gasoline dual fuelling has the potential to improve methane’s combustion. The fundamental combustion characteristics of a methane-gasoline Dual Fuel (DF) blend needs further investigation. In the current experimental study, the relationship between laminar flame velocity and Markstein length, with the ratio of gas to liquid in a DF blend has been investigated using spherical flames in a constant volume combustion vessel. A binary blend of primary reference fuels (PRF95) was used as the liquid fuel. Methane was added to PRF95 in three different energy ratios 25%, 50% and 75%. Values of the stoichiometric laminar flame velocities and Markstein lengths are measured at pressures of 2.5, 5, 10Bar and a temperature of 373K. It has been found that with a 25% increase in the DF ratio, the Markstein length is reduced by 15%, 21%, 32% at a pressure of 2.5, 5 and 10Bar respectively whereas at the same pressures the laminar flame velocity is reduced by 2%, 3% and 5%. The flame evolution at the early stages of combustion is found to be faster with an increase in the DF ratio, and gradually as the flame develops it becomes slower.

Fundamental Combustion Characteristics of Lean and Stoichiometric Hydrogen Laminar Premixed Flames Diluted With Nitrogen or Carbon Dioxide

In this work, the laminar combustion characteristics of H2/N2/air (H2/CO2/air) were systematically investigated under different hydrogen ratios (40–100%) and equivalence ratios (0.4–1.0) in a closed combustion vessel using the spherical expanding flame method associated with Schlieren technology. The unstretched laminar burning velocities were compared with data from previous study, and the result indicates that excellent agreements are obtained. Numerical simulations were also conducted using GRI3.0 and USC II mechanisms to compare with the present experimental results. The Markstein length for H2/inert gas can be decreased by decreasing the equivalence ratio and hydrogen ratio. The results indicate that the H2/inert gas premixed flames tend to be more unstable with the decrease of equivalence ratio and hydrogen ratio. For H2/N2 mixture, the suppression effect on laminar burning velocity is caused by modified specific heat of mixtures and decreased heat release, which result in a decreased flame temperature. For H2/CO2 mixture, the carbon dioxide has stronger dilution effect than nitrogen in reducing laminar burning velocity owing to both thermal effect and chemical effect.

DME/Methane混合燃料を用いた過給HCCI燃焼特性に関する研究

An investigation was made of the fundamental combustion characteristics of a supercharged Homogenous Charge Compression Ignition (HCCI) engine operating on a gaseous fuel blend of dimethyl ether and methane. Besides measuring the in-cylinder pressure, spectroscopic techniques were used to measure light absorbance and light emission intensity. Exhaust gas samples were also analyzed with a Fourier transform infrared gas analyzer. The experimental results revealed that varying the input heat energy and the mixing ratios of the gaseous fuels caused the in-cylinder pressure to rise sharply, making stable engine operation difficult. However, it was found that applying supercharging while keeping the input heat energy constant moderated the pressure rise during combustion, though the in-cylinder pressure rose overall. As a result, that made it possible to avoid knocking and also to obtain cleaner exhaust emissions. Consequently, the results of this study demonstrate that applying supercharging according to the engine load is a key factor in controlling HCCI combustion.

高温酸素燃焼条件における浮き上がり火炎の基礎燃焼特性

高温酸素燃焼条件における浮き上がり火炎の基礎燃焼特性

Thermal characterization of coal/straw combustion under air/oxy-fuel conditions in a swirl-stabilized furnace: A CFD modelling

Global warming is mainly due to increase of CO2 from fossil fuel based power generation in developed countries. The level of CO2 increase in the atmosphere is alarming. International effort imposed some strict obligations to control these emissions. To reduce the greenhouse gas emissions, shifting from coal based production to biomass resources is a must. The present study investigates the combustion behaviour of biomass (straw) compared with the coal combustion in air and O2/CO2 mixtures. Reference air-fuel and oxy-fuel (25%, 30% and 35% O2) cases were considered maintaining a constant thermal load of 30 kW in a semi-technical scale once-through swirl-stabilized furnace. The main objective of this study is to illustrate the impact on the combustion characteristics including flame temperature, burnout, and emissions for pure coal to pure biomass (straw) combusted in air and oxy-fuel atmospheres. Good agreement between the results obtained in this numerical work and results reported in literature was observed. This work has shown that significant changes occur to the fundamental combustion characteristics for straw when burned in the O2/CO2 atmosphere compared to air firing case. Comparatively higher flame temperatures were observed for oxy-firing case. The CO levels are predicted to decrease in the downstream section during oxy-fuel combustion compared to air-firing flames due to O2 availability. The burnout is reliably advanced during oxy-fuel combustion to 99.8% than air firing.

Fundamental Combustion Characteristics of Ethanol/Liquid Oxygen Rocket Engine Combustor with Planar Pintle-type Injector

Throttling technology is a key technology for future space transportation system. One of the most important components of the rocket engine for thrust control is the propellant injection system. The pintle injector is one of the most promising candidates due to its simple structure and combustion stability. However, the number of studies on the pintle injector is limited and detailed phenomena of the pintle injector has not been clarified. Therefore, a combustion test using an ethanol/liquid oxygen rocket engine is carried out with a planar pintle injector in a rectangular combustor. Fundamental combustion characteristics such as characteristic exhaust velocity efficiency are acquired at the combustion pressure of 0.45 MPa and O/F=1.25 to 1.80. C* efficiency increases as the equivalence ratio increases up to unity. Flame images during combustion at 0.45 MPa and O/F=1.75 were acquired. Luminous flame was not observed in throughout all of the region except for the vicinity of the faceplate. However, strong luminous flame was observed at the time of ignition. The combustion chamber used in this research can simulate one of the recirculation zones observed in actual combustor with pintle-type injectors, which contributes to the combustion stability. However, the recirculation zone formed at the top of the injector is not simulated appropriately due to the existence of the lower wall of the combustor.

Combustion characteristics and optimization of CPG (compressed producer gas) in a constant volume combustion chamber

PG (producer gas) from biomass gasification can be used either in SI (spark ignited) or CI (compressed ignited) engine. Present study evaluates fundamental combustion characteristics of SI CPG (compressed producer gas) in a CVCC (constant volume combustion chamber) at various equivalence ratios (ϕ) using optical technique. The combustion characteristics were compared to common fuels: gasoline, CNG (compressed natural gas) and LPG (liquefied petroleum gas). The amount of air required for each ϕ was calculated based on fixed mass of gasoline injected. For optimization study, CPG was tested at different MIF (mass increase factors): MIF = 0% (minimum configuration), MIF = 25% (middle configuration) and MIF = 50% (maximum configuration) by using DOE (design of experiment). ANOVA (analysis of variance) was conducted and the values of experimental reliability were 0.9775 and 0.9875% for flame speed and peak pressure, respectively. From the experiments, the flame speed and peak pressure of CPG were found to be 3.15 m/s and 312.09 kPa, respectively. These were lower compared to gasoline, LPG and CNG. Optimization analysis shows that CPG peak pressure is comparable to gasoline and achieve its maximum flame speed at ϕ = 1.1 and MIF = 35%.

Effects of hydrogen addition on combustion characteristics of n-decane/air mixtures

To explore the possibility of simultaneously extending the lean extinction limit and reducing the emission levels with hydrogen addition, a computational study is performed to investigate the effects of hydrogen addition on the fundamental combustion characteristics of n-decane/air mixtures. It is found that a small amount of hydrogen addition can significantly promote the reactivity of n-decane/air mixtures, leading to shortened ignition delays at high temperatures, increased laminar flame speeds, and reduced extinction residence times. The results on emissions show that the addition of hydrogen leads to a reduction in CO emission index under fuel rich conditions, while NO emission index increases with increasing hydrogen addition for all the conditions examined. The extent of the hydrogen addition effects on different combustion responses at varying pressures has also been investigated. In addition, sensitivity analysis has been conducted to identify the key reactions that are responsible for the enhanced reactivity associated with hydrogen addition. The present results further demonstrate that with the aid of hydrogen addition, leaner and hence cleaner combustion can be achieved without compromising static flame stability.

Fundamental combustion characteristics of Jatropha oil as alternative fuel for gas turbines

To investigate the combustion characteristics of Jatropha pure oil (JPO) and Jatropha methyl ester (JME) as an alternative fuels for gas turbines, combustion experiments were conducted at atmospheric pressure employing an air-assist pressure swirl atomizer. As well as the NOx and CO concentrations and smoke number in the exhaust gas, the flame radiation intensity was measured using the two-color method. The results show that the flame radiation intensity and the soot emission decrease with increasing mixing ratio of JPO or JME to diesel fuel. It was also found that the flame radiation intensity has a strong correlation with the mean diameter of the fuel spray for all fuels. The CO emission for JPO is higher than those for JME and diesel fuel in the case of low flame temperature.

The Combustion Characteristics of Biomass Syngas from High Temperature Air, Entrained Flow and Circulating Fluidized Bed Gasifiers

The study has been performed to determine fundamental combustion characteristics of syngas. Three technologies were selected to produce the syngas; High Temperature Agent Gasifier (HTAG), Entrained Flow Gasifier (EFG) and Circulating Fluidized Bed Gasifier (CFBG). Although the material used for production of syngas was the same, wood biomass, the compositions of syngas obtained were different. The adiabatic flame temperatures were determined at different air to fuel ratio. The maximum adiabatic temperature for HTAG, EFG and CFBG syngas at stoichiometric condition were 1846 K, 2250 K and 2234 K respectively. It has been observed that combustion of CFBG syngas produces more nitrogen oxide (NOx) than when using syngas of EFG. The high NOx in CFBG is caused by the high methane content, which increases the adiabatic flame temperature to 2200 K at stoicheometric condition. The lowest NOx emission was observed in HTAG syngas. The adiabatic temperature increased linearly with the preheating temperature, whilst oxygen enrichment increased the adiabatic temperature. It has been concluded that syngas produced from EFG and CFBG are better candidate as gaseous fuel in combustion chamber than HTAG syngas.

Chemical kinetic study of a novel lignocellulosic biofuel: Di-n-butyl ether oxidation in a laminar flow reactor and flames

The combustion characteristics of promising alternative fuels have been studied extensively in the recent years. Nevertheless, the pyrolysis and oxidation kinetics for many oxygenated fuels are not well characterized compared to those of hydrocarbons. In the present investigation, the first chemical kinetic study of a long-chain linear symmetric ether, di-n-butyl ether (DBE), is presented and a detailed reaction model is developed. DBE has been identified recently as a candidate biofuel produced from lignocellulosic biomass. The model includes both high temperature and low temperature reaction pathways with reaction rates generated using appropriate rate rules. In addition, experimental studies on fundamental combustion characteristics, such as ignition delay times and laminar flame speeds have been performed. A laminar flow reactor was used to determine the ignition delay times of lean and stoichiometric DBE/air mixtures. The laminar flame speeds of DBE/air mixtures were measured in the stagnation flame configuration for a wide rage of equivalence ratios at atmospheric pressure and an unburned reactant temperature of 373K. All experimental data were modeled using the present kinetic model. The agreement between measured and computed results is satisfactory, and the model was used to elucidate the oxidation pathways of DBE. The dissociation of keto-hydroperoxides, leading to radical chain branching was found to dominate the ignition of DBE in the low temperature regime. The results of the present numerical and experimental study of the oxidation of di-n-butyl ether provide a good basis for further investigation of long chain linear and branched ethers.

Combustion characteristics of LPG and biodiesel mixed fuel in two blending ratios under compression ignition in a constant volume chamber

This study aims to investigate the combustion characteristics of mixed fuel of liquefied propane gas (LPG) and biodiesel under compression ignition (CI) in an effort to develop highly efficient and environmentally friendly mixed fuelbased CI engines. Although LPG fuel is known to be eco-friendly due to its low CO2 emission, LPG has not yet been widely applied for highly efficient CI engines because of its low cetane number and is usually mixed with other types of CI-friendly fuels. In this study, a number of experiments were prepared with a constant volume chamber (CVC) setup to understand the fundamental combustion characteristics of mixed fuel with LPG and biodiesel in two weight-based ratios and exhaust gas recirculation (EGR) conditions. The results from the current investigations verify the applicability of mixed fuel of LPG and biodiesel in CI engines with a carefully designed combustion control strategy that maximizes the benefits of the mixed fuel. Based on the results of this study, ignition is improved by increasing the cetane value by using higher blending ratios of biodiesel. As the blending ratios of biodiesel increased, CO and HC decreased and CO2 and NOx increases.

Performance investigation of Fischer–Tropsch kerosene blends in a laboratory-scale premixed flame burner

The increasing use of synthetic fuels, such as Fischer–Tropsch (FT), in transportation, requires an accurate, in-depth knowledge of their fundamental combustion characteristics. However, synthetic fuels, being complex hydrocarbon mixtures, have not been standardized in terms of physical and thermochemical properties and are mostly treated by adapting appropriate surrogate mixtures. Surrogate mixture determination does not always take into account detailed fuel composition and this can lead to inaccurate prediction of fundamental fuel properties, such as molecular weight and heat of formation. In this direction, the present work presents a systematic approach for characterizing similar FT blends and assessing their performance in a simple configuration. A methodology based on the detailed composition of each examined blend is followed here, providing accurate estimates of the above fuel properties. The determined heating values are used in temperature and emission measurements of several flames of FT fuels stabilized in a laboratory-scale premixed burner. A total of six fuels are assessed: a neat, paraffinic FT blend, four FT blends with varying aromatic and naphthenic content and a pure n-decane.

Combustion characteristics of an SI engine fueled with H2–CO blended fuel and diluted by CO2

This paper presents further results of the study on fundamental combustion characteristics of gaseous fuels simulated for a biogas produced through a biomass gasification process with a catalyzer. The main work focuses on combustion characteristics of H2–CO blended fuel and the effect of CO2 dilution on it in a spark-ignition engine under the condition of WOT, MBT and a constant speed of 1500 rpm. Equivalence ratio were limited to lower than 0.8 in order to avoid excessive high combustion temperature to damage the engine, and lean conditions were maintained during the experiment to get acceptable economy and emissions. The results show that the BMEP decreases with an increase in dilution rate. The COV of IMEP is lower than 10% under most conditions, while H2 and CO2 have the opposite influence on brake thermal efficiency. CO2 dilution combustion could induce to remarkable decreasing in NOx emission with little decrease in brake thermal efficiency, which benefits for biomass gaseous fuel application. If 500 ppm of NOx emission and 26% of brake thermal efficiency could be viewed as accepted level, the accepted operation range of H2–CO mixture have been obtained.