Heat Flux Burner Research Articles

A comprehensive kinetic model for dimethyl ether and dimethoxymethane oxidation and NOx interaction utilizing experimental laminar flame speed measurements at elevated pressure and temperature

Laminar flame speeds of dimethyl ether and dimethoxymethane at pressures from 1 to 5 bar and initial temperatures from 298 to 373 K were determined experimentally using a constant volume spherical vessel and a heat flux burner setup. This study is the first to report dimethoxymethane laminar flame speeds at a pressure higher than 1 bar. Using these experimental data along with data available in the literature, a new kinetic model for the prediction of the oxidation behavior of dimethyl ether and dimethoxymethane in freely propagating and burner stabilized premixed flames, in shock tubes, rapid compression machines, flow reactors, and a jet-stirred reactor has been developed. The experimental results from the present work and literature are interpreted with the help of the derived kinetic model. This newly developed reaction mechanism considers the redox chemistry of NOx to accommodate the influence of the oxygen level on the onset of fuel conversion and interconversion of NO and NO2. The current model suggests that an increased O2 level promotes the HO2 production, which in turn leads to the formation of OH radicals, which promotes the combustion of the fuel/air mixture under lean conditions. The increase of OH radical concentrations is mainly via the NO/NO2 interconversion reaction channel, NO+HO2=NO2+OH, NO2+H=NO+OH, CH3OCH3+NO2=CH3OCH2+HONO, followed by the thermal decomposition of HONO. This work extends the kinetic database and helps to improve the understanding of dimethyl ether and dimethoxymethane combustion behavior. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2.

Temperature dependence of the laminar burning velocity for n-heptane and iso-octane/air flames

The heat flux method is advantageous for obtaining adiabatic stretch-less flame and measuring laminar burning velocity, SL, with low uncertainty. However, its implementation is sometimes hampered by the instability, manifested as cellularity of the flame stabilized over a flat perforated burner. This paper summarizes the approaches of flame cellularity abatement on the heat flux burner, which are implemented in the present study for measuring burning velocities of n-heptane and iso-octane/air flames. The combination of approaches helped to effectively overcome the cellularity at the fuel-rich side of the tested flames, and the SL was measured at unburnt temperatures Tu=298K-358K and equivalence ratios ϕ=0.7-1.6, at atmospheric pressure, with the SL uncertainty being evaluated. Numerical simulations were carried out using LLNL mechanism, Chaos mechanism and Luong171 mechanism, and the results agree well with the experimental data. From the obtained experimental and numerical SL data, the temperature coefficients α in SLSL0=TuTu0α as well as the overall activation energy, Ea, were derived. It was noted that for n-heptane and iso-octane/air flames, the tendencies of the α and Ea against ϕ resemble those for methane, ethane, and propane/air flames. Distinct over-rich flame structures were observed and discussed for n-heptane and iso-octane/air flames around ϕ≥1.5. Moreover, extrapolation proced/ure of the SLmeasurements was validated using analytical presentation of the heat flux method sensitivity, s vs. cpα∗-1, and other parameters involved in the data processing, which may help to improve the accuracy of future experiments.

Safety parameters for oxygen-enriched flames

The utilization of low-quality gaseous fuel from biomass gasification and the abundance of oxygen-rich streams obtained as a by-product of nitrogen-air separation by membrane technology has incentivized the development of sustainable oxygen-enriched combustion technologies in the last decades. However, a dearth of experimental and numerical analysis addressing the reactivity and safety aspects of these mixtures at initial low temperatures can be observed in the current literature.In this work, the heat flux burner was adopted for the measurement of the laminar burning velocity of methane in oxygen enriched air at different equivalence ratios. Results were compared with numerical data obtained by means of detailed kinetic mechanisms developed at the University of Bologna and the Gas Research Institute (GriMech3.0). Simplified correlations for the estimation of the laminar burning velocity with respect to the oxygen content at any equivalence ratio were developed, tested and evaluated.An elemental reaction-based function was found appropriate for the estimation of the overall reactivity of the investigated mixtures. Besides, numerical analyses were performed to characterize the flame structures in terms of temperature and product distribution under several initial conditions. These results gave further insights into the reaction mechanisms of gaseous fuels in the case of oxygen-enriched air, highlighting potential bottlenecks for kinetic model refinements. Eventually, relevant safety parameters were estimated, in particular the flammability range of the fuel/oxidant mixture, in terms of lower and upper flammability limits.

Experimental and numerical study of the effect of elevated pressure on laminar burning velocity of lean H2/CO/O2/diluents flames

Laminar burning velocities (SL) of H2/CO/O2/diluents (He or N2) were reported at lean conditions (ϕ = 0.5–1.0), elevated pressures (1–11 bar) and a wide range of H2-CO volumetric ratios. A new high-pressure rig based on specially designed heat flux burner was applied to obtain planar, adiabatic and stretch-less flames used for laminar burning velocity measurements. Five popular syngas oxidation mechanisms were tested against the obtained SL data and showed satisfactory agreement even at high pressures up to 11 bar. SL decreases as pressure increases while the negative pressure dependence of mass burning rate is experimentally and numerically found for syngas with high diluent content in the oxidizer or high hydrogen content in the fuel, which indicated the negative dependence of the overall reaction order on adiabatic flame temperature. Sensitivity and rate of production analyses indicated that the sensitivity coefficients and HO2 mole fraction increase to the maximum at the pressure of the minimum overall reaction order and then decreases, which leads to larger uncertainty on predicting SL at higher pressures. The flame front height represented by the peak OH* chemiluminescence location also shows a non-monotonic variation trend as pressure increases and can serve as a target for mechanism validation.

Arrangement and three-dimensional analysis of cooling wall in 1000 MW S–CO2 coal-fired boiler

The Supercritical carbon dioxide (S–CO2) Brayton cycle is considered as a promising alternative to traditional steam Rankine cycle due to its high efficiency and compactness. However, in coal-fired power system, the S–CO2 temperature at cooling wall entrance is newly recognized quite high, leading to an over-temperature crisis. A coupled model of combustion and S–CO2 heat transfer was established to predict cooling wall temperature of a 1000 MW S–CO2 Brayton coal-fired boiler. Based on the module arrangement in S–CO2 boiler, the “cold S–CO2-hot fire matching and cascaded temperature control” principle in 1D model was proposed to reduce the cooling wall temperature. Three methods were examined including the low-temperature fluid matching high heat flux burner region, the counterflow, and more cold fluid in circulation drained to match high heat flux zone. The results show that the optimal arrangement can significantly reduce the temperature in overheated region 12–44 °C and eliminate the local hot spot. In addition, the 3D model was developed to obtain the maximum wall temperature and the uneven temperature in the circumferential direction. The employment of spiral cooling wall could alleviate circumferential unevenness and reduce wall temperature. The present work can provide important guidance to design of S–CO2 Brayton coal-fired power system.

Data Consistency of the Burning Velocity Measurements Using the Heat Flux Method: Syngas Flames

This work is aimed at presenting a comparison and detailed analysis of the existing experimental data of laminar burning velocities obtained for syngas flames using the heat flux method. The consistency of these data is essential in the development, validation, and optimization of combustion mechanisms. The core of the approach is the evaluation of any variability or inconsistency among the data obtained under the same experimental conditions. The measurements have been compared both against each other and against model predictions using three detailed kinetic mechanisms available in the literature, including an updated model from the authors. Some questionable conditions have been experimentally revisited using an improved heat flux burner presented for the first time. The possible sources of uncertainty have been analyzed and discussed, and recommendations have been given toward the reduction of experimental uncertainties. Furthermore, at all experimental conditions scrutinized, the updated version of the kinetic mechanism was found in better agreement with consistent data sets compared to the previous version.

Experimental and numerical evaluation of low-temperature combustion of bio-syngas

Environmental regulations have strongly incentivized the development of alternative technologies and renewable sources for the energy supply, including bio-syngas and low-temperature combustion. However, accurate estimation methods for low-temperature chemistry of these mixtures are still missing. Hence, experimental data with the limited impact of fluid dynamics aspects are strongly required. To this aim, the heat flux burner has been adopted in this work for the measurements of the laminar burning velocity. Data were compared to evaluate the accuracy of the kinetic mechanism developed at the University of Bologna (KIBO) and exiting mixing rules. Hirasawa's correlation has been found as the most reliable empirical correlation. Furthermore, the KIBO model has been validated and compared with existing models for the investigated conditions. These results have allowed for the evaluation of the effects of fuel composition on the preferable reaction paths and on the NOx production rate.

Low temperature combustion of methane/alkenes mixtures

The use of methane is commonly adopted as thermal diluent and chemical inhibitor in the partial oxidation of alkenes, in order to avoid the occurrence of unwanted reactions and full oxidation. Moreover, methane-alkene mixtures are commonly present in several industrial applications such as petrochemical processes.In this work, experimental data for pure ethylene and propylene flames in presence of air were collected, as first. Then the effect of alkenes addition on methane premixed flames was investigated. These fuel compositions were investigated by means of a heat flux burner at different gaseous compositions. The measurements were compared with experimental data retrieved from current literature as well as with the results of a detailed kinetic mechanism.The data collected in this work suggest the existence of two chemical regimes, resulting in a methane-dominated kinetic, in case of alkene addition lower than 20 %v/v, and alkene-dominated kinetic, in case of higher alkenes content. The production of soot precursors for methane combustion was found to be significantly increased by the addition of propylene. Finally, the maximum reactivity, expressed in terms of laminar burning velocity, undergoes a non-negligible shift toward a richer composition of methane in the presence of alkene.The agreement between the experimental data and numerical predictions allowed for further evaluation of the chemical interactions, product and heat profiles as a function of the initial composition. A criterion based on the flame structure was posed and successfully implemented to identify the temperature values giving unsafe or undesired conditions in the epoxidation processes.

Numerical and experimental investigation of the laminar burning velocity of biofuels at atmospheric and high-pressure conditions

Premixed flame characteristics play a crucial role in most advanced combustion applications, such as gas turbines, aircraft combustors or internal combustion engines. The laminar burning velocity is one key parameter to determine the stabilisation and propagation of premixed flames. This study examined the influence of high pressures on the laminar burning velocity of biofuels and the correlation between laminar burning velocity and pressure. The investigated biofuels were pure ethanol as the state-of-the-art surrogate, pure iso-butanol as a possible alternative and several blends of ethanol/iso-octane. Due to the usage of two methods to measure the laminar burning velocity – the Heat Flux burner and the closed combustion vessel – a comparison was added for atmospheric conditions. The measured laminar burning velocities were conducted for equivalence ratios from 0.7 to 1.3, a pressure range from 1 bar(a) to 15 bar(a) and 373 K. The results were further compared to existing numerical mechanisms and literature data.The numerical mechanisms reveal prediction performances in the range of 0.2 cm/s (0.4%) and 5.2 cm/s (16.2%) for dedicated mechanisms and equivalence ratios between 0.9 and 1.1. The two methods show deviations of the laminar burning velocities between 2.3 cm/s (4.9%) and 6.7 cm/s (11.4%).

The effect of a hydrogen addition to the premixed flame structure of light alkanes

The effect of a hydrogen addition to the combustion chemistry of light alkanes was evaluated by the experimental and numerical determination of the laminar burning velocity of methane/propane/hydrogen mixtures in air at different equivalence ratios and fuel compositions by using a heat flux burner. The experimental results have been compared with additional data selected for the main experimental rig commonly adopted in current literature. The obtained set of data was used for the validation of the detailed kinetic model developed by the University of Bologna. Subsequently, an in-depth analysis on the reaction system was carried out in order to evaluate the effects of a low hydrogen content on the reaction paths, concentrations profiles of the main combustion products and temperature profiles with respect to the distance from the burner. Thus, additional information was obtained by means of a sensitivity analysis, evaluating the relative relevance of each reaction on the laminar burning velocity and comparing obtained the results for all the investigated conditions.

Laminar burning velocities of low calorific and hydrogen containing fuel blends

The laminar burning velocity is a fundamental property of a reactive fuel-oxidizer mixture, varying with composition, pressure and initial temperature. These values are important for validation of reaction mechanisms and the specific design of industrial burners. There are several experimental methods to measure laminar burning velocity, e.g. the bunsen flame method, the spherically expanding flame method, the stagnation flame method and the flat flame burner method, which also includes the heat-flux burner method. The accuracy of the different methods could be enhanced over the last years but there are still uncertainties of up to 30 % depending on method, boundary condition and fuel composition. Furthermore, for a lot of fuels, especially fuel blends, there is a lack of data.In this study, the heat-flux burner method was applied to measure laminar burning velocities of low calorific fuels and hydrogen containing fuel blends. These fuel blends are of major interest since the hydrogen concentration in the gas grid system could in future to a significant value. For example, within the “power-to-gas” concept, some new technologies arise to use renewable energy to produce hydrogen and take it as “fuel”. On the other hand, the change of the gas composition also changes the combustion properties, for example burning velocity, heating value and ignition delay. Therefore, different low calorific fuels and hydrogen containing fuel are tested within a range of equivalence ratios from 0.7 to 1.4 for initial temperatures of 298 K up to 363 K for atmospheric conditions.

Effects of CH4 Content on NO Formation in One-Dimensional Adiabatic Flames Investigated by Saturated Laser-Induced Fluorescence and CHEMKIN Modeling

Experiments applying saturated laser-induced fluorescence (LSF) technology were performed focusing on the influence of equivalence ratio, syngas mixture contents, on NO formation in H2/CO/CH4/CO2/N2/O2 premixed flat flames supplied by a heat flux burner. Experimental data were extracted to validate calculation by CHEMKIN software. Both experiments and CHEMKIN calculation draw one conclusion that NO mole fraction in CH4–air flame peaks at stoichiometric and ϕ 1.3, due to thermal NO and prompt NO routes. All mechanisms applied can well-predict the NO mole fraction at the fuel lean side but failed at the fuel rich side. NO mole fraction in syngas flame is propotional to CH4 ratio as shown in the experimental data, but all the mechanisms failed to predict it with some even having a wrong tendency. A rate of NO production and sensitivity analysis suggests that thermal and prompt NO routes play different roles in each mechanism, and modifications to mechanisms are required to improve NO concnetration prediction...

Investigation of NO formation in premixed adiabatic laminar flames of H2/CO syngas and air by saturated laser-induced fluorescence and kinetic modeling

A series of experiments were performed to investigate the relationship between syngas composition (H2/CO/CO2/N2/O2) and NO formation. Laser-saturated fluorescence measurements of NO in a premixed syngas flat flame from a heat flux burner were recorded for various fuel compositions, dilution ratios and equivalence ratios. Quantitative measurements were compared with the predictions from CHEMKIN software using four different chemical mechanisms, i.e., GRI mech 3.0, Chemical Reaction Engineering and Chemical Kinetics 1407, GDF-Kin 3.0, and the Mendiara and Glarborg mechanism; each mechanism included reaction subsets for NOx formation. These models were validated against a CH4 flame; the GDF-Kin 3.0 mechanism produced the best agreement with measurements. NO measurements for CO/H2 syngas fuel with 50% CO2 dilution at various equivalence ratios found the maximum NO production near stoichiometry. The majority of NO produced for these conditions is predicted to come from the combination of the NNH and N2O pathways. Investigation of NO production during combustion of different CO/H2 ratio syngas diluted with 60% N2 showed that increasing the H2 fraction decreased the total NO concentration. However, normalizing the NO production rate by the fuel mass consumption rate showed H2 produces more NO per gram of fuel consumed. Predictions of temperature and NO rate of production along the flame axis showed that the syngas with high H2 content had a flame front closer to the burner exit and NO production rate greater than the high CO case. Dilution of CO/H2 syngas fuel with N2 and CO2 showed that CO2 has a stronger reduction in NO emission than N2. GDF-Kin 3.0 predicted that dilution with CO2 caused a greater reduction in flame temperature than the same volume of N2. Predicted NO rate of production and reaction sensitivity analysis predicted that the Zel’dovich pathway was dominant for undiluted syngas. However, as the flame temperature reduced, the Zel’dovich pathway was inhibited to a greater extent than the N2O and NNH pathways so that, at high dilution, NO production was driven by the N2O and NNH pathways.

Heat transfer and flame stabilization of laminar premixed flames anchored to a heat-flux burner

Measurement of the burning velocity of unstretched laminar hydrogen/air premixed flames suffers from large uncertainties owing to the highly diffusive nature of hydrogen that can give rise to flame instability. This paper reports on a numerical study of the structures and stability of laminar premixed CH4/O2/CO2 flames and H2/O2/N2 flames anchored to a heat-flux burner using a high-order numerical method with detailed chemical kinetic mechanisms and detailed transport properties. The aim is to elucidate the effect of the flow and temperature inhomogeneity generated by the burner plate holes on flame structures and burning velocity. Heat transfer flux between the burner plate and the surrounding gaseous mixture is investigated under various standoff distances and burner plate temperatures. The burning velocity and the detailed flow, temperature and species distributions in flames with a zero net heat flux between the flames and the burner plate are analyzed. It is found that for the methane flames, when the standoff distance is sufficiently small, the burner can essentially suppress the intrinsic flame instability, but the plate holes can give rise to flame wrinkles of the size of the holes. At high standoff distances, the non-uniformity of the flow from the burner plate holes has a minor effect on the flame surface wrinkling; however, large-scale cellular structures can appear on the flame surface due to intrinsic flame instability. For the studied methane flames the effect of non-uniformity of the flow from the burner plate holes on the burning velocity is fairly small. For the studied hydrogen flames the burner plate could not totally suppress the intrinsic flame instability. The intrinsic flame instability can give rise to a significant increase in the flame surface area and mean burning velocity, with more than 25% increase in the burning velocity.

Fiber Laser Intracavity Spectroscopy of hot water for temperature and concentration measurements

The feasibility of temperature and concentration measurements using near-IR (similar to 1.5 mu m) water spectra obtained by fiber laser intracavity spectroscopy was evaluated. The spectra were registered with water vapor heated in a tubular oven at temperatures between 1000 and 1300 K and in adiabatic flames where temperatures were above 1800 K. Adiabatic flames of methane were stabilized on the heat flux burner. For temperature and concentration evaluation, the observed spectra were fitted by simulated spectra calculated utilizing the HITEMP database. Several discrepancies between HITEMP data and the experiments leading to significant errors in evaluation were found. After small corrections to the database better, accuracy of the temperature (+/- 70 K) and concentration (+/- 20 %) measurements is obtained. A more precise spectroscopic assignment is needed to improve the accuracy of the results. (Less)

Effects of Exhaust Gas Dilution on the Laminar Burning Velocity of Real-World Gasoline Fuel Flame in Air

The effects of exhaust gas recirculation (EGR) on the laminar burning velocity of a commercial gasoline fuel-air mixture have been studied both numerically as well as experimentally at atmospheric pressure. The experiments have been performed using the heat flux burner method. For numerical simulation, a binary mixture of 95% iso-octane and 5% n-heptane (PRF95) is assumed to be the surrogate for the real world gasoline fuel. The numerical simulations have been carried out using a skeletal mechanism comprising of 171 species and 861 reactions for primary reference fuels. A correlation of laminar burning velocity of the commercial fuel is suggested based on experimental data for different EGR dilutions. The results show that the reduction in the laminar burning velocity caused by the EGR dilution can be recovered by the increase in the unburnt gas temperature. However, the rise in the unburnt gas temperature results in a trivial rise in the adiabatic flame temperature. The increase in the laminar burning ve...

Laminar premixed flat non-stretched lean flames of hydrogen in air

Laminar burning velocity of lean hydrogen + air flames at standard conditions is still a debated topic in combustion. The existing burning velocity measurements possess a large spread due to the use of different measurement techniques and data processing approaches. The biggest uncertainty factor in these measurements comes from the necessity to perform extrapolation to the flat flame conditions, since all of the previously obtained data were recorded in stretched flames. In the present study, laminar burning velocity of lean hydrogen + air flames and its temperature dependence were for the first time studied in stretch-free flat flames on a heat flux burner. The equivalence ratio was varied from 0.375 to 0.5 and the range of the unburned gas temperatures was 278–358 K. The flat flames tended to form cells at adiabatic conditions, therefore special attention was paid to the issue of their appearance. The shape of the flames was monitored by taking OH* images with an EM-CCD camera. In most cases, the burning velocity had to be extrapolated from flat sub-adiabatic conditions, and the impact of this procedure was quantified by performing measurements in H2 + air mixtures diluted by N2. The effect of extrapolation was estimated to be of negligible importance for the flames at standard conditions. The measured burning velocities at 298 K showed an important difference to the previously obtained literature values. The temperature dependence of the burning velocity was extracted from the measured results. It was found to be in agreement with the trends predicted by the detailed kinetic modeling, as opposed to a vast majority of the available literature data.

An experimental and modeling study of the combustion of tetrahydrofuran

Tetrahydrofuran (THF) is a well suited starting point fuel to study the combustion chemistry of saturated cyclic esters that are being considered as promising bio-fuels. To better understand the combustion chemistry of THF, laminar low-pressure premixed flame structure, atmospheric adiabatic laminar burning velocities, and high-pressure ignition delay times were investigated. The structure of laminar premixed low-pressure (6.7kPa) argon-diluted (78%) flames of THF were studied at three equivalence ratios (0.7, 1.0 and 1.3) using on-line gas chromatography analyses. The results consist of temperature and mole fraction profiles (about 40 species) measured as a function of the height above the burner. Ethylene, propene, formaldehyde, acetaldehyde, and dihydrofurans were observed as important intermediates. Aromatic species were detected in very low amounts. The adiabatic laminar burning velocities of THF–air mixtures were measured using the heat flux burner method at atmospheric pressure (initial temperatures from 298 to 398K, at equivalence ratios from 0.55 to 1.60). The maximum burning velocity of THF was comparable to that of ethanol and diethyl ether. The ignition delay times of THF–oxygen–argon mixtures were measured behind reflected shock waves (temperatures from 1300 to 1700K, pressures around 8.5atm, mixtures containing 0.25–1% of fuel for equivalence ratios of 0.5–2.0). A new detailed kinetic model for THF combustion was developed using a combination of automatic generation (EXGAS), Evans–Polanyi correlations (for H-abstraction kinetic data), and CBS-QB3 theoretical calculations (for unimolecular initiation, H-abstraction and β-scission kinetic data). An overall good agreement between simulations and the present experimental results has been found. The main THF reaction pathways under flame conditions have been identified from flow rate analyses.

Effect of H2/CO ratio and N2/CO2 dilution rate on laminar burning velocity of syngas investigated by direct measurement and simulation

Laminar burning velocities of syngas/air premixed flames, varying with H2/CO ratio (from 5/95 to 75/25) and N2 or CO2 dilution rate (from 0% to 60%), were accurately measured using a Teflon coated Heat Flux burner and OH-PLIF based Bunsen flame method. Experiments were carried out at atmospheric pressure and room temperature, with fuel/air equivalence ratios ranging from fuel-lean to fuel-rich. Coupled with experimental data, three chemical kinetic mechanisms, namely GRI-Mech 3.0, USC Mech II and Davis H2–CO mechanism, were validated. All of them can provide good prediction for the laminar burning velocity. The laminar burning velocity variations with H2 and dilution gas contents were systematically investigated. For given dilution gas fraction, the laminar burning velocity reduction rate was enhanced as H2/CO ratio increasing. Effects of the syngas components and equivalence ratio variation on the concentrations of radical H and OH were also studied. It appears that there is a strong linear correlation between the laminar burning velocity and the maximum concentration of the H radical in the reaction zone for syngas. This characteristic is exclusively different from that in methane air premixed flame. These findings indicated that the high thermal diffusivity of the H radical played an important role in the laminar burning velocity enhancement and affected the laminar burning velocity reduction rate under dilution condition.

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H2/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H2/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N2 diluted synthetic H2 and H2/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.