Flame Temperature Rise Research Articles

Construction of boron-based double-layer core-shell structure composites and their combustion energy release characteristics

Although boron (B) powder has a high energy density, the high boiling point oxide layer (B2O3) on the surface seriously hinders the combustion of internal B particles, which further limits the use of B powder. In order to solve this issue, polydopamine (PDA) was used as a sandwich layer (sandwich layer used to connect solid-liquid interface) between the original B powder and liquid perfluoropolyether (PFPE), and then a double-layer core-shell structure B@PDA@PFPE was prepared through the electrostatic self-assembly method. Herein, PDA not only helps remove naturally occurring B2O3, but also provides more active sites and improves the reactivity of B and PFPE. The results indicate that the B@PDA@PFPE has excellent combustion flame and energy release characteristics due to the superior dispersion and reactivity of PFPE. Compared to that of the original B powder, the results of thermal decomposition and combustion heat of B@PDA@PFPE with 30%PFPE discloses that the oxidation temperature drops by about 19.5 ℃, the combustion efficiency rises by about 22.3 %, and the flame expansion rate and flame temperature rise by approximately 40 times and 95.6 ℃, respectively. Therefore, the construction of double-layer core-shell structure effectively promotes the combustion of B powder. This work provides a new strategy for the application of B powder.

Experimental study, dimensional analysis and an integral model for horizontal buoyant turbulent jet fires under opposing wind

This work demonstrates a comprehensive model for horizontal turbulent jet flame geometries under opposing wind, for which few data or models can be found in the literature. This situation can be a practical scenario of offshore drilling platform with a horizontal flare under opposing wind. The opposing wind pushes the jet flame back, which eventually turns around and upwards, pointing finally downstream in the windward direction. The flame geometries are characterized by four parameters including horizontal length and vertical height from nozzle exit to the location of farthest point that flame envelope could reach before the flame turns around as well as horizontal length and vertical height from nozzle exit to the location of flame tip. Experiments are carried out for horizontal jets discharged from circular nozzles having diameters of 3, 5, 7 and 9 mm and employing propane as fuel with opposing wind speeds from 0.35 to 1.08 m/s. The locations of flame tip and the flame farthest point decrease with increasing opposing wind for a constant heat release rate. Dimensional analysis is derived from a physical model considering the momentum of fuel jet, flame buoyancy due to the flame temperature rise, opposing wind momentum and normalized flow rate at stoichiometric conditions at flame tip. The normalized flame geometry parameters based on the proposed model are well represented by two characteristic length scales derived by accounting for the interaction between excess jet-wind momentum and flame buoyancy as well as that between opposing wind momentum and flame buoyancy. In addition, the normalized mass flow rate at stoichiometric conditions determines the total length of flame trajectory. Moreover, an integral model is deduced by an air entrainment model and top hat profiles for the mass and momentum conservations to predict well the flame geometries of horizontal jet fires under opposing wind.

Effect of altitude on vertical temperature distribution and longitudinal smoke layer thickness in long tunnel fires

SummaryBased on large eddy simulation, a series of long tunnel fire experiments with different heat release rates (HRRs) and altitudes were carried out. The vertical temperature and thickness of fire smoke are studied. The simulation results show that the higher the altitude, the lower the flame temperature rise, while the change of smoke plume temperature rise is opposite. The movement of smoke in the tunnel can be divided into four regions, and the smoke layer thickness in the longitudinal direction of the tunnel corresponds to the latter three regions. The thickness in Region II increases along the longitudinal direction, the thickness in Region III is a constant value, and the thickness in Region IV increases along the longitudinal direction. Besides, the change of altitude only has an effect on the smoke layer thickness in Region IV. Then, by considering the altitude, HRR, and smoke layer thickness, and using dimensional analysis, an empirical formula for predicting the smoke layer thickness under the influence of altitude in Region IV was established.

Linear control of thermoacoustic oscillations with flame dynamics modeled by a level-set method

This article focuses on co-simulation of the coupled nonlinear dynamics of flame heat release rate, acoustics, and feedback control in an active thermoacoustic oscillation control system. This work is motivated by an extensive existing body of literature showing the potential of closed-loop active control to suppress thermoacoustic oscillation. Linear algorithms such as linear quadratic Gaussian (LQG) control are often used for thermoacoustic oscillation control, with the important caveat that flame heat release rate oscillation is often highly nonlinear. This creates a need for a tool that can co-simulate the coupled nonlinear thermoacoustic dynamics with linear control. The main contribution of this article is the development of a tool capable of co-simulating nonlinear heat release dynamics using a level-set formulation, together with linear acoustics plus a linear feedback controller. The article demonstrates this framework on an LQG-controlled wedge flame in a unidimensional Rijke tube. The nonlinear thermoacoustic model is formed in a modified level-set solver by connecting the linear acoustics to the original nonlinear flame dynamics via a velocity convection equation. This framework succeeds in preserving the nonlinearity of the thermoacoustics in the Rijke tube without the LQG control. For the flame-driven Rijke tube in this article, LQG control is successful in suppressing the nonlinear thermoacoustic oscillation. In addition, the simulation based on the framework is useful in elucidating the impact of factors such as flame location, flame temperature rise, and the timing of the onset of LQG control on the instability suppression performance and energy needs.

Forced turbulence affected auto-ignition and combustion modes under engine-relevant conditions

Auto-ignition phenomena are paid more attention due to their close relation with abnormal combustions in advanced internal combustion engines. Previous investigations on intensive auto-ignition and knocking phenomena have been mainly conducted under standard flow conditions, whereas the role of turbulence is not fully clarified under engine-relevant conditions. In this study, forced turbulence affected auto-ignition and combustion modes were investigated in a novel rapid compression facility fueled with isooctane/air mixtures. In combination with instantaneous pressure acquisition, high-speed photography and infrared imaging were employed for auto-ignition initiation and reaction wave propagation. The combustion modes were comparatively investigated under standard flow and forced turbulence conditions. Experimental results show that standard flow scenarios are dominated by a thermo-chemical mechanism. With the elevation of intake pressure and thereby energy density, sequential auto-ignition induced normal combustion, end-gas auto-ignition induced mild knocking, and super-knock are observed in sequence, with obvious cool flame temperature rise and two-stage auto-ignition. Under forced turbulence conditions, the adiabatic core starts to be destroyed by the colder fluid from wall boundary layers, leading to lower temperatures and larger thermal stratifications. Such that prolonged auto-ignition timing, reduced burning rate, and attenuated cool flame are observed. Energy density plays an important role in strong knocking initiation, and there are some increases in the demarcation under forced turbulence conditions. However, mixture reactivity still plays the first-order significance in intensive auto-ignition and knocking formation. The current work helps to provide useful insights into the nature of auto-ignition and knocking combustion and the gaps between fundamental platforms and practical engines.

Investigation of the effect of barium-based additive on smoke and NOx emissions of a diesel engine fueled with conventional and biodiesel fuels

Nowadays, increasing energy demand and environmental and air pollution issues have spread the use of alternative fuels such as biodiesel in diesel engines. While the advantages of biodiesel that are comparable to petroleum-based diesel fuels and low unburned HC, carbon monoxide and smoke (soot) emissions make the use of this fuel environmentally significant; high NOx emissions are still a problem to be solved. The formation of decreased smoke reducing radiative heat transfer from combustion chamber in biodiesel engines and increase in formation of NOx due to rise in flame temperature are among the reasons reported for high NOx in the literature. In this study, in a diesel engine using biodiesel, soot emissions produced by petroleum diesel fuel were reduced to biodiesel level by using metallic additive (Ba), and NOx emissions were compared. When the amount of Ba in the fuel was increased, there was a significant decrease in the smoke emissions, but in this case, NOx emissions decreased slightly, while they were expected to increase. When NOx emissions of diesel and biodiesel were compared for the same or similar smoke emissions, it was seen that high NOx emissions were achieved with biodiesel. As a result, although Ba additive reduced smoke emissions, it was seen that its effect on NOx formation was not very clear.

LES of Subsonic Reacting Mixing Layers

We study a class of chemically reacting, spatially evolving, subsonic mixing layers via large eddy simulations (LES). A primary goal is to assess the inflow conditions, numerical methods, and physical models requirement to reproduce experimental results on molecular mixing and effects of inflow conditions in high-Reynolds number mixing layers: here, we target experiments performed by Slessor et al. (J. Fluid Mech. 376, 115–138 1998). The streams forming the mixing layer carry small amounts of hydrogen and fluorine, initiating a hypergolic reaction upon mixing at large Damkohler number. In this regime, product formation and temperature rise in the flow is mixing limited. The chemical compositions considered for this study correspond to low levels of heat release and results in adiabatic flame temperature rise of 171K and 267K. Both reacting and non-reacting simulations are performed with the Vreman sub-grid scale model (Vreman Phys. Fluids 16(10), 3670–3681 2004). A grid resolution study is done and comparisons are made with the available experimental data. To mitigate dispersive errors and ensure boundedness in species mass fractions that occur in simulations of non-premixed combustion, non-linear scaling limiters are used for reconstructing species densities during flux evaluation. The simulations show good agreement of the velocity and temperature rise profiles with experiment, and reveal differences in the flow field attributed to changes in the inflow conditions.

Numerical simulation of premixed methane/air and synthesis gas/air flames for turbulence swirling jet

This paper reports on the investigation of combustion features of turbulent swirling methane/air and synthesis gas/air flames via using modern CFD-approaches. For methane/air mixture the equivalence ratio Φ was 0.7 which is near to the lean blow off limit. Two regimes with 1:1 and 1:2 ratios of hydrogen and carbon monoxide in the fuel mixture have been considered for synthesis gas combustion. For validation of numerical models, the reliable experimental data has been used. It is found that LES approach demonstrates good agreement with the experimental data. Comparison between methane/air and synthesis gas/air flames demonstrates that in the latter case the size of recirculation zone and the flame temperature are decreasing. But, addition of carbon monoxide in synthesis gas/air mixture leads to flame temperature rise.

Evaluating lower flammability limit of flammable mixtures using threshold temperature approach

A flammable gas could be ignited only if its concentration in air is over a threshold concentration, which is known as the lower flammability limit (LFL). Therefore, predicting LFLs of flammable gases is indispensable for safely handling such flammable gases. The Le Chatelier’s mixing rule for the LFL is the prevailing method for estimating the LFLs of mixtures with multiple flammable components. In this work, a novel derivation for this rule is proposed based on the threshold temperature concept. It is found that the important assumption required to reach this mixing rule is that the adiabatic flame temperature rises at LFL are approximately the same for each flammable component. As the adiabatic flame temperature rise is in a sense the required energy gap to initiate a combustion, it should not change while adding/removing inert gas into/from the system. Therefore, it is also the required perquisite to predict the LFLs of inertized mixture or the LFLs in oxygen atmosphere.

An alternative approach to accommodate detailed ignition chemistry in combustion simulation

In the current work, we utilize a recently developed staged Livengood–Wu (L–W) correlation to evaluate and predict the two-stage ignition behaviors of different primary reference fuels (PRFs) under homogeneous reciprocating engine conditions, to demonstrate its capability in capturing detailed ignition kinetics in combustion simulation. Based on the understandings of the global and detailed kinetics of low-temperature chemistry, simplified Arrhenius-based global expressions were developed for both ignition stages, and a linear correlation was applied for the cool flame temperature rise at the end of the first stage, for PRFs with various octane number (ON) 0, 20, 60 and 87 under constant volume conditions. For their performance under homogeneous charge compression ignition (HCCI) conditions, it is shown that for the same engine speed, the inlet temperature range allowing for two-stage ignition shrinks with increased ON numbers, and eventually two-stage ignition turns into single-stage ignition with sufficiently high ON numbers. Based on the ignition database, the combustion phasing for all PRFs under a wide HCCI operation range are predicted with the staged L–W integral method with satisfactory accuracy, including the first- and second-stage ignition timing as well as the cool flame temperature rise. The sensitivities of each of the engine operation parameters are successfully captured, further demonstrating the applicability of the staged L–W method for realistic fuels with low temperature chemistry and its potential applications for engine combustion control. This study also provides an alternative approach of accommodating low temperature ignition kinetics in combustion simulation, especially useful for those under variable thermodynamic conditions.

The Analysis of Nitrogen Oxides Formation During Oxygen - Enriched Combustion of Natural Gas

Abstract This paper presents the study of oxygen–enriched combustion of natural gas and its impact on nitrogen oxides emission. The research were performed on two experimental stands, i.e. combustion chambers with an industrial swirl burner of maximum power equal to 90 kW and 10 kW. The investigation includes the influenced of oxygen enhanced within the range between 21% and 30%. Furthermore, the role of temperature during the oxygen enrichment was analysed. The results of the research showed that with the rise in oxygen concentration in the air the concentration of nitrogen oxides also increases what is directly related to the rise in flame temperature as well as the addition of oxygen.

On the crossover temperature and lower turnover state in the NTC regime

This work investigates the thermodynamic and chemical kinetic behavior of the lower stationary point, denoted as the lower turnover state, of the NTC curve near the low temperature regime, where the ignition delay exhibits a local minimum. It is found that the ignition delay varies Arrheniusly with the turnover temperature under various pressures, and that the turnover temperature, Tlower, and the initial pressure, P0, are related and can be correlated by another Arrhenius dependence, as P0 ∼ exp(−Ea/Tlower). These two behaviors have been systematically investigated using detailed mechanisms for typical n-alkanes exhibiting NTC, namely n-butane and n-heptane. It is found that the first and second stage ignition delays are approximately equal to each other at the lower turnover states under various pressures, such that the total ignition delay shares similar temperature dependence with the first stage ignition delay. To further characterize the second ignition stage, the cool flame temperature rise at the end of the first stage is investigated and shown to decrease linearly with increasing initial temperature. A crossover temperature is then defined at the state where the cool flame temperature rise disappears. This state can be correlated with a constant α, which denotes the ratio between OH production from the low-temperature chemistry and the total fuel consumption. It is shown that the second stage ignition delay correlates well with the reaction rate constant for the H2O2 decomposition. Analytical expressions are then derived based on these kinetic insights on the two stages, which well reproduce the lower turnover temperature under different pressures and provide further insights into the two-stage ignition at the turnover states. Comparison of the present results with the literature experimental data of the n-heptane ignition delay time shows good agreement.

Effects of Hydrogen Addition on the Entropy Generation of Biogas Conventional Combustion

Biogas has a great potential to be applied for heat and power generation throughout the world due to its availability from various resources. However, one of the most important barriers of biogas utilization development is its low calorific value. In order to increase the performance of biogas in industrial application, hydrogen enriched biogas could be substituted. In this paper a set of numerical simulations were conducted to estimate the variation of entropy generation in hydrogen enriched biogas flames due to hydrogen addition to the fuel. Reynolds Averaged Navier Stokes with a second order turbulence closure and laminar flamelet combustion model was applied to compute energy fields and flow in the flame. It was found that hydrogen enrichment resulted in an augmentation in the entropy generation rate of the biogas conventional flame. Such increase could be attributed to the increase in irreversibilities due to biogas flame temperature rise.

Entropy Generation in Turbulent Swirl-Stabilized Flame: Effect of Hydrogen Enrichment

A set of numerical simulations were conducted to estimate the change in entropy generation in swirl stabilized CH4/air flames due to H2 addition to the fuel. A robust finite volume computational model solving the Reynolds Averaged Navier Stokes with a first order turbulence closure and laminar flamelet combustion model was used to compute the flow and energy fields in the flame. Hydrogen enrichment resulted in an increase in the entropy generation rate of the flame. Such increase was attributed to the increase in heat transfer irreversibilities due to flame temperature rise.

Characterization of nanocrystalline Mg0.6Zn0.4Fe2O4 soft ferrites synthesized by glycine-nitrate combustion process

In this study, Mg–Zn ferrite with the chemical formula of Mg0.6Zn0.4Fe2O4 is synthesized through a modified combustion synthesis using glycine as fuel and metal (Mg, Zn and Fe) nitrates as reactants. The technique, known as glycine-nitrate process, involves exothermic decomposition of a viscous liquid, prepared by thermal dehydration of an aqueous solution containing metal nitrates and glycine. The product powders produced at seven different molar ratios of glycine to nitrate (G/N ratio), varying from 0.37 to 0.75, are agglomerates of fine particles whose typical diameter are several tens of nanometers. Thermodynamic modeling of the combustion reaction indicates that as the fuel-to-oxidant ratio increases, the amount of gases produced and the adiabatic flame temperature rise. X-ray diffraction shows that samples crystallize in a spinel-type structure in all reactions. The morphology of the powders is examined using field emission scanning electron microscopy and transmission electron microscopy. Through magnetic measurements conducted by a vibrating sample magnetometer, the maximum saturation magnetization (46emu/g) is found to occur at the highest G/N ratio.

Promotion and inhibition of a hydrogen—oxygen flame by the addition of trimethyl phosphate

This paper gives results of numerical modeling of a laminar hydrogen—oxygen flame doped with trimethyl phosphate at various pressures and compositions of the combustible mixture. The calculations were performed using the PREMIX and CHEMKIN-II software packages. It was found that phosphorus-containing additives promoted the flame at subatmospheric pressures and inhibited it at atmospheric pressure. Kinetic analysis showed that catalytic recombination reactions were responsible for both phenomena. In the case of subatmospheric pressures, the promoting effect and its enhancement with increasing additive concentration were related to a flame temperature rise in the chemical-reaction zone due to catalysis of the recombination reactions by phosphorus-containing compounds. Increasing the additive concentration led to an increase in both the rate of the branching reaction H + O2 = OH + O and the rate of the chain termination reaction, but the increase in branching reaction rate prevails, resulting in an increase in the flame velocity. In the case of atmospheric-pressure flames, where the reaction-zone temperature is close to the adiabatic equilibrium value, the additive led to an increase in the rate of decay of active flame species and, hence, to a decrease in the flame propagation velocity with increasing additive concentration.

Heat Exchange in the Hearth of a Blast Furnace Operating with Combined Blast Parameters.

Heat exchange occurs most intensively in the hearth of the blast furnace (BF) and determines metal quality, furnace productivity and operating parameters. The heat and chemical energy needed for the BF process is generated in the raceways. This paper is dedicated to the advancement of the heat exchange theory in the hearth of a BF operating with combined blast. The heating processes of products of melting in the raceway and in the liquid bath of the hearth and the effect of pulverized coal injection (PCI) on heat exchange in the oxidizing zone and its extension are studied. An increase in furnace productivity requires additional heat power for the heating of pig iron and slag and therefore a rise in flame temperature. PCI increases radiation in the raceways, thus permitting BF operation with a lower flame temperature. These theoretical conclusions are confirmed by calculations for two BFs in the Ukraine and Spain and by statistical analysis of the operation of the majority of BFs in EU countries. At one BF in Germany the effect of PCI and other combined blast parameters on the raceway extension was investigated using the laser technique.

A computational study of sooting limits in laminar premixed flames of ethane, ethylene, and acetylene

The onset of sooting in laminar premixed flames was examined computationally. Three series of atmospheric fueloxygennitrogen flames of ethane, ethylene, and acetylene were simulated using a detailed chemical kinetic model of soot particle nucleation and growth. It was found that the critical equivalence ratios for soot appearance, both the absolute values and temperature dependencies, can be predicted fairly close to the experimental observations, even when no reactions for the oxidation of aromatics by hydroxyl are included in the mechanism. The analysis of the computational results strongly suggests that the appearance of soot is controlled by two factors, concentration of acetylene and growth of polycyclic aromatics, and the latter is limited by the rise in flame temperature towards the end of the main reaction zone. The present results and conclusions are in agreement with the principal findings of previous modeling studies.

Effects of heat release in a turbulent, reacting shear layer

Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to 940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean temperature rise profiles, normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.

The measure of the inhibition of quenched premixed flames

It is demonstrated that the measure of the inhibition of a quenched flame whose burning velocity is maintained constant (by heat transfer to a flame holder) is a rise in flame temperature on inhibition. This is in contrast to the measure of the inhibition of an adiabatic flame which is a change in burning velocity at constant temperature. This behavior is illustrated for the case of NaHCO 3 inhibition of a quenched CH 4/air flame, φ = 1.2, burning at 10 cm/sec.