Double Flame Research Articles

Phytic acid/cellulose nanofibers modified layered double hydroxides flame retardants: Synthesis, properties and applications

Phytic acid/cellulose nanofibers modified layered double hydroxides flame retardants: Synthesis, properties and applications

Experimental Study on the Combustion Characteristics of Ultralow-Concentration Methane in Divergent Porous Media Burner

ABSTRACT To improve the stable combustion characteristics of ultralow-concentration methane (Concentration of 3.5%-5.0%), a series of combustion experiments were carried out by filling alumina spheres with different particle sizes (i.e. 5 mm, 10 mm and 15 mm) in a divergent burner, and the effects of divergent angle (i.e. 10°, 20° and 30°) and pore distribution on the flame propagation and stable combustion characteristics of ultralow-concentration methane were revealed. The results show that larger particle sizes and methane flow rates lead to the double flame surface phenomenon, and increasing angle and decreasing particle size can help to improve the preheating efficiency of the burner. Moreover, the inlet flow rates corresponding to the steady combustion of ultra-low concentrations methane are between 0.225 m/s and 0.308 m/s, the increase in inlet velocity, particle size, and angle will reduce the stability of the flame and make the shape of the flame surface developed in the order of “V→W→∧.” The burner with 10 mm particles and 30° angle has a wider stable combustion range, and the burner filled with smaller pore distribution help to improve the utilization of the lower concentration methane premixed.

Quantification of the hetero-contact formation process for a CuO/CeO2 hetero-aggregate model system prepared by double flame spray pyrolysis

In hetero-aggregates, different materials form sintered hetero-contacts, which lead to new material properties. In this study, the influence of process parameters on the formation of hetero-contacts in Double Flame Spray Pyrolysis (DFSP) was quantified. The mixing of jets was monitored by Particle Image Velocimetry (PIV) measurements as well as by thermocouples and compared to the single flame setting. Overlap of the jets was evaluated using tracers to visualize the mixing zone. Ten CuO/CeO2 model material samples were prepared with different flame inclination angles, nozzle distances and different particle volume fractions. Mixing on hetero-aggregate level was quantified using Convolutional Neural Network (CNN) evaluations of STEM images, Temperature-Programmed Reduction (TPR) and Differential Centrifugal Sedimentation (DCS) disk centrifugation to determine cluster sizes and hetero-coordination numbers as mixing quantifiers. Here, good correlations between the differently measured quantifiers were observed.

Double flame dynamics in hotspot ignition

Hotspot ignition is a key problem in fundamental flame initiation, combustion control and prevention of abnormal combustion phenomena. Depending on the characteristics of the hotspot and the thermochemical condition of the reactive mixture, subsequent reaction front initiated from a hotspot can vary substantially. The current paper focus on the dynamics and chemistry for one of the most complicated scenarios - hotspot induced double flames, which include a cool and a hot flame segment. This work adopts a recently developed compressible reacting flow simulation code COGNAC to investigate the initiation and propagation of double flame dynamics from hotspot ignition in a one-dimensional spherical coordinate. The results have shown that under atmospheric pressure, double flame initiation is not feasible for sufficiently small hotspot. Under elevated pressures, double flame can be initiated within a much wider hotspot window, exhibiting complex flame dynamics, such as variations in cool and hot flame bifurcation behaviors and their initiation locations. Analysis on thermochemical structure and dynamics of double flame has shown that the interaction of cool and hot flames in a double flame structure is inherently intermutual, in that the leading cool flame enhances trailing hot flame speed through reform of the unburnt mixture, while the trailing hot flame initiation affects the leading cool flame via thermal expansion effects.

Full-scale experimental study on combustion characteristics and smoke temperature of double-source fires in different tunnels

Through a series of full-scale fire tests conducted in three different tunnels, the common characteristics of combustion and smoke temperature of double pool fires in actual operation tunnel environments are investigated. The evolution process of the coupling effects of shielding entrainment and thermal feedback enhancement is revealed. It is found that the ventilation state and the fire source spacing can alter the dominant relationship between the shielding entrainment effect and the thermal feedback enhancement effect, resulting in different evolution patterns of combustion characteristics and smoke flow patterns of double pool fires in tunnels. The heat release rate increases by 4 times when the double pool fire flames merge under the longitudinal ventilation compared with the double pool fire flames without fusion under the natural ventilation. The flame tilt angles of double pool fires were determined through the image processing method, and the prediction model of the flame inclination angle of downstream fire sources was established, considering fire source spacing. At the same time, the characteristic of temperature measured by Fiber Bragg Grating in double-source fire scenarios was investigated, developing a model to predict the longitudinal temperature rise distribution. It is found that the maximum temperature rise prediction model based on steady-state fire has poor applicability in the weak-plume fire scenario of full-scale tunnels. The analysis of the smoke flow patterns shows that the two-stage ventilation is more suitable for smoke control compared to full longitudinal ventilation in tunnel double-source fire accidents, especially for ensuring the safety of individuals trapped in the area between the two fire sources.

Experimental Study on Double Jet Flames with Different Nozzle Diameters

Double jet flames are commonly observed in the energy industry. A facility consisting of a double jet flames apparatus, was designed to experimentally simulate the interaction of double jet fires with different spaces and nozzle diameters. It has been found that the exit momentum, nozzle diameters and nozzle spacing significantly affect the flame behavior. The experimental results show that the flame merging probability (Pm) of double jet fires with different nozzle diameters can be fitted with dimensionless parameters that couple the Froude number(Fr) and the distance between fire sources. Additionally, it has been observed that the difference in height between the two flames can be used as a parameter to determine the correlation between flame height and the distance to the nozzle center.

Usage of multi-extruded nozzles for hydrogen fuel mixing in the double cavity flame holder in the existence of the shock generator

Improvement of the fuel mixing inside the cavity flame-holder is important for the development of this injection system in scramjet engines. This article presents significant flow results on the mixing role of multi-extruded nozzles inside dual cavity flame holders with a single shock generator. A three-dimensional model of the dual cavity with injectors located on the inclined rear wall is produced to simulate real flow physics associated with proposed jet configurations. The impression of air injection from the inclined rear wall is also examined by computational modeling of the compressible flow inside the dual cavity. Based on the achieved results, the injection of multiple extruded nozzles enhances fuel diffusion in the cavity when the shock generator is applied. Our findings disclose that the injection of the fuel from the inclined rear wall results in more efficient fuel mixing inside the dual cavity flame holder. Mixing of the annular jet is increased about 14% via backward air jet inside the cavity.

Fire performance, toxicity, and thermal diffusivity of wastepaper mixed with magnesium hydroxide, depending on the particle size of expandable graphite

As the construction of high-rise buildings increases to solve residential spaces caused by high density population concentration, the occurrence of fire accidents in high-rise buildings is also increasing. The primary cause of fire damage in high-rise buildings is the spread of fire caused by exterior wall finishes. Therefore, it is essential to develop semi-non-combustible finishes that can be applied to high-rise buildings to prevent the spread of fire due to exterior wall finishes. To address this issue, numerous studies are being conducted to develop flame retardant finishing materials that reduce heat release rate and total heat release. A double flame retardant mixed waste paper with expandable graphite and magnesium hydroxide was manufactured to improve the fire performance of cellulose building finishing materials. Total heat release (THR), CO, and CO2 Generation changes were measured using a cone calorimeter, and thermal diffusion rate was measured through the LFA 1000 experiment. The correlation of total heat release, CO generation, CO2 generation, and mass reduction rate by variation of expandable graphite's mesh size at double flame-retardant waste paper were secured through a cone calorimeter test. The thermal diffusion rate data of the specimen were secured through the LFA 1000 experiment. Through experiments, it has been confirmed that specimens using specific expandable graphite particles can be utilized as fire-resistant finishing materials in construction, ensuring fire resistance performance.

A Numerical Study on the Ignition and Flame Propagation for Locally Stratified Methanol/n-Heptane Dual-Fuel Mixtures

ABSTRACT Flame propagation under thermal and compositional stratifications is relevant to a wide range of applications in combustion devices. In this work, numerical studies are conducted to investigate the multi-stage ignition process and flame propagation in the stratified methanol/n-heptane-air dual-fuel mixture under engine-like conditions. Firstly, the effects of the initial gas temperature of the oxidizer (TOX) and fuel stream on the ignition process and combustion modes are investigated. The results show that TOX affects the early-stage ignition, and a fuel-rich stabilization mode is observed at low temperatures. For example, at 800 K, both cool and hot flames are initiated at the regions close to the most reactive mixture fraction, . A transition from the fuel-lean to fuel-rich and then back to the fuel-lean regions in the mixture fraction space is observed at higher temperatures. Then, hot flames propagate into the methanol/air premixture, forming quasi-steady hot-flame fronts in the fuel-rich regions at TOX = 800 and 900 K owing to the lack of oxygen and low adiabatic flame temperatures. At 1000 K, another flame front propagates into the n-heptane/air mixtures owing to enhanced reaction rate at higher local temperatures. By increasing TOX, a balance owing to the gas expansion is observed at 1020 K, and a spontaneous ignition mode appears at temperatures above 1050 K owing to the high reactivity and early ignition of the methanol/air premixture. Secondly, the effect of compositional stratification on the ignition and flame structures is investigated. Compared with the double flame structure under thermal and compositional stratification conditions, only a single flame structure is observed for those cases with uniform temperatures. A high temperature in the fuel stream region accelerates the decomposition of n-heptane, resulting in the complete consumption of oxygen.

Numerical studies on the ignition and propagation for spherically expanding premixed cool flames under gravitational conditions

Cool flame speed is a fundamental parameter, which requires special consideration in terrestrial measurement due to the slow propagation of the cool flame, the intrusion of gravity, and multiple flame regimes (cool, hot, and double flames). The current study employs multiscale numerical simulations with and without gravity to investigate premixed spherical cool flame ignition and propagation. One-dimensional simulations without gravity are adopted to study the effects of initial ambient temperature T0, pressure P0, equivalence ratio ϕ, and ignition kernel temperature TH on feasible conditions for an isolated cool flame measurement, emphasizing the intrinsic couplings among the cool flame, hot flame, and autoignition. Besides considering the minimum and maximum ignition energies for low-temperature ignition (LTI), the results highlight the impacts of the auto-ignition upstream and hot flame downstream of the cool flame on speed measurement. Comprehensive effects of T0, P0, and ϕ are evaluated in order to obtain accurate flame speed versus stretch rate data for extrapolation. Two-dimensional simulations focus on the cool flame deformation with buoyancy. Buoyancy induces a toroidal vortex, causing the spherical cool flame to be mushroom-like at high Richardson numbers, a non-dimensional metric used to evaluate the relative importance of buoyancy. The flame deformation at different ambient temperatures and equivalence ratios are observed. Detailed analyses of the flow field, the deformed flame structure, and the effects of stretch and diffusion are performed. In addition, three equivalent flame radius definitions are proposed and compared to extrapolate cool flame speed. Different from the previous studies on the gravitational hot flames, the current results identify the flame volume-based radius (i.e., Req,V) to be the best one for extrapolating non-spherical cool flame speed, with which cool flames faster than 8 cm/s can be measured within 5% uncertainty. Finally, a systematic strategy is proposed to guide the experimental design. The current work reveals unique cool flame dynamics under gravity conditions and helps to measure the cool flame speed in a terrestrial environment.

Forced ignition of premixed cool and hot DME/air flames in a laminar counterflow

Recently, low-temperature chemistry (LTC) and LTC-induced cool flames have attracted extensive interest since they can substantially influence the ignition process and accelerate the subsequent hot flame propagation. However, it is unclear how the flow affects the forced ignition of a cool flame and its subsequent transition to a hot flame. In this study, we conduct transient 2D simulations for the forced ignition processes of premixed cool and hot flames in a laminar, axisymmetric, counterflow configuration with well-defined flow field. Different ignition energies and strain rates are used to ignite and stabilize cool and/or hot flames in a counterflow. The critical conditions for the ignition of cool and hot flames are identified, and the cool flame kernel quenching and its transition to hot flame are discussed. It is found that the ignition energy determines the highest temperature and thereby controls the thermal runaway process. The strain rate influences the flame kernel propagation and the transition from cool flame to hot flame. A large strain rate may quench the cool flame, while a small strain rate may lead to the transition from cool flame to hot flame due to the sufficiently long residence time. Therefore, cool flame ignition and stabilization can only be achieved within a certain range of strain rates. Furthermore, an ignition regime diagram in terms of ignition energy and strain rate is proposed, and four regimes are identified: (I) only cool flame, (II) only hot flame, (III) ignition failure, and (IV) transition from cool flame to hot flame (with appearance of double flame structure). These results provide new insights into how the flow affects the cool flame ignition and transition.

Large eddy simulation of spray combustion using the spray flamelet/progress variable model: Further extension and validation

The classical flamelet combustion model assumes that the combustion system consists of two homogeneous inlet streams for oxidizer and fuel. It usually requires extension and validation for configurations with different types of inlet streams. In the current work, the recently developed two-phase spray flamelet/progress variable (TSFPV) model is further extended and validated on the configurations of Delft spray flame AII and Sydney piloted ethanol spray flame EtF2, representing the systems with a single inlet and three inlets, respectively. The strategies for generating the spray flamelet library are adjusted, and an extended model is developed for modeling flame AII. Large eddy simulations of these two flames are conducted to evaluate the accuracy of the TSFPV model. The results of flame AII show that some droplets penetrate into the flame zone, and significant droplets/flame interactions occur. The flame exhibits a typical double flame structure. The predictions of gas phase temperature are in good agreement with the experimental measurements, which indicates that the current model can better consider the influence of droplet evaporation and the temperature reduction caused by interphase heat loss. The results of flame EtF2 also achieve good agreement with experimental data according to radial profiles of axial droplet velocities and gas phase temperature. Overall, the TSFPV model is well validated and exhibits sound performance on different flame configurations, indicating its widespread applicability and generalizability.

Experimental research on self-initiation process of rotating detonation wave by high-temperature ethylene-rich gas

This study investigates an experimental process of high-temperature ethylene-rich gas and air for rotating detonation. After abandoning the pre-detonator method, the high-temperature ethylene-rich gas was injected and collided with the outer wall to create a localized high temperature and pressure region that ignited the gas mixture to form the deflagration state. Whether it is a single deflagration flame or double deflagration flame, it is eventually transformed through the deflagration-to-detonation transition (DDT) process into a double detonation wave. With propagation and collision of the double waves, the injection pressure increases, which results in shortening the self-initiation delay time. The equivalence ratio, gas temperature, and mass flow rate affect the success ratio of self-initiation. Rotating detonation engine (RDE) can be successfully initiated in the equivalence ratio range of 0.82 to 1.69. Equivalence ratio and gas temperature increase the propagation velocity of rotating detonation wave (RDW), for maximum RDW velocity of 1324.61 m/s, the gas temperature is 1021.3 °C. At the double wave collision point, a new weak RDW appears that decreases the RDE operating stability and a periodic quenching and re-initiation of RDW phenomenon occurs at that point.

Development of an analytical strategy for the determination of trace lead in hibiscus tea extract by double slotted quartz tube assisted flame atomic absorption spectrometry after manganese ferrite based dispersive solid phase extraction

In this study, a manganese ferrite (MnFe2O4) based dispersive solid phase extraction method (MF-DSPE) was proposed for the determination of lead (Pb) ions in hibiscus tea by double slotted quartz tube flame atomic absorption spectrometry (SQT-FAAS). The MnFe2O4 nanoparticles used as sorbent were produced in high yields using a green and friendly microwave synthesis procedure. The limit of detection (LOD) was found to be 14.2 µg/L under the optimal conditions. Recovery experiments were performed to assess the applicability of the developed method using hibiscus tea as test sample matrix. High recovery results were obtained by applying the matrix matching calibration method. Thanks to the developed MF-DSPE-SQT-FAAS method, the detection sensitivity for lead was improved by about 108-folds according to the comparison of the slopes of the calibration plots of the FAAS and MF-DSPE-SQT-FAAS systems.

A facile strategy for constructing lightweight, fire safety and compression resistance poly(vinylalcohol) aerogels with highly-efficient expansible graphene oxide/layered double hydroxides hybrid synergistic flame retardant

Poly(vinyl alcohol) (PVA) aerogels with excellent environmentally friendly properties have been considered to replace undegradable polymer foams. However, due to highly flammable, hydrophilic, and worse compression resistance performance, PVA aerogels have always been excluded from practical. Herein, a fire safety and compression resistance PVA/expansible graphene oxide (EGO)/Layered double hydroxides (LDHs) (PGL) aerogel was prepared via the freeze-drying method and electrostatic adsorption of flame retardant. The ice crystals from aerogels were sublimated and left a mass of tree-like pore tunnel structures. Meantime, the compound of EGO and LDHs rendered PGL aerogels high compressive strength of 6.0917 MPa (at 80% of strains), a high specific modulus of 19.16 m2/s2, and an ultra-low density of 0.059 g/cm3. Especially, the as-prepared PGL aerogels showed heat release reduced by 55.4%, smoke release reduced by 54.3%, and the limiting oxygen index reaching up to 31%. Moreover, LDHs also enhanced the interface with PVA/EGO resulting in hydrophobic performance improvement. The proposed enhancements mechanism suggested that (i) chemical reactions between EGO and PVA matrix; (ii) a mass of negative potential sites from the interface of PVA/EGO composites made LDHs sheets easily adsorbing; (iii) oxygen-containing groups from EGO and LDHs absorbed mass of heat during combustion; (iv) the compact char residues on the surface of aerogels acting as barriers suppressed smoke and prevented PVA matrix from further combustion. Therefore, electrostatic adsorption as a facile production process was paved for meeting the compression resistance, flame-retardant, heat-insulating, and smoke-suppressed requirements of PVA aerogels in this work.

Geometrical features and global radiant heat of double turbulent jet flames

In energy industry, double jet fires are often reported to occur with a large release momentum at the leakage exit. The available literature considered the double buoyancy-controlled jet flames, and also neglected the restrictive effect of ground or tank surface. In this paper, an experimental setup is built to model the interaction of double turbulent jet fires with different spacings. Different nozzle diameters and exit velocities are chosen to achieve the transition from flame buoyancy to exit momentum for the dominance of turbulent jet flames. Comparative tests are also conducted between with and without a plate near the nozzle exit. It is found that the flame buoyancy, exit momentum and nozzle spacing significantly affect the flame merging behavior. A new dimensionless parameter coupling the Froude number (Fr) and the spacing is proposed to well fit the merging probability (Pm) of double jet flames dominated by the flame buoyancy (Fr < 105 for propane) and the exit momentum (Fr ≥ 105 for propane), respectively. The interaction of double jet flames leads to a less lift-off height than a single jet flame, and the lift-off height can also be reduced by a plate near the nozzle exit. A correlation coupling the spacing, the exit diameter and Froude number, is developed for the mean flame height of double jet fires. The distributions of radiant heat flux are intensively measured around the double jet flames, which can help to calculate the radiative fraction. The radiative fraction of double jet flames at the full merging state (Pm = 1) follows the law of a single jet flame, and it decreases at the intermittent (0 < Pm < 1) and no merging (Pm = 0) states, as compared to that of a single jet flame. In addition, the available line source radiation model of a single jet flame is revised to predict the thermal radiation of double jet flames in the three different merging states.

Material Discrimination in Nanoparticle Hetero‐Aggregates by Analysis of Scanning Transmission Electron Microscopy Images

AbstractHetero‐contacts are interfaces between different materials at the nanoscale leading to novel functional properties. In hetero‐aggregates, primary particles of at least two different materials are mixed at primary particle or cluster level. Double flame spray pyrolysis (DFSP) is a versatile technique for the controlled synthesis of such materials. Characterization of hetero‐aggregates by scanning transmission electron microscopy (STEM) requires acquisition and evaluation of many aggregate images in order to derive statistically significant results. Usually, STEM energy dispersive X‐ray spectroscopy (EDXS) is used to acquire elemental maps providing the material distribution of the primary particles within hetero‐aggregates. However, the acquisition of a single EDXS map takes up to several minutes. For this reason, determination of material types of primary particles from the intensity in high‐angle annular dark field STEM images alone is desirable. These images can be acquired within a couple of seconds. In the present work, a method is suggested which allows for achieving this objective. It can be applied to distinguish materials with a significant difference in their atomic number and hence sufficient material contrast in the STEM images.

Flemish Immigration and Geoffrey of Monmouth's De gestis Britonum

Flemish Immigration and Geoffrey of Monmouth's <i>De gestis Britonum</i>

Effects of water vapor addition on downstream interaction in CO/O2 counterflow premixed flames

The effects of H2O addition on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1. When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low strain rate, the lean-lean and rich-rich extinction boundaries show strong and weak interactions similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low strain rate, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As the strain increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as strain rate increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L + XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2 + 1.0% H2O) flames with the increase of strain rate, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.

Two-dimensional simulation of cool and double flame formation induced by the laser ignition under shock-tube conditions

The laser ignition-induced spherical cool and double flame’s initiation, propagation, and transition are studied using a two-dimensional simulation with a detailed mechanism using n-heptane/O2/Ar/He mixture. The current study’s primary goal is to reveal how the radicals and flow field produced by the laser ignition lead to cool, hot, or double flame formation and to determine the flame propagation modes. It is found that after the laser spark, the over-driven shock wave results in vortex pairs, which significantly distorts the temperature field and the spatial distribution of the radicals. A torus-like hot flame is ignited at the ignition kernel center instantaneously. The results show that the laser-ignited hot flame can transit to a cool or double flame depending on the laser pulse energy. On the one hand, the hot flame extinguishes under a small laser ignition energy condition, at which the radicals at the hot flame front are transported by the vortex pair and lead to a cool flame formation. On the other hand, when the laser spark energy is large, a transient premixed double flame structure is observed. The double flame can be localized, and the appearance location is primarily affected by the spatial distribution of laser-induced radicals. The cool flame can coexist with a trailing hot flame for milliseconds, dramatically accelerating the hot flame propagation and fundamentally impacting the flame speed interpolation. Computational singularity perturbation (CSP) analysis identifies key reactions that belong to typical low-temperature chemistry in the double flame zone. A comparative simulation case with an initial temperature above the low-temperature chemistry regime confirms that no double flame structure can be formed under high-temperature conditions. The present study provides essential insight and guidance for the flame speed measurement using laser ignition under engine-relevant conditions.