Small Flame Research Articles

Fuel density effect on near nozzle flow field in small laminar coflow diffusion flames

Flow characteristics in small coflow diffusion flames were investigated with a particular focus on the near-nozzle region and on the buoyancy force exerted on fuels with densities lighter and heavier than air (methane, ethylene, propane, and n-butane). The flow-fields were visualized through the trajectories of seed particles. The particle image velocimetry technique was also adopted for quantitative velocity field measurements. The results showed that the buoyancy force exerted on the fuel as well as on burnt gas significantly distorted the near-nozzle flow-fields. In the fuels with densities heavier than air, recirculation zones were formed very close to the nozzle, emphasizing the importance of the relative density of the fuel to that of the air on the flow-field. Nozzle heating influenced the near-nozzle flow-field particularly among lighter fuels (methane and ethylene). Numerical simulations were also conducted, focusing specifically on the effect of specifying inlet boundary conditions for fuel. The results showed that a fuel inlet boundary with a fully developed velocity profile for cases with long tubes should be specified inside the fuel tube to permit satisfactory prediction of the flow-field. The calculated temperature fields also indicated the importance of the selection of the location of the inlet boundary, especially in testing various combustion models that include soot in small coflow diffusion flames.

Characterization of operating conditions of two residential wood combustion appliances

This work presents the main physical characteristics and operating conditions of a fireplace and a stove commonly used in Portugal for domestic heating based on biomass combustion. The fuel used was wood from pine (softwood) and eucalyptus (hardwood). The general trend during the batch combustion of wood logs was characterized by three main periods: i) a first period of fuel heating followed by fuel drying and initial steps of devolatilization without the existence of a visible flame, ii) a second period characterized by devolatilization, ignition, combustion of volatiles and char, during which a vigorous flame is observed, and iii) a third period, mainly identified by the combustion of the char, during which there are only localized small visible flames over the char particles. Each of these periods is characterized by specific fuel consumption rate, flue gas temperature and flue gas composition. The main differences between the wood combustion conditions in the fireplace and in the stove include: i) a lower flue gas temperature in the fireplace, ii) a higher combustion flue gas flow rate in the fireplace, iii) a higher rate of fuel consumption in the stove, and iv) higher CO, total hydrocarbon and particle emission factors during the combustion of wood in the fireplace. Differences between hardwood and softwood combustion were also pointed out.

Large-eddy simulation of ethanol spray combustion using a SOM combustion model and its experimental validation

In this paper, large-eddy simulation of ethanol spray-air combustion was made by using an Eulerian–Lagrangian approach, a sub-grid-scale kinetic energy stress model, and a filtered finite-rate combustion model with a sub-grid scale reaction rate, called second-order moment (SOM) combustion model, proposed by our research group. The simulation accounts for the sub-grid turbulence–spray–combustion interactions and the coupling between the temperature fluctuations and species concentrations fluctuations. The simulation results are validated in detail by experiments. It is found that the LES-SOM statistical results are in better agreement with the experimental results than those obtained by LES accounting for only the filtered finite-reaction rate (LES-FA) neglecting the SGS reaction rate, and are in much better agreement than those obtained by RANS modeling using the most complex PDF equation combustion model with detailed chemistry. The instantaneous LES results show the earlier production of the coherent structures in the two-phase jet combustion than that in the single-phase jet combustion. The instantaneous temperature map indicates the existence of small flame islands, expressing the droplet-group combustion, which is not observed in single-phase jet combustion.

The influence of layered, spherical, and tubular carbon nanomaterials' concentration on the flame retardancy of polypropylene

The characteristic influences of increasing concentrations of graphene, expanded graphite (EG), carbon black (CB), and multiwall carbon nanotubes (MWNT) are investigated on pyrolysis, reaction to small flame, burning behavior, and on electrical, thermal, and rheological properties of flame retarded polypropylene (PP-FR). The property-concentration dependency is different for the various material properties, as threshold, linear, and leveling off functions were observed. Increasing concentrations of carbon nanoparticles resulted in a decrease in the electrical resistivity of the polymer by crossing the percolation threshold. The developing nanoparticle network changes melt flow behavior for small shear rates, increases thermal conductivity and therefore, affects the UL 94 classification and oxygen index. The onset temperature of PP decomposition is shifted to temperatures up to 37°C higher; the peak heat release rate is reduced by up to 74% compared to PP-FR. Both effects leveled off with increasing particle concentration. Among the four carbon nanomaterials tested, graphene presents superior influence on composite properties over the tested concentration range and outperforms commercial CB, MWNT, and EG. POLYM. COMPOS., 36:1230–1241, 2015. © 2014 Society of Plastics Engineers

Combustion behavior of oak wood (Quercus mongolica L.) modified by 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU)

Abstract The modification of wood with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) is an efficient strategy to improve its dimensional stability and durability during its service period. In this study, oak wood (Quercus mongolica L.) was treated with DMDHEU and the combustion behavior of the treated wood was examined. The bench-scale thermogravimetry and cone calorimetry showed that the treatment with DMDHEU alone enhanced the flammability of wood, but a combined treatment with DMDHEU and traces of magnesium chloride as a catalyst lowered the flammability to the level of untreated wood. In full-scale fire tests for flooring materials, it could be confirmed that both the untreated and treated woods have similar combustion behavior and can be classified as Dfl-S1 (capable of resisting a small flame and, for a certain period, a heat flux attack; the total smoke production is limited). Accordingly, further fire-retardant treatments may be required if DMDHEU-treated wood is used in public areas.

Phosphorus-containing thermotropic liquid crystalline polymers: a class of efficient polymeric flame retardants

The flame retardation of polymeric materials can be achieved by addition of small-molecule flame retardants. However, traditional small molecule flame retardants exhibit practical drawbacks during application, such as migration problems, the deterioration of polymer performance, as well as potential persistence, bio-accumulation and toxicity (PBT), etc. High molecular weight molecules have been found to be less accessible by living organisms and so have an automatically lower PBT profiles than small molecules. Phosphorus-containing thermotropic liquid crystalline polymers have proven to be a class of efficient polymeric flame retardants, which can overcome the aforementioned drawbacks of small molecules and have potential industrial applications to replace some of the existing small molecule flame retardants. The recent relevant developments are reviewed in this article.

Robust superhydrophobic surfaces from small diffusion flame treatment of hydrophobic polymers

When pure perfluorinated acrylic or natural wax-perfluorinated acrylic polymer blend coating surfaces are treated with small diffusion flames for a short duration, deposition of superhydrophobic nanostructured carbonaceous films is observed. The carbonaceous films display a good degree of surface binding which cannot be removed by impinging and rolling water droplets. The resultant surfaces have very low droplet roll-off angles and remarkable resistance to saturation against fast impacting water streams as well as to disintegration in ultrasonic bath treatment.

Performance evaluation of bromofluoropropene in extinguishing liquid fuel spray fires

AbstractLiquid fuel spray fires emit high radiation heat fluxes, posing great threat to humans. The study of suitable agents and techniques for extinguishing this particular type of fire is of great importance. In this study, degradable 2‐bromo‐3,3,3‐trifluoropropene (BTP), a new clean fire extinguishing agent, was tested for its effectiveness in extinguishing three types of liquid fuel spray fires, namely diesel, gasoline, and ethanol. Bench‐scale experiments were conducted in a 6 × 5 × 3 m compartment with natural ventilation. The liquid fuels sprayed at varying pressures were ignited by a small open flame and then extinguished by a portable BTP extinguisher. Results showed that BTP of less than 60.0 g could extinguish all liquid fuel spray fires of 0.20 to 1.0 MPa in less than 2.0 s. The results also showed that when compared with fire sparked by gasoline and diesel, it is significantly easier to put out ethanol spray fires because of its high flame temperature and low flame power. Based on well‐established fire suppression theories and experimental results, the detailed mechanism of how BTP functions as an extinguishing agent in the suppression of liquid fuel spray fires will be discussed. Copyright © 2013 John Wiley & Sons, Ltd.

Flame-retarded bisphenol A polycarbonate/silicon rubber/bisphenol A bis(diphenyl phosphate): Adding inorganic additives

Various inorganic additives belonging to four different groups: layered materials, metal hydroxides, metal oxides/carbonate and metal borates are investigated in bisphenol A polycarbonate/silicon rubber/bisphenol A bis(diphenyl phosphate) (PC/SiR/BDP) to improve flame retardancy. The pyrolysis, reaction to small flame and fire behaviour of the blends are characterised and structure–property relationships discussed. Among the added layered materials, talc functions as an inert filler with potential for commercialisation, whereas organically modified montmorillonite (LS) enhances decomposition. PC/SiR/BDP + talc and PC/SiR/BDP + LS reinforce the char and induce a flow limit. The different dispersion and location of boehmite (AlO(OH)) nano-particles and Mg(OH) 2 micro-particles determine the impact on performance. PC/SiR/BDP + Mg(OH) 2 shows additional hydrolysis and thus reduced flame retardancy. AlO(OH) is embedded in SiR and thus behaves as an inert filler. Both additives worked as smoke suppressants. Using selective filling with nano-particles is proposed as an interesting route for flame retardancy in PC/SiR blends. Adding metal oxides and carbonate (MgO, CaCO 3 and SiO 2 ) changes the decomposition pathways of PC/SiR/BDP, worsening the fire performance of PC/SiR/BDP. CaCO 3 harbours the potential to intumescence, even though an early collapse of the char structure occurred. Adding hydrated metal borates, CaB, MgB and ZnB, changes the pyrolysis and flame retardancy action. Smoke suppression occurs; LOI is improved as well as UL 94 classification. ZnB performs better than MgB and CaB. The comprehensive study, also based on systematic material variation, delivers valuable guidelines for future development of flame-retarded multi-component PC blends.

Numerical Simulation of Effects of Gravity on the Micro Jet Flame in the Confined Space

A microscale ceramic tube was used as a burner jet, and a coaxial jet flame was established in the confined space between the ceramic tube and the outer quartz glass tube. The effects of gravity on the small jet flame characteristics in the confined space were investigated numerically. Comparisons between the experimental data and the numerical data showed that characteristics of the small jet flame structure and temperature field were in good agreement. It verified the accuracy of the numerical simulation. Numerical simulations of flame characteristics at zero gravity level were performed. The results show that the gravity level has a greater influence on the flame width than that on the flame length. The chemical reaction rate is larger under the condition of normal gravity than that of zero gravity.

Carbon black, multiwall carbon nanotubes, expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites

Herein, we examine the influence of adding functionalized graphene (FG), distinct expanded graphites and carbon nanofillers such as carbon black and multiwall carbon nanotubes on mechanical properties, morphology, pyrolysis, response to small flame and burning behavior of a V‐2 classified flame‐retarded polypropylene (PP). Among carbon fillers, FG and multilayer graphene (MLG) containing fewer than 10 layers are very effectively dispersed during twin‐screw extrusion and account for enhanced matrix reinforcement. In contrast to the other fillers, no large agglomerates are detected for PP‐FR/FG and PP‐FR/MLG, as verified by electron microscopy. Adding FG to flame‐retardant PP prevents dripping due to reduced flow at low shear rates and shifts the onset of thermal decomposition to temperatures 40°C higher. The increase in the onset temperature correlates with the increasing specific surface areas (BET) of the layered carbon fillers. The reduction of the peak heat release rate by 76% is attributed to the formation of effective protection layers during combustion. The addition of layered carbon nanoparticles lowers the time to ignition. The presence of carbon does not change the composition of the evolved pyrolysis gases, as determined by thermogravimetric analysis combined with online Fourier‐transformed infrared measurements. FG and well‐exfoliated MLG are superior additives with respect to spherical and tubular carbon nanomaterials. Copyright © 2013 John Wiley & Sons, Ltd.

Flame retardancy through carbon nanomaterials: Carbon black, multiwall nanotubes, expanded graphite, multi-layer graphene and graphene in polypropylene

Herein we investigate the influence of carbon additives with different particle sizes and shapes on the flame retardancy and mechanical properties of isotactic polypropylene. Thermally reduced graphite oxide (TRGO) and multi-layer graphene (MLG250), consisting of few graphene layers, are compared with spherical, tubular and platelet-like carbon fillers such as carbon black (CB), multiwall nanotubes (MWNT) and expanded graphite (EG). The different morphologies control the dispersion of the carbon particles in PP and play a key role in structure–property relationships. Uniformly dispersed CB, MLG250 and TRGO shift the onset temperature of PP decomposition to temperatures around 30 °C higher, induce a flow limit in the composites' melt viscosity and change drastically their fire behaviour. The prevented dripping and significantly increased heat absorption result in decreased time to ignition and hardly any change in the reaction to a small flame. Under forced-flaming conditions reductions in the peak heat release rate of up to 74% are achieved due to the formation of a protective layer of residue during combustion. The described effects of carbon nanomaterials on the properties of PP composites are most pronounced for well-exfoliated graphenes, making them preferable to less exfoliated, micron-sized expanded graphite or conventional spherical and tubular carbon nanoparticles.

Flame retardancy of starch–based biocomposites — aluminum hydroxide-coconut fiber synergy

The use of coconut fiber (CF) agricultural waste was considered as an environmentally friendly and inexpensive alternative in flame retarded biocomposites. To decrease the high content of aluminum trihydrate (ATH) required, the thermal decomposition (thermogravimetry), flammability [oxygen index (LOI) and UL 94 test] and fire behavior (cone calorimeter) of a combination of CF and ATH were investigated in a commercial blend of thermoplastic starch (TPS) and cellulose derivatives. CF induced some charring activity, slightly decreasing the fire load and burning propensity in cone calorimeter test. ATH decomposes endothermically into water and inorganic residue. Significant fuel dilution as well as a pronounced residual protection layer reduces the fire hazards. Replacing a part of ATH with coconut fibers resulted in improved flame retardancy in terms of ignition, reaction to small flame, and flame-spread characteristics [heat release rate (HRR), fire growth rate (FIGRA), etc.]. The observed ATH and CF synergy opens the door to significant reduction of the ATH contents and thus to interesting flame retarded biocomposites.

Proposed Fire Screening Tests on Plastic Foams with a Cone Calorimeter in Hong Kong

Many big building fires happened in the past involved burning furniture foam. In some places such as Hong Kong, foams treated with fire retardant additives are only required to test their ignitability under a small flame. It was proposed earlier that the behaviour of furniture foam in flashover fires should be assessed more extensively by a cone calorimeter under high radiative heat fluxes. In this paper, this screening approach is further studied using five samples, including three different pillow foams, one mattress foam and one expensive fire-safe sofa foam. Both thermal and smoke aspects were studied in a cone calorimeter under heat fluxes of 20 kWm-2, 30 kWm-2, 50 kWm-2 and 70 kWm-2. It was observed that all the foam samples were burnt under heat fluxes above 30 kWm-2. Only one pillow foam sample was not ignited under a low incident heat flux of 20 kWm-2, but it morphed into a mixture of solid and liquid. Under heat fluxes above 30 kWm-2, the fire-safe foam burnt vigorously; the burning showed no differences from foam without fire retardant. The fire-safe foam even released more smoke and toxic gases with higher carbon monoxide concentration. From the above cone calorimeter test, foams commonly used in the market can only withstand small match fires to delay ignition time. Foam products should be protected by additional provisions, such as an external cover, to protect them against big fires.

Mixing Enhancement in a Compact Trapped Vortex Combustor

Previous studies on a single-cavity, compact trapped vortex combustor concept showed good flame stability for a wide range of flow conditions. However, achieving good mixing between cavity products and mainstream flow was still a major challenge. In the present study, a passive mixing enhancement strategy of using inclined struts along with a flow guide vane is presented and experimentally tested at atmospheric pressure conditions. Results show excellent mixing and consequently low values of the combustor exit pattern factor in the range of 0.1 and small flame lengths (5–7 times the main-duct depth). The pressure drop is small in the range of 0.35%, and NOx levels of the order of 1–2 ppm are achieved. The flame stability is excellent, and combustion efficiency is reasonable in the range of 96%. The effectiveness of the proposed strategy is explained on the basis of in-situ OH chemiluminescence images and prior numerical simulations of the resulting complex flow field. The flow guide vane is observed to lead to a counterclockwise cavity vortex, which is conducive to the rise of cavity combustion products along the inclined struts and subsequent mixing with the mainstream flow.

The Effect of Radiation on Oxy-Fuel Combustion Characteristics in Microchannels

The importance of thermal radiation in heat transfer mechanism in many micro combustion systems has been well identified in the past few years. There is currently lack of quantitative understanding on the radiation heat transfer in relatively small scale laminar diffusion flames in microchannels. In the present study a two dimensional model is considered to investigate the effects of radiation on oxy-fuel combustion characteristics in microchannels. The discrete-ordinates radiation model is used for the study. It is observed that excluding radiation model results in the over-prediction of combustion temperatures in the micro-reactor. It has also been observed that the overall reaction rate and its peak value increase when accounting for radiative heat transfer, despite the decrease in temperature caused by radiation. Therefore, it is important to incorporate a radiation heat transfer model in combustion micro-systems in order to predict their characteristics accurately.

Superhydrophobic and electroconductive carbon nanotube-fluorinated acrylic copolymer nanocomposites from emulsions

It is shown that when short multi-walled carbon nanotube (MWCNT) dispersed methanol–toluene mixtures including the azeotrope are mixed with an aqueous fluoroacrylic copolymer latex dispersion (Capstone ST-100), stable and surfactant free MWCNT suspended polymer emulsions can be prepared. These emulsions are sprayed on various surfaces such as glass, paper and metals to form electrically conductive polymer nanocomposite coatings. Thermal curing of the spray-cast films slightly above the melting point of the fluorinated acrylic latex, transforms them into superhydrophobic films with static water contact angles exceeding 150°. Alternatively, thermal treatment by impinging flame heating by small diffusion flames for a few seconds, also transforms the films into superhydrophobic state with very high static water contact angles exceeding 165° and very low droplet roll-off angles less than 5°. During this process, amorphous carbon nanobeads are deposited on the nanocomposite surfaces changing their surface topology. Superhydrophobicity of the flame treated nanocomposites can even withstand water hammering from impacting liquid streams with impact speeds close to 7m/s. The nanocomposites remain electrically conductive at all times even after conversion into superhydrophobic state either by thermal curing or by flame treatment with an electrical conductivity of 25S/m. The ohmic conductivity shows superior resistance against long delay times during repeated bias sweeps. The ease of preparation and stable ohmic electrical properties of these coatings can make them very suitable for applications such as corrosion protection for metal surfaces, conductive electrode materials for electrochemical energy conversion devices and lab-on-a-chip systems.

Characteristics of stabilized spinel cathode powders obtained by in-situ coating method

TiO2-coated LiMn2O4 cathode powders are prepared using an in situ spray pyrolysis process. The TiO2-coated LiMn2O4 powders have spherical aggregated structures of nanometer-sized primary particles. The mean sizes of the primary and secondary TiO2-coated LiMn2O4 powders are 55 and 880 nm, respectively. The transmission electron microscopy images of the LiMn2O4 primary particles show a single-crystalline and well-faceted structure. The single-crystalline LiMn2O4 primary particles are uniformly coated with an amorphous TiO2 layer. An immediate reaction of titanium tetraisopropoxide with oxygen forms a small flame at the exit of an alumina tube located in the center part of a quartz reactor. Sudden collisions between TiO2 vapor and submicron-sized composite powders of the Li and Mn components occur to form the TiO2-coated cathode powders. The discharge capacities of the TiO2-coated LiMn2O4 cathode powders are 126 and 109 mAh g−1 in the first and 170 cycles at a current density of 1 C. The capacity retentions of the pure and TiO2-coated LiMn2O4 powders are 69% and 86% of the initial capacity after 170 cycles.

Evaluation of simple models of flame radiation in the frame of fire propagation

Simple models for radiative transfer evaluation in the frame of flame propagation are discussed and evaluated through comparisons with a complete absorbing–emitting volume flame model. The cases of the solid flame model and of the optically thin flame are considered as they are usually involved in fire propagation studies. It is shown that the solid flame model provides satisfactory results in some applications, but only for flames with sufficiently high optical thickness (above three at least). However, its inability to model the intensity distribution when shifting from the normal to the flame surface and its inaccuracy when addressing the case of low or even moderate optical thicknesses are two weaknesses that should be kept in mind when using this model. Regarding the optically thin flame model, it is demonstrated that its use should be restricted to really low optical thicknesses, far below one, which makes the application of such models quite unrealistic unless for small scale flames. The true absorbing–emitting flame treatment should be preferred, with moderate supplementary cost regarding computational integration, if combined with dedicated methods like a reciprocal Monte Carlo Method as presented here for example.

425 各種燃焼方式でのマイクロフレームの燃焼特性(エネルギー有効利用(2))

The burning techniques are used in numerous regions from the heating furnace, the gas turbine and the boiler to the small burning receptacles. Retaining stable combustion is an important burning technique for using effective energy. In recent years, it is reported that a small flame (which is called micro flame) is formed on the tip of nozzle in atmospheric pressure. This flame has some characteristics of combustion that are different from the ones of conventional diffusion flame. On the other hand, the combustion method using the low temperature plasma at atmosphere pressure attracts attention and has been studied. It is reported that the optical emission measured at the discharge area between the electrodes shows that the plasma has CH radical. It is possible to enhance plasma-assisted combustion with these radicals. In this study the micro flame for methane, hydrogen and fue gas diluted with nitrogen is formed by using the burner nozzle whose various inner diameters are less than 1mm. for diffusion combustion, premixed combustion and plasma-assisted combustion. The flame configuration and the stability limit of diffusion combustion are compared with these of plasma-assisted combustion in the case of micro flame.