Mixing Flame Research Articles

Flammability and thermal properties of bitumen with aluminium trihydroxide and expanded vermiculite

Effects of aluminium trihydroxide (ATH), expanded vermiculite (EVMT), organic expanded vermiculite (OEVMT) and their mixtures (ATH/EVMT and ATH/OEVMT) on flame retardancy of bitumen were investigated. The flammability and thermal properties of the bitumen with different flame retardants were evaluated by limiting oxygen index, thermogravimetry and differential scanning calorimetry tests. Results show that the mixed flame retardant of ATH and EVMT shows a much more significant influence on the flame retardancy of bitumen than the single one. This unusual flame retardancy arises from the positive synergetic effect between the ATH and EVMT components in this mixed flame retardant, which provide the water vapor and protective barrier during the burning test of bitumen. The thermal analysis reveals that the addition of mixed flame retardant (ATH and EVMT) improves the thermal degradation behavior of bitumen drastically, which also confirms the synergetic effect of ATH and EVMT on the flame retardancy of bitumen.

The mechanism of flame and smoke retardancy of asphalt mortar containing composite flame retardant material

The study was carried out to analyze the flame and smoke retardancy of asphalt mortars containing composite flame retardant material and the mechanism of which by the help of limit oxygen index, thermogravimetry, difference scanning calorimetry and smoke density test. The results showed that the LOI of asphalt mortar with M-FR can increase by 5.9. Thermogravimetry-Difference Scanning Calorimetry (TG-DSC) analyses illustrated that M-FR can both increase the decomposition temperature and decrease the total decomposition because of the combined effect of MiberIII mineral fiber and mixed flame retardant. Meanwhile, it can play the role of cooling by a great number of endothermic reactions, especially in the temperature range from 400°C to 500°C. Max smoke density (MSD), smoke density rating (SDR) were proposed to evaluate the smoke retardancy. The value of MSD and SDR, together with test phenomena, demonstrated that M-FR can reduce the smoke production.

Study of Heattransfer During Piercing Process of Oxyfuel Gas Cutting

The heat transfer parameters, the local heat transfer coefficient, α, and the gas temperature adjacent to the plate, TG, are nearly unchanged with time during preheating for oxyfuel gas cutting. A genetic algorithm (GA)-based identification technique for α and TG is proposed. The validity of the proposed technique and the accuracy of the identified parameters are examined by comparing the measured and calculated plate back face temperatures during spot heating tests. Hydrogen-LP mixed gas and LPG are used as preheating gases in these tests. It is considered that the plate temperatures during preheating for piercing can be calculated by using α and TG identified in spot heating tests. The minimum piercing time is estimated by calculating the time until the plate heating face temperature reaches the kindling temperature. The validity of the above assumption is examined by comparing the estimated and measured minimum piercing times for hydrogen-LP mixed gas and LPG. As a result, the following are found: 1) The plate temperatures during spot heating tests calculated by using the identified heat input parameters agree well with the ones measured. This demonstrates the accuracy of the identified parameters and the validity of the proposed heat transfer simulation technique. 2) The heat flux of a hydrogen-LP mixed gas flame around the preheating gas ejection hole is 40 % greater than that of a LPG flame, while the total calorific value of hydrogen-LP mixed gas is 25 % lower than that of LPG. This result shows that it is not appropriate to evaluate the thermal effect of the preheat flame only from the total calorific value alone. 3) The calculated time for the plate face temperature to exceed the steel’s kindling temperature almost agrees with the minimum piercing time observed in piercing tests. For the first time, it is possible to anticipate the piercing time by numerical simulation.

Flammability and rheological behavior of mixed flame retardant modified asphalt binders

The flammability and rheological behavior of asphalt modified by mixed flame retardants was investigated using differential scanning calorimetry and dynamic shear rheometer, respectively. The mixed flame retardants used consisted of decabromodiphenyl ether, antimony trioxide and aluminium trihydroxide with the ratio of 3:1:1 by mass. Experimental results indicated that the inclusion of 6 wt% mixed flame retardants improves the flame retardancy significantly by consuming the released heat of asphalt in a temperature range of 300–500 °C. The close values of the activation energy obtained by fitting Arrhenius model demonstrate flame retardant modified asphalt exhibits similar thermal susceptibility to base asphalt in the temperature range of 30–100 °C. It can foresee that the content of flame retardant is expected to increase with respect to better flame resistance if required based upon this research.

Experimental investigation of related properties of asphalt binders containing various flame retardants

The improved flame retardancy of asphalt binders containing various kinds of flame retardants including antimony trioxide, decabromodiphenyl ether (EBPED), aluminium trihydroxide (ATH) and zinc borate (ZB) under different additive concentrations was investigated by limited oxygen index (LOI) and differential scanning calorimetry (DSC) test. To assess the effects of the type and concentration of the flame retardants on rheological properties of asphalt binders, the softening point, penetration, ductility, storage stability and viscosity were tested. The thin film oven test (TFOT) was used to investigate the effect of short-term oxidation on flame-resistant asphalt binders. Experimental results indicated that all of selected flame retardants had a relatively small effect of rheological properties of the asphalt binders, but a great effect on flame retardancy. Asphalt binder with LOI of 25.9 was obtained by adding 6 wt% mixed flame retardants of EBPED: antimony trioxide: ZB=3:1:1 by mass. Asphalt binder was transformed from inflammable materials into flame retardant materials. Therefore, the asphalt binder with both good flame retardancy and rheological properties is a novel road functional material to meet two respects' demands as flame retardant materials and road materials at the same time.

The Two-Dimensional supersonic flow and mixing with a perpendicular injection in a Scramjet Combustor

A numerical investigation has been performed on supersonic mixing of hydrogen with air in a Scramjet (Supersonic Combustion Ramjet) combustor and its flame holding capability by solving Two-Dimensional full Navier-Stokes equations. The main flow is air entering through a finite width of inlet and gaseous hydrogen is injected perpendicularly from the side wall. An explicit Harten-Yee Non-MUSCL Modified-flux-type TVD scheme has been used to solve the system of equations, and a zero-equation algebraic turbulence model to calculate the eddy viscosity coefficient. In this study the enhancement of mixing and good flame holding capability of a supersonic combustor have been investigated by varying the distance of injector position from left boundary keeping constant the backward-facing step height and other calculation parameters. The results show that the configuration for small distance of injector position has high mixing efficiency but the upstream recirculation can not evolved properly which is an important factor for flame holding capability. On the other hand, the configuration for very long distance has lower mixing efficiency due to lower gradient of hydrogen mass concentration on the top of injector caused by the expansion of side jet in both upstream and downstream of injector. For moderate distance of injector position, large and elongated upstream recirculation can evolve which might be activated as a good flame holder.

PSR-based microstructural modeling for turbulent combustion processes and pollutant formation in double swirler combustors

The present study numerically investigates the fuel-air mixing characteristics, flame structure, and pollutant emission inside a double-swirler combustor. A PSR (Perfectly Stirred Reactor) based microstructural model is employed to account for the effects of finite rate chemistry on the flame structure and NO formation. The turbulent combustion model is extended to nonadiabatic flame condition with radiation by introducing an enthalpy variable, and the radiative heat loss is calculated by a local, geometry-independent model. The effects of turbulent fluctuation are taken into account by the joint assumed PDFs. Numerical model is based on the non-orthogonal body-fitted coordinate system and the pressure/velocity coupling is handled by PISO algorithm in context with the finite volume formulation. The present PSR-based turbulent combustion model has been applied to analyze the highly intense turbulent nonpremixed flame field in the double swirler combustor. The detailed discussions were made for the flow structure, combustion effects on flow structure, flame structure, and emmission characteristics in the highly intense turbulent swirling flame of the double swirler burner.

Navier-Stokes numerical simulation of supersonic hydrogen-fluorine combustion in CW chemical lasers

Results are presented of numerical simulations of nonpremixed supersonic combustion with an accountof nonequilibrium vibration energy exchange and stimulated radiation in the continuous-wave (CW) HF chemical lasers with different fuel/oxidant mixing devices. The main goal of the study was to obtain better insight into the mechanism of coupling between the processes in the combustion region (laser cavity) and in the supersonic supply system (nozzle array). In order to take into account this effect, which is caused by the upwind influence of the cavity processes on the nozzle array flow, a coupled one-domain approach is used, i.e., a simultaneous Navier-Stokes simulation of both nozzle and cavity flows. The results of the computations clearly show that coupling or, more exactly, the feedback mentioned above, is explained mainly by the fuel and oxidant nozzle flows interaction due to propagation of the pressure disturbances from one nozzle to another through the nozzle array base recirculation zone and subsonic part of the nozzle boundary layers. It causes significant deformation of the mixing zone and flame configuration and turns out to be crucial when the difference between the nozzle exit pressure levels is large. In this case, separation of the nozzle boundary layer may occur, which leads to the ignition of the mixture inside the nozzle array. The results illustrate also the influence of the regime parameters and the design of the nozzle array on the flow wave pattern, flame configuration, structure of the base recirculation zones, and the laser output characteristics.

Considerations for ion density and temperature determinations in flames doped with alkali metals

Various methods are discussed for determining the ion density (ions ml −1), equivalent to the free electron concentration [e −], and associated temperature T f in a flame plasma. The methods are illustrated using a conical, pre-mixed, fuel-rich, CH 4-O 2 flame at atmospheric pressure. Ions are sampled along the flame axis through a nozzle into a mass spectrometer. The measurements involve both total ion collection through the sampling nozzle and mass-spectrometric signals of individual ionic species. The flame can be doped with alkali metals by spraying an aqueous solution of their hydroxides or salts as an aerosol into the pre mixed flame gas using an atomizer. The alkali metals can be added singly or as a mixed solution of several metals. Analytical expressions involving equilibrium constants are derived for the concentrations of metallic atoms in equilibrium with their hydroxides and atomic cations as a function of T f. These expressions for different metals can be combined to yield [e −] and T f in terms of the ratios of the atomic ion signals and the solution concentrations. In principle, these alkali metals, measured either as a mixture or singly, can yield both [e −] and T f. In practice, if either T f or [e −] is known from independent measurements, two alkali metals, measured either as a mixture or singly, provide values of the other quantity [e −] or T f. Experimental limitations of the methods are illustrated and discussed. In particular, it is shown that the ion signal measured through a sampling nozzle is not proportional to the ion density in the flame plasma. Formulae for calculating equilibrium constants for thermal ionization and hydroxide formation as a function of temperature in the range 1900–2500 K are given for Li, Na, K, Rb, and Cs; the Rb formulae are based on comparative measurements of alkali metal mixtures.

Flow structure in near-nozzle region of gas jet flames

A flow visualization study of the near-nozzle regions of a cold jet, an attached flame, and a lifted flame of propane jet exiting a contoured nozzle with uniform velocity profile in quiescent air is presented. The observed flowfields and celerities of the coherent structures in the three cases are compared. The results show that com- bustion reactions occur away from both jet boundary and cores of coherent structures. The presence of flame retards the growth of the coherent structures and increases their celerity substantially. HE interaction of flames and flow structure in combus- tion systems has been the subject of several investiagations in the past. Many of these studies were concerned with pre- mixed flames. In recent years, however, considerable effort has been devoted to understand the flame/flow interaction in reacting plane shear layers13 and axisymmetric gas jets.47 Such studies are motivated by the necessity to understand the roles of coherent structures in the mixing rates.89 This infor- mation is needed to improve the burning characteristics of dif- fusion flames and to develop theoretical models for their prediction. A review of the uses of coherent structures was presented recently by Coles.10 An understanding of the detailed flow structure and its interaction with reaction zones is also crucial to unfold the stability mechanism of diffusion flames, including liftoff and blowout phenomena, which has been a topic of controversy in recent years.11'12 Here, we present some results from a flow visualization study in the near-nozzle region of propane gas jet flames where the flowfield is essen- tially an axisymmetric shear layer. The results not only con- firm the presence of large-scale structures in the flames but also reveal some characteristic features of their behavior in the presence of combustion. We focused our attention on the near-nozzle region in order to understand 1) the changes in flow structure of cold jets caused by the presence of a surrounding burner-attached flame, and 2) the liftoff and reattachment processes of flames.

Prediction of a three rectangular jet turbulent mixing and diffusion flame

Prediction of a three rectangular jet turbulent mixing and diffusion flame

Distribution of atomic hydrogen in the combustion products of preliminarily mixed and diffusional hydrogen-oxygen flames

Distribution of atomic hydrogen in the combustion products of preliminarily mixed and diffusional hydrogen-oxygen flames

On recirculating flow in an opposed jet flame holder

The analysis of ducted jets with recirculation has received much attention in recent yearsle4 because of its potential application in high speed combustion systems to obtain enhanced mixing and stable flame. In an opposed-jet Same stabilization system, a small jet of high velocity gas flows in a direction opposite to the confined mainstream of premixed combustible mixture. The stagnation zone created by the interaction of the jet and the mainstream provides the required locally decelerated region and the recirculation of the hot products of combustion is effected due to the jet entrainment in the confined mainstream. Since its inception as a potential flame holder,’ experimental workrU7 has been reported on the opposed-jet system which reveal some gross-behaviour of the flow pattern, but theoretical research on such a flow system for predicting the flow parameters is, however, meagre. A complete analysis of the flow-field demands the simultaneous solution of the relevant differential equations governing the conservation of mass, momentum and energy using the appropriate mathematical model for the different physical and chemical processes involved. The simple ‘integral methods’ proposed by a group of Soviet research workersse7 for the prediction of the flow field for isothermal and also the reacting situation for the opposed-jet system are seriously limited by the boundary layer assumptions which are not strictly valid in recirculating flows. It is only recently that a general numerical method has been available for solving the elliptic partial differential equations for turbulent, recirculating flows. Here, a simplified mathematical model of the flow of a non-reacting turbulent jet spreading into a ducted, uniform, counterflowing stream is proposed. The numerical method of Gosman et ~1.~ is applied to predict the flow parameters in the isothermal situation of an opposed-jet flame holder. Experimental investigations are also carried out for comparison of experimental results with the prediction to validate the prediction procedure.

Pulverized coal combustion: The influence of flame temperature and coal composition on thermal and fuel NO x

A laboratory combustor was used to investigate the factors that influence the conversion of fuel nitrogen in coal during coal combustion. Fuel NO was isolated by experimentation utilizing Argon/Oxygen/Carbon Dioxide mixtures as the oxidant, and care was taken to compare cases with air at matched conditions. For both well mixed and slowly mixed flame types, fuel NO contributed over 75% of the total NO emissions for all conditions examined. Fuel NO was insensitive to temperature changes except when the adiabatic flame temperatures were above 2480°K (4000°F). At the highest adiabatic flame temperature, 2580°K (4200°F), a 10% increase in fuel NO was observed. Four different coals and one coal char were investigated. Fuel NO could not be correlated with fuel nitrogen content alone, even though aerodynamic conditions were kept constant. Fuel nitrogen conversion to NO during pulverized char combustion was 12–16% at a stoichiometric ratio of 1.15 compared to 28% for a pulverized coal of the same nitrogen content. Furthermore, in contrast to the coal results, NO emissions from char combustion were not greatly influenced by changes in injector design. The implication is that although conversion of fuel nitrogen to NO may be relatively low during the char burnout regime of coal combustion, the residual “char NO” may be especially resistant to abatement by modifications of the burner aerodynamics.