Flame Heat Research Articles

Transverse-load, strain, temperature, and torsion sensors based on a helical photonic crystal fiber.

In this Letter, we demonstrate a fabrication method of a helical photonic crystal fiber (HPCF) and an inflated HPCF (IHPCF) by use of an inflation-assisted hydrogen-oxygen flame heating technique. The transverse load, strain, temperature, and mechanical torsion properties of the HPCF and IHPCF were investigated experimentally to develop high-sensitivity sensors. The experimental results show that the transverse-load sensitivity could be greatly enhanced by means of enlarging the size of the air holes in the IHPCF; that is, the transverse-load sensitivity, i.e., 15.50 nm/(N·mm-1), of the IHPCF is two times higher than the transverse-load sensitivity of the HPCF, i.e., 4.45 nm/(N·mm-1). Moreover, both the HPCF and IHPCF exhibit high strain, temperature, and torsion sensitivities. Hence, such an HPCF/IHPCF could have great potential in sensing applications.

The Effect of CuO on the Thermal Behavior of the High-Energy Combustion Agent of the Al/MnO2 System

In this work, the effect of CuO addition into the high-energy combustion agent of Al/MnO2 system was studied. First, the combustion experiments of five samples with different contents had been carried out, in which CuO was found capable of influencing the flame ejection to a great extent. Then, in order to find out the underlying reasons, CuO effects on the thermal behavior of Al/MnO2 system were analyzed via theoretical calculations of Gibbs free energy and enthalpy changes. In addition, field emission scanning electron microscopy (FE-SEM) that could characterize the mixture morphology and thermogravimetric-differential scanning calorimetry (TG-DSC) that could indentify the exothermic and endothermic reactions and measure mass change were carried out. Finally, on the basis of all experimental findings, it was suggested that addition of CuO into Al/MnO2 system could result in dramatic increase of gas content throughout the reaction and the consequent high pressure. Also, speed of flame injection and heat released in the high-temperature area would thus be conducive to the continuous exothermic behavior of the reaction.

Numerical investigation of flame appearance and heat flux and in a deep-throttling variable thrust rocket engine

Much attention has been drawn to liquid-propellant rocket engine with thrust that can be varied on demand in recent years. A GO2/kerosene deep-throttling variable thrust rocket engine using the newly proposed gas-liquid pintle injector is presented. Three-dimensional numerical simulations are conducted to investigate the flame appearance and heat flux distribution of the engine. The SST k-omega model is used for modeling turbulence and single-step finite-rate kinetics are used for modeling combustion. A grid sensitivity study is performed to examine the validity of the simulation results. The effects of total mass flow-rate, kerosene droplet injection velocity and kerosene droplet size distribution on flame appearance and heat flux distribution of the combustor are studied in detail. Three entirely different flame appearances are observed and it correspondingly causes different heat flux distributions. Details of the flow field structure and mass fraction contour inside the combustor are discussed to explore the cause of heat flux distribution. The work will provide a reference for the design of thermal protection system of the variable-thrust rocket engine operating with a wide range of space.

Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow

The stability limit of a supersonic ethylene jet flame in a fuel-rich hot coflow was examined by investigating the influence of the injection pressure, which was varied from 2.0 atm to 4.5 atm, and of the equivalence ratio of the coflow, which was varied from 1.2 to 1.6. The flames were investigated with time-resolved chemiluminescence and schlieren images, as well as a large-eddy simulation of combustion. The results show that, with increasing injection pressure, the flame state changes from stable to unstable and blow-off, and the flame brush thickness, heat release, and height of coflow decrease. The flame stability limits decrease as the equivalence ratio of coflow increases. Lastly, a large-eddy simulation was performed to investigate the mechanism of flame stabilization, and the numerical simulation results are in good agreement with the experimental results. It was found that the stability of a supersonic flame is affected by the chemical time scale and flow time scale.

Numerical and Theoretical Evaluations of a Full-Scale Compartment Fire with an Externally Venting Flame

Understanding of the physics and mechanisms of externally venting flames from enclosure fires with parallel sidewalls at the opening is fundamental to investigating fire spread of U-shape building facade. This work aims to identify the impact of separation distance of sidewalls on facade flame height, heat flux and sustained internal combustion. A series of numerical simulations is conducted in a full-scale compartment with 3.6 m in length/width and 3.1 m in height. An external facade wall is measured 6 m in height and 3.6 m in width. A pyrolysis surface with an area of 3.2 × 3.2 m2 is placed in the middle of the enclosure with a theoretical heat lease rate varying from 1 to 8 MW. The semi-empirical correlations, derived from a reduced cubic enclosure, are evaluated for a full scale facade fire for identifying the flame height dependence on the separation distance of sidewalls. It is found that an enhanced buoyancy-induced flow between sidewalls leads to an increase of the flame height by a factor of 50% and a rise of the peak of radiation heat flux by a factor of 20% as compared to a flat facade fire. The height of externally venting flame is sensitively reduced by a factor of 40% to 50% with an increase of the normalized sidewalls distance by opening width up to 3. The predicted flame height is more pronounced with an increase of flame height by a factor of 40% for a full scale facade than for a small scale one due to turbulence development. The predicted internal HRR is in quantitative agreement with the one from the correlation for a global equivalence ratio below 3. The predicted temperatures are in good agreement with the experimental data except in the intermittent-like regions where an over-prediction of the temperature by a factor of 50% is found due to a reduction in accuracy of LES combustion model.

Numerical prediction of the Flame Describing Function and thermoacoustic limit cycle for a pressurised gas turbine combustor

ABSTRACTThe forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements.

Effect of Oxygen on the Burning Behavior of Liquid and Solid Fuels in a Large-Scale Calorimeter

The purpose of this experimental study was two-fold: first, to explore and understand the effects of oxygen availability on the combustion of liquid and solid fuels; second, to provide data for comparison with CFD models. Experiments were conducted in the controlled-atmosphere calorimeter of IRSN, called CADUCEE, varying the oxygen concentration in the oxidizing stream and the size of the fire. Polymethylmethacrylate (PMMA) and heptane were used as fuels. Results are found to be in good agreement with the literature data. As the oxygen level decreases, the mass loss rate and flame heat feedback decrease, as well as the flame height and maximum flame temperature, for both fuels whatever the sample size. For heptane pool fires, temperature measurements in the liquid layer reveal a decrease in heat transfer at the fuel surface and inside the fuel with the oxygen molar fraction. For PMMA, the radiative and convective contributions to the total heat flux remain nearly constant, with about 65% and 35% respectively, regardless of sample size and oxygen concentration.

Nonlinear mode transition mechanisms of a self-excited Jet A-1 spray flame

Low-frequency combustion instabilities arise in an aero-engine gas turbine combustor under idle/sub-idle conditions, particularly when pilot-mode non-premixed combustion is sustained. Despite the fundamental importance of such instabilities, little is known about how they are initiated and grow in the system. Here, we present nonlinear mode transition processes using several analysis methods, including Fourier/Hilbert transforms, phase portraits, spectrograms, low-order analytic modeling, and high-speed flame visualization methods. Our results demonstrate that a non-premixed Jet A-1 spray flame yields an intermediate-amplitude, quasi-periodic L1 mode oscillation at 50 Hz for a low pilot equivalence ratio, and the frequency gradually increases with increasing fuel flowrates. Subsequent to a critical point (103 Hz), the system undergoes a discontinuous mode transition, giving rise to the formation of an extremely large pressure oscillation with an L2 mode structure at 244 Hz. Several key triggers were found to induce the mode shift: (i) generation of a high intensity pulse in the flame's heat release rate due to the spontaneous ignition of unburned reactant mixtures, (ii) emergence of a large-scale vortical structure and its interaction with a partially premixed flame front, and (iii) development of self-sustained limit cycle oscillations driven by periodic convection of a hot spot – a mechanism known as entropy wave propagation. Our study identifies the occurrence of a large-amplitude peak followed by a local minimum intensity, analogous to the activation energy concept, as an essential step for entry into a new state with large-amplitude limit cycle oscillations.

A novel reactivity index for SI engine fuels by separated weak flames in a micro flow reactor with a controlled temperature profile

The reactivities of n-heptane, n-pentane, PRF50, PRF80, iso-pentane and iso-octane were investigated by separated weak flames in a micro flow reactor with a controlled temperature profile at a pressure of 500 kPa. At this elevated pressure condition, low-temperature reactions in the cool flame and blue flame were enhanced. High temperature reactions in the hot flame became weaker and independent of the base fuel as they mainly consist of CO and hydrogen–oxygen reactions. By conducting simulations, it was found that the ratio of heat that is released in the blue flame to the total released heat is a measure that well describes the reactivity of a given fuel. This ratio was named the Blue flame Heat Contribution Index (Blue flame-HCI). It is primarily governed by the molecular structure that controls the low-temperature isomerization reaction RO2 ⇌ QOOH. This step is important, as the QOOH, in turn, reacts with O2 to form OOQOOH, which decomposes to keto–hydroperoxide and OH, where the keto–hydroperoxide immediately dissociates and releases another OH. This formation of two additional OH radicals controls the chain branching at low temperatures. As straight-chain alkanes have more pathways for the isomerization reaction to form QOOH, n-heptane and n-pentane showed higher reactivity than the branched iso-pentane and iso-octane. In the experiments, formaldehyde (CH2O) concentrations were measured in the cool flame to evaluate the fuel reactivity using the newly introduced FormAldehyde Index (FAI). The FAI and the Blue flame-HCI were in good agreement with each other and described well the fuel reactivity. Furthermore, the FAI showed a linear correlation with the critical compression ratio as the reactivity of a given fuel is governed by low-temperature chain branching that is initiated by the isomerization reaction.

Synergistic Flame Retardant Effect of an Intumescent Flame Retardant Containing Boron and Magnesium Hydroxide.

In this study, to develop an organic/inorganic synergistic flame retardant and to reduce the dosage and cost of flame retardants, organic/inorganic synergistic flame retardants, hexakis(4-boronic acid-phenoxy)-cyclophosphazene (CP-6B), and magnesium hydroxide (MH) were chosen. The flame retardant properties of CP-6B/MH in epoxy resin (EP) were discussed. EP/CP-6B/MH had better flame retardancy and heat resistance compared with EP/CP-6B and EP/MH. A limiting oxygen index of EP/3.0%CP-6B/0.5%MH of 31.9% was achieved, and vertical burning V-0 rating was achieved. Compared with EP, the cone calorimeter dates of EP/CP-6B/MH decreased. CP-6B/MH inhibited combustion and did little to damage mechanical properties. Besides, the flame retardant mechanism was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and pyrolysis–gas chromatography–mass spectrometry. CP-6B/MH exerted good synergistic effects.

Embedded flame heat flux method for simulation of quasi-steady state vertical flame spread

Flame spread has been an important but unsolved problem for decades as its numerical solution requires simultaneous accurate solutions in solid and gas phases. In the current study, a numerical model involving gas-phase Large Eddy Simulation (LES) and comprehensive solid pyrolysis model was built as the first step of the research with a sufficient resolution for reproducing the flame spread rates in a set of upward flame spread experiments on birch rods. In the current conditions, the flame spread is dominated by the convective heat transfer and the predicted spread rates are thus sensitive to the heat transfer model. The convective and radiative heat fluxes during steady state were then extracted from the model results and used to improve the accuracy of coarse mesh simulations by embedding them as thermal boundary conditions for the solid phase. Three alternative techniques were investigated: Normalizing the heat flux profile with the length of the pyrolysis zone (fully coupled technique) resulted in accurate spread rates only if the initial pyrolysis zone was specified precisely and the gas phase resolution remained sufficiently fine. In the second, uncoupled technique, the length of the pyrolysis zone was fixed. Accurate steady state flame spread rates were then achieved regardless of the mesh resolution and the initial length of the pyrolysis zone. Finally, the most promising technique involved a normalized heat flux profile within the pyrolysis zone and a generalized heat flux function within the preheating zone. Correct spread rates were achieved with up to ten times increase in cell size regardless of the initial pyrolysis zone properties.

Investigation on building eave effect of fire-induced ejected plume from a room window and its heat flux imposing upon the facade wall

This paper investigates building eave effect of fire-induced ejected plume from a room window and its heat flux imposing upon the facade wall. Based on a 1:8 cubic model and Froude modeling, a series of reduced scale experiments are set up. The window dimensions and total heat release rate due to a room fire are changed during experiments. The eave upon the facade wall, made of noncombustible fiber board is installed horizontally above the window top, varying with different vertical heights (distance of the eave to the window top) and lengths (direction normal to the facade wall). The facade flame height is recorded by a CCD camera, while the heat fluxes imposing upon the facade wall are collected by five water-cooled heat flux gauges. Results show that the ejected flame with horizontal eave can be distinguished into the “free flame stage”, the “flame horizontal spread stage” and the “flame overflow stage”. The visible flame height and heat flux within the “flame overflow stage” are detailed discussed, revealing the physical mechanism of the vertical height and length of the horizontal eave effects. Finally, an implicit correlation in regarding with the visible flame height, the updated flame extension area beneath the horizontal eave as well as the vertical height of the horizontal eave to the window top is established. Taking the horizontal eave conditions into consideration, the heat flux correlation is well corrected and proposed. The results would provide theoretical basis for the fire-induced thermal protection in high buildings.

Twist-direction-dependent orbital angular momentum generator based on inflation-assisted helical photonic crystal fiber.

A novel twist-direction-dependent high-order orbital angular momentum (OAM) generator based on an inflated helical photonic crystal fiber (IHPCF) was demonstrated by use of an inflation-assisted hydrogen-oxygen flame heating technique. Compared with the helical photonic crystal fiber, the IHPCF exhibits a perfect transmission dip without distinct splits, thus generating high-quality OAM±6 modes. The helical phase of the generated OAM modes is dependent on the twist direction of the IHPCF and independent of the polarization state of the input light. In addition, the polarization state of the generated OAM modes is the same as that of the input light.

An experimental study of the detailed flame transport in a SI engine using simultaneous dual-plane OH-LIF and stereoscopic PIV

Understanding the detailed flame transport in IC engines is important to accurately predict ignition and combustion phasing, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport in technical systems are required to guide model development and validation.This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for spark-ignition engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV are used to acquire 3D measurements of unburnt gas velocity (U⇀gas), flame displacement speed (SD) and overall flame front velocity (U⇀Flame) during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed, stoichiometric isooctane-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, SD exhibits and inverse relationship with flame curvature and a strong correlation between negative SD and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in context of IC engines. Flame wrinkling is more severe at the higher RPM, which results in a broader SD distribution towards higher positive and negative velocities. Spatially-resolved distributions of U⇀gas and SD are presented to describe in-cylinder locations where either convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings provide a first insight into the detailed flame transport within a technically relevant environment and are designed to support the development and validation of engine simulations.

Synchronous full-field measurement of temperature and deformation based on separated radiation and reflected light

Optical method has been playing an important role in measuring temperature and deformation at elevated temperatures. Here we propose a simple and effective iterative algorithm to realize accurate and simultaneous measurement of the full-field temperature and deformation of specimens subjected to flame heating at temperatures up to 1000 °C. The algorithm separates the optical images of specimen surface into radiant and reflected parts. The radiant part is used to calculate the full-field temperature by an improved two-color method, and the reflected part is used to obtain the deformation of the specimen surface based on high-temperature digital image correlation method. Experimental validation shows that the proposed method can effectively eliminate the mutual interference between the radiation and reflected light, displaying great potential for synchronous measurement of temperature and deformation with sufficient accuracy by using one camera.

Effects of heat release and imposed bulk strain on alignment of strain rate eigenvectors in turbulent premixed flames

The impact of combustion heat release and bulk compressive strain on local alignment between the flame front normal and strain rate eigenvectors (principal strain rates) are investigated using simultaneous laser-induced fluorescence imaging of OH and tomographic particle image velocimetry in turbulent premixed CH4/O2/N2 counterflow flames with Karlovitz numbers (Ka) of 1.3 and 2.7. The bulk strain rate imposed by the counterflow introduces a preferential alignment of the most compressive principal strain rate, s3, with the flame normal, n. Dilatation induced by heat release acts as a competing mechanism that promotes the alignment of the most extensive principal strain rate, s1, with n. In the counterflow flames, the preferential s3-n alignment prevails and remains dominant across the entire flame. This alignment stands in stark contrast to observations from previous studies in turbulent Bunsen flames or flames in isotropic turbulence, indicating the significance of bulk strain rate in determining local strain-flame alignment. The effects of increasing turbulence intensity on strain rate-flame front alignment are twofold; on the one hand, turbulence diminishes the s3-n preferential alignment that is associated with the bulk strain field by increasing flame surface wrinkling and reducing the tendency of the flame front normal and s3-eigenvectors to align with the axis of the counterflow. On the other hand, turbulence reduces the impact of heat release and enhances preferential s3-n alignment approximately 1 mm ahead of the flame front, reflecting the characteristic alignment of compressive strain and scalar gradients in turbulent non-reacting flows. The effects of bulk strain are also observed in strain rate alignment statistics based on the fluctuating velocity fields, although the impact is less pronounced than for the statistics based on the full velocity fields.As a result of this complex interplay between heat release, turbulence, and bulk strain rate, the flame-tangential strain rate is on average extensive, and the flame-normal strain rate is dominantly compressive except for an approximately 0.8 mm wide region near the flame front where it is extensive due to dilatation. The compressive bulk strain in the counterflow was also shown to compress the length scales over which the strain rate and its alignment are affected by flame heat release. The present finding is important for developing turbulent flame models to accommodate the effect of bulk strain rate that is inherently associated with practical burner geometries, and the length scale dependence of the bulk strain effect could be a consideration for determining the cutoff scale in the context of large-eddy simulations.

Factors influencing the fire-resistance of epoxy compositions modified with epoxy-containing phosphazenes

The fire-resistance (State Standard GOST 28157-89, analogue of UL-94 test) of epoxy compositions based on D.E.R.-330 resin, isomethyltetrahydrophthalic anhydride and new epoxy-containing aryloxycyclotriphosphazenes was studied. Thermogravimetric analysis and microstructural studies of the coke residue formed during combustion were carried out. It has been determined that resistance to combustion of the cured compositions increases significantly as grows phosphazenes content in them. This is due both to the increase in the amount of porous coke residue formed during combustion (which acts as a barrier to flame propagation and heat transfer from the flame to the sample), and to the increase in the size of the pores formed therein. The obtained data can be used to create tough and flame-resistant composites for microelectronics, aviation and other industries.

The role of precessing vortex core in two combustion regimes: Numerical simulation studies

Large Eddy simulation (LES) with finite rate chemistry was used to investigate the combustion dynamics in a lab-scale PRECCINSTA combustion chamber. Transient three dimensional numerical simulations were carried out at two different thermal powers (10 kW and 35 kW) with a fixed equivalence ratio of 0.7. The predicted results were compared with the experimental data and good agreements were found between them. In the cold flow field under both conditions, a precessing vortex core (PVC) in the inner shear layer (ISL) existing between the swirling jet and the inner recirculation zone (IRZ). However, two different flow and combustion dynamics were observed when combustion occurred. At thermal power of 10 kW, there was a V-shaped flame and the combustion of the flame was stable. The PVC disappeared and the vortices arrangement was symmetrical in the ISL. However, at 35 kW, there was a M-shaped flame with a PVC in the ISL and combustion instability triggered. In depth analysis of the characteristics of flow, temperature and heat release field, we found that the flame surface was wrinkled periodically by the PVC which enhanced the mixing between the cold fresh gas and hot burned products. Then, the mixture was ignited locally and heat release was rapid in the middle of the combustion chamber. These effects were directly related to the periodic vortices motion which was induced by PVC. It was confirmed that the influence of PVC on flame surface and heat release is an important factor for triggering the combustion instability at thermal power of 35 kW. The zone division based on different roles of flow/flame and thermoacoustic coupling was also discussed to illustrate the combustion instabilities caused by PVC.

Flame Structures and Heat Transfer Characteristics of an Impinging Flame on Ammonia Combustion

Flame Structures and Heat Transfer Characteristics of an Impinging Flame on Ammonia Combustion

Characteristics of Flat-Wall Impinging Spray Flame and Its Heat Transfer under Small Diesel Engine-Like Condition. 3th Report: Effect of Oxygen Concentration

To investigated heat transfer phenomena inside the combustion chamber wall in diesel engines, we investigated the effects of spray/flame impingement on transient heat flux to the wall. By using a constant volume vessel under engine-like condition, surface heat flux of the wall at spray/flame impingement was measured with three thin film thermocouple heat flux sensors. Fuel was injected using a single-hole injector with a 0.133 mm diameter nozzle. Under these conditions, spray evaporates, then burns near the wall. In order to investigate the relation between diesel flame and wall heat loss, two colour-method was applied to observe local flame temperature distribution. The effect of oxygen concentration on the heat transfer was investigated parametrically. The results showed that the ratio of oxygen concentration influenced the combustion behaviour, which has significant effect on flame temperature and flame contact area that gave rise to further heat loss on the wall.