Steady Flame Research Articles

Silicon alloying enhances fast heating rate combustion of aluminum particles

Alloys and composites of aluminum (Al) have shown promise in regulating and enhancing particle combustion for energy generation applications. Recent work on aluminum-silicon (Al-Si) spherical alloy particles has shown improved combustion at low heating rates through enhanced diffusion accompanying a lower melting temperature. This study extends reactivity analysis to higher heating rates, comparing oxidation of Al-Si with Al. Flame speeds of Al-Si powder mixed with molybdenum trioxide (MoO3) powder (Al-Si+MoO3) exhibited a faster transition to steady propagation relative to Al+MoO3. Bomb calorimetry experiments revealed up to 5.8% greater early temperature rise for Al-Si powder. Rapid steady propagation for Al-Si mixture and faster temperature rise for Al-Si particles were attributed to accelerated kinetics evidenced in thermal equilibrium analysis of the mixtures using a differential scanning calorimeter (DSC). Larger DSC exotherms in the early stages of oxidation (i.e., 480–720 °C) correlated with early heat release and promoted steady flame propagation for Al-Si+MoO3 compared with Al+MoO3. Furthermore, in multiple heating rate DSC studies, Al-Si+MoO3 ignited at a lower heating rate (i.e., 15 ⁰C/min) than Al+MoO3 (i.e., 20 ⁰C/min). Both mixtures ignited at temperatures less than 625 °C which is above the melting temperature of Al-Si (574 °C) and below Al (659 °C). Thus, alloy particle fuels with enhanced diffusion-controlled kinetics promote steady flame propagation and show promise for energy generation applications. This is especially promising for technologies driven by enhanced condensed phase combustion such as some propellant additives or applications dependent on reliable burn rates such as pyrotechnic time delay formulations and primers.

Effects of curvature on flame acceleration in micro channels

The effects of channel curvature on the flame propagation are examined for premixed flames in micro channels. Previous studies have shown that curved channels can enhance the flame stretching, which can significantly increase the flame propagation speed. In this study, the effect of flame stretching was excluded by the narrow channel limit in order to study the effects of channel curvature on the flame propagation. An asymptotic approach was first extended to include the effects of channel curvature. Accordingly, the two-dimensional problem was reduced to an effective one-dimensional problem with a clear physical meaning of curvature. A parametric study of curvature was then performed to investigate its effects on the flame propagation. It was found that curvature inhibits both the convection and diffusion processes. An increase in the curvature results in a significantly reduced flame thickness and a significantly increased reaction rate. Furthermore, the initial flame propagation speed is smaller in the channel with a larger curvature, but its growth rate is higher under the same ignition condition. With a large channel curvature, the flame has a smaller propagation speed of the steady compressibility-driven flame and a shorter distance to reach the steady state.

Effects of curvature on soot formation in steady and unsteady counterflow diffusion flames

A numerical study has been conducted in order to understand the effects of flame curvature on soot formation in laminar non-premixed flames. For the fundamental understanding, the canonical configuration of a counterflow diffusion flame is employed as it exhibits the essential combustion physics associated with non-premixed flames. Numerical results for the steady counterflow ethylene flames confirmed that the response of soot in curved flames is governed by the intricate coupling between flow convection, soot kinetics, and differential transport of soot impacting the rates of soot formation sub-processes. The sensitivity of the soot formation in curved flames to the strain rate variation is also analyzed to understand their competing effects. Furthermore, the dynamic response of soot formation to the unsteady curvature is investigated by imposing harmonic oscillations to the curvature. Numerical results revealed that for an increase in the frequency of curvature oscillations, the soot formation gets attenuated and amplitudes of the induced oscillations exhibit large phase-shift with respect to imposed fluctuations. The higher characteristic time scales of larger-sized particles proved to control the overall dynamic response of soot under unsteady fluctuations of curvature. To examine the dynamic response of soot formation under a more complex case of flow-flame-soot interaction, the unsteady analysis of flames is extended by subjecting them to the simultaneous strain rate and curvature oscillations. The correlation between imposed frequencies and variability of soot formation response under fluctuating strain rate and curvature is elaborated.

Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.

A nonlinearly kinematic model of the asymmetrically turbulent premixed slit flame subjected to two-way harmonic disturbances

Transversal and longitudinal combustion instabilities typically occur in gas turbines and rocket engines. Quantifying the nonlinear flame dynamic response of turbulent premixed flames subjected to the two-way disturbances and mean bulk flows is significant to understand the complex nonlinear combustion instability phenomena. A Two-way Nonlinear Unforced Flame Speed Modeling (2W-NUFSM) approach was proposed in this study. The turbulent effects were accounted for in the flame propagation speed term and the steady flame term in the kinematic equation of the flame front; an expansion method was then used to obtain the nonlinear solutions corresponding to the transverse and longitudinal acoustic disturbances. The turbulent effect on the flame consumption speed was also considered in the heat release rate calculation of the perturbed flame. Validations of this approach were conducted by comparing its results to the former credible results obtained from other methods and good agreements were obtained. The influence of turbulent effects on the flame dynamic response in an asymmetrical premixed flame was investigated in detail. The coupling effect of the turbulence and harmonic flow disturbances on the flame nonlinear dynamic response was also quantified.

Study on combustion characteristic of premixed H2/C3H8/air and working performance in the micro combustor with block

Combustion characteristics and thermal performance of premixed H2/C3H8/air in a micro planar combustor with block insert are experimentally and numerically tested. Effects of hydrogen blended fraction, equivalence ratio, flow rate and block width on the flame stability and heat transfer are discussed and analyzed. The results indicate that hydrogen addition is beneficial to the combustion stability in the micro combustor, and the lowest hydrogen blending mole fraction of the premixed H2/C3H8/air steady flame is obtained by experimental study. The combustion stability range is the widest in the combustor with a 0.7 mm block inserted at a high fuel rate. Moreover, the wall temperature is deeply influenced by hydrogen blending ratio and block size, that is, the combustor with appropriate block width and H2/C3H8 blended burning gains a higher radiation temperature. Then, effects of the hydrogen fraction on the reaction pathways are analyzed, and the fluxes of OH and H production by the key reactions such as H + O2 = OH + O and H2 = OH + H are calculated, where flame location and reaction area are significantly affected. It also finds that the combustor C0.7 fueled with 90% hydrogen gains the highest mean wall temperature of 1150.3 K and radiation power of 27.9 W under the same chemical energy input Ec = 90.6 W.

Flame propagation mode transition of premixed syngas-air mixtures in a closed duct

Knowledge regarding the transformation of the flame propagation dynamics is of importance to predict the safety issues of explosion disasters. The current work experimentally investigated the premixed syngas-air flame propagating in a 2 m duct with a set of obstacles, focusing on the flame propagation mode transition with the change of hydrogen volume fraction (φ). Three modes were identified as the φ increases: steady flame propagation, oscillating flame propagation, and end-gas autoignition. The steady flame propagation mode occurred when φ = 0, where the slow oxidation of pure CO determines its propagation feature. The flame became unsteady and oscillated violently with a slight addition of hydrogen. Flame oscillation resulted from its interaction with pressure wave, and the strength of the pressure wave directly impacted the severity of oscillation. The autoignition appeared when φ ≥ 0.4, resulting from the secondary reflected pressure wave interacted with the region that the thermodynamic state was highly disturbed. What is more, the autoignition kernel migrated from the region ahead of the flame front to the end-wall region when φ ≥ 0.6. The reason is that syngas with higher φ has a lower temperature threshold for explosion and a shorter ignition delay.

A technique to establish liquid ethanol tubular combustion by dual swirl

This study proposes a compact dual swirl combustor to establish a steady liquid ethanol tubular flame, in which tangential air is injected through two stages. The outer intense swirl induces reversed flame on the surface wall of a concentric copper tube, and the central liquid fuel is heated to vaporize through the inner swirling air inside the copper tube. By enhancing the heat transfer process and promoting the mixing of the reactant, the overall heat output approaches 10 kW, much higher than the original combustor. With temperature measurement and heat transfer analysis, the flame stabilization mechanism has been elucidated. Two kinds of typical flame structures, a tubular flame and an anchored conical flame, have been obtained under various operating conditions. As the flow rate of inner air gradually increases, plenty of the heat aiming to vaporize the liquid fuel is taken off by the air flow, and small droplets occasionally appear downstream the nozzle; furthermore, the global equivalence ratio (Φgl) at the nozzle exit gradually decreases, and an anchored flame appears, which interrupts the fuel mixing with outer swirling air, resulting in soot formation in some cases. Plenty of tests illustrates that as Φgl is below 11, the flame anchoring will appear.

3D simulations of spinlike flames in Co/Al multilayers with enhanced conduction losses

Reactive Co/Al multilayers are uniformly structured materials that may be ignited to produce rapid and localized heating. Prior studies varying the bilayer thickness (i.e., sum of two individual layers of Co and Al) have revealed different types of flame morphologies, including: (a) steady/planar, (b) wavy/periodic, and (c) transverse bands, originating in the flame front. These instabilities resemble the “spin waves” first observed in the early studies of solid combustion (i.e., Ti cylinder in a N2 atmosphere), and are likewise thought to be due to the balance of heat released by reaction and heat conduction forward into the unreacted multilayer. However, the multilayer geometry and three-dimensional (3D) edge effects are relatively unexplored. In this work, a new diffusion-limited reaction model for Co/Al multilayers was implemented in large, novel 3D finite element analysis (FEA) simulations, in order to study the origins of these spinlike flames. This reaction model builds upon previous work by introducing three new phase-dependent property models for: (1) the diffusion coefficient, (2) anisotropic thermal conductivity tensor, and (3) bulk heat capacity, as well as one additional model for the bilayer-dependent heat of reaction. These novel 3D simulations are the first to predict both steady and unsteady flames in Co/Al multilayers. Moreover, two unsteady modes of flame propagation are identified, which depend on the enhanced conduction losses with slower flames, as well as flame propagation around notched edges. Future work will consider the generality of the current modeling approach and also seek to define a more generalized set of stability criteria for additional multilayer systems.

Fast and Calibration-Free Temperature Imaging of Dynamic Flames via Wavelength Modulated Lateral Shearing Interferometry

Temperature distributions of dynamical flames were measured by using a proposed calibration-free wavelength modulated lateral shearing interferometry. A distributed feedback (DFB) laser was modulated in the wavelength as a sawtooth profile. The wrapped flame phase in the images was obtained pixel by pixel by calculating the phase shift between the interference light intensity profiles before and after flame ignition. Phase unwrapping and recovery were adopted to solve the flame phase distribution. The refractive index distribution was reconstructed with the help of Inverse Abel transform. The temperature imaging was further achieved by the ideal gas law and Gladstone-Dale model. Numerical simulations were conducted for performance evaluations. The flame phase, refractive index and temperature distributions were reconstructed with errors of 0.0094, 0.0154 and 0.0958, respectively. Standard deviations were in the magnitude of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> at the presence of Gaussian random noises and showed the noise immunity of the proposed method. In actual experiments, reconstructed temperature distribution of a steady flame generated on a Bunsen burner agreed well with the thermocouple measurements. A dynamical process of flame extinguishment was portrayed at one kilo frames per second. Temporal variation of the flame temperature distribution well reflected the extinguishing process. The entire measurement process did not require prior calibration of the modulated wavelength, and the imaging speed was more than ten times faster than the existing phase modulated interferometer. The proposed method can be used for on-line evaluation of dynamic flames.

Effects of thermal expansion, strain rate, and gravity force on the steady propagation of a premixed flame in semi-closed channels

The flame dynamics of steady propagation of a premixed flame in semi-closed channels is investigated. The hydrodynamic model takes into account impacts of channel width, thermal expansion, straining effect, and downward gravity force on the flame front. The steady flame shape and its velocity field could be determined by numerical iteration of a weakly nonlinear front equation and linearized governing equations with Fourier transform, respectively. Results reveal that the introduction of thermal expansion and gravity force enhance the flame curvature and increase the burning velocity, while the straining effect suppresses the flame bending processes. In addition, a non-monotonic dependence of the burning velocity on the Peclet number is further demonstrated. It reveals that the thermal expansion and strain effect would have impacts on the burning velocity as well.

A layer-peeling method for signal trapping correction in planar LII measurements of statistically steady flames

This work presents a layer-peeling (LP) algorithm to correct the signal trapping effect in planar laser-induced incandescence (LII) measurements of soot volume fraction. The method is based on measurements of LII signals captured by an intensified CCD camera at a series of parallel planes across a diffusion flame. A method based on presumed function (PF) of soot volume fraction is also proposed for comparison. The presented methods are numerically tested based on synthetic LII signals emitted from a simulated axisymmetric laminar diffusion flame using the CoFlame code. Numerical results showed that the LP method is able to correct the signal trapping effect, even for fairly large optical thicknesses and in a wide range of detection wavelengths. The correction decreases the relative errors induced by neglecting the trapping effect considerably. The signal trapping effect correction is less important for the determination of integrated soot quantities such as radially integrated soot volume fraction or total soot loading. Planar LII measurements were carried out and calibrated in order to test the method experimentally in a coflow flame. The LP, PF and a simplified analytical (SA) model were compared. The results indicate that the differences in soot volume fraction of 1 ppm or about 15% are obtained in zones of maximum soot loading of 6.5 ppm when the trapping effect is accounted for. Also, the LP and SA methods were found computationally efficient and accurate compared to the PF method. Although the study was performed in a canonical laminar axisymmetric flame, the proposed method can be applied to any statistically steady 3D flame.

An empirical characteristic scaling model for freely-propagating lean premixed hydrogen flames

Lean premixed hydrogen flames are thermodiffusively unstable due to the high mobility of the fuel, which leads to localised acceleration and thinning of the flame. Consequently, the one-dimensional steady unstretched laminar flame speed and thermal thickness are not representative of multi-dimensional flames, and the extent of the disparity is strongly dependent on reactant conditions. This paper presents an empirical model to predict local as well as characteristic freely-propagating values of flame speeds and thicknesses that can be evaluated from one-dimensional simulations (in Cantera for example). It was found that the thermodiffusive response was well characterised in terms of the second-order instability parameter (ω2) that arises from classical linear stability analysis. This instability parameter depends strongly on Zel’dovich number, and presents non-monotonic behaviour in pressure/temperature/equivalence-ratio space. In particular, there is a surface where ω2 attains a local peak, and different characteristic correlations are found either side of the surface. Specifically, the thermodiffusive response (over the range of ω2 considered) is stronger and more unpredictable (greater uncertainties) on the higher-pressure side of this peak surface. The empirical model is inferred from a large dataset of two-dimensional freely-propagating flames over a broad range of reactant conditions. PDFs of local flame speed and thickness are used to define freely-propagating characteristic values as the corresponding mean surface value, implicitly supplying a mean local burning enhancement factor I¯0. Local flame speeds are then correlated with curvature and strain rate through JPDFs to identify an appropriate Markstein number. The resulting model consists of expressions for characteristic flame speeds, thicknesses and Markstein numbers in terms of the instability parameter ω2, all of which can be evaluated based solely on inexpensive one-dimensional calculations.

Reactive Structures of Ammonia MILD Combustion in Diffusion Ignition Processes

Reactive structures have been analyzed, when ammonia is used as a fuel, in a steady 1D counterflow diffusion flame layer, mimicking diffusion ignition processes. The characterization has been carried out in a wide range of feeding parameters under Moderate or Intense Low-oxygen Dilution (MILD) combustion conditions. Both the Hot-Fuel-Diluted-Fuel (HFDF) and Hot-Oxidant-Diluted-Fuel (HODF) configurations were studied to analyze the main effects of the inlet feeding conditions on the oxidative structures. The reaction zone has been analyzed in terms of temperature and heat release profiles in the mixture fraction space for various ranges of inlet parameters, using a standard code and a validated chemical kinetic scheme. Several features of the reaction zone have been recognized as reported also in previous works for hydrocarbon flames. They were used as discriminative for the achievement of various combustion regimes. In particular, the flame thickening process and the absence of correlation between the maximum heat release and the stoichiometric mixture fraction were analyzed to build maps of behaviors. The latter were reported on an inlet preheating level-temperature increase plane for fixed values of the bulk strain rate and system pressures. Another relevant feature previously reported with hydrocarbons in the literature, in Hot Diluted Diffusion Ignition (HDDI) processes under MILD conditions, was the pyrolysis depression. The latter characteristic has been not observed when ammonia is used as a fuel, for the operative conditions here investigated. Indeed, the heat release profiles do not show the presence of negative heat release regions. The results obtained for the HFDF configuration are strongly dependent on the system pressure level. Finally, the HODF condition has been also analyzed for ammonia at the atmospheric pressure. Boundaries of the combustion regimes and reactive structure features showed several differences between HFDF and HODF cases with respect to the inlet parameters.

Impact Assessment of Flammable Gas Dispersion and Fire Hazards from LNG Tank Leak

This study conducts an impact assessment of flammable gas dispersion and fire hazards from LNG tank leak. The release source model is used to estimate LNG release rate. A CFD (computational fluid dynamics) based 3D model is established to simulate dispersion behavior of flammable gas from the phase transformation of LNG. Subsequently, a FDS (fire dynamics) based model is built to simulate the pool fire due to LNG tank leak. The impact of gas dispersion and fire on personnel and assets is assessed based on simulation results, which can provide a theoretical basis and method support for major accident assessment of tank leakage in large LNG receiving station. The results show that the dispersion of flammable gas from LNG tank leak has an obvious stage characteristic. The flammable gas reached a steady state around 300 s, and the corresponding coverage area is about 16250 m2. The pool fire simulations indicate that the steady flame is formed at 20 s. The flames flow along the wind, and the maximum temperature of the fire reaches 670°C, and the maximum thermal radiation reaches 624 kW/m2. According to the fire damage criteria, the pool fire from LNG tank leak may pose a serious threat on the safety of adjacent assets and personnel.

Numerical study of flame spread in a narrow flow duct in microgravity – effects of flow confinement and radiation reflection

A three-dimensional numerical study is performed to investigate concurrent-flow flame spread over thin solid fuels in microgravity. The model considers the burning scenarios of a recently concluded ISS microgravity experiment, Confined Combustion. Cellulose based thin samples are burned in a small flow duct. The height of the flow duct and the radiation reflectance of the duct wall are varied. Flame development and steady spread flame characteristics are compared with the experimental results at various duct heights. The numerical results demonstrate that the confinement imposed by the duct walls accelerates the flow during the combustion thermal expansion, enhancing the conductive heat transfer to the solid samples. When the duct height is below a critical height, the flow confinement limits oxygen supply to the flame, and the duct wall acts as a conductive heat sink. As a result of the interplay of these effects, the flame spread rate and pyrolysis length first increase and then decrease as the duct height decreases. Eventually, the flame fails to spread at a quenching duct height. In addition, side-leading concave (two-teeth fork shaped) flames are observed below the critical duct height. This flame shape increases the flame surface area and facilitates oxygen transport to the combustion zone. When the duct wall reflectance varies, a higher reflectance yields a longer pyrolysis length and a faster spread rate. This is due to enhanced heat input to both the solid sample surface and the gaseous flame. This effect is most significant for medium duct heights. At large duct heights, the duct wall is far from the flame and the sample. At small duct heights, while flame spread rate increases with the wall reflectance, the pyrolysis and flame length remain similar as combustion is limited by oxygen supply.

Pore-scale flame dynamics in a one-layer porous burner

Novel design of one-layer porous burner for experimental studies of the flame front dynamics at pore scale level is proposed. Results of the experimental and numerical studies of non-stationary flame front behavior at pore scale are discussed and compared at a qualitative level. The results demonstrate that upstream propagation of the combustion wave can be accompanied by the oscillations of two different types. The first type is the FREI-like pulsations, which mechanism is identical to the flames with repetitive extinction and ignition in narrow channels with external heating. The second type is the small-amplitude oscillations, which, apparently, have the same nature as the transitional oscillatory combustion mode between the steady flame and fully-developed FREI pulsations. This type of unstable flame behavior differs by existing of the flame over the entire oscillation period without extinction. It is found that these two types of flame oscillations clearly separates by oscillation frequencies and amplitudes. Typical frequencies of FREI-like pulsations are about tens of hertz, while for small amplitude oscillations it has an order of hundreds of hertz. It was shown that both types of oscillations can coexist inside the porous media at the same time but at different pore channels. Moreover, the mixed oscillations representing by fully developed FREI-like pulsations alternating with small-amplitude oscillations are frequently observed.

The thickened flame approach for non-premixed combustion: Principles and implications for turbulent combustion modeling

Modeling turbulent non-premixed combustion remains a challenge in the context of Large Eddy Simulation (LES) in complex geometries and for realistic conditions, taking into account all physical phenomena impacting the flame such as heat loss, dilution, or liquid fuel atomization and evaporation. In this work, the Thickened Flame concept, which allows to resolve the flame front on the LES grid while preserving the consumption speed, and initially derived for premixed combustion, is adapted to diffusion flames. It is demonstrated that the concept holds for these flames, with however, a different formulation of the model due to but their specific nature and properties. In particular, in the high-Damköhler regime, the thickening factor is applied only to the diffusion coefficients. The behavior of thickened diffusion flames is illustrated on laminar steady strained flames for both simple and complex chemistry, showing how the Thickened Flame concept applies. Based on these results, an expression for the thickening factor related to mesh coarsening is derived. For a complete turbulent combustion model, the thickening factor should also describe the sub-grid scale flame-turbulence interaction, which is left for future work.

Upward flame spread and self‐induced buoyant blow‐off over two‐sided thin fabric at different inclination angles

SummaryA series of upward flame spread experiments over two‐sided long thin flax fabric sheets were conducted at different inclination angles from horizontal (0°) to vertical (90°). With the inclination increased to a critical angle, the flame on the lower side of the 4 and 6 cm wide samples can be blown off spontaneously. After the lower‐side blow‐off, the remained flame on the upper side spread steadily to the end of the sample. To clarify the controlling mechanism, the flame spread rates and flame geometry characteristics were investigated and compared in different inclination angles. The maximum flame spread rate and flame length monotonically increased with the inclination angle under the assistance of self‐induced buoyancy. However, the steady flame spread rate and flame length on the remained side dropped significantly after one‐side extinction. The lower side flame was blown off by the large buoyant flow velocity induced from the critical flame length, which is controlled by the small Damköhler number. Flame thickness and flame tilt angle were analyzed at varied inclinations to help understand the heat transfer from the flame to the unburnt fuel surface. Two opposite effects of self‐induced buoyancy were proposed at different ranges of inclination: the thermal assistance regime at the lower inclination angle and the kinetical extinguishing regime at a higher inclination angle. They are dominated by the heat transfer from the flame and local fluid dynamic around the flame base, respectively.

Confined combustion of polymeric solid materials in microgravity

Microgravity experiments are performed to study the effects of confinement on the burning behavior of polymeric solid materials. Flat, 100 × 22 × 1 mm PMMA samples are burned in concurrent air flow in a small flow duct aboard the International Space Station. Three different burning scenarios are examined, double-sided, single-sided, and parallel samples. In the first two scenarios, single samples are burned on both sides and on one side, respectively. Flat baffles are placed parallel to the sample to confine the available space for combustion. The distance between the baffle and the sample (H) is varied in different tests. In each test, imposed flow is reduced in steps and steady flame spread is achieved at each flow speed until the flame quenches. The results show that at the same confined condition, steady state flame length and spread rate are proportional to flow speed over the range tested. When confinement increases (or H decreases), the flame spread rate and flame length increase first and then decrease. In addition, the quenching flow speed decreases and then increases with decreasing H. These results suggest that the confinement can increase or decrease solid fuel flammability depending on conditions. In the third burning scenario, two PMMA samples are placed parallel to each other separated by a distance H. Twin flames are observed and combustion is confined between the two samples. Among the three tested burning scenarios, twin flames have the largest flame length and burning rate at the same confinement level (H). This is because the thermal interaction between the twin flames enhances the heat feedback to the solid fuel and reduces the relative heat loss to the surrounding flow duct. Comparing single- and double-sided flames with the same baffle-sample distance, the spread rate of a single-sided flame is slightly less than half of that of a double-sided flame. This is due to the halved pyrolysis area exposed to the flame and heat loss on the back side of the sample. Optimal transport of oxygen to the flames also plays a role.