Increase In Flame Length Research Articles

Experimental and numerical study of combustion in pipe cutting operations with hydrogen-blending gas

This study investigated the flame combustion of gas-containing pipe cutting operations by combining experimental and numerical simulation techniques. Based on the empirical flame length formula and experimental and numerical simulation flame length data, a flame length formula for gas outlet with large aspect ratio was proposed. In the experimental phase, cutting experiments were conducted with cutting slit lengths of 1–11 cm for pipe pressures of 10–550 Pa and hydrogen doping ratios of 0%, 10%, and 20%. The flame lengths were then measured. A hydrogen-doped natural gas combustion model based on the vortex dissipation conceptual model and Grimech-3.0 mechanism was established to simulate the experimental conditions. The flame lengths derived from the numerical simulation were in good agreement with the experimentally measured flame lengths, with an average error of 7.02%. This indicates that the reliability of the numerical model can be verified. The effects of hydrogen doping ratio, tube pressure, and cutting slit length on flame length and thermal radiation range were investigated using the control variable method. The results demonstrated that hydrogen doping reduced flame length and flame length away from the flame. Furthermore, an increase in pipe pressure and single cut length also resulted in an increase in flame length. It was observed that under normal operating conditions, a safe intensity of thermal radiation could be maintained within the operating distance when the operator wore protective clothing.

Upslope fire spread and heat transfer mechanism over a pine needle fuel bed with different slopes and winds

The surface fire spread involved slope and wind effects are significantly important in wildland fires, while very limited attention has been paid on the heat transfer mechanism, especially for different fire line conditions. This work experimentally evaluates the effects of slope angle (0˚ to 30˚) and wind velocity (0 to 2 m/s) on spread of surface fire over a pine needle fuel bed. Flame length, flame angle, rate of fire spread, fuel consumption rate and heat flux are acquired and quantitatively analyzed. Two different fireline shapes of sheet and inverted ‘v’ are observed and assumed for different wind velocities and slope angles, respectively. The flame length, rate of spread, mass loss rate and heat flux increase with the increasing wind velocity and slope angle, while flame angle and fuel consumption efficiency vary in the opposite way. The extreme fire can be observed with the external manifestations of a steep increase in flame length, rate of spread and heat flux under conditions of larger wind velocity and slope angle, which is similar to an eruptive fire. In addition, the heat transfer of convection and radiation heat flux is quantitatively analyzed with wind velocity and slope angle to explore heat flux control mechanism. Moreover, the modified models of radiation heat flux considering fire line shape are presented, which demonstrates good effectiveness in predicting radiation heat flux by comparing the calculated values and experimental data.

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.

Effects of porosity and area density on upward flame spread characteristics over thin flax fabric

Few investigations have systematically addressed the porosity effects of upward flame spread over fabric fuels, although the porosity is a special property for fabric fuels. The present paper studies the porosity and area density effects on upward flame spreading using 160.0 cm tall and 8.0 cm wide flax fabric samples with various porosities and area densities. The flame shape, flame length, flame spread rate, ignition time, standoff distance and surface temperature distribution are obtained and analyzed. The major findings are summarized as follows: as the porosity increases and corresponding area density declines, the flame spread rate and flame length increase, whereas the ignition time decreases, which is because the oxygen can reach the fuel surface in the pyrolysis region more easily and, subsequently, the heat flux received by the virgin fuels increases. The two parameters of flame standoff distance and surface temperature in the preheating region can be applied to characterize the heat flux received by the virgin fuels. Generally, when the porosity increases and the corresponding area density decreases, the flame standoff distance and the surface temperature at the same distance from pyrolysis front increase, which reveals that the heat flux received by the virgin surface increases.

Turbulent flames investigation of methane and syngas fuels with the perspective of near-wall treatment models

The near-wall treatment models play an important role in accurate turbulent flow simulation, particularly the ones close to the boundary layer. In the study, three k−ε epsilon near-wall treatment models, such as Enhanced Wall Treatment, Standard Wall Functions, and Scalable Wall Functions, have been numerically studied to verify the experimental results of methane combustion performed by Brookes and Moss within a laboratory-scale rig. Then, the turbulent flame of methane investigated at various wall distance (y∗) ranges specifically 0 < y∗ < 1, 1 < y∗ < 11.225, 11.225 < y∗ < 30, and 30 < y∗ < 500 to determine the effects of the mesh structures and y∗ on the numerical results. Following the validation of the near-wall treatment models and y∗, the turbulent combustion flame of methane and three syngas fuels were numerically investigated at the same thermal power, 8.6 kW. H2/CO ratios of the syngas 1, syngas 2, and syngas 3 are 2.36, 1.26, and 0.63, respectively. The maximum flame temperatures inside the chamber depend on the type of fuel, fuel species, and the percentage of the species. The results of the work show that Enhanced Wall Treatment presents better prediction as compared to the others, not only at 0 < y∗ < 1 but also at the other wall distance ranges. Standard Wall Functions demonstrates approximately the same results as compared to Scalable Wall Functions. Besides, the NO emissions in the flue gas of the syngas fuels are higher than methane due to higher flame temperatures and unburned hydrocarbon species. Furthermore, increment the H2/CO ratio for syngas fuels causes an increase in flame length, diameter, and temperature as well as NO emissions.

Large eddy simulation of hydrogen piloted CH4/air premixed combustion with CO2 dilution

Large eddy simulation (LES) of constant adiabatic temperature, hydrogen-piloted, turbulent lean premixed methane/air jet flames with varying amounts of CO2 addition are reported. Constant adiabatic temperature is achieved by increasing the fuel flow rate slightly to account for the higher specific heat of CO2 compared to N2. Such flames are relevant to low NOx gas turbines with high hydrogen content fuels and Exhaust Gas Recirculation (EGR). A newly designed burner called Piloted Axisymmetric Reactor Assisted Turbulent (PARAT) flame burner was utilized. The operating conditions in the experiment were selected to highlight the kinetic effects of CO2 addition by matching the Reynolds numbers, Lewis numbers and adiabatic flame temperatures. The LES simulations utilize a finite rate chemistry solver with DRM19 combustion mechanism with adaptive zoning and a dynamic structure turbulence model. A five-level adaptive mesh refinement (AMR) improves the velocity and temperature gradient resolution. The LES predicts the experimentally observed increase in flame length with CO2 levels caused by a decrease in the turbulent flame speed. The computational results also capture the experimentally observed departure from the thin flame limit and a collapse of the root mean square (RMS) versus mean temperature profiles for the three levels of CO2. The flame structure analysis showed super-equilibrium CO concentrations because of non-equilibrium chemistry effects caused by the external addition of CO2.

Experimental study on combustion characteristics and lean blow-out limits of non-premixed oxy-methane flames in a porous-plate reactor

The combustion characteristics and lean blow-out (LBO) limits of non-premixed oxy-methane flames in a face-to-face two-porous-plates reactor are experimentally investigated in this study. The reactor mimics the process of oxygen permeation and oxy-fuel combustion inside high-temperature membrane reactors (HTMRs). The fuel (CH4) was supplied together with CO2 in the main longitudinal stream while the oxygen was supplied via the upper and lower rectangular porous plates. The experiments were conducted over ranges of equivalence ratio (0.5, 0.7 and 1.0), CO2 percentage in the oxidizer mixture (0, 27, 35, 55 and 70%), and fuel firing rate (0.5, 0.75, 1.0, 1.5, 2, 2.5 and 3 kW). At fixed CO2%, there was insignificant increase in flame length while increasing the equivalence ratio. The flame luminous intensity was found to be decreasing while increasing CO2%. Increase in the LBO limit was obtained with the increase in the equivalence ratio at fixed fuel firing rate. The minimum limit of O2% to allow for a visible flame was found to be about 21% (79% CO2) at stoichiometric condition. The increase in fuel firing rate decreased CO2% at blow-out condition due to attainment of the required flow speed that overwhelmed the increase in flame speed.

Numerical simulations of microgravity ethylene/air laminar boundary layer diffusion flames

Microgravity ethylene/air laminar boundary layer diffusion flames were studied numerically. Two oxidizer velocities of 250 and 300 mm/s and three fuel injection velocities of 3, 4, and 5 mm/s were considered. A detailed gas-phase reaction mechanism, which includes aromatic chemistry up to four rings, was used. Soot kinetics was modeled by using a pyrene-based model including the mechanisms of nucleation, heterogeneous surface growth and oxidation following the hydrogen-abstraction acetylene-addition (HACA) mechanism, polycyclic aromatic hydrocarbon (PAH) surface condensation and soot particle coagulation. Radiative heat transfer from CO, CO2, H2O and soot was calculated using the discrete ordinate method (DOM) coupled to a wide-band correlated-k model. Model predictions are in quantitative agreement with the available experimental data. Model results show that H and OH radicals, responsible for the dehydrogenation of sites in the HACA process, and pyrene, responsible for soot nucleation and PAH condensation, are located in a thin region that follows the stand-off distance. Soot is produced in this region and, then, is transported inside the boundary layer by convection and thermophoresis. The combustion efficiency is significantly lower than 1 and is reduced as the flow residence time increasing, confirming that these sooting micro-gravity diffusion flames are characterized by radiative quenching at the flame trailing edge. In particular, this quenching phenomenon explains the increase in flame length with the oxidizer velocity observed in previous experimental studies. The effects of using approximate radiative-property models, namely the optically-thin approximation and gray approximations for soot and combustion gases, were assessed. It was found that the re-absorption and the spectral dependence of combustion gases and soot must be taken into account to predict accurately temperature, soot volume fraction, flame geometry and flame quenching.

Flammability profiles associated with high-pressure hydrogen jets released in close proximity to surfaces

This paper describes experimental and numerical modelling results from an investigation into the flammability profiles associated with high pressure hydrogen jets released in close proximity to surfaces. This work was performed under a Transnational Access Agreement activity funded by the European Research Infrastructure project, H2FC.The experimental programme involved ignited and unignited releases of hydrogen at pressures of 150 and 425 barg through nozzles of 1.06 and 0.64 mm respectively. The proximity of the release to a ceiling or the ground was varied and the results compared with an equivalent free-jet test. During the unignited experiments concentration profiles were measured using hydrogen sensors. During the ignited releases thermal radiation was measured using radiometers and an infra-red camera. The results show that the flammable volume and flame length increase when the release is in close proximity to a surface. The increases are quantified and the safety implications discussed.Selected experiments were modelled using the CFD model FLACS for validation purposes and a comparison of the results is also included in this paper. Similarly to experiments, the CFD results show an increase in flammable volume when the release is close to a surface. The unstable atmospheric conditions during the experiments are shown to have a significant impact on the results.

Laminar premixed flame fuel consumption rate modulation by shocks and expansion waves

The main contributing effects leading to premixed laminar flame fuel consumption rate changes following variable strength shock or expansion wave passage were studied analytically and numerically. The effects were separated into two groups: gas compression, or one-dimensional effects, and flame front distortion, or two-dimensional effects. The first group was examined analytically using a one-dimensional flame model and solution of shock and expansion wave refraction. Two-dimensional effects were examined numerically using both shock and expansion wave passage through a sinusoidally perturbed propane-air laminar flame. Flame fuel consumption amplification due to the gas compression increased rapidly (from 1.3 to 24 times for Mach 1.1–1.7 shocks), while flame length amplification peaked (∼15 times) at a given shock strength (∼Mach 1.3). Thus gas compression became the dominant effect for stronger shocks. In contrast to the shocks, a slower reaction kinetic produced by the expansion wave passage allowed for a significantly longer period of flame length growth. This resulted in a temporary burning rate increase above initial values. The negative chemical kinetic effect of the weak expansion wave passage was temporarily offset by the flame length increase. The overall reaction rate amplification was up to four times stronger in fast/slow refractions. Also, the observed values of the flame stretch are reported in the paper.

Effects of imperfect premixing coupled with hydrodynamic instability on flame propagation

The problem of flame propagation in imperfectly premixed mixtures—mixtures of reactants with variable composition—is considered in this numerical study. We carry out two-dimensional direct numerical simulations of a flame propagating in a globally lean fuel-oxidizer mixture with imposed velocity and composition fluctuations of various intensities. The configuration adopted is that of a flame front interacting with spatially evolving fluctuations, and the characteristic scales of the domain and of the fluctuations imposed are significantly larger than the characteristic thickness of the flame, to account for important flame dynamics such as the hydrodynamic instability. One-step chemistry and Fick’s diffusion law are considered, along with unity Lewis number assumption for all the species. It is observed, in agreement with previous results, that relatively weak fluctuations in composition alone may lead to a large increase in flame length and burning rate. The hydrodynamic instability caused by gas expansion, catalyzed by the composition fluctuations interacting with the flame, is found to be responsible for the flame length enhancement. It is observed as well that the relative importance of this effect diminishes as the velocity fluctuations present become more intense, and that composition fluctuations have a small impact on flame length for these cases. It is additionally found that, with increasing intensity of composition fluctuations, there is eventually a reduction of burning rate per unit length of flame which leads, consequently, to a weak reduction of overall burning rate for the largest velocity fluctuation intensities covered by this study.

Buoyancy effects in strongly pulsed turbulent diffusion flames

The effects of buoyancy in pulsed turbulent jet diffusion flames were investigated by conducting experiments in both microgravity and normal gravity. In all cases the flames were fully modulated; that is, the fuel flow was completely shut off between pulses. Unheated ethylene fuel was injected using a 2-mm-diameter nozzle into a combustor with an oxidizer coflow at ambient pressure. Microgravity conditions ( 10 − 4 g ) were achieved for 2.2 s in drop tower tests. Flames with short injection times and high duty cycle exhibit a marked increase in the ensemble-averaged flame length due to the removal of buoyancy. For other injection conditions, including steady state injection, the flame length is not strongly impacted by buoyancy. The significant increases in flame length with injection duty cycle are consistent with the duty cycle near the flame tip of microgravity flames exceeding that of normal gravity flames. The celerity of isolated compact flame puffs is approximately 40% less in microgravity than in normal gravity. An analytical argument indicates that duty cycle near the flame tip can significantly exceed that at injection due to the combination of a puff growth and the decrease in the celerity of the flame puffs with downstream distance. This effect is predicted to be significantly greater in the absence of buoyancy and for shorter injection times, in qualitative agreement with the experiments. The cycle-averaged centerline temperatures were generally higher in the microgravity flames than in normal gravity, especially at the flame tip where the difference was as much as 200 K.

Effects of Coflow on Turbulent Flame Puffs

Pulsed, turbulent jet diffusion flames in an air coflow of variable strength were examined experimentally. In all cases, the flames were fully modulated, that is, the fuel flow was completely shut off between pulses. Isolated puffs of unheated ethylene fuel were injected using a 2-mm-diam-nozzle into a combustor with an air coflow at 1-atm pressure. For short injection times (τ<50 ms), compact, pufflike structures were generated. The mean flame length of these puffs was at least 51% less than that of a steady-state, that is, nonpulsed, flame for the same injection Reynolds number. More elongated flame structures, with a flame length closer to that of steady-state flames, occurred for longer injection times of up to 300 ms. The addition of coflow generally causes an increase in the mean flame length. For short injection times (r < 50 ms), this resulted in an increase in flame length of up to 27% for a coflow strength of U cof /U jet = 0.02. The fractional increase in the flame length due to coflow of pulsed flames with longer injection times, as well as steady flames, was significantly less. The mean flame length for the flame with the coflow duct generally exceeded that of the corresponding free flame, even for the case of zero coflow. The amount of coflow required to achieve a given increase in mean flame length is quantitatively consistent with a scaling argument developed as part of this investigation.

Structure and Flame Length of Fully-Modulated, Turbulent Diffusion Flames

The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unhealed natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated “cigar-shaped” structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presented. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.

Experimental criteria for the determination of fractal parameters of premixed turbulent flames

The influence of spatial resolution, digitization noise, the number of records used for averaging, and the method of analysis on the determination of the fractal parameters of a high Damkohler number, methane/air, premixed, turbulent stagnation-point flame are investigated in this paper. The flow exit velocity was 5 m/s and the turbulent Reynolds number was 70 based on a integral scale of 3 mm and a turbulent intensity of 7%. The light source was a copper vapor laser which delivered 20 nsecs, 5 mJ pulses at 4 kHz and the tomographic cross-sections of the flame were recorded by a high speed movie camera. The spatial resolution of the images is 155 × 121 μm/pixel with a field of view of 50 × 65 mm. The stepping caliper technique for obtaining the fractal parameters is found to give the clearest indication of the cutoffs and the effects of noise. It is necessary to ensemble average the results from more than 25 statistically independent images to reduce sufficiently the scatter in the fractal parameters. The effects of reduced spatial resolution on fractal plots are estimated by artificial degradation of the resolution of the digitized flame boundaries. The effect of pixel resolution, an apparent increase in flame length below the inner scale rolloff, appears in the fractal plots when the measurent scale is less than approximately twice the pixel resolution. Although a clearer determination of fractal parameters is obtained by local averaging of the flame boundaries which removes digitization noise, at low spatial resolution this technique can reduce the fractal dimension. The degree of fractal isotropy of the flame surface can have a significant effect on the estimation of the flame surface area and hence burning rate from two-dimensional images. To estimate this isotropy a determination of the outer cutoff is required and three-dimensional measurements are probably also necessary.

Laminarization of turbulent flames in rotating environments

The stability limits for transition from laminar to turbulent flow in free boundary layers can be substantially increased by imposing an external centrifugal force field on the boundary layer. For the case of an axial jet introduced into a rotating cylindrical flow, the centrifugal forces can have a stabilizing effect and impede transverse motion of fluid. The centrifugal forces act against the turbulent viscous forces with a consequent reduction in the entrainment of surrounding fluid into the boundary layer. For the case of burning jets, strong density gradients are set up which further influence the stability of the flow. When the density increases radially outward within a centrifugal force field, a stable radial density stratification is set up which again impedes turbulent mixing. The combination of these two stabilizing forces leads to the “laminarization” of flames and results in an increase in the length of the diffusion flames. Experiments were carried out in three systems. In the first, a rotating wire-mesh screen generated a free vortex with a central fuel-gas jet diffusion flame. The second system consisted of a vertical stationary cylindrical tube mounted on a variable swirl generator with a central burning gaseous fuel jet. The third system used was an isothermal model of the second system with a helium jet replacing the fuel jet. Measurements of temperature, gas concentration, velocity, and turbulence characteristics show that the imposition of a rotating flow field on a turbulent diffusion flame results in increase in flame length, reduction of the rate of spread of the flame, and laminarization of the boundaries of the flame. A modified Richardson number is proposed as a criterion for laminar-turbulent transition stability.