Tubular Flame Research Articles

Rapidly mixed combustion of hydrogen/oxygen diluted by N2 and CO2 in a tubular flame combustor

In this study hydrogen flames have been attempted in a rapidly mixed tubular flame combustor for the first time, in which fuel and oxidizer are individually and tangentially injected into a cylindrical combustor to avoid flame flash back. Three different cases were designed to examine the effects of fuel and oxidizer feeding method, diluent property, oxygen content and equivalence ratio on the characteristics of hydrogen flame, including the flame structure, lean extinction limit, flame stability and temperature. The results show that by enhancing mixing rate through feeding system, the range of equivalence ratio for steady tubular flame can be much expanded for the N2 diluted mixture, however, at the oxygen content of 0.21 (hydrogen/air) the steady tubular flame is achieved only up to equivalence ratio of 0.5; by decreasing oxygen content, the lean extinction limit slightly increases, and the upper limit for steady tubular flame establishment increases significantly, resulting in an expanded tubular flame range. For CO2 diluted mixture, the stoichiometric combustion has been achieved within oxygen content of 0.1 and 0.25, for which the burned gas temperature is uniformly distributed inside the flame front; as oxygen content is below 0.21, a steady tubular flame can be obtained from the lean to rich limits; and the lean extinction limit increases from 0.17 to 0.4 as oxygen content decreases from 0.21 to 0.1, resulting in a shrunk tubular flame range. Laminar burning velocity, temperature and Damköhler number are calculated to examine the differences between N2 and CO2 diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment.

Measurement and simulation of partially-premixed cellular tubular flames

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H2O and temperature primarily observed in locations with moderate and high mole fractions of CO2. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.

Flame instabilities and flame cell dynamics in opposed nonpremixed tubular flames with radiative heat loss

The flame instabilities and flame cell dynamics of opposed nonpremixed tubular flames near radiation-induced extinction limits are investigated using the linear stability analysis and 2-D detailed numerical simulations with three different initial conditions (IC): the perturbed IC, the C-shaped IC, and the asymmetric IC. From the linear stability analysis and 2-D simulations with the perturbed IC for different Damköhler numbers, Da, it is found that the flame response to the initial perturbation near the 1-D radiation-induced extinction limit, DaE, R, is divided into four different regimes exhibiting different oscillatory and diffusive-thermal (D-T) instability characteristics. The radiation effects on flame structure are identified by examining the transport budgets of flame cells at the stretch-induced extinction limit, DaE, S, and the 2-D radiation-induced extinction limit, DaE, P. From 2-D simulations with the C-shaped IC, however, it is found that once the D-T instability occur near DaE, R, the number of flame cells remains constant and the flame cell size keeps being reduced with increasing Da until global extinction occurs, which indicates that flame cells can survive far beyond DaE, R by reducing their size to compensate for significantly-large radiative heat loss. When a tubular flame with the C-shaped IC is initially located beyond DaE, R, two identical edge flames can develop and propagate toward each other, leading to extinction by a head-on collision. However, a rotating flame cell can be observed from a tubular flame with the asymmetric IC because a relatively-weak flame cell is quenched prior to the head-on collision. Finally, the flame instability characteristics of opposed tubular flames with extremely-large radiation intensity are identified; high-stretched tubular flames are also affected by radiative heat loss such that the oscillatory instability occurs even at low Da and the D-T instability does for all tubular flames that survive within the combustible regime.

The impact of thermal diffusion on the structure of non-premixed flames

It is well understood how the flame structure of laminar non-premixed counterflow flames is influenced by differential diffusion, which is driven by species gradients, and corresponding models are available for physical and flamelet space. Thermal diffusion, driven by temperature gradients, is another process that leads to changes in the flame structure and can cause species separation due to differences in the thermal diffusion coefficients. In the present work, a systematic study quantifying the impact of thermal diffusion on the flame structure of non-premixed flames is carried out for five fuel-diluent-oxidizer systems. We also investigate the influence of curvature and strain and present a method for easily incorporating differential diffusion and cross-diffusional effects in flamelet modeling. The study is carried out for one-dimensional counterflow flames, tubular flames and a laminar two-dimensional coflow flame. Examples of flame structures are examined and compared to results obtained with an extended flamelet model, accounting for thermal diffusion and non-unity Lewis number diffusion.

Non-premixed swirl-type tubular flames burning liquid fuels

Tubular flames represent a canonical combustion configuration that can simplify reacting flow analysis and also be employed in practical power generation systems. In this paper, a theoretical model for non-premixed tubular flames, with delivery of liquid fuel through porous walls into a swirling flow field, is presented. Perturbation theory is used to analyse this new tubular flame configuration, which is the non-premixed equivalent to a premixed swirl-type tubular burner – following the original classification of premixed tubular systems into swirl and counterflow types. The incompressible viscous flow field is modelled with an axisymmetric similarity solution. Axial decay of the initial swirl velocity and surface mass transfer from the porous walls are considered through the superposition of laminar swirling flow on a Berman flow with uniform mass injection in a straight pipe. The flame structure is obtained assuming infinitely fast conversion of reactants into products and unity Lewis numbers, allowing the application of the Shvab–Zel’dovich coupling function approach.

Effects of internal flue gas recirculation rate on the NOx emission in a methane/air premixed flame

This study fundamentally investigates the effects of internal flue gas recirculation (IFGR) on the combustion characteristics of methane/air mixtures, particularly on the NOx formation. To induce IFGR, a prototype tubular flame burner with internal recirculation has been made, in which 12 recirculation paths are designed. The IFGR rate is controlled by varying the number (n) of opening recirculation paths (n = 0-12). To quantify IFGR rate and furthermore evaluate its influence on the NOx formation, the recirculated gas volume is determined by measurements of velocity distribution with a Particle Image Velocimetry (PIV) system. The results illustrate that by increasing n, the flame length increases while flame luminosity weakens, steady tubular flame range in equivalence ratio shrinks, and the flame temperature decreases. Temperature of recirculated burned gas drops significantly before diluting the fresh reactants. Detailed observations show that the measured IFGR rate is about 13% (convert to STP) when all the recirculation paths are opened (n = 12), in which the NOx concentration significantly decreases to almost a half. Owing to IFGR, the local oxygen mole fraction and equivalence ratio in the reaction zone decrease, which directly reduce the flame temperature and oxygen atom concentration, resulting in a significant decrease of thermal type NO formation. Variations of species concentrations with IFGR rate have been examined, and the obtained relations provide a useful guide for the operation of small boilers adopting IFGR, which can reduce NOx emission without great cost and space.

Laser Measurement on Oxy-fuel Flame with Optically Accessible Tubular Flame Burner

Laser Measurement on Oxy-fuel Flame with Optically Accessible Tubular Flame Burner

Oxy-fuel combustion of methane in a swirl tubular flame burner under various oxygen contents: Operation limits and combustion instability

Oxy-fuel combustion of methane was conducted in a rapidly mixed tubular flame burner, as oxygen content (mole fraction) was varied from 0.25 to 1.0. The fuel and oxidizer were injected from individual slits, and the slit widths were varied to promote mixing. Temporal variation of flame structure and acoustic pressure fluctuations were simultaneously measured by using a high speed video camera and a microphone respectively to examine unsteady combustion. Detailed results show that when the oxygen mole fraction was below 0.5, a steady flame can be established in a wide range from lean to rich; above 0.6, combustion-driven oscillation occurred by increasing the equivalence ratio to around 1.0. By reducing fuel slit width to half of the oxidizer slit and assigning CO2 to both fuel and oxidizer slits to maintain the same, high injection velocities, steady tubular combustion range was much expanded. However, when the oxygen mole fraction exceeded 0.7, steady combustion evolved into periodic oscillations by gradually increasing equivalence ratio; around stoichiometry the flame became steady, and thereafter evolved into unsteady state in the rich condition. Based on pressure fluctuation measurements and frequency spectral analyses, high frequency (around 2000Hz) combustion-driven oscillation was observed. This study shed lights on the operation limits of steady oxy-methane tubular combustion under various oxygen contents. For combustion under ultrahigh oxygen contents (0.7–1.0), the inlet width should be much thinner, and the fuel and oxidizer streams be maintained at the same, high injection velocities to achieve steady tubular flame around the stoichiometry.

CFD investigation on mixing in a rapidly mixed tubular flame burner

Computational fluid dynamics is used to simulate a mixing process in a rapidly mixed tubular flame burner (RTFB). The effect of several parameters such as the swirl number (S), the velocity ratio (α) and the injector arrangements (N1 and N2) is investigated. The mixing process is identified for a variation of the swirl number (from 0.23 to 5.44) via the Lagrangian discrete phase model. The validation of the numerical results is performed by confronting the predicted particle trajectory, the tangential velocity and the mixing layer thickness results to the experimental data. By means of the validated model, a mathematical correlation between the mixing coefficient and the different geometric parameters characterizing the RTFB is established, enabling the prediction of the mixing time for any RTFB design.

Effect of the Number of Injectors on the Mixing Process in a Rapidly Mixed Type Tubular Flame Burner

Three-dimensional simulations are performed to study the non-reactive mixing process in a rapidly mixed type tubular flame burner (RTFB). The current work examines the effect of the number of injectors (N= 2, 4 and 6) on the mixing process by focusing on three criterions (Flow structure, local swirl intensity and mixing layer thickness). The Discrete Phase Model (DPM) is used to track the particle trajectories. Validation of the numerical results is carried out by comparing the predicted particle trajectories, central recirculation zone (CRZ) and tangential velocity results to the experimental data. It is concluded that the model offers a satisfactory prediction of the flow field in a RTFB. Numerical results show that, for the same geometrical swirl number (Sw) and the same Reynolds number (ReT), the increasing of the number of injectors enhances the mixing process by generating a larger reverse flow and reducing the mixing layer thickness. It is also concluded that the local swirl intensity along of the RTFB can be correlated in terms of geometric swirl number and number of injectors.

Numerical Study on the Combustion Characteristics of Ultra-lean and Ultra-diluted Methane/air Tubular Flames - Comparison with Counterflow Twin Flame -

Numerical Study on the Combustion Characteristics of Ultra-lean and Ultra-diluted Methane/air Tubular Flames - Comparison with Counterflow Twin Flame -

管状火炎炉筒面における旋回流の熱伝達特性

管状火炎炉筒面における旋回流の熱伝達特性

Radial / Tangential Modes of Resonant Oscillatory Combustion in Tubular Flame Burners

Radial / Tangential Modes of Resonant Oscillatory Combustion in Tubular Flame Burners

Effects of Damköhler Number on Methane/Oxygen Tubular Combustion Diluted by N2 and CO2

To fundamentally elucidate the mixing and its effects on the characteristics of methane/oxygen flame in a rapidly mixed tubular flame burner, experiments were conducted under various oxygen mole fractions and flow rates. Two inert gases of nitrogen and carbon dioxide were used, respectively. The inert gas was added to both the oxidizer and fuel slits to maintain the oxidizer/fuel injection velocity ratio near unity. Based on flow visualization, the mixing process around injection slits and that in the axial downstream were discussed. The Damköhler number (Da1), defined as the ratio of molecular mixing time to reaction time, was selected as a parameter to quantitatively examine the criterion for the establishment of tubular flame from low to ultrahigh oxygen mole fractions (0.21–0.86). The mixing around slit exit determined the tubular flame establishment. Due to a flow time between two neighboring injection slits of fuel and oxidizer, part of the fuel was mixed in the downstream swirling flow, resulting in luminous helical structures. Hence, the Damköhler number (Da2), defined as the flow to the reaction time ratio, was examined. Detailed observations indicated that when Da2 was smaller than unity, the flame was uniform in luminosity, whereas the flame was nonuniform when Da2 ≥ 1. The value of Da2 was about 1.5 times as Da1; however, they correspond to different mixing zones and Da2 can be more easily calculated. The differences in flame stability between N2 and CO2 diluted combustion were also studied.

Experimental and numerical study of H2-air non-premixed cellular tubular flames

Experimental and numerical simulation results are presented for the first time of H2 fueled non-premixed cellular tubular flames. Non-intrusive measurements of temperature and major species were obtained with Raman scattering. Numerical simulations were performed with a 2D fully-implicit primitive variable finite difference code that includes multicomponent transport and detailed chemical kinetics. Good agreement is found for all flame cases. The experimental cell temperatures and the shapes of the flame cells are well predicted. Less significant discrepancies are observed in the character of the extinction zones and are discussed. Overall, non-premixed cellular tubular flames are found to be valuable experimental and numerical targets for fundamental combustion research.

Flamelet budget and regime analysis for non-premixed tubular flames

A recently developed flamelet model for non-premixed combustion accounting for curvature effects is examined. The work focusses on the curvature-induced differential diffusion in flame-tangential direction that is not included in classical flamelet formulations. Non-premixed tubular hydrogen and hydrocarbon counterflow flames are computed in physical and in composition space for various stretch rates. These flames were studied experimentally in previous works (Hu et al., 2007; Hu and Pitz, 2009) and allow a detailed investigation of stretch and curvature effects, while retaining a well-defined one-dimensional flame structure. First, the influence of curvature effects on the flame structure of a non-premixed tubular hydrogen flame is assessed by comparing flamelet solutions with and without curvature to the physical space solution. For the selected flame configuration, curvature-induced differential diffusion leads to a flame temperature deviation of 11% compared to a planar counterflow flame with the same scalar dissipation rate profile. Thereafter, the curvature terms are examined in a budget analysis of the flamelet equations. Finally, hydrogen and hydrocarbon tubular flames are placed in an extended regime diagram that allows non-premixed flame structures to be characterized with respect to curvature effects. The regime diagram contains the classical flamelet regime (Regime I) and the curvature-affected flamelet regime (Regime II) enclosing a novel transition zone, where curvature effects are not dominant, but already influence the flame structure significantly. It is shown that the hydrogen flames (Lef=0.34) fall into the transition zone while the hydrocarbon flames (Lef=0.88−1.49) reside in the classical flamelet regime with negligible curvature influence.

CO2 diluted propane/oxygen combustion in a rapidly mixed tubular flame burner

Propane/oxygen combustion diluted by CO2 was attempted in a rapidly mixed tubular flame burner to eliminate hazards of flame flash back. The flame structure and stability were systematically examined with oxygen mole fraction varying from ultralow to ultrahigh in the oxidizer. To enhance mixing, a tubular flame burner with a thin inlet was utilized, and in addition, CO2 was added also to the fuel slit as well as the oxidizer slit. The results show that a steady and uniform tubular flame can be obtained from lean to rich when the oxygen mole fraction was no more than 0.50, in which the tubular flame was established at ultralow oxygen mole fraction of 0.18, and the steady flame range in equivalence ratio gradually expanded with increasing oxygen mole fraction. By increasing oxygen mole fraction to 0.60, the tubular flame became non-uniform in structure; above 0.70, the tubular flame at the stoichiometry was not obtained. To obtain stoichiometric combustion at a higher oxygen mole fraction, the width of fuel slit was halved while that of the oxidizer slit was doubled to make the width ratio approach the stoichiometry of propane/oxygen (1/5). However, a tubular flame failed to be established above the oxygen mole fraction of 0.70 owing to flame anchoring at the exit of oxidizer slit. To increase both the fuel and oxidizer injection velocities, the width of oxidizer slit was also reduced, and steady tubular combustion at the stoichiometry was achieved up to oxygen mole fraction of 0.80. In the case of stoichiometric propane/oxygen combustion, a flame was anchored at each exit of slit, resulting in turbulent combustion. For combustion under ultrahigh oxygen mole fractions, a tubular flame was established at the very lean condition, and evolved into turbulent combustion through an oscillation region with increasing the equivalence ratio.

Numerical simulation of premixed H2–air cellular tubular flames

The detailed flame structure of laminar premixed cellular flames in the tubular domain is simulated in 2D using a fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Numerical results for H2/air flames are presented and compared against spatially resolved experimental measurements of temperature and chemical species including atomic H and OH. The experimental results compare well for flame structure and cell number, despite the numerical model under-predicting the peak temperature by 200 K. Numerical experiments were performed to assess the ability for cellular tubular flames to impact experimental and numerical investigations of practical flames. The cellular flame structure is found to provide a highly sensitive geometry that is useful for validating diffusive transport modelling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. the Soret effect) that is suitable for practical combustion codes.

GS0602 管状火炎を用いた希釈可燃限界導出法の研究

We generally define flammability limits by using a one-dimensionally propagating flame confined in a special burner developed in United States Bureau of Mines. But it is difficult to make absolutely the same equipment in laboratories. On the other hand, tubular flame is easy to be formed and it has quasi-one-dimensional character. We conducted numerical calculations with a radiation model and determined diluent limits of various flames of a low-grade fuel; fuel mixture of CO_2 and CH_4, and that of N_2 and CH_4. As a result, we found that the method of deriving diluent limits by using tubular flames is valid as a measuring method alternative to the one-dimensionally propagating flames.

112 急拡大部を設けた管状火炎バーナの振動燃焼(OS1 燃焼,エネルギー変換,熱利用(4),研究討論セッション)

Combustion oscillation is investigated for a rapidly-mixed tubular flame burner to clarify what kind of influence abrupt pipe expansion causes. The OH^* chemiluminescence of the tubular flame is detected for the fluctuation of heat release by an optical wave length demultiplexer. It is found from the experimental results that the sound pressure level of the rapidly-mixed type is lower than that of the premixed type by about 20 dB near stoichiometric condition. This is why heat release oscillation, which increasing the sound pressure level in the premixed type, doesn't exist in the rapidly-mixed type.