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Highlights • Deterministic tubular flame is obtained between oxygen content of 0.13 and 0.35. • Quantitative relation between flame length and laminar burning velocity is studied. • Flame structure and operation limits are determined for EGR and oxyfuel combustion. Abstract Premixed C 3 H 8 /O 2 combustion has been investigated by adopting N 2 , CO 2 and their mixture (50% N 2 -50% CO 2 by volume) as the diluents, respectively. The oxygen content (mole fraction) in the oxidizer is varied from 0.13 to 0.35, which covers the operating conditions of EGR and OEC. The flame structures, temperatures, burning velocities and NO x emissions are examined under various oxygen contents. Detailed observations show that a steady, uniform tubular flame can be obtained over a wide range from lean to rich. By increasing oxygen content, the flame diameter increases slightly; the flame length decreases significantly, which is almost inversely proportional to laminar burning velocity. During decreasing the burning velocity to less than about 20 cm/s, a turning point appears and the flame length becomes extensively large. With the same oxygen content and equivalence ratio, by replacing half or all of the N 2 diluent with CO 2 , the flame temperature and burning velocity decrease significantly. To obtain the same adiabatic flame temperature, the CO 2 diluted mixture requires the highest oxygen content among the three diluents, however, its measured flame temperature is also highest. In the N 2 -CO 2 diluted combustion, owing to thermal and chemical effects of CO 2 , the NO x emission reduces remarkably comparing with N 2 dilution; under the same flame temperature, the NO x concentration reduces to almost 1/5 of that diluted by N 2. The lean extinction limit raises by increasing CO 2 content in the diluent, while the limit decreases with increasing oxygen content. [ABSTRACT FROM AUTHOR]

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 N 2 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 CO 2 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 N 2 and CO 2 diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment. [ABSTRACT FROM AUTHOR]

This study fundamentally investigates the effects of internal flue gas recirculation (IFGR) on the combustion characteristics of methane/air mixtures, particularly on the NO x 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 NO x 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 NO x 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 NO x emission without great cost and space. [ABSTRACT FROM AUTHOR]

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 CO 2 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 2000 Hz) 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. [ABSTRACT FROM AUTHOR]

Propane/oxygen combustion diluted by CO 2 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, CO 2 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. [ABSTRACT FROM AUTHOR]

An inherently safe technique of rapidly mixed tubular flame combustion, in which fuel and oxidizer are separately injected from their own slits, has been extended to CO 2 diluted methane/oxygen combustion. Two different cases are investigated. When CO 2 is added only to the oxidizer slit, a stable tubular flame is obtained for a wide range of equivalence ratios with the oxidizers of oxygen mole fraction between 0.21 and 0.50. However, once the oxygen mole fraction exceeds 0.60, the stable flame is obtained within a small equivalence ratio range near the lean extinction limit, and the stable range becomes narrower and narrower with increasing the oxygen mole fraction. When CO 2 is added also to the fuel slit so as to maintain the oxidizer/fuel injection velocity ratio near unity, and the flow rates of fuel and oxidizer are increased, the stable range is dramatically widened and a stable flame at stoichiometry can be obtained up to the oxygen mole fraction of 0.86, which yields adiabatic flame temperature around 2950 K. The Damköhler number, defined as the mixing to reaction time ratio, is examined to discuss the criterion for the establishment of stable tubular flame. It is found that when CO 2 is added only to the oxidizer slit, stable flame can be obtained for D a < 1, whereas not for D a > 1 due to formation of diffusion flames near the fuel slits. When CO 2 is added also to the fuel slit, however, stable flame can be established even for D a ⩾ 1 as well as D a < 1. Detailed observations indicate that when D a is adequately smaller than unity, the flame is uniform in luminosity, whereas the flame is non-uniform when D a is near or larger than unity. It is considered that the diffusion flame formation at the fuel slits is prohibited due to the high injection velocities, which gives a chance of fuel/oxidizer mixing, however, the tubular flame obtained is of non-uniform structure due to insufficient mixing time when D a ⩾ 1. [ABSTRACT FROM AUTHOR]

To fundamentally elucidate the requirement for an inherently safe technique of rapidly mixed type tubular flame combustion, experiments have been made to investigate (1) the mixing process of fuel and oxidizer, and (2) the appearances of methane flames under various oxygen mole fractions. Three optically accessible quartz burners of different slit widths were made for measuring the mixing layer thickness with a PIV system. Under various rates of flow of the oxidizer to the fuel, a boundary layer type flow is recognized to dominate the mixing of fuel and oxidizer around the exit of the injection slit, namely the mixing layer thickness is inversely proportional to the square root of mean injection velocity. Using two stainless steel burners, combustion tests were conducted with the oxidizers of oxygen/air mixtures. To quantitatively investigate the requirement for tubular flame establishment, the Damköhler number, which is the ratio of characteristic mixing time to characteristic chemical reaction time, has been discussed in detail. The mixing time was calculated according to estimated mixing layer thickness, while the chemical reaction time was computed with the Chemkin code. The Damköhler number has proved to be a useful measure for success/failure of tubular flame combustion. When the Damköhler number is larger than unity, chemical reaction starts before complete fuel/air mixing and the tubular flame fails to be established; when the Damköhler number is much smaller than unity, the fuel and the oxidizer are completely mixed before the onset of reaction, resulting in successful tubular flame combustion. The results confirm our hypothesis in a previous study. Furthermore, based on the concept of Damköhler number, the minimum flow rate to achieve the tubular flame combustion could be estimated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mixing is well analyzed based on flow visualizations in the tubular flame burner. [•] Flow reversal is induced in the tubular flame burners of swirl number above 0.69. [•] A higher swirl number and a larger inlet velocity are preferred to enhance mixing. [•] Injection velocities should be the same to promote mixing. [Copyright &y& Elsevier]

Abstract: An inherently safe technique of rapidly mixed type tubular flame combustion has been applied to pure oxygen combustion. The flame characteristics under various oxygen mole fractions are experimentally investigated with use of burners of different slit widths, and in addition, with optically accessible quartz burners. To fundamentally understand the requirements for rapidly mixed pure oxygen tubular flame combustion, the characteristic reaction times were calculated with Chemkin code, while the characteristic mixing times were estimated based on the width of mixing layer determined. Results show that the rapidly mixed type tubular flame combustion has been obtained under oxygen-enriched and even pure oxygen conditions. At high oxygen mole fraction, diffusion flames are anchored at the exits of the fuel slits, which restrains mixing between fuel and oxidizer, resulting in a failure of tubular flame combustion. By reducing the slit width, however, the diffusion flame is inhibited and a uniform tubular flame can be obtained from 0.11 to 0.18 in overall equivalence ratio. To quantify the mixing process, the flow field was visualized, and it has been found that there exist two types of flows, a boundary layer type flow near the exits of the slits close to the wall and an axisymmetric potential flow around the axis of rotation. Based on the mixing layer width of the boundary layer type flow, the Damkohler numbers are obtained and it is clearly shown that when the Damkohler number is less than unity, the mixing is completed before the onset of reactions, resulting in establishment of tubular flame combustion. [Copyright &y& Elsevier]

• A compact, robust and clean combustion technique is proposed to burn liquid ethanol. • A steady ethanol/air tubular flame is established in a dual swirl combustor up to 10 kW. • Flame anchoring appears as global equivalence ratio at fuel nozzle exit is below 11. 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 (Φ gl) 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 Φ gl is below 11, the flame anchoring will appear. [ABSTRACT FROM AUTHOR]

• Swirl tubular flame burner yields a wider operation limit than the slit burner without rotation. • Heating rate enhances with swirl number via increase of heat transfer area or turbulence. • Turbulent mixing under cold flow evolves to flow laminarization under combustion condition. • Propose a formula employing swirl number to accurately predict the heating rate. This study experimentally investigated the heating process of an axially induced cold gas stream in a swirl tubular flame established through tangential fuel/air pre-mixture injection, aiming to clarify the effects of swirl motion on the heat and mass transfer enhancement. Tubular flame burners with geometry swirl numbers (S) of 0.35 to 1.4 were employed in addition to a conventional slit-type burner to systematically study the flame structure, stability, temperature distribution and flow field. The results show that in comparison to the slit burner without rotation, the swirl tubular flame burner provides a much wider range of stable flame from lean to rich limits even under large flow rates. The heating rate of the inner cold stream by the outer burned gas stream increases as the swirl number raises. Cold flow visualization illustrates that the interface between the two streams is rolled up and instability appears at the interface for the larger swirl numbers of S = 0.7 and 1.4, resulting in enhancement of mixing rate. At large flow rates the two streams are well mixed under cold flow condition, however, they are separated and ordered under combustion condition in the upstream, probably resulted from the burned gas expansion outside the central gas stream. While their mixing rapidly enhances as S raises to beyond 0.7. A simple theoretical analysis illustrates that the enhancement of the heating rate with a rotational motion can be quantitatively evaluated by replacing the numeric unity in Eq. (1) with the value of S + 1. The enhancement seems to be through an increase of heat transfer area at low rotational velocities, or increases of heat and mass transfer rates at high rotational velocities. [ABSTRACT FROM AUTHOR]