Tubular Burner Research Articles

Effect of Porosity on Combustion Performance in Packed Bed Porous Media

This study investigates the effect of packed bed material porosity and air-to-fuel ratio on the combustion stabilization of a premixed gaseous mixture. An experimental work was carried out in a single-layer concept of a packed bed on a constant cross-sectional area tubular burner. Two types of materials, Alumina (Al2O3) and Zirconia (ZrO2), with different porosities, namely 0.36, 0.4, 0.44, and 0.46, were tested. The results showed that porosity has a significant effect on the position of the reaction zones. As porosity decreases, the reaction zone moves downstream of the packed bed. The excess air ratio does not affect the position of the reaction zone but has an impact on the temperature distribution inside the porous medium. The packed bed material affects the volume of the reaction zone and the temperature distribution inside the porous media, where Zirconia has a reaction zone volume higher than Alumina. The concentration of NOx was reduced with increasing porosity. Zirconia media exhibits a lower level of NOx emission compared to Alumina. For an excess air ratio of 1.6, the maximum NOx values were 22.5 and 17.5 ppm for Alumina and Zirconia, respectively.

Measuring the burnout rate of flammable liquids

The article discusses the express method proposed by the authors for experimental measurement of the mass burnout rate of liquid combustible substances. Describes the experiments carried out with various liquids, which impregnated products made of porous heat-resistant material in the form of spheres or flat cylindrical "tablets" of different diameters. These products were placed on digital scales and set on fire. A video camera connected to a computer recorded the readings of the scales and simultaneously monitored the combustion process, fixing the value of the current time t. When processing the video file, the dependence of the mass of the liquid burning on the sample on time M = f(t) was plotted. On this graph, a linear section was allocated to measure the mass loss rate, and the specific mass burnout rate of the liquid ṁ was calculated from it. The results were compared with data obtained by other researchers in experiments using various methods for determining ṁ: in particular, with the methods of "stationary sphere", "suspended drop", burnout in a tubular burner, and also from a flat free surface.

H₂-CO₂-공기 관상 화염 측정을 위한 라만 분광학 데이터 정리

An approach of reducing spontaneous Raman spectroscopy data, based on the matrix inversion method which logically removes undesired sources of the Raman scattering signals including cross-talk due to the signals from other species, is discussed. The approach contains extensive calibration of temperature-dependent system response. The Raman spectra of several cold and hot calibration flames and their resulting calibration matrix are elaborated. The structure of H₂-CO₂-air tubular flames, in terms of major species (CO₂, O₂, N₂, H₂O, and H₂) concentrations and temperature, was successfully measured with the optical technique. The Raman spectra data used in this study are taken from a uniquely designed experimental apparatus – a tubular burner and Raman spectroscopy system at Vanderbilt University. The characteristic uncertainties for chemical species are approximately ±2% by mole fraction for hot products and ±0.5% for room temperature reactants. The approach described here for hydrogen/air tubular flames may be adapted to hydrocarbon/air tubular flames when other necessary major species, for example, CH₄ and CO for methane/air tubular flame, are considered in the whole process.

Potassium release during pulverized biomass combustion in a tubular burner investigated by TDLAS

The ignition and effect of different treatments of pulverized biomass on potassium release were monitored online using visible light detection and calibration-free TDLAS techniques in a tubular burner, with the following main conclusions. High-speed cyclonic flow enhanced the transport effect around the particles, allowing for the rapid separation and diffusion of volatile fractions from biomass particles, as well as the rapid but transient release of potassium. There were no visible envelope flames or trailing flames around the burning particles. High-speed photography also confirmed the homo-heterogeneous ignition. The release of water-soluble potassium was significantly postponed under an O2/CO2 atmosphere compared with the air atmosphere, shifting from a quick and instantaneous model to a continuous and slow-release one. In comparison, the O2/CO2 atmosphere had little suppression effect on the exchangeable and organic potassium release process. In both air and O2/CO2 combustion, the water-soluble potassium release was the fastest, followed by organic potassium and exchangeable state potassium. The release ratio of atomic potassium, the proportion of released atomic potassium in initial total potassium in particles, from water-washed and NH3Ac-washed rice husk is higher than that from rice husk combustion, under O2/CO2 and O2/N2 combustion conditions. The release process of potassium enriched in char takes a long time but has a low release rate. The inhibitory effect of the O2/CO2 atmosphere on the potassium release during rice husk char combustion was relatively weak compared with rice husk combustion.

Ethanol spray tubular flame established in a swirling air flow

This study proposes an efficient technique to burn ethanol spray in an intense swirling air flow and investigates the mechanism of flame lift-off. On the basis of typical tubular burners for gaseous fuels, ethanol spray was axially induced and mixed with the tangentially injected air under room temperature, yielding the ethanol spray tubular flames under various operating conditions. The results show that from an ultra-lean condition of global equivalence ratio of 0.1 to the rich condition, two typical flames, namely attached and lifted tubular flames, were established. Then, the structure of the ethanol spray tubular flame was specified by temperature measurements, OH-PLIF imaging, indicating its overwhelming aerodynamic and thermal stability with low pollutant emissions. Meanwhile, a flue gas analyzer was used to characterize the gas composition in the hot exhaust. It is found that the burner can achieve very low emissions of both CO and NOx, illustrating high combustion completeness under both attached and lifted flame conditions. To give a better understanding and make fully utilization of spray tubular flames, a parametric study was carried out to investigate the effects of the tangentially swirling air flow, the flow rate and oxygen concentration of atomizing gas, and the flow rate of liquid ethanol. Generally, an attached flame is lifted under a higher tangential velocity of air flow; increasing oxygen concentration of the atomizing gas flow leads to the height decrease of the flame lift-off, even the reattachment of the lifted flame; by raising the ethanol flow rate to exceed a critical value, the flame also lifts off. Furthermore, the evaporation time of droplets, residence time and chemical reaction time were calculated to quantify the flame lift-off behavior. The lifted flame can be established only when the evaporation time is larger than both the reaction time and the residence time.

Experimental investigation on lift-off, blowout and drop-back in partially premixed LPG open flames in tubular burner

The higher pollutant level in non premixed combustion and safety issues pertaining to premixed combustion can be counteracted by partially-premixed mode of combustion. The partially premixed flames (PPF) exhibit the benefits of both premixed and non premixed flames. PPF enhances complete combustion leading to reduced soot formation and hence lower emission. However, the equivalence ratio plays an important role in the stability of such flames. This paper reports the experimental investigation on the flame characteristics and stability of partially premixed LPG-air flames in tubular burner. The stability curve obtained for the base case without any secondary flow shows that the velocity at lift-off, drop-back and blowout increases with increasing equivalence ratio. In the presence of secondary co-flow air, the lift-off and blow off velocity decreases compared to base case indicating poor stability due to extensive flame stretch leading to aerodynamic quenching. The experimental results show that the velocity of flow at lift off, blow out and drop back are higher in the presence of secondary swirl air than the base case. Co-swirl air increases the stability due to better mixing at the flame base with increased residence time. Flame stability deteriorates with co-flow air as co-flow strains the flame boundary due to flame stretch.

Computational analysis on the stability and characteristics of partially premixed butane-air open flames in tubular burner

Partially premixed combustion is one of the developing areas of combustion research that has the advantages of both premixed and diffusion mode of combustion. The present work involves a computational study on the stability and characteristics of partially premixed butane-air flames. The effect of operating parameters like fuel-air ratio, primary aeration, and the presence of co-flow and co-swirl on the stability and flame characteristics has been studied. The simulation results show that the height of the flame decreases with an increase in primary aeration and also in the presence of a co-swirl stream. It has also been found that the stability of flames increases with co-swirl air but deteriorates with the presence of the co-flow air. The flame temperature increases with primary aeration and it has been observed that the peak flame temperature shifts away from the burner mouth for lower primary aeration. It has been observed that the flame stability improves with co-swirl air which is attributed to the re-circulation zone created due to the swirl motion which acts as a heat source. The poor stability in the presence of co-flow air is attributed to flame stretching and aerodynamic quenching of the stretched flamelets. The lift-off velocity and the stable operating range increases with equivalence ratio and also with co-swirl air.

Experimental study on the stability and flame characteristics of LPG flames in O2/N2 and O2/CO2 environment

The current energy crisis and the escalating concern over pollution level necessitate the development of various low emission and clean technologies for efficient utilization of fossil fuels. These technologies offer the benefits of using fossil fuels continuously without harming the environment. Oxy fuel combustion technology is one of the promising low emission combustion technologies having an additional advantage of being a better retrofit option compared to other clean technologies. In the present study, the stability and flame characteristics of LPG flame in a tubular burner has been investigated with oxygen as the oxidizing gas and nitrogen or carbon dioxide as the balance gas. The experiments were conducted to study the effect of primary aeration and the effect of balance gas on the stability and flame characteristics of air and oxy flames. The experimental results from the preliminary experiments show that the flame height decreases with increase in primary aeration for both air and oxy cases and the premixed blue zone increases with primary aeration. The flames observed for oxy cases are dull and weak at all equivalence ratio (or primary aeration). The measured axial flame temperature indicated that the temperature of oxy flames is lower than that of the air flames for the same primary aeration. The data obtained from the stability tests show that the oxy flames can withstand relatively lower port velocity than its air counterpart. Therefore, the stable operating range of the given burner design is lower when CO2 is used as the balance gas. These differences in the stability and flame characteristics is attributed to the difference in the thermo physical properties like molar heat capacity, mass diffusivity and thermal conductivity of the balance gas CO2 and N2.

Potassium Release During Pulverized Biomass Combustion in a Tubular Burner Investigated by TDLAS

Potassium Release During Pulverized Biomass Combustion in a Tubular Burner Investigated by TDLAS

Filtered Rayleigh scattering measurements of temperature in cellular tubular flames

Steady, premixed tubular flames that produce cellular flames due to low Lewis number can assist in validating diffusional properties for numerical models. The lean hydrogen-air mixture creates conditions of low Lewis number that have shown to result in cellular flames inside tubular burners due to preferential diffusion. Filtered Rayleigh scattering (FRS) is used to achieve two-dimensional temperature measurements across perpendicular planes in the cellular tubular flame. Different FRS strategies, a pulsed Nd:YAG laser with and without Fabry–Perot etalon as well as a continuous wave laser, are contrasted for consistency and uncertainty in the tubular flame measurements. The FRS temperature profiles are found to be consistent with previous spontaneous Raman scattering temperature measurements but differ from previous direct numerical simulation results.

Effects of temperature-time history on the flame synthesis of nanoparticles in a swirl-stabilized tubular burner with two feeding modes

This work examines the effect of temperature-time history on the flame synthesis of titania nanoparticles using a specially designed swirl-stabilized tubular burner with two kinds of precursor feeding modes, the tangential inlet and the central jet inlet. When the Damköhler number is less than unity, a rapidly mixed type tubular flame is well-formed, which is chosen to be the condition for flame synthesis. The ex situ characterization of the synthesized TiO2 nanoparticles shows that the particle samples from the tangential inlet feeding mode are mainly anatase (up to 84.3%) with a mean primary particle size of 13.0 ± 3.2 nm, whereas those from the central jet inlet feeding mode are large polydisperse rutile (up to 74%) nanoparticles with a mean size of 16.9 ± 8.8 nm. The significant differences in particle size and crystal phase are attributed to the varied temperature-time histories of two feeding modes, as the residence time of nanoparticles in high-temperature zone of the central jet inlet mode is much longer with larger variation than that of the tangential inlet feeding mode. Finally, the well-dispersed Pd/TiO2 nanocatalysts are generated by elaborately feeding palladium acetate through the central jet inlet and titanium tetraisopropoxide through the tangential inlets, respectively, to better match the temperature-time histories with the decomposition properties of two different precursors. The study demonstrates the potential of this swirl-stabilized burner for producing tailored single- and multi-component nanomaterials for a variety of applications.

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.

Experimental study on temperature variation in a porous inert media burner for premixed methane air combustion

The detailed axial temperature variations of the porous media burner during startup and switch-off processes were experimentally measured and analyzed for the premixed methane air combustion. A tubular burner filled by alumina pellets with diameters of 10 mm and 4.3 mm, respectively, was used as the porous media burner to study the effect of the burner inlet gas flow rate, equivalence ratio, and diameter of alumina pellets on the combustion temperature distribution along the burner axis and the combustion wave velocity. The results showed that the superadiabatic combustion could be achieved for the combustion wave velocity greater than zero, and the critical equivalence ratio corresponding to the superadiabatic combustion was bigger for the smaller pellet diameter or higher inlet air flow rate. This study can provide the useful information for the design and operation of the porous media burner, and is significant to understand the combustion phenomena in the porous media burner.

Effect of Low Frequency Burner Vibrations on the Characteristics of Premixed Flames

Mechanical vibrations in a burner are found to affect the characteristics of premixed flames. In the present experimental investigation, the effects of axial vibrations of a tubular burner on the characteristics of laminar premixed methane-air flames are investigated. The effect of the frequency and amplitude of the burner vibrations on the flame height oscillations are studied. At a low frequency of around 10 Hz, no effect of vibrations on flame characteristics is observed. At frequencies higher than 10 Hz, the amplitude of flame height oscillations is found to increase with increase in both frequency and amplitude of burner vibrations for the range of frequencies and amplitudes considered.

Laminar flame speed of soy and canola biofuels

In this article, the flame speed values determined experimentally for laminar premixed flames of the vapors of two biofuels in air are presented. The laminar flame speed is a fundamental thermochemical property of fuels, and is essential for analyzing the flame propagation in practical devices, even those employing turbulent flames. The fuels obtained from transesterification of soy and canola oils are tested. Also, the diesel flames are studied to serve as a baseline for comparison. The experiments are performed with a tubular burner; pre-vaporized fuel is mixed with hot air and is ignited. The flame speed is determined at fuel-equivalence ratios of 1; 1,1 and 1,2 by recording the geometry of the flame. The experimental results show that the flame speed of biofuels is lower by about 15% than that of diesel. Also, the maximum value of flame speed is obtained at an equivalence ratio of approximately 1,1.

An Experimental Investigation of Flammability Limits and Autoignition Temperatures of Petrofuels and Biofuels in a Tubular Burner

A set of experiments to determine the flammability limits of pure and binary blends of fuels was carried out in a combustion tube operated at atmospheric pressure. The lowest and highest volumetric fraction of vapor fuels that allowed flame propagation when the air/fuel mixture was submitted to a high voltage spark with energy higher than the minimum ignition energy defined the lower and upper flammability limit, respectively. The obtained experimental results for liquefied petroleum gas (LPG), acetone, ethylic alcohol, gasoline, diesel, and bioethanol are in close agreement with results reported in the literature. Diagrams of flammability limits of mixtures of gasoline/bioethanol and diesel/biodiesel were found within the range of composition of standard commercial blends of these fuels available for market in Brazil. The autoignition temperature and the influence of temperature on flammability limits of pure ethanol and a commercial mixture with 4% of biodiesel and 96% of diesel were experimentally determined.

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.

Rapid Characterization of Radiation and Pollutant Emissions of Biodiesel and Hydrocarbon Liquid Fuels

As a result of decreasing petroleum supplies, new fuel sources, such as transesterified biofeedstock based oils and their blends with petroleum diesel fuels, have emerged with potential to partially replace conventional diesel and gasoline fuels. Although these fuels have shown some promising results in engine studies, their basic combustion properties have not been well documented. Also, research is underway to develop new fuels from other sources or by altering their molecular structure to be fungible with conventional fuels. Thus, there is a need for tests to characterize the combustion and emission properties of these new liquids, which are available only in small quantities at the research and development stage. This paper deals with a technique that meets those goals. The fuel was prevaporized and mixed with air and burnt in a tubular burner (9.5 mm inner diameter) at atmospheric pressure under laminar conditions. A pilot methane/air flame was used as the ignition source. The test conditions were so chosen that the measured properties could be attributed primarily to the fuel chemical structure. Several liquid fuels were tested, including commercially available petroleum-based No. 2 diesel fuel, canola methyl ester (CME B100) biodiesel, kerosene, methanol, toluene, and selected alkanes. The radiative heat flux from the flames was measured using a wide-angle pyrheliometer; the emissions from the flames were sampled to measure the concentration of CO, CO2, and NO. The measured radiant heat fraction values and the emission indices of NO and CO of both petroleum-derived and biofuels agreed well with those found in literature; thus, the feasibility of this method to rapidly characterize the combustion and emission properties of new liquids, such as biofuels, is demonstrated.

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H 2, CH 4 or C 3H 8 with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N 2, He, Ar or CO 2. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H 2/N 2, concave flame curvature towards the fuel suppresses cellular instabilities.

Structural study of non-premixed tubular hydrocarbon flames

Tubular non-premixed flames are formed by a uniquely designed opposed tubular burner. Structural measurements of hydrocarbon flames are conducted using the laser-induced Raman scattering technique. Temperature and major species concentrations are recorded for flames produced by 30% CH 4/N 2 and 15% C 3H 8/N 2 burning against air. Numerical simulations of these flames with detailed chemistry show good agreement between the measured and simulated results. By comparing the numerical results of the tubular curved flames to those of the opposed-jet planar flames, it is shown that flame curvature towards the fuel stream strongly effects the temperature (±80 K) of flames with low fuel Lewis number (15% H 2/N 2, Le f = 0.41). The effect of curvature on flames with high (15% C 3H 8/N 2, Le f = 1.51) and near-unity (30% CH 4/N 2, Le f ≅ 1) fuel Lewis numbers is much less.