Emission characteristics and axial flame temperature distribution of producer gas fired premixed burner

This paper presents the emission characteristics and axial flame temperature distribution of producer gas fired premixed burner. The producer gas fired premixed burner of 150 kW capacity was tested on open core throat less down draft gasifier system in the present study. A stable and uniform flame was observed with this burner. An instrumented test set up was developed to evaluate the performance of the burner. The conventional bluff body having blockage ratio of 0.65 was used for flame stabilization. With respect to maximum flame temperature, minimum pressure drop and minimum emissions, a swirl angle of 60° seems to be optimal. The experimental results also showed that the NO_x emissions are inversely proportional to swirl angle and CO emissions are independent of swirl angle. The minimum emission levels of CO and NO_x are observed to be 0.167% and 384 ppm respectively at the swirl angle of 45–60°. The experimental results showed that the maximum axial flame temperature distribution was achieved at A/F ratio of 1.0. The adiabatic flame temperature of 1653 °C was calculated theoretically at A/F ratio of 1.0. Experimental results are in tune with theoretical results. It was also concluded that the CO and UHC emissions decreases with increasing A/F ratio while NO_x emissions decreases on either side of A/F ratio of 1.0.     

Influence of fuel fraction gradient on triple flame velocity in plain and axis-symmetrical channels

Dynamics of laminar triple flame investigated numerically for the different mixture degrees. One-step methane–air chemistry adequate to reach and lean mixture combustion was accepted. Velocity of triple flame is determined as a function of methane concentration logarithm gradients μ = d(ln Y₁)/dx (characterizing mixing degree). It is found that maximum velocity of the triple flames correspond to the value of the methane concentration logarithm gradients μ ≈ 1000 m^?1 for plain and μ ≈ 2000 m^?1 for axis-symmetrical channels. The maximum velocity of triple flame in plain and axis-symmetrical channels in the case of non-gradient incoming gas flow is about twice bigger than normal laminar flame velocity S_f ≈ 2.1S_l.     

The non-linear thermo-acoustic response of a small swirl burner

The non-linear response of a swirl stabilised, lean premixed flame (CH₄/air) was determined by forcing the flame acoustically at frequencies between 40 and 200 Hz with increasing amplitude. Measuring the chemiluminescent emission from OH¹ with a photodiode sensor and calculating the flame transfer function, a linear response to increasing amplitude was observed at 40 and 60 Hz for all amplitudes with an equivalence ratio ? = 0.56. However, between 80 and 200 Hz the flame response exhibited non-linear characteristics for r.m.s velocity fluctuations greater than 20% of the mean flow velocity. With ? = 0.48, even 60 Hz became non-linear. Phase-locked Particle Image Velocimetry and Intensified CCD imaging were deployed at three amplitudes for detailed study of the flame and flow field response to forcing. At low frequencies the flow field was characterised by a pulsating inner recirculation zone, whilst at all frequencies the outer recirculation zone was modified by vortices rolling up the annular jet. As the forcing amplitude was increased, the effect on the flame shape became more pronounced, with large variations in flame volume at low frequencies and flame extinction due to stretching of the flame around the roll-up vortices at the higher frequencies. The results indicate different driving mechanisms behind the flame response at low and high frequencies. At low frequencies the flame response is governed by equivalence ratio fluctuations due to the ‘stiff’ fuel system and the volumetric fluctuations of the input air. At the higher frequencies the response is governed by flow field features such as vortex roll-up.     

Effects of slip boundaries on thermal convection in 2D box using lattice Boltzmann method

This work studied the thermal convection under various slip boundary conditions in a 2D box with aspect ratio equal to two. The slip parameter is the normalized tangential momentum accommodation coefficient (TMAC, 0 ? σ ? 1). The results show that the slip boundary conditions of vertical side walls (σ_v) and horizontal plates (σ_h) will affect the pattern selections of the flow and temperature fields. When σ_h < 0.02, the pattern is the one-roll mode for all σ_v. When σ_h ? 0.02 and σ_v ? 0.1, the fluids prefer the two-roll mode where two rolls make the fluids to move upwards in the middle of the box. While σ_h ? 0.02 and σ_v ? 0.2, the fluids prefer the other two-roll mode which makes the fluid to move downwards in the middle of the box.     

Unsteady stagnation-point flow over a plate moving along the direction of flow impingement

Unsteady plane and axisymmetric stagnation flow of an incompressible viscous fluid on the body that moves along the oncoming flow with a time-dependent velocity is studied in this work. Similarity solutions for the full Navier–Stokes equations are obtained and the results of the flow velocity, shear stress and stream lines are reported for both plane two dimensional flow and axisymmetric flow. The results show that the features of the similar boundary flow highly depends on a characteristic parameter β. There exists a critical value β_c below which no similarity solution to the flow is found. When β_c < β < 0, two solution branches exist and different flow patterns appear for each branch. Flow with monophonically growing velocity, reversed flow and flow with S-shaped velocity are obtained for various values of β. The boundary layer thickness of the plane and axisymmetric flows is tabulated, the streamlines of the flow are demonstrated, and the shear stress over the boundary layer is also discussed.     

Numerical calculation of local entropy generation in a methane–air burner

This study considers numerical simulation of the combustion of methane with air including 21% oxygen and 79% nitrogen in a burner and the numerical solution of the local entropy generation rate due to the high temperature and velocity gradients in the combustion chamber for various fuel flow rates (from 5 to 10 lpm). Swirling air flow is also used to burn the methane more efficiently. The effects of equivalence ratio (ϕ) and swirl number (S) on the combustion and entropy generation rate are investigated for different (consecutive) equivalence ratios (from 0.5 to 1.0) and swirl numbers (from 0 to 0.3). The numerical calculation of combustion is performed individually for these cases with the help of the Fluent CFD code. The volumetric entropy generation rate distributions and the other thermodynamic parameters are calculated numerically by using the results of the combustion calculations. The maximum values of the rates of reaction-1 and -2 decrease with the increase of ϕ. In the case of ϕ < 1, complete combustion occurs, and the combustion in the case of ϕ = 1 is very close to the complete combustion state. In the case of no swirl, the entropy generation rate decreases exponentially with the increase of ϕ in the cases of high Q_f, whereas they have quadratic profiles having their minimum values in cases of low Q_f. In terms of the entropy generation rates, the optimum equivalence ratios for Q_f = 5, 6, 7, >7 lpm in the case of S = 0 and Q_f = 10 lpm in the case of S = 0.3 are obtained as ϕ = 0.66, 0.8, 0.86, 1.0 and 0.92, respectively.     

Transient flame spread during convective ignition of solid fuel in a sudden-expansion combustor

The convective ignition of solid fuel (PMMA) in a sudden-expansion combustor (Re_h = 6200, u₀ = 22 m/s, [O₂] ～ 11.7%, T₀ = 810 °C) is investigated from the perspective of flame–vortex interactions. Three phases of the transient flame spread are identified via the diagnostics of flow visualization and particle image velocimetry (PIV). The dominance of small/large vortices is revealed, respectively, in the pre-/post-ignition regimes, which demonstrates the small-to-large vortex transformation due to heat release. Attributed to the decreased characteristic reaction time and enhanced mixing, the first ignition is observed at the downstream end of the fuel, after which a primitive flame is formed and initiates the opposed flame spread. During the spread, the rolling behavior of flame kernels are considered to be dominated by the small eddies. The combined effects of broken vortices and continuing pyrolysis introduce the periodical extinction–reignition around the reattachment region. At the final phase, the entrainment of flame kernels into the shear layer is facilitated by the large shedding vortices, and a sustained diffusion flame is established. The study not only provides novel insights into the convective ignition of solid fuel in the separation–reattachment flow, but also serves as a basis for the advancement of ignition control.     

Effects of dilution with carbon dioxide on the laminar burning velocity and flame stability of H2–CO mixtures at atmospheric condition

The objective of this investigation was to study the effect of dilution with CO₂ on the laminar burning velocity and flame stability of syngas fuel (50% H₂–50% CO by volume). Constant pressure spherically expanding flames generated in a 40 l chamber were used for determining unstretched burning velocity. Experimental and numerical studies were carried out at 0.1 MPa, 302 ± 3 K and ? = 0.6–3.0 using fuel-diluent and mixture-diluent approaches. For H₂–CO–CO₂–O₂–N₂ mixtures, the peak burning velocity shifts from ? = 2.0 for 0% CO₂ in fuel to ? = 1.6 for 30% CO₂ in fuel. For H₂–CO–O₂–CO₂ mixtures, the peak burning velocity occurred at ? = 1.0 unaffected by proportion of CO₂ in the mixture. If the mole fraction of combustibles in H₂–CO–O₂–CO₂ mixtures is less than 32%, then such mixtures are supporting unstable flames with respect to preferential diffusion. The analysis of measured unstretched laminar burning velocities of H₂–CO–O₂–CO₂ and H₂–CO–O₂–N₂ mixtures suggested that CO₂ has a stronger inhibiting effect on the laminar burning velocity than nitrogen. The enhanced dilution effect of CO₂ could be due to the active participation of CO₂ in the chemical reactions through the following intermediate reaction CO + OH ? CO₂ + H.     

Convection and instability of thermocapillary flow in a liquid bridge subject to a non-uniform rotating magnetic field

A three-dimensional liquid bridge is considered in this study to numerically investigate the effects of an external non-uniform rotating magnetic field (RMF) on the thermocapillary flow in semiconductor melt under microgravity. Simulations are carried out to examine the convection and instability features of the thermocapillary flow over a range of Marangoni numbers (Ma = 15–50) under a non-uniform RMF. The present results show that applying an external non-uniform RMF enhances the maximum tangential velocity and depresses the maximum axial velocity. As a consequence, an approximately axisymmetric flow is maintained in the melt under the effect of the non-uniform RMF, which is beneficial for growing high quality crystal. Further investigation of the thermocapillary flow subject to different non-uniform RMFs (corresponding to Taylor numbers Ta = 3.8 × 10²–1.86 × 10⁴ and Rotating Reynolds number Re_ω = 2.2 × 10⁴) reveals that the thermocapillary convection may undergo a transition from the approximately axisymmetric steady flow to a periodically oscillatory flow for Ma above a critical value. The critical Ma generally increases with the intensity of the non-uniform RMF.     

Hydrodynamics and heat transfer in swirl flow under conditions of one-side heating. Part 1: Pressure drop and single-phase heat transfer

The paper gives the basic results of experimental investigation of hydrodynamics and heat transfer in heat-absorbing devices of the ITER thermonuclear reactor, which are subjected to one-side heating. The entire array of experimental data is obtained in the following range of parameters of water flow: pressure p = 0.7–2.0 MPa, mass flux G = 340–25,000 kg/(m² s), inlet water temperature T_in = 15–60 °C. The experiments are performed with turbulent swirl flows of water for twisted tapes with the flow swirl coefficient k = 0.90, 0.66, 0.49, 0.39, 0.25, 0.19, and 0, as well for test sections without a tape. Given in the first part of the paper are the data on pressure drop and single-phase convective heat transfer. Appropriate calculation formulas are derived, which reliably generalize the experimental data.     

Streamwise interaction of burning drops

The spatial distribution of drops and their interactions are influential parameters in spray combustion. Most available researches on this subject were about lateral spacing or were performed in micro-gravity. Studies about upstream/downstream convective interaction of burning drops are scarce. In this study, drop strings of different spacing were investigated in a high-temperature oxidizing environment for their flame transition, flame width variation and drop evaporation rate. The flame transition showed that along the flow direction, the drop flame initially located ahead of the drop, became a spherical envelope flame, then moved behind the drop, and finally burned as a wake flame. It was found that a drop string with an initial drop spacing (S_i) of 2.5 or 5 was surrounded by a bulk flame tube, exhibited local group burning and soot layer. In addition, for S_i = 2.5, spacing instability and collision merging of the burning drops occurred; the wake flame stretched away from the drop could attach to and stabilize on the rear drop. In the experiment, for all cases, most of drops in the string were not surrounded by the flame. For S_i < 30, the drop evaporation rate was lower than that of a single drop. For 30 < S_i < 75, the drop evaporation rate was higher than that of a single drop. The interaction of drops diminished if S_i was more than 75.     

A simple analytical model to study and control azimuthal instabilities in annular combustion chambers

This study describes a simple analytical method to compute the azimuthal modes appearing in annular combustion chambers and help analyzing experimental, acoustic and large eddy simulation (LES) data obtained in these combustion chambers. It is based on a one-dimensional zero Mach number formulation where N burners are connected to a single annular chamber. A manipulation of the corresponding acoustic equations in this configuration leads to a simple dispersion relation which can be solved by hand when the interaction indices of the flame transfer function are small and numerically when they are not. This simple tool is applied to multiple cases: (1) a single burner connected to an annular chamber (N = 1), (2) two burners connected to the chamber (N = 2), and (3) four burners (N = 4). In this case, the tool also allows to study passive control methods where two different types of burners are mixed to control the azimuthal mode. Finally, a complete helicopter chamber (N = 15) is studied. For all cases, the analytical results are compared to the predictions of a full three-dimensional Helmholtz solver and a very good agreement is found. These results show that building very simple analytical tools to study azimuthal modes in annular chambers is an interesting path to control them.     

Experimental and kinetic modeling study of methyl butanoate and methyl butanoate/methanol flames at different equivalence ratios and C/O ratios

Premixed laminar methyl butanoate/oxygen/argon and methyl butanoate/methanol/oxygen/argon flames were studied with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry at 30 torr (4.0 kPa). Three flames were investigated in the experiment: MB (methyl butanoate) flame F1.54 (? = 1.54, C/O = 0.479), MB flame F1.67 (? = 1.67, C/O = 0.511) and MB/methanol flame F1.67M (? = 1.67, C/O = 0.479). By measuring the signal intensities at different distances from the burner surface, the mole fraction profiles of intermediates are derived. Experimental results show that the flame front shifts downstream and peak mole fractions of intermediates increase remarkably with the increase of equivalence ratio for pure MB fuel. When methanol is added, the peak mole fractions of most intermediates including those of soot precursors decrease remarkably at the same equivalence ratio, while peaks of soot precursors vary little (only slightly decreasing) at same C/O ratio. It is concluded that the formation of soot precursors is more sensitive to C/O ratio than to equivalence ratio. Besides, more CO₂ is produced near the burner surface in MB flame than that in MB/methanol flame, and this validates an early production of CO₂ in methyl ester oxidation. In addition, a modified MB detailed mechanism is used to model flame structure, and improved agreements between the experimental and predicted results are realized. Based on the simulation results, reaction flux and sensitivity are analyzed for CO₂ and C₃H₃, respectively.     

The turbulent burning velocity of iso-octane/air mixtures

Turbulent burning velocities of iso-octane air mixtures have been measured for expanding flame kernels within a turbulent combustion bomb. High speed schlieren images were used to derive turbulent burning velocity. Turbulent velocity measurements were made at u′ = 0.5, 1.0, 2.0, 4.0, 6.0 m/s, equivalence ratios of 0.8, 1.0, 1.2, 1.4 and pressures of P = 0.1, 0.5, 1.0 MPa. The turbulent burning velocity was found to increase with time and radius from ignition, this was attributed to turbulent flame development. The turbulent burning velocity increased with increasing rms turbulent velocity, and with pressure; although differences were found in the magnitude of this increase for different turbulent velocities. Generally, raising the equivalence ratio resulted in enhanced turbulent burning velocity, excepting measurements made at the lowest turbulent velocity. The results obtained in this study have been compared with those evaluated for a number turbulent burning velocity correlations and the differences are discussed.     

Three-dimensional thermocapillary–buoyancy flow of silicone oil in a differentially heated annular pool

In order to understand the characteristics of thermocapillary–buoyancy flow, we conducted a series of unsteady three-dimensional numerical simulations of thermocapillary–buoyancy flow of 0.65cSt silicone oil (Prandtl number Pr = 6.7) in an annular pool with different depth (d = 1–11 mm) heated from the outer wall (radius r_o = 40 mm) and cooled at the inner cylinder (r_i = 20 mm) with an adiabatic solid bottom and adiabatic free surface. Simulation conditions correspond to those in the experiments of Schwabe [D. Schwabe, Buoyant–thermocapillary and pure thermocapillary convective instabilities in Czochralski systems, J. Crystal Growth 237–239 (2002) 1849–1853]. Simulation results with large Marangoni number predict three types three-dimensional flow patterns. In the shallow thin pool (d = 1 mm), the hydrothermal wave characterized by curved spokes is dominant. In the deep pools (d ⩾ 5 mm) the three-dimensional stationary flow appears and this flow pattern corresponds to the Rayleigh-Benard instability, which consists of pairs of counter-rotating longitudinal rolls. When 2 mm ⩽ d ⩽ 4 mm, the hydrothermal wave and three-dimensional oscillatory flow coexist in the pool and travel along the same azimuthal direction with the same angular velocity. The critical conditions for the onset of three-dimensional flows were determined and compared with the experimental results. The characteristics of three-dimensional flows were discussed.     

An experimental study on the effects of a new swirl generator on thermal performance of a circular tube

This study experimentally focuses on the effects of a swirl generator on the thermal performance of a heat exchanging tube. The applied swirl generator is a helically twisted tube with a five-lobe cross section. As the main outcome, the thermal performance of the test tube equipped with the swirl generator are evaluated using the heat transfer rate in the form of Nusselt number and pressure drop in the form of friction factor. Water is used as the working fluid in the experiments performed for different Reynolds numbers from 6000 to 30,000. The different values of twist-angle (90 ≤ θ ≤ 360) and length (2 ≤ l ≤ 4) are investigated as the main geometrical parameters of the swirl generator. The results show that the swirl generator offers an enhancement up to 85% in the Nusselt number and an increase up to 52% in the friction factor. Therefore, the swirl generator presents a thermal performance up to 1.65. This study presents some correlations to predict the Nusselt number and the friction factor of the test tube equipped with the swirl generator.     

Computed NOx emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NO_x emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NO_x emission of H₂/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NO_x are formed through Zeldovich (or thermal), NNH and N₂O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N₂O routes also are the primary NO formation paths, a large amount of N₂ will be reformed from NNH and N₂O species. For hydrogen-rich syngas flames, the NO formation from NNH and N₂O routes are lesser, where NO can be dissipated through the reactions of NH + NO → N₂ + OH and NH + NO → N₂O + H more actively. At a rather low pressure (0.01 atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5–10 atm), NO is then converted back to N₂ through reversed N₂O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO₂, H₂O, and N₂ on NO emissions for H₂/CO syngas flames were studied. The hydrogen-lean syngas flames with H₂O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH + O → NH + NO. But for hydrogen-rich syngas flames with CO₂ dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route.     

Hydrothermal preparation and electrochemical properties of Gd3+ and Bi3+, Sm3+, La3+, and Nd3+ codoped ceria-based electrolytes for intermediate temperature-solid oxide fuel cell

The structure, the thermal expansion coefficient, electrical conductivities of Ce_0.8Gd_0.2?xM_xO_2?δ (for M: Bi, x = 0–0.1, and for M: Sm, La, and Nd, x = 0.02) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (28–59 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300–1400 °C. The maximum conductivity, σ_700 °C around 4.46 × 10^?2 S cm^?1 with E_a = 0.52 eV, is found at x = 0.1 for Bi-co-doping. Among various metal-co-dopings, for x = 0.02, the maximum conductivity, σ_700 °C around 2.88 × 10^?2 S cm^?1 with E_a = 0.67 eV, is found for Sm-co-doping. The electrolytic domain boundary (EDB) of Ce_0.8Gd_0.1Bi_0.1O_2?δ is found to be 1.2 × 10^?19 atm, which is relatively lower than that of the singly doped samples. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.6 × 10^?6 K^?1 for the CeO₂, 12.1 × 10^?6 K^?1 for Ce_0.8Gd_0.2O_2?δ, and increase with co-doping to 14.2 × 10^?6 K^?1 for Ce_0.8Gd_0.18Bi_0.02O_2?δ. The maximum power densities for the single cell based on the codoped samples are higher than that of the singly doped sample. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes.     

Study of interaction of entrained coal dust particles in lean methane–air premixed flames

This study investigates the interaction of micron-sized coal particles entrained into lean methane–air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0–25 μm, 53–63 μm, and 75–90 μm, is used. The effects of different coal dust concentrations (10–300 g/m³) entrained into the mixture of methane–air at three lean equivalence ratios, ?, of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust–methane–air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53–63 μm and 75–90 μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0–25 μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.     

Prediction of the critical condition for flame acceleration over wood surface with different sample orientations

To understand the inclination effects on flame spread over wood surface, a set of flame spread experiments were carried out for different sample orientation angles from ?50° to 20° in the Hefei Plain (at the altitude of 50 m) and in the Tibetan plateau (at the altitude of 3658 m). At both altitudes, a transition zone was found at 10–20° orientation for flame spread rate, the preheated length and flame angle from the horizontal. The transition zone was an external manifestation of the change of flame spread from steady state to acceleration. A new relationship of Π_c = 1 was established to predict the occurrence of acceleration based on theoretical analysis. Experimental data at the two altitudes suggested that the critical value of Π_c is about 1.1–1.2, which has a good agreement with the theoretical value.