No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Furnace Wall Temperature Investigation
Abstract
It is desired to keep the outer metal walls of a heat transfer medium (HTM) furnace warm enough to prevent corrosion. A computational study was carried out in order to assess the normal and lowest possible sheet metal temperatures. Various combustion models, radiation parameters, and operating conditions were considered. Field-measured values matched CFD results closely. It was found that the walls were sufficiently warm under all reasonable modeling approaches and conceivable operating circumstances.
ResearchGate has not been able to resolve any citations for this publication.
The high temperature air combustion performance of natural gas in an industrial furnace with a swirling burner was numerically modeled. A Beta function PDF (Probability Density Function) combustion model was selected to simulate the gas combustion combined with the Reynolds Stress Model (RSM) to simulate the turbulent flow. The radiation was simulated by a Discrete Ordinates method. The NO chemistry was simulated by thermal NO model. The simulation was performed at inlet air oxygen fraction 8% and the total air excess ratio 1.1 for natural gas. The effect of preheated air temperature on NO emission, temperature, O2 and CO distribution in the furnace was investigated. Results showed that thermal NO emission increased when the preheated air temperature increased from 1073 K to 1473K. When the preheated air temperature increased, both of the maximum and averaged temperature in the furnace increased. The oxygen was consumed by the formation of thermal NO at higher inlet air temperature and the fuel was not fully burnt out.
Loss mechanisms in a scallop shrouded transonic power generation turbine blade passage at realistic engine conditions have been identified through a series of large-scale (typically 12 million finite volumes) simulations. All simulations are run with second-order discretization and viscous sublayer resolution, and they include the effects of viscous dissipation. The mesh (y+ near unity on all surfaces) is highly refined in the tip clearance region, casing recesses, and shroud region in order to fully capture complex interdependent flow physics and the associated losses. Aerodynamic losses, in order of their relative importance, are a result of the following: separation around the tip, recesses, and shroud; tip vortex creation; downstream mixing losses, localized shocks on the airfoil; and the passage vortex emanating from under the shroud. A number of helical lateral flows were established near the upper shroud surfaces as a result of lateral pressure gradients on the scalloped shroud. It was found that the tip leakage and passage losses increased approximately linearly with increasing tip clearance, both with and without the effect of the relative casing motion. For each tip clearance studied, scrubbing slightly reduced the tip leakage, but the overall production of entropy was increased by more than 50%. Also the overall passage mass flow rate, for a given inlet total pressure to exit static pressure ratio, increased almost linearly with increasing tip clearance. In addition, it was also found that there was slight positive and negative lift on the shroud, depending on the tip clearance. At the lowest tip clearance of 20 mils there was a negative lift on the shroud. In the 200-mil tip clearance case there was a positive lift on the shroud. The relative motion of the casing contributed positively to the lift at every tip clearance, affecting more at the lowest tip clearance where the casing is closest to the blade tip. Lastly, it was found that the computed entropy generation for the stationary 80-mils case using the SKE turbulence model was close to that of the 80-mils scrubbing case using the RKE turbulence model. In light of the proposed mechanisms and their relative contributions, suggested design considerations are posed.
Porous burners offer potential for ultra-lean methane emission mitigation by combustion. In these systems, heat recirculation between the porous medium and the fuel stream leads to enhanced combustion behaviour. In this research, conductive, convective and radiative heat transfer models were added in the commercial Computational Fluid Dynamics (CFD) code ANSYS CFX, to describe the interaction between the porous solid and the fluid. Relatively detailed skeletal kinetic mechanisms were implemented and a stiff chemistry solver was used to account for the differing chemical and fluid dynamics timescales. Results from test cases are presented to illustrate the model performance and to highlight some computational issues.
Beginning from a state of hydrostatic equilibrium, in which a heavy gas rests atop a light gas in a constant gravitational field, Rayleigh-Taylor instability at the interface will launch a shock wave into the upper fluid. We have performed a series of large-eddy simulations which suggest that the rising bubbles of light fluid act like pistons, compressing the heavy fluid ahead of the fronts and generating shocklets. These shocklets coalesce in multidimensional fashion into a strong normal shock, which increases in strength as it propagates upwards. The simulations demonstrate that the shock Mach number increases faster in three dimensions than it does in two dimensions. The generation of shocks via Rayleigh-Taylor instability could play an important role in type Ia supernovae.
In large eddy simulation of turbulent flow, because of the spatial filter, inhomogeneity and anisotropy affect the subgrid stress via the mean flow gradient. A method of evaluating the mean effects is to split the subgrid stress tensor into ‘rapid’ and ‘slow’ parts. This decomposition was introduced by L. Shao et al. [Phys. Fluids 11, No. 5, 1229–1248 (1999; Zbl 1147.76495)] and applied to a priori tests of existing subgrid models in the case of a turbulent mixing layer. In the present work, the decomposition is extended to the case of a passive scalar in inhomogeneous turbulence. The contributions of rapid and slow subgrid scalar flux, both in the equations of scalar energy and scalar flux, are analyzed. A priori numerical tests are performed in the turbulent Couette flow with a mean scalar gradient. Results are then used to evaluate the performances of different popular subgrid scalar models. It is shown that the existing models can not well simulate the slow part and need to be improved in future works.
Time-averaged predictions from unsteady solutions of the two-dimensional Navier–Stokes equations are contrasted with Reynolds-averaged results for a reacting flow problem in a high pressure combustor. The goal is to determine whether the two-dimensional unsteady approximation can be useful as an engineering analysis in problems for which time-averaged quantities are of primary interest. The conditions are taken from an experiment in which non-premixed gaseous oxygen and hydrogen were injected into a combustion chamber through coaxial channels. The resulting flowfield is dominated by a large recirculation zone arising from the back-step created by the injector. The results of steady and time-averaged, unsteady solutions are strikingly different. The unsteady simulation produces strong unsteady structures whose time-averaged results lead to a much wider flame zone, a different recirculation zone structure, and a substantially different wall heat flux than those obtained with a steady RANS procedure. The time-averaged calculations yield the correct combustor chamber pressure and compare considerably more favorably with heat flux measurements than do the RANS results. The two-dimensional approximation, however, overstates the unsteady vortex roll up and precludes large scale mixing across the axis of symmetry, thereby giving deficient predictions near the centerline. Overall, the present results indicate that capturing large-scale unsteady characteristics can provide more accurate predictions of recirculation dominated reacting flows and suggest that two-dimensional, time-averaged solutions represent a potentially useful engineering tool for problems of this nature while also serving as a precursor for full three-dimensional simulations.
Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and cannot correctly predict the rich flammability limit. However, the best choice of rate parameters suitably reproduces rich and lean flammability limits as well as the dependence of the flame speed on pressure and equivalence ratio for all of the fuels examined. Two-step and quasi-global approaches also yield information on flame temperature and burned gas composition. However, none of the simplified mechanisms studied accurately describes the chemical structure of the flame itself.
The formation of self-excited pressure oscillations in technical combustion systems depends on the dynamical behaviour of the flames used. One goal of future combustor development is the prediction of combustion instabilities during the design process. The first aim of the work was to calculate the flame transfer function of forced flames with CFD codes using 'Unsteady-Reynolds-Averaged-Navier-Stokes' approaches. Pulsed isothermal turbulent jets as well as pulsed premixed jet flames with different thermal loads (20/40/60 kW) were modelled using forcing frequencies of up to 200 Hz. Three different versions of the κ-ε turbulence model and a 'Turbulent-Flamespeed-Closure' combustion model were applied. A variation of discretisation schemes and turbulent diffusion allowed the estimation of numerical errors. An important mechanism driving combustion instabilities is the interaction of the flame with large-scale ring vortices. The second aim was to investigate how the interaction works to get more insight and understanding of this phenomenon. The results obtained were validated against experimental data.
A 3D computational fluid dynamics investigation of particle-induced flow effects and liquid entrainment from an industrial-scale separator has been carried out using the Eulerian-Lagrangian two-way coupled multiphase approach. A differential Reynolds stress model was used to predict the gas phase turbulence field. The dispersed (liquid) phase was present at an intermediate mass loading (0.25) but low volume fraction (0.05). A discrete random walk method was used to track the paths of the liquid droplet releases. It was found that gas phase deformation and turbulence fields were significantly impacted by the presence of the liquid phase; these effects have been parametrically quantified. Substantial enhancement of both the turbulence and the anisotropy of the continuous phase by the liquid phase was demonstrated. It was also found that a large number (&1000) of independent liquid droplet release events were needed to make conclusions about liquid entrainment. Known plant run conditions and entrainment rates validated the numerical method.
A moving-deforming grid study was carried out using a commercial computational fluid dynamics (CFD) solver, FLUENT® 6.2.16. The goal was to quantify the level of mixing of a lower-viscosity additive (at a mass concentration below 10%) into a higher-viscosity process fluid for a large-scale metering gear pump configuration typical in plastics manufacturing. Second-order upwinding and bounded central differencing schemes were used to reduce numerical diffusion. A maximum solver progression rate of 0.0003 revolutions per time step was required for an accurate solution. Fluid properties, additive feed arrangement, pump scale, and pump speed were systematically studied for their effects on mixing. For each additive feed arrangement studied, the additive was fed in individual stream(s) into the pump-intake. Pump intake additive variability, in terms of coefficient of spatial variation (COV), was >300% for all cases. The model indicated that the pump discharge additive COV ranged from 45% for a single centerline additive feed stream to 5.5% for multiple additive feed streams. It was found that viscous heating and thermal/shear-thinning characteristics in the process fluid slightly improved mixing, reducing the outlet COV to 3.2% for the multiple feed-stream case. The outlet COV fell to 2.0% for a half-scale arrangement with similar physics. Lastly, it was found that if the smaller unit's speed were halved, the outlet COV was reduced to 1.5%.
A large-scale parametric air–water test stand (AWTS) study involving more than 40 evaluations was carried out for the purposes of three-stream airblast reactor feed injector characterization and optimization; a subset of seven air stream combinations is discussed here. The role of CFD as a supplement to, or a replacement for, air–water testing is of great industrial interest. To this end a set of CFD simulations was carried out to complement the AWTS study. Pressure responses, spray opening characteristics near the feed injector face, and spray distribution were primary measures for both the AWTS and CFD programs. It was found that, over the range of variables studied, there was somewhat of a match between CFD and AWTS results. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS (frequencies between 75 and 600Hz), and an interesting transition in spray bursting character occurred at moderate inner air flows. The oscillatory flow pattern mimics prior work in terms of the energy of the fluctuations, but the fact that the present fluctuations occur at an order of magnitude lower frequency is apparently related to the comparatively low gas/liquid momentum ratio in the current study. Overall, it is shown that the CFD method contained herein can be used to supplement, but not replace, air–water testing for said injector configuration.
Principles of mathematical models as tools in engineering and science are discussed in relation to turbulent combustion modeling.A model is presented for the rate of combustion which takes into account the intermittent appearance of reacting species in turbulent flames. This model relates the rate of combustion to the rate of dissipation of eddies and expresses the rate of reaction by the mean concentration of a reacting specie, the turbulent kinetic energy and the rate of dissipation of this energy. The essential features of this model are that it does not call for predictions of fluctuations of reacting species and that it is applicable to premixed as well as diffusion flames.The combustion model is tested on both premixed and diffusion flames with good results.Special attention is given to soot formation and combustion in turbulent flames. Predictions are made for two C2 H2 turbulent diffusion flames by incorporating both the above combustion model and the model for the rate of soot formation developed by Tesner et al., as well as previous observations by Magnussen concerning the behavior of soot in turbulent flames.The predicted results are in close agreement with the experimental data.All predictions in the present paper have been made by modeling turbulence by the k-∈ model. Buoyancy is taken into consideration in the momentum equations. Effects of terms containing density fluctuations have not been included.
A computational fluid dynamics (CFD) modeling study has been carried out. The study involved gaseous fuel combustion with associated chemical reactions, radiative heat transfer, and turbulence. The three different combustion environments that were adopted experimentally in a 100 kW drop-tube firing unit were examined. One air-fired and two oxy-fuel-fired cases [21 vol % O2 for one combustion case (OF21) and 27 vol % O2 for the other combustion case (OF27)] were investigated. A swirl injection system was used to achieve the flame stability of the turbulent non-premixed combustible gases. A modified eddy breakup (EBU) model was used with appropriate empirical coefficients for propane combustion reactions. The irreversible single-step and reversible multi-step reaction mechanisms were considered. The overall agreement of the CFD results with the available measured data was reasonable. The data compared were the temperature distributions and the species concentrations (CO2, CO, and O2) at the most intensive combustion locations in the furnace. The luminous appearance and temperature levels of the OF27 flame were relatively close to the reference (air-fired) flame. This was due to a reduced volumetric flow rate and an increase in the O2 concentration in the gas mixture. The carbon dioxide concentrations for both oxy-fuel-fired scenarios were around 8 times higher than that of the air-fired combustion case. The results obtained with the multi-step chemistry mechanism showed improved agreement, particularly in the flame zone. The concentration of CO was lower in the OF21 case. The unburnt fuel in the air-fired and OF27 cases was less than that of the OF21 case because of the low oxygen concentration used in the latter combustion case. This study can provide a basis for the future investigation of combustion characteristics in a large-scale furnace under oxy-fuel-firing conditions
Calcined coke is an important material for making carbon anodes for smelting alumina to aluminum. Calcining is an energy intensive industry and a significant amount of heat is wasted in the calcining process. Efficiently managing this energy resource is tied to the profit margin and survivability of a calcining plant. To help improve the energy efficiency of the calcining process, a 3D computational model is developed to gain insight of the thermal-flow and combustion behavior in the calciner. Comprehensive models are employed to simulate the moving petcoke bed with moisture evaporation, devolatilization, and coke fines combustion with a conjugate radiation-convection-conduction calculation.
This book has been more than 20 years in gestation; its lineage can be traced back to Barrie's lecturing at the University of Surrey in the late 1970s and early 1980s and Peter's first combustion course, provided internally to Rugby Cement ' s engineers in 1981.
Oxy-coal combustion exhibits different characteristics of combustion, flow and heat transfer from those of air-coal combustion, due to the high concentration of CO2 and H2O in the product gases. Using computational modeling, this study investigated the combustion and wall heat flux (WHF) of a 100 MWe boiler under air- and oxy-coal combustion conditions. The boiler had 12 swirl burners installed on the front wall for thermal input of 284 MWth. Flame temperatures and corresponding WHF in oxy-coal combustion increased linearly as O2 concentration increased from 24% to 30%. The case with 28% O2 achieved the same level of WHF with that of air-coal combustion, which had a similar adiabatic flame temperature. Due to the lower heat capacity, the gas temperature above the burner region lowered more rapidly in air-coal combustion than in oxy-coal combustion. The proportion of char converted by CO2 and H2O increased from approximately 8% in air-coal combustion to 19–23% in oxy-coal combustion. The increased rates of endothermic gasification reactions by CO2 and H2O lowered the temperature in the internal recirculation zone during oxy-coal combustion. This retarded char oxidation upstream of the flames.
The weighted sum of gray gases model postulates that the total emissivity and absorptivity may be represented to any desired degree of accuracy by the sum of a gray gas emissivity (or absorptivity) weighted with a temperature dependent factor. The gray gas emissivity is expressed in terms of an absorption coefficient and product of the absorbing gas partial pressure and path length. The weighting factors are generally given by polynomials in gas temperature with associated polynomial coefficients. For absorptivity, a second polynomial for the surface irradiation temperature must be introduced. Absorption and polynomial coefficients are reported for total emissivity and absorptivity for carbon dioxide, water vapor, and mixtures of these gases. A regression scheme was employed to fit the model to property values of total emissivity and absorptivity obtained from the exponential wide band model.
The features of the internal and external flows in high-speed vehicles with a magnetohydrodynamic air-intake ensuring additional deceleration of the supersonic flow are considered. Preliminary investigations carried out earlier showed that this MHD flow control makes it possible significantly to increase the gasdynamic component of the vehicle thrust. However, there are significant negative effects, mainly the development of an additional vehicle drag force associated with the magnetic field. Thus, there arises a complex of interrelated problems with opposite effects on the resulting characteristics of the vehicle. In the present study these questions are investigated both on the basis of developed physicomathematical models and numerical methods and by means of the combined optimization of the internal duct profile and the external configuration of the vehicle. It is shown that a strategy for improving the vehicle characteristics can only be chosen by simultaneously analyzing the features of the internal (magnetohydrodynamic) and external (gasdynamic) flows.
In this paper, a comprehensive computational fluid dynamics (CFD) modelling study was undertaken by integrating the combustion of pulverized dry lignite in several combustion environments. Four different cases were investigated: an air-fired and three different oxy-fuel combustion environments (25vol.% O2 concentration (OF25), 27vol.% O2 concentration (OF27), and 29vol.% O2 concentration (OF29) were considered. The chemical reactions (devolatilization and char burnout), convective and radiative heat transfer, fluid and particle flow fields (homogenous and heterogenous processes), and turbulent models were employed in 3-D hybrid unstructured grid CFD simulations. The available experimental results from a lab-scale 100KW firing lignite unit (Chalmer’s furnace) were selected for the validation of these simulations. The aerodynamic effect of primary and secondary registers of the burner was included through swirl at the burner inlet in order to achieve the flame stability inside the furnace. Validation and comparison of all the combustion cases with the experimental data were made by using the temperature distribution profiles and species concentration (O2, CO2, and H2O) profiles at the most intense combustion locations of the furnace. The overall visualization of the flame temperature distributions and oxygen concentrations were presented in the upper part of the furnace. The numerical results showed that the flame temperature distributions and O2 consumptions of the OF25 case were approximately similar to the reference combustion case. In contrast, in the OF27 and OF29 combustion cases, the flame temperatures were higher and more confined in the closest region of the burner exit plane. This was a result of the quick consumption of oxygen that led to improve the ignition conditions in the latter combustion cases. Therefore, it is concluded that the resident time, stoichiometry, and recycled flue gas rates are relevant parameters to optimize the design of oxy-fuel furnaces. The findings showed reasonable agreement with the qualitative and quantitative measurements of temperature distribution profiles and species concentration profiles at the most intense combustion locations inside the furnace. These numerical results can provide useful information towards future modelling of the behaviour of pulverized brown coal in a large-scale oxy-fuel furnace/boiler in order to optimize the burner’s and combustor’s design.
NOx formation during the combustion process occurs mainly through the oxidation of nitrogen in the combustion air (thermal NOx) and through oxidation of nitrogen with the fuel (prompt NOx). The present study aims to investigate numerically the problem of NOx pollution using a model furnace of an industrial boiler utilizing fuel gas. The importance of this problem is mainly due to its relation to the pollutants produced by large boiler furnaces used widely in thermal industrial plants. Governing conservation equations of mass, momentum and energy, and equations representing the transport of species concentrations, turbulence, combustion and radiation modeling in addition to NO modeling equations were solved together to present temperature and NO distribution inside the radiation and convection sections of the boiler. The boiler under investigation is a 160MW, water-tube boiler, gas fired with natural gas and having two vertically aligned burners.The simulation study provided the NO distribution in the combustion chamber and in the exhaust gas at various operating conditions of fuel to air ratio with varying either the fuel or air mass flow rate, inlet air temperature and combustion primary air swirl angle. In particular, the simulation provided more insight on the correlation between the maximum furnace temperature and furnace average temperatures and the thermal NO concentration. The results have shown that the furnace average temperature and NO concentration decrease as the excess air factor λ increases for a given air mass flow rate. When considering a fixed value of mass flow rate of fuel, the results show that increasing λ results in a maximum value of thermal NO concentration at the exit of the boiler at λ=1.2. As the combustion air temperature increases, furnace temperature increases and the thermal NO concentration increases sharply. The results also show that NO concentration at exit of the boiler exhibits a minimum value at around swirl angle of 45°.
Burning velocities of mixtures of air with methonol, isooctane, and indolene (RMFD303) have been measured using the constant volume bomb method for fuel-air equivalence ratios φ = 0.8-1.5 over the pressure and temperature ranges p = 0.4–50 atm and T = 298–700K. The effect of adding simulated combustion products to stoichiometric isooctane-air mixtures was also studied for diluent mass fractions f = 0−0.2. Over the range studied, the results can be fit within ± 10% by the functional form , where Su0 depends on fuel type and equivalence ratio and α and β depend only on equivalence ratio. In overlapping ranges, the results agree well with those previously reported.
Unsteady flow features of a plant-scale (>1.5 m diameter) cyclone-ejector system have been studied numerically and validated experimentally. Complexity arises from the fact that the transient pressure field within the Lapple cyclone governs the operation of the annular ejector, and vice versa. Eight geometric configurations for improving the system operation were evaluated. Simple geometric changes were shown numerically to make operational improvements while incrementally improving particle collection efficiency. It was also found that compressible, time-dependent CFD results were extremely sensitive to the pressure discretisation approach and to the differential Reynolds Stress pressure strain formulation.
An isothermal, incompressible, swirling flow in three generic combustor configurations featuring different inflow structure with respect to the circumferential velocity type (both configurations with annular and central swirling jet were considered), combustor confinement (in terms of expansion ratio ER) and swirl intensity (S) was studied computationally by means of Reynolds Averaged Navier-Stokes method (RANS), using second-Moment Closure (SMC) models. The work focuses on the investigation of the combined effects of the above-mentioned flow parameters on the mixing between a swirling annular jet (representing air stream) and the non-swirling inner jet (representing fuel) within a combustor. Both the basic high-Reynolds number SMC model, modified to account for the non-linearities in the pressure scrambling and dissipation processes, and a low-Reynolds number SMC model, accounting separately for the viscous and non-viscous wall blockage, were applied. Inflow conditions are computationally generated, rather than prescribed. In the course of the inflow data generation, a number of the ''equilibrium'' swirling flows in the concentric annulus and pipe geometries, from which coaxial and central swirling jets expand into a combustion chamber, are computed. The SMC results reproduced all important mean flow and turbulent features in good agreement with available experimental and LES data.
A one-dimensional and unsteady model for the gasification/combustion of thick wood is developed and coupled with a two-dimensional CFD model for the gaseous phase processes, solved by the commercial code CFX4.4. Simulations of the combustion of single wood logs, as obtained with the coupled solid- and gas-phase models and with a solid-phase model alone, are compared with experimental measurements. Acceptable agreement is obtained in the first case, whereas the solid-phase model alone highly underestimates the conversion time and overestimates the mean mass loss rates. A two-dimensional formulation of the equations for the solid-phase does not affect significantly the global parameters.
Today, CFD is a common analysis tool for combustion chambers. A difficult task is still the accurate prediction of minor and intermediate species such as CO. The difficulty arises because the time scales associated to CO production and oxidation are of the same order as the turbulent mixing process, but most turbulent combustion models assume that the chemistry is either very fast or relatively slow in comparison to the time scales of turbulence and mixing. The Intrinsic Low Dimensional Manifold (ILDM) method based on mixture fraction and two reaction progress variables is used to generate an ILDM chemistry table. The method removes fast reactions but includes inter-medium and slow reaction paths. A presumed PDF-ILDM model for turbulent combustion is implemented in the commercial CFD code CFX-5, which models the reaction progress through a reduced set of reaction progress variables. During the CFD analysis all thermo-chemical data is obtained from a pre-integrated chemistry table. A comparison with a partially premixed jet flame shows that the PDF-ILDM model performs much better than other more commonly used combustion models. In particular the good CO predictions are encouraging and indicates the potential of the model for more complex applications.
Modern low-emission premix combustion systems are often susceptible to combustion instabilities. Active instability control (AIC) systems are commonly used to attenuate those oscillations. For the control authority of AIC systems the effective amplitude and phase of the fuel modulation at the fuel outlet are as critical as the proper injection position. In typical cases the modulation of the fuel at the location of the actuator can be fundamentally different in amplitude and phase from the modulation of the fuel flow at the fuel outlet. In addition to the well known effects stemming from the acoustics and Mach number of the fuel system, the fuel flow in the fuel system is also modulated by the oscillation of the pressure in the combustor in case of combustion instabilities. The superposition of the upstream modulation by the actuator and the modulation downstream by the combustion instability can result in an unexpected behaviour of the fuel injection, from total compensation of the modulation to very high oscillations in the resonant case accompanied by drastic phase shifts. This paper describes the influence of the secondary fuel modulation due to the combustion instability on the control authority of AIC systems on the basis of theoretical considerations and measurements for an atmospheric test rig with a natural gas fired swirl burner.
Five different flame states are identified in a compact combustion chamber that is fired by a 30 kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure. These flame states include a nozzle-attached tulip shaped flame, a nonattached torroidal-ring shaped flame (SSF) suitable for very low NOx emission in a gas turbine combustor and a Coanda flame (CSF) that clings to the bottom wall of the combustion chamber. Flame state transition is generated by changing the swirl number and by pre-mixing the combustion air with 70% of the natural gas flow. The flame state transition pathways reveal strong hysteresis and bifurcation phenomena. The paper also presents major species concentrations, temperature and velocity profiles of the lifted fame state and the Coanda flame and discusses the mechanisms of fame transition and stabilization.
Tunable diode laser (TDL) absorption sensors of water vapor are attractive for temperature, gas composition, velocity, pressure, and mass flux measurements in a variety of practical applications including hydrocarbon-fueled combustion systems. Optimized design of these sensors requires a complete catalog of the assigned transitions with accurate spectroscopic data; our particular interest has been in the 2ν1, 2ν3, and ν1+ν3 bands in the near-IR where telecommunications diode lasers are available. In support of this need, fully resolved absorption spectra of H2O vapor in the spectral range of 6940–7440cm−1 (1344–1441nm) have been measured as a function of temperature (296–1000K) and pressure (1–800Torr), and quantitative spectroscopic parameters inferred from these spectra compared to published data from Toth, HITRAN 2000 and HITRAN 2004. The peak absorbances were measured for more than 100 strong transitions at 296 and 828K, and linestrengths determined for 47 strong lines in this region. In addition to reference linestrengths S(296K), the air-broadening coefficients γair(296K) and temperature exponents n were inferred for strong transitions in five narrow regions, near 7185.60, 7203.89, 7405.11, 7426.14 and 7435.62cm−1 that had been targeted as attractive for future diagnostics applications. Most of the measured results, determined within an accuracy of 5%, are found to be in better agreement with HITRAN 2004 than with earlier editions of this database. Large discrepancies (>10%) between measurements and HITRAN 2004 database are identified for some of the probed transitions. These new spectroscopic data for H2O provide a useful test of the sensor design capabilities of HITRAN 2004 for combustion and other applications at elevated temperatures.
The work is concentrated on the formulation and validation of integral models within RANS framework for the numerical prediction of the premixed and partially premixed flames occurring in gas turbine combustors. The premixed combustion modelling is based on the BML approach coupled to the mixing transport providing variable equivalence ratio. Chemistry is described by means of ILDM model solving transport equations for reaction progress variables conditioned on the flame front. Multivariate presumed PDF model is used for the turbulence-chemistry interaction treatment. Turbulence is modelled using the second moment closure (SMC) and the standard κ-ε model as well. The influence of non-gradient turbulent transport is investigated comparing the gradient diffusion closure and the solution of the scalar flux transport equations. Different model combinations are assessed simulating several premixed and partially premixed flame configurations and comparing results to the experimental data. The proposed model provides good predictions particularly in combination with SMC.
A 45? eight flat bladed swirler was investigated with eight equispaced fuel holes 20 mm downstream of the swirler exit on the outer wall of the swirler discharge duct. The swirler had a dump flow expansion from the swirler discharge duct (diameter d) to the combustor cylinder (diameter D) with an expansion ratio D/d of 1?84. The axial swirler and the dump flow expansion were typical of the size used in industrial low NOx swirler combustors with 60% of the combustion air flow through the swirler. The axial swirler had a 25 mm diameter central hub that was used to house a central fuel injector with eight radial outward injector holes. The results showed that with outer swirler wall fuel injection, ultra low NOx emissions were obtained with natural gas and propane at 600 K and 740 K inlet temperatures and atmospheric pressure combustion. Central fuel injection was shown to have superior flame stability and inferior NOx emissions and was a good location for a pilot flame to assist in power turndown flame stability requirements.
Two-dimensional burner-flame stability is discussed with arbitrary gas expansion. Density variations are allowed for by fully coupling the continuity and momentum equations. The flame is assumed to be close to a porous-plug-type flameholder so that the conventional hydrodynamic zone upstream of the flame cannot be included. Instead, the flow is assumed to obey a Darcy-type law within the holder, relating pressure gradient and velocity. It is shown that the influence of the holder and the acceleration due to gravity are important factors governing the onset of cellularity in porous-plug burner flames. Further, the balance of the transverse and longitudinal Darcy constants used to describe the upstream hydrodynamic zone within the holder have a vital effect on stability predictions. Experimental observations are confirmed by the theory presented.
Radiation is the principal mode of heat transfer in furnaces. Models for gaseous radiative properties have been well-established for air combustion. However, there is uncertainty regarding their applicability to oxy−fuel conditions. In this paper, a new and complete set of weighted sum of gray gases model (WSGGM) is derived, which is applicable to computational fluid dynamics (CFD) modeling of both air−fuel and oxy−fuel combustion. First, a computer code is developed to evaluate the emissivity of any gas mixture at any condition using the exponential wide band model (EWBM), and the calculated results are validated in detail against data in the literature. Then, the validated code is used to generate emissivity databases for representative air- and oxy-firing conditions, for each of which a refined WSGGM with new parameters is derived. The practical way to implement the model to CFD simulations of combustion systems is given. Finally, as a demonstration, the new model is implemented to CFD modeling of two furnaces of very different beam lengths. The CFD results are compared to those based on the widely used WSGGM in the literature, from which some useful guidelines on oxy−fuel modeling are recommended.
A mixedness-reactedness flamelet combustion model coupled with a comprehensive radiation heat transfer model based on the
discrete transfer method of solution of the radiative transport equation is applied for the simulation of a 3MW non-swirling
turbulent non-premixed natural gas flame in the experimental furnace at the International Flame Research Foundation. In the
calculation, turbulence is represented by the standard k − ε and a differential Reynolds-stress model. Predictions are compared with measurements of mean gas velocity, temperature, major
species concentrations and incident radiation wall flux. The radiative mixedness-reactedness flamelet combustion model, irrespective
of the model for turbulence, is able to reproduce the basic structure of the experimental flame, which is stabilised downstream
of the burner nozzle. In the near burner region, encompassing the non-reacting lift-off zone, good quality predictions are
obtained using both the turbulence models, whereas further downstream, within the combusting zone of the jet, the Reynolds-stress
turbulence model generates better predictions at and about the furnace axis. The nitric oxide (NO) formation via the thermal-
and prompt-NO routes was also calculated and compared with in-flame and flue-gas NO data. The measured NO level at the furnace
exit is well reproduced in the calculation, however discrepancies exist near the burner where NO concentrations around the
furnace axis are overpredicted.
This paper presents a three dimensional numerical simulation with experimental validation of a gas-fired self-regenerative crucible furnace. Turbulence, radiation and chemical reactions are simulated using the software Gambit V2 and Fluent V6.2. Different combustion models are used to assess their effects on the numerical results. Aerodynamics, temperature fields, species profiles and emissions are compared with the experimental data. The results indicate that k–ε RNG model predicts the formation of two concentric swirls: the first one elevating up to the top of the furnace and the second one going down and reaching the outlet. In addition, it was found that is important to inject the fuel using certain vertical inclination of the nozzle in order to obtain a longer and flater flame. Finally, the use of PDF mixture fraction model for combustion causes overprediction of both temperature and CO, while Finite Rate/Eddy Dissipation model is rougher for temperature and species prediction.
The present work is focused on a new procedure for the determination of emission from combustion processes, which allow using very detailed and comprehensive reaction schemes, on the basis of the results obtained from CFD computations. This procedure is validated in the case of high swirled confined natural gas diffusion flames. The experimental data refer to the work developed within the German TECFLAM cooperation concerning a swirl burner fed with natural gas characterized by 150 kW thermal load and 0.8 equivalence ratio (TECFLAM webpage, www.tu-darmstadt.de/fb/mb/ekt/tecflam; Schmittel et al., Proceedings of the Combustion Institute 28 (2000) 303–309).
The order of accuracy and error magnitude of node- and cell-centered schemes are examined on representative unstructured meshes and flowfield solutions for computational fluid dynamics. Specifically, we investigate the properties of inviscid and viscous flux discretizations for isotropic and highly stretched meshes using the Method of Manufactured Solutions. Grid quality effects are studied by randomly perturbing the base meshes and cataloguing the error convergence as a function of grid size. For isotropic grids, node-centered approaches produce less error than cell-centered approaches. Moreover, a corrected node-centered scheme is shown to maintain third order accuracy for the inviscid terms on arbitrary triangular meshes. In contrast, for stretched meshes, cell-centered schemes are favored, with cell-centered prismatic approaches in particular showing the lowest levels of error. In three dimensions, simple flux integrations on non-planar control volume faces lead to first-order solution errors, while second-order accuracy is recovered by triangulation of the non-planar faces.
Laser diagnostic and flow visualization techniques have previously been applied to several reacting and isothermal swirling jets. Resolved flow features include the presence of vortex breakdown and unsteady behavior. As such, turbulent flames stabilized on the present (laboratory scale) burner bear similitude to those in larger (industrial) swirl combustors. With this in mind, the impact of vortex breakdown and unsteady behavior on flame stability continues to require further study. Such understanding can be gained through applying non-intrusive laser diagnostics to flow conditions covering a broad range of flame stability characteristics. This paper presents the results of one such investigation. Laser Doppler Velocimetry (LDV) measurements are acquired (along the centreline) to ascertain the presence of time periodicity and downstream recirculation. Unsteady behavior is identified through the spectra of axial velocity data whilst negative axial velocities delineate vortex breakdown. This information is augmented with observations of visible flame length and discussed in relation to established flame stability characteristics. Findings indicate that swirl numbers, over which improvements in flame stability occur, coincide with conditions leading to unsteady behavior and shorter flames. Because downstream flow reversal is not resolved at all such conditions, information available indicates flow unsteadiness is a clearer contributing factor, compared to vortex breakdown, on the observed stability characteristics. Shorter flames coupled with improvements in flame stability have significant implications on the design and operation of swirl combustors.
This study investigates the formation process of soot particles in turbulent flows within fuel-rich natural gas flames. Research on carbon formation from relatively simple hydrocarbons under real process conditions and harsh environments allows new insights into the general understanding of soot formation. Experimental work was undertaken on a small-scale gas furnace based on the design of a modern oil-fired carbon black furnace. The combustion system investigated is fired by a nonpremixed, swirl stabilized, confined methane-air flame. The study comprises establishing conditions under which the production of soot particles takes place, progressing toward quantified analysis. A large number of process parameters have been investigated, and the relative role of incomplete combustion and thermal decomposition in the process of carbon particulate formation has been illustrated. Maximum solid carbon formation was realized at maximum air temperature and maximum furnace temperature. Experimental investigations, using carbon particle concentration measurements coupled with on-line gas analysis showed that slightly fuel rich conditions are governed by incomplete combustion only, whereas richer combustion systems are governed by thermal decomposition processes. Both processes are found to be strongly temperature dependent, whereby an increase in temperature reduces particle production in the former process, but enhances it in the latter. A 3-D simulation of the simultaneous processes of incomplete fuel combustion and fuel decomposition in turbulent combustion systems, using a novel integration of fluid mechanics, chemical kinetics, and a standard soot model, facilitates discrimination between the two main sources of carbonaceous particulate matter, and indicates reasonable agreement with experimental trends.
Scaling and development of low-swirl burners for low-emission furnaces and boilers
- R Cheng
- D Yegian
- M Miyasato
- G Samuelsen
- C Benson
- R Pellizzari
- P Loftus
Cheng, R., Yegian, D., Miyasato, M., Samuelsen, G., Benson, C.,
Pellizzari, R. and Loftus, P. (2001) 'Scaling and development
of low-swirl burners for low-emission furnaces and
boilers', Proceedings of the Combustion Institute, Vol. 28,
No. 1, pp.1305-1313.
New Method of Computation of Radiation Heat Transfer in Combustion Chambers
- N Shah
Shah, N. (1979) New Method of Computation of Radiation Heat
Transfer in Combustion Chambers, PhD Thesis (Imperial
College of Science and Technology, London).
Simulation of combustion and thermal-flow inside a petroleum coke rotary calcining kiln, part 1: process review and modeling
- Z Zhang
- T Wang
Zhang, Z. and Wang, T. (2010) 'Simulation of combustion and
thermal-flow inside a petroleum coke rotary calcining kiln,
part 1: process review and modeling', ASME Journal of
Thermal Science and Engineering Applications, Vol. 2, No. 2,
pp.1-8.