Conditional moment closure calculations of a swirl-stabilized, turbulent nonpremixed methane flame

As natural gas production and usage continues to increase, displacing oil and coal, there is an escalating requirement to ensure that natural-gas burning equipment performs as cleanly and efficiently as possible to allow the environmental advantages of this fuel to be realized. Presented are encouraging results obtained from the three-dimensional, elliptic, CMC modeling of a low-swirl-stabilized nonpremixed flame of methane, which are representative of such technologies. Calculations were based upon the solution of the three-dimensional fluid-flow equations supplemented with a Reynolds stress and scalar flux second-moment closure, and the chemistry applied to represent mean production rates of species was a 16-step reduced mechanism. Predictions of species in both mixture fraction and real space display a level of conformity with experimental data that is encouraging for these methods in both a qualitative and a quantitative sense. Shortcomings of the modeling procedures are discussed in light of the results, and suggestions are made for future investigation.     

Using the tabulated diffusion flamelet model ADF-PCM to simulate a lifted methane-air jet flame

Two formulations of a turbulent combustion model based on the approximated diffusion flame presumed conditional moment (ADF-PCM) approach [J.-B. Michel, O. Colin, D. Veynante, Combust. Flame 152 (2008) 80-99] are presented. The aim is to describe autoignition and combustion in nonpremixed and partially premixed turbulent flames, while accounting for complex chemistry effects at a low computational cost. The starting point is the computation of approximate diffusion flames by solving the flamelet equation for the progress variable only, reading all chemical terms such as reaction rates or mass fractions from an FPI-type look-up table built from autoigniting PSR calculations using complex chemistry. These flamelets are then used to generate a turbulent look-up table where mean values are estimated by integration over presumed probability density functions. Two different versions of ADF-PCM are presented, differing by the probability density functions used to describe the evolution of the stoichiometric scalar dissipation rate: a Dirac function centered on the mean value for the basic ADF-PCM formulation, and a lognormal function for the improved formulation referenced ADF-PCMχ. The turbulent look-up table is read in the CFD code in the same manner as for PCM models. The developed models have been implemented into the compressible RANS CFD code IFP-C3D and applied to the simulation of the Cabra et al. experiment of a lifted methane jet flame [R. Cabra, J. Chen, R. Dibble, A. Karpetis, R. Barlow, Combust. Flame 143 (2005) 491-506]. The ADF-PCMχ model accurately reproduces the experimental lift-off height, while it is underpredicted by the basic ADF-PCM model. The ADF-PCMχ model shows a very satisfactory reproduction of the experimental mean and fluctuating values of major species mass fractions and temperature, while ADF-PCM yields noticeable deviations. Finally, a comparison of the experimental conditional probability densities of the progress variable for a given mixture fraction with model predictions is performed, showing that ADF-PCMχ reproduces the experimentally observed bimodal shape and its dependency on the mixture fraction, whereas ADF-PCM cannot retrieve this shape.     

Detailed investigation of a pulverized fuel swirl flame in CO2/O2 atmosphere

A novel approach to oxycoal flame stabilization has been developed at the Institute of Heat and Mass Transfer at RWTH Aachen University [D. Toporov, M. Förster, R. Kneer, in: Third Int. Conf. on Clean Coal Technologies for Our Future, Cagliari, Sardinia, Italy, 15-17 May 2007]. The swirl burner design and its operating conditions have been adjusted in order to enforce CO formation thus stabilizing the flame and obtaining a full burnout at levels of O₂ content in the O₂/CO₂ mixture similar to those in air. The paper presents results of detailed numerical and experimental investigations of a stable oxy-fired pulverized coal swirl flame (type-2) obtained with a 21 vol% O₂ concentration. The combustion tests were performed in a vertical pilot-scale furnace (100 kW_th) in the framework of the OXYCOAL-AC research project aiming to develop a membrane-based oxyfuel process. The experimental results concerning gas velocities, gas and particle temperatures, and gas compositions are presented and discussed, focusing on the underlying mechanisms as well as on the aerodynamics of the oxycoal flame. A comparison between measurements and simulations has shown the validity of the numerical method used. The reported data set can be used for validation of numerical models developed for prediction of oxyfuel combustion.     

Design and development of a SPMB (self-aspirating, porous medium burner) with a submerged flame

This work reports design and development of a SPMB (self-aspirating porous medium burner) for replacing the self-aspirating, CB (conventional gaseous fuel, free flame burners), which are widely used in heating process of SMEs (small and medium scale enterprises) in Thailand but they have relatively low thermal efficiency of about 30 percent. Design of the SPMB relies on the same important characteristics of the CB, i.e. using the same mixing tube and the same fuel nozzle. The SPMB is formed by a packed bed of alumina spheres. The pressure drop across the packed bed, diameter of particles and a combustion chamber diameter are estimated by Ergun’s equation in combination with Pe (Peclet number). The SPMB yields a submerged flame with an intense thermal radiation emitted downstream. An output radiation efficiency as high as 23 percent can be achieved at relatively high turn-down ratio of 2.65 and firing rate ranging from 23 to 61 kW. The SPMB shows a more complete combustion with relatively low CO emission of less than 200 ppm and acceptably high NO_x emission of less than 98 ppm as compared with the CB throughout the range of firing rate studied, suggesting the possibility of the SPMB in replacing the CB.     

Flame sheet model for the burning of a low-volatility liquid fuel in a low-permeability medium under low rates of strain

In this work we analyze a diffusion flame established in a low-permeability medium. A low-strained impinging jet of oxidant against a pool of low-volatility liquid fuel is the considered geometry. Owing to the differences on the transport properties of gas, liquid and solid, the problem presents physical processes occurring in different length scales. Hence, we perform an asymptotic analysis in order to obtain the profiles of temperature and species concentration in each length scale. As a result of the low-permeability feature of the medium, the velocity field is determined mainly by the gradient pressure (Darcy equation). The viscous effects become confined into small regions near the stagnation-point and the liquid–fuel interface. The effects of porosity, fuel Lewis number, strain-rate and liquid–fuel volatility on the flame temperature, flame position and vaporization rate are discussed. It is shown that the low-permeability medium is necessary in order to sustain the vaporization process of the low-volatility liquid fuel, as it enhances the heat transfer to the fuel reservoir. This model is valid for high rates of interphase heat exchange and low rates of strain.     

Properties of premixed hydrogen/propane/air flame in ceramic granular beds

This study investigated how various H₂ concentrations of fuel mixed with a ceramic granular bed (CGB) affect the propagation characteristics of a premixed C₃H₈/air flame. The results showed that at low firing rates (Γ < 1.5 kJ/s), as Γ increased, adding H₂ exerted little effect on the flame temperature and absolute flame front velocity (S_ab); however, at high Γ (Γ > 1.5 kJ/s), as Γ increased, adding H₂ caused a significant decrease in the flame temperature and a substantial increase in S_ab. Thus, heat transfer was not obvious, and the flame reaction zone was relatively broad. Although adding H₂ moved the flame reaction zone upstream, the heat transfer mechanism reduced the effect that resulted from adding H₂ under low Γ conditions. Because of the characteristics of H₂, the flame reaction zones moved upstream, causing the flame thickness and heat loss to increase. Adding H₂ decreased the flame temperature and increased S_ab.     

Impact of injection conditions on flame characteristics from a parallel multi-jet burner

This numerical study systematically investigates the influence of initial injection conditions of reactants on flame characteristics from a parallel multi-jet burner in a laboratory-scale furnace. In particular, varying characteristics from visible flame to invisible Moderate or Intense Low-oxygen Dilution (MILD) combustion is explored. Different parameters examined include the initial separation of fuel and air streams (S), air nozzle diameter (D_a), fuel nozzle diameter (D_f), and air preheat temperature (T_a). The present simulations agree qualitatively well with previous measurements reported elsewhere for two reference cases investigated by experiment. A number of new and significant findings are then deduced from the simulations. For instance, all S, D_a and D_f are found to play significant roles in achieving a proper confluence location of air and fuel jets for establishing the MILD combustion. Particularly, varying D_a is most effective for controlling the combustion characteristics. It is also found that the stability limits of the non-premixed MILD combustion varies with different combustor systems and inlet reactant properties. Moreover, for the first time, several analytical approximations are obtained that relate the flue-gas recirculation rate and the fuel-jet penetration to D_a, D_f, S and also reactant properties.     

Radiation effects on MHD flow of Maxwell fluid in a channel with porous medium

This paper describes the heat transfer analysis with thermal radiation on the two-dimensional magnetohydrodynamic (MHD) flow in a channel with porous walls. The upper-convected Maxwell (UCM) fluid fills the porous space between the channel walls. The corresponding boundary layer equations are transformed into ordinary differential equations by means of similarity transformations. The resulting problems are solved by employing homotopy analysis method (HAM). Convergence of the derived series solutions is ensured. The effects of embedded parameters on the dimensionless velocity components and temperature are examined through plots. The variation of local Nusselt number is also analyzed.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Enhanced thermal stratification in a liquid storage tank with a porous manifold

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 L and a height-to-radius ratio of 4 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.     

Mixed convection boundary layer flow from a vertical flat plate embedded in a porous medium filled with nanofluids

The steady mixed convection boundary layer flow past a vertical flat plate embedded in a porous medium filled with nanofluids is studied using different types of nanoparticles as Cu (cuprom), Al₂O₃ (aluminium) and TiO₂ (titanium). The model used for the nanofluid is the one which incorporates only the nanoparticle volume fraction parameter. The basic partial equations are reduced to an ordinary differential equation which is solved numerically for some values of the volume fraction and mixed convection parameters. It is shown that the solution has two branches in a certain range of the parameters. The effects of these parameters on the velocity distribution are presented graphically.     

Visualization of heat flow in a vertical channel with fully developed mixed convection

A study on visualization of heat flow in three channels with laminar fully developed mixed convection heat transfer is performed. The first channel is filled with completely pure fluid; the second one is completely filled with fluid saturated porous medium. A porous layer exists in the half of the third channel while another half is filled with pure fluid. The velocity, temperature and heat transport fields are obtained both by using analytical and numerical methods. Analytical expression for heat transport field is obtained and presented. The heatline patterns are plotted for different values of Gr/Re, thermal conductivity ratio, Peclet and Darcy numbers. It is found that the path of heat flow in the channel strongly depends on Peclet number. For low Peclet numbers (i.e., Pe = 0.01), the path of heat flow is independent of Gr/Re and Darcy numbers. However, for high Peclet numbers (i.e., Pe = 5), the ratio of Gr/Re, Darcy number and thermal conductivity ratio influence heatline patterns, considerably. For the channels with high Peclet number (i.e., Pe = 5), a downward heat flow is observed when a reverse flow exits.     

Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method

The flame spray method for preparing spherical powders is a technique in which solid particles are forced to fuse into spheres by exposing them directly to the combustion flame, and has already been applied to producing ceramic (metal) spherical powders. With this technique, it is critical to analyze quantitatively the fusing and sphering characteristics of solid particles, and thus the distributions of gas velocity and temperature of the flame gas have to be calculated first. However, little research has been conducted regarding this subject. In this study, a combustion model is formulated to calculate the profiles of gas velocity and temperature for the fuel gas flame jetted from the commonly used coaxial double-pipe nozzle. The calculation agrees roughly with the flame shape observed in experiments, implying the possibility of using this model to calculate the heat transfer from flame gas to solid powders. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 201–213, 1996     

Unsteady mass transfer in mixed convective heat flow from a vertical plate embedded in a liquid-saturated porous medium with melting effect

This paper numerically studies the transient mass transfer in mixed convective heat flow with melting effect from a vertical plate in a liquid saturated porous medium in the presence of aiding external flow. The governing equations are transformed into the non-dimensional form by using pseudo similarity coordinate (ζ) and dimensionless time (ξ). The resulting two dimensional boundary value problem (BVP) is then solved by the method of lines (MOLs) with the central finite difference and Newton's iteration to obtain the entire numerical solutions for all transient process from the initial stage (ξ = 0) to the final state (ξ = 1). The results show the rate of dynamic mass transfer at the solid–liquid interface is reduced with increasing the melting strength. In addition, the response time and the rate of the dynamic mass transfer for aiding buoyancy are respectively shorter and faster than those for opposing buoyancy from the transient molecular diffusion to the steady mixed convection in a porous medium with melting effect.     

DNS of a premixed turbulent V flame and LES of a ducted flame using a FSD-PDF subgrid scale closure with FPI-tabulated chemistry

Two complementary simulations of premixed turbulent flames are discussed. Low Reynolds number two-dimensional direct numerical simulation of a premixed turbulent V flame is first performed, to further analyze the behavior of various flame quantities and to study key ingredients of premixed turbulent combustion modeling. Flame surface density, subgrid-scale variance of progress variables, and unresolved turbulent fluxes are analyzed. These simulations include fully detailed chemistry from a flame-generated tabulation (FPI) and the analysis focuses on the dynamics of the thin flame front. Then, a novel subgrid scale closure for large eddy simulation of premixed turbulent combustion (FSD-PDF) is proposed. It combines the flame surface density (FSD) approach with a presumed probability density function (PDF) of the progress variable that is used in FPI chemistry tabulation. The FSD is useful for introducing in the presumed PDF the influence of the spatially filtered thin reaction zone evolving within the subgrid. This is achieved via the exact relation between the PDF and the FSD. This relation involves the conditional filtered average of the magnitude of the gradient of the progress variable. In the modeling, this conditional filtered mean is approximated from the filtered gradient of the progress variable of the FPI laminar flame. Balance equations providing mean and variance of the progress variable together with the measure of the filtered gradient are used to presume the PDF. A three-dimensional larger Reynolds number flow configuration (ORACLES experiment) is then computed with FSD-PDF and the results are compared with measurements.     

Non-Darcy mixed convection from a horizontal plate embedded in a nanofluid saturated porous media

The problem of steady mixed convection boundary-layer flow over an impermeable horizontal flat plate embedded in a porous medium saturated by a nanofluid is numerically studied. The model used for the nanofluid incorporates only the effect of the volume fraction parameter. The surface of the plate is maintained at a constant temperature and a constant nano-particle volume fraction. The resulting governing partial differential equations are transformed into a set of two ordinary (similar) equations, which are solved using the bvp4c function from Matlab. A comparison is made with the available results in the literature, and the present results are in very good agreement with the known results. A representative set of numerical results for the reduced heat transfer from the plate, dimensionless velocity and temperature profiles is graphically and tabularly presented. Also, the salient features of the results are analyzed and discussed.     

Natural convection heat and mass transfer from a vertical truncated cone in a porous medium saturated with a non-Newtonian fluid with variable wall temperature and concentration

This work presents a boundary-layer analysis about the natural convection heat and mass transfer near a vertical truncated cone with variable wall temperature and concentration in a porous medium saturated with non-Newtonian power-law fluids. A coordinate transform is used to obtain the nonsimilar governing equations, and the transformed boundary-layer equations are solved by the cubic spline collocation method. Results for local Nusselt numbers are presented as functions of power-law indexes, surface temperature and concentration exponents, buoyancy ratios, and Lewis numbers. The heat and mass transfer rates of the truncated cones with higher surface temperature and concentration exponents are higher than those with lower exponents. Moreover, an increase in the power-law index of fluids tends to decrease the heat and mass transfer from a vertical truncated cone in a porous medium saturated with non-Newtonian power-law fluids.     

Couette flow through a porous medium of a high prandtl number fluid with temperature-dependent viscosity

The velocity and temperature profiles for an impulsively started Couette flow have been derived for a fluid with a high and strongly temperature-dependent viscosity when the flow takes place through a porous medium. The steady as well as the transient state flows are discussed and the influence of the medium permeability is assessed. In the steady state the presence of the porous medium causes higher maximum temperatures only for sufficiently low permeabilities and in all cases significantly lower velocity profiles. The flow development times tend to be higher for the low permeabilities only for high Nahme numbers. For the transient state it is noticeable that, in all cases, the velocity develops much more rapidly than the temperature and that the presence of the porous medium accelerates this tendency. Finally the medium permeability produces skew temperature profiles and higher temperature time gradients at all times.     

Double diffusion from a vertical truncated cone in a non-Newtonian fluid saturated porous medium with variable heat and mass fluxes

This paper studies the double diffusion flow over a vertical truncated cone with variable heat and mass fluxes in a porous medium saturated with non-Newtonian power-law fluids. A coordinate transformation is used to obtain the nonsimilar governing equations, and the transformed boundary layer equations are then solved by the cubic spline collocation method. Results for local surface temperature and concentration are presented as functions of power-law indexes, exponents for variable heat and mass fluxes, buoyancy ratios, and Lewis numbers. The local surface temperature and concentration of the truncated cone decrease as the exponents for variable heat and mass fluxes are increased. Moreover, a decrease in the power-law index of fluids tends to decrease the local surface temperature and concentration of the truncated cone.     

Double diffusion from a horizontal cylinder of elliptic cross section with uniform wall heat and mass fluxes in a porous medium

This work studies the double diffusion near a horizontal cylinder of elliptic cross section with uniform wall heat and mass fluxes in a fluid-saturated porous medium. A coordinate transformation is used to obtain the non-similar governing boundary layer equations. The transformed equations are solved numerically by an efficient cubic spline collocation method. Results for the local surface temperature and the local surface concentration are presented as functions of the Lewis number, the buoyancy ratio, and the aspect ratio when the major axis of the elliptical cylinder is vertical (slender orientation) and horizontal (blunt orientation). As the Lewis number is increased, the local surface concentration decreases while the local surface temperature increases. Moreover, an increase in the buoyancy ratio tends to decrease both the local surface temperature and the local surface concentration.