P. Koutmos

A study of the turbulent structure of a two-dimensional diffusion flame formed behind a slender bluff-body

An experimental investigation of confined turbulent reacting wake flows past a 2D square cylinder with planar fuel-jet injection into the vortex formation region is presented. Measurements of mean and turbulent velocities and temperatures and a sample of results on related statistics, obtained with laser velocimetry and thin digitally compensated thermocouples throughout the wake region for two Reynolds numbers and for a range of fuel-to-air velocity ratios (FAVR), are discussed. Counterpart isothermal flows with and without air-jet injection are also documented to facilitate a discussion of the effect of combustion on the turbulent wake aerodynamics. The study has shown that 2D slender bluff-body stabilized diffusion flames differ from their axisymmetric counterparts in that they produce longer recirculation zones and flame lengths. Their peak temperature and turbulence levels are located at the forward stagnation point, away from the burner face. In contrast to axisymmetric geometries, large-scale activity and periodic shedding are here drastically suppressed at medium and low FAVR operation. These intensify as the flame length shortens and global extinction conditions are approached, in a fashion similar to premixed bluff-body stabilized flames. On the basis of measured mean vorticity and turbulent intensity distributions within the recirculation region, a mechanism of vortex shedding suppression is suggested for the reacting wake. Vortex strengths, the double vortex structure, and the linear flame length dependence on FAVR are more similar for both geometries.

A study of turbulent diffusion flames formed by planar fuel injection into the wake formation region of a slender square cylinder

The present work describes the experimental and numerical investigation of turbulent reacting wake flows formed by planar fuel-jet injection into the wake formation region of a confined two-dimensional square cylinder. Complementary studies of the counterpart isothermal air-injected wakes facilitated a discussion on the effect of injection and reaction on near and far wake development. Detailed measurements of turbulent velocities, temperatures, and statistics were obtained by laser velocimetry and thin, digitally compensated thermocouples for a number of short, ultralean, low fuel-air velocity ratio (FAVR) flames. The measurements highlighted the principal characteristics of slender bluff-body diffusion flame stabilization and identified differences and similarities with respect to axisymmetric bluff-body flame configurations regarding entrainment, vortex, and flame lengths, temperature and turbulence distributions, and large-scale vortex activity. In the numerical work, large eddy simulations of the reacting wake flows were performed employing a partial equilibrium/two-scalar exponential PDF combustion model applied at the subgrid level. Statistical independence of the joint PDF scalars was relaxed and the appropriate SGS moment equations were solved. The subgrid scale motions were modeled with a first-order closure using the solution of an equation for the SGS energy. Comparisons between simulations and measurements indicated the ability of the model to reproduce the experimentally observed variations in the mean and turbulent fields for a range of FAVR values and two Reynolds numbers. The method resolved important large-scale features of the isothermal and reacting flows, thus allowing a more effective exploitation of the combustion model and clearly out-performed a standard k-e/β -PDF procedure.

Isothermal and non-premixed turbulent reacting wake flows past a two-dimensional square cylinder

Abstract The present work describes the study of turbulent wake flows past two-dimensional square cylinders with and without (air or fuel) jet injection into the vortex formation region under isothermal and reacting conditions. An experimental investigation provided detailed measurements of turbulent velocities, temperatures and related statistics obtained by Laser Velocimetry and thin digitally compensated thermocouples. Both isothermal, plane or air-injected and reacting wakes were measured to assess the effect of combustion on the wake aerodynamics.In the numerical work the periodic isothermal wakes and the quasi-periodic reacting wakes were successfully calculated with a 2D time-dependent Navier–Stokes procedure that encompasses aspects from both the LES formalism and the conventional k − e procedures. Within the framework of this approach large scale active structures are distinguished both on the basis of their character deterministic or random as well as on their size while taking into account the percentage contribution of the vortical structure energy in the total fluctuating energy budget.

Time‐dependentcomputations of turbulent bluff‐body diffusion flames close to extinction

A two dimensional time‐dependent Navier Stokes formulation that encompasses aspects from both the LES formalism and the conventional k‐e approaches was employed to calculate a range of reacting bluff‐body flows exhibiting high or low level large scale structure activity. Extensive regions of local flame extinction found in these bluff‐body flame configurations were treated with a partial equilibrium/two‐scalar exponential PDF combustion submodel combined with a local extinction criterion based on a comparison of the turbulent Damkohler number against the ratio of the scalar scale to the reaction zone thickness. A dual‐mode description, burning/ non‐burning, of combustion provided the local gas state. Comparisons between calculations and measurements indicated the ability of the method to capture all the experimentally observed variations in the momentum and reactive scalar mixing fields over a range of operating conditions from the lean to the rich blow‐out limit.

A Study of Slender Bluff-Body Reacting Wakes Formed by Concurrent or Countercurrent Fuel Injection

The work presents an investigation of turbulent propane flames stabilized by planar injection across the span of a square cylinder, either from its leading face against the approach flow or directly into its vortex formation region. The non-premixed or partially premixed reacting wakes were studied by regulating the fuel injection level and position. Turbulent velocities, temperatures, CH*, flame images, and exhaust emissions were measured using laser velocimetry, digitally compensated thermocouples, chemiluminescence imaging, and gas analysis. Lean and ultra-lean fuel/air velocity ratios of 0.36 and 0.23 were investigated under concurrent and countercurrent injection at a Reynolds number, based on the square burner diameter, of 5700. Large eddy simulations were undertaken using the dynamic Smagorinsky model, the eddy dissipation concept, and an 11-step global mechanism for propane combustion and NOx. The methodology helped to elucidate some aspects regarding the interaction of the flame front with the vortex formation region, the impact of heat release on wake development, and the flame behavior as lean blow-off (LBO) was approached, under both forms of fuel injection. Differences between the partially premixed planar wake and other types of non-premixed or fully premixed configurations were also exposed and discussed. At the initial stages of fuel reduction toward blow-off, the simulations suggested a more efficient stabilization of the flanking reacting fronts within the side vortices of the counterinjected square burner by comparison to axisymmetric counterparts. However, as the two reactive layers were progressively retracted, the large scale asymmetric vortex shedding was reinstated at the flame trailing edges and had a detrimental effect on overall stability leading to a more rapid approach to LBO in the final stages.

Simulations of fuel injection and flame stabilization in the wake formation region of a slender cylinder

The characteristics of turbulent propane flames established by planar fuel-jet injection from the center of a slender cylinder and stabilized in its wake formation region are presented and discussed. Isothermal and reacting investigations addressed the effects of combustion on large-scale vortex shedding and facilitated an examination of the impact of fuel injection and heat release on the near-wake aerodynamics. The turbulent velocity and temperature fields were measured with laser velocimetry and thin digitally compensated high-temperature thermocouples over a range of fuel injection ratios and two Reynolds numbers or 8520 and 14,285. A multilayered vorticity distribution, a threefold decrease of the mean and root-meansquare cross-stream entraining velocity, and a fourfold elongation of the primary recirculation, composed of a complex system of multiple vortices, were some of the effects of combustion on near-wake development. The study provided information on the operation of this cylinder-type burner and exposed operational differences and similarities in relation to axisymmetric and other slender bluff-body stabilizers. Two-dimensional large-eddy simulations of the wakes were also performed, employing a partial equilibrium scheme and a two- (correlated)-scalar exponential probability density function (PDF) turbulence/chemistry model which was applied at the subgrid level. An anisotropic subgrid eddy viscosity derived from scale similarity between resolved and subgrid fluctuations together with two equations for the turbulence kinetic and scalar energies completed the subgrid closure and supplied the scalar covariances in the PDF formulation. The present investigations suggested that favorable agreement between computations and experiments was achieved with respect to identified trends in several important performance parameters such as entrainment rates, recirculation and flame lengths, temperature and turbulence distributions, and large-scale vortex structure activity, supporting the extension of the present reactive model to include the effects of three dimensionality.

Investigation of Disk-Stabilized Propane Flames Operated under Stratified and Vitiated Inlet Mixture Conditions

AbstractThe work describes an experimental investigation of propane flames established through fuel injection and gradual premixing with preheated and vitiated oxidizing air, within a double-cavity arrangement, formed along a cylinder and two concentric disks. Ignition of the inlet stratified and vitiated mixture and flame stabilization is established at the recirculation region of the afterbody disk. Measurements of mean velocities, temperatures, OH* and CH* chemiluminescence and gas analysis provided information for the interpretation of the relative variations in flame structure and burner performance. The study illustrates some aspects regarding the influence of the preheated/vitiated conditions on the combustion of the inlet stratified fuel-air mixture profile, with respect to flame front stabilization, disposition, and burner emissions. Some important differences in the vitiated flame structure and topology are discussed in comparison to the unvitiated case characteristics, in an initial effort to e...

Performance Evaluation of a Model Swirl Burner under Premixed or Stratified Inlet Mixture Conditions

AbstractThis work presents a comparative study of the performance between fully premixed and stratified propane flames stabilized in a disk burner configuration operating with a swirl coflow over a range of stoichiometric to ultra-lean conditions. A selection of radial equivalence ratio gradients up to the fully premixed case are regulated by staged premixing of propane and air within a double cavity formed along three concentric disks. Flame stabilization is established at the recirculation region of the afterbody disk. Measurements of temperatures, flame chemiluminescence images of OH* and CH*, and gas analysis provided information for the interpretation of the relative variations in flame structure and burner performance. The study elucidates some aspects regarding the influence of the variable inlet fuel–air profile and its interaction with the swirling coflow on the stabilization and disposition of the toroidal flame front. The results help to delineate important differences in the emission performan...

A study of partial extinction and reignition effects in turbulent non-premixed jet flames of CH4 and CO/H2/N2 with a two-scalar reactedness-mixture fraction presumed PDF model

A method for modeling the effects of finite-rate chemistry such as partial extinctions and reignitions is developed and used to compute turbulent CO/H2/N2 or CH4-air-piloted jet diffusion flames close to extinction. The method is combined with a two-dimensional Large Eddy Simulation procedure employing a partial equilibrium/two-scalar exponential Probability Density Function (PDF) combustion submodel applied at the subgrid scale (SGS) level. Subgrid motions are modeled with a first-order closure utilizing an anisotropic subgrid eddy-viscosity and two equations for the subgrid scale turbulent kinetic and scalar energies. Statistical independence of the joint PDF scalars is here avoided and the required moments are obtained from an extended scale-similarity assumption. Extinction is determined by comparing the local Damkohler number against a ‘critical’ local limit related to the Gibson scalar scale and the reaction zone thickness. The post-extinction regime is treated with a reactedness progress variable computed from a Lagrangian transport equation and a two-scalar reacledness-mixture fraction presumed PDF. Comparisons with well documented turbulent flame data suggested the ability of the presented methodology to describe many experimental trends and variations of the momentum and reactive scalar mixing fields at flame conditions close to extinction.

Simulations of local extinction phenomena in bluff-body stabilized diffusion flames with a Lagrangian reactedness model

Two-dimensional large-eddy simulations of bluff-body stabilized flames of methane and propane, exhibiting significant finite-rate chemistry effects, are presented. A partial equilibrium/two-scalar exponential probability density function (PDF) combustion submodel is applied at the subgrid level. Subgrid scale motions are modelled with a first-order closure employing an anisotropic subgrid eddy-viscosity and two equations for the subgrid turbulent kinetic and scalar energies. Statistical independence of the joint PDF scalars is avoided and the necessary moments are obtained from an extended scale-similarity assumption. Extinction is accounted for by comparing the local turbulent Damkohler number against a ‘critical’ local limit related to the Gibson scalar scale and the reaction zone thickness in mixture fraction space. The post-extinction regime is modelled via a Lagrangian transport equation for a reactedness progress variable which follows a linear deterministic relaxation to its mean value (interaction...