D. Papailiou

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.

The structure of two-phase grid turbulence in a rectangular channel: an experimental study

Measurements in two-phase, water‐air bubble, grid turbulence have been conducted in a vertical water channel of square cross section. The water flow was directed upwards. A 30 mm mesh grid located at the entrance of the test section generated, in the absence of the dispersed phase, a nearly isotropic turbulent flow field occupying the central part of the channel. Air bubbles were injected in the flow from the grid. Velocity measurements of the liquid phase were obtained using Laser Doppler Velocimetry. The measured flow characteristics included mean velocity and statistical quantities such as turbulence intensity, probability distribution function, skewness and flatness factors, autocorrelation, macroscale and power spectra, for both the longitudinal and transverse velocity components. Local void fraction was measured with an optical probe as well as with hot film anemometry. Bubble velocity and size were estimated with photographic techniques. Presented measurements illustrate the upwards development of void fraction distribution at successive cross sections of the channel. The observed distributions indicate the presence of a mechanism responsible for capturing and redistributing the bubbles. Accordingly, physical mechanisms possibly responsible for the observed phenomenon such as a Segre‐Silberberg eAect, modified for deformable bubbles in the presence of buoyancy, or the action of large eddies present in the flow are discussed. The velocity field has been obtained on a cross-section at a distance of 30 mesh from the grid at a constant water volumetric flow rate. Based on the obtained experimental results, the influence of the dispersed phase on the initially isotropic turbulence field is identified and the physical processes responsible for the observed changes are discussed. The presence of the second phase appears to introduce inhomogeneities in the void fraction and flow property distributions leading to the emergence of two spatial regions controlled by diAerent physical processes. Also, two distinct regimes in the measured quantities are observed, corresponding to low and high values of gas flow rate ratio. The described changes result in a significant departure from initial isotropy, followed by a tendency towards returning to a state characterised by nearly normal probability

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.

Statistical characteristics of a turbulent free-convection flow in the absence and presence of a magnetic field

Abstract Recorded signals of temperature fluctuations in a mercury turbulent-free convection layer in the presence and absence of a magnetic field were analysed. The statistical quantities obtained from the analysis were the flatness, skewness and intermittency factors. The distributions of these statistical quantities indicate the presence of large vortex structures in the layer and their strong participation in the mixing and transfer flow mechanisms. Also, there is evidence that a drastic change in the large scale structures occurs at transition, caused by the presence of the magnetic field.

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.

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.

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...

Investigation of the Effects of Base Injection on the Flow Field Past a Square Cylinder

The complex turbulent flow produced downstream of a cylindrical or square bar has frequently been considered as a model problem due to its fundamental physical importance as it encompasses interactions among anisotropic regions, large scale recirculations, unstable shear layers and periodic or quasi-periodic energetic shedding of vortices (Bearman, 1984; Roshko, 1993). Fundamental understanding of the turbulent processes involved in a range of generic bluff-body configurations as well as other complex flow/geometry derivative combinations of practical importance is necessary to improve methods of prediction and allow better control or exploitation of these flow features.