J. C. F. Pereira

Experimental and theoretical investigation of backward-facing step flow

Laser-Doppler measurements of velocity distribution and reattachment length are reported downstream of a single backward-facing step mounted in a two-dimensional channel. Results are presented for laminar, transitional and turbulent flow of air in a Reynolds-number range of 70 Re Although the high aspect ratio of the test section (1:36) ensured that the oncoming flow was fully developed and two-dimensional, the experiments showed that the flow downstream of the step only remained two-dimensional at low and high Reynolds numbers. The present study also included numerical predictions of backward-facing step flow. The two-dimensional steady differential equations for conservation of mass and momentum were solved. Results are reported and are compared with experiments for those Reynolds numbers for which the flow maintained its two-dimensionality in the experiments. Under these circumstances, good agreement between experimental and numerical results is obtained.

Measurements of turbulent and periodic flows around a square cross-section cylinder

Laser-Doppler measurements of the velocity characteristics are presented for the turbulent flow around a square cross-section cylinder mounted in a water channel for Re=14000. The study involved spectral analysis and digital filtering of the LDV data obtained behind the cylinder. The purpose of the measurements is to separate and quantify the turbulent and the periodic, non-turbulent, motions of the wake flow, in order to improve knowledge of the nature of the fluctuations in the near-wake region of two-dimensional bodies. The results show, for example, that in the zone of highest velocity oscillations the energy associated with the turbulent fluctuations is about 40% of the total energy.

The plane symmetric sudden-expansion flow at low Reynolds numbers

Detailed velocity measurements and numerical predictions are presented for the flow through a plane nominally two-dimensional duct with a Symmetric sudden expansion of area ratio 1:2. Both the experiments and the predictions confirm a symmetry-breaking bifurcation of the flow leading to one long and one short Separation zone for channel Reynolds numbers above 125, based on the upstream channel height and the maximum flow velocity upstream. With increasing Reynolds numbers above this value, the short separated region remains approximately constant in length whereas the long region increases in length. The experimental data were obtained using a one-component laser-Doppler anemometer at many Reynolds number values, with more extensive measurements being performed for the three Reynolds numbers 70, 300 and 610. Predictions were made using a finite volume method and an explicit quadratic Leith type of temporal discretization. In general, good agreement was found between measured and predicted velocity profiles for all Reynolds numbers investigated.

Computation of turbulent evaporating sprays with well-specified measurements: a sensitivity study on droplet properties

Abstract An extensive numerical study was carried out for a confined evaporating spray in a turbulent, heated gas flow using a published well-defined experimental dataset. The Eulerian-Lagrangian stochastic models were employed for spray calculations wherein the gas turbulence was modeled using the second-moment transport model for the Reynolds stresses and heat-flux vectors, and the droplet dispersion was modeled using the Lagrangian stochastic models with or without temporal correlations. Two fashions of the infinite-conduction-evaporation model were studied, both of which have taken into account the variable gas-film properties by the 1 3 - rule . Numerical results for the droplet phase, i.e., the mean diameters, mass fluxes, mean and fluctuating velocities were presented and discussed by comparison with the experimental data. The sensitivity of various droplet properties to the number of droplet trajectories at the inlet, the drift correction approaches for the improvement of mass-flux predictions, and the evaporation models was investigated in terms of the well-defined experimental dataset. Results show that the droplet mean velocities are generally not sensitive to all the factors considered, that droplet r.m.s. velocities downstream are sensitive to the number of trajectories, that the droplet mass-flux accumulation near the centreline can be substantially improved by using a new drift correction approach, and that mass-flux predictions are sensitive to the evaporation models.

Experimental and numerical investigation of flow oscillations in a rectangular cavity

The unsteady, incompressible, moderate Reynolds number flow past a rectangular cavity is experimentally and numerically investigated. Laser-Doppler anemometry, flow visualization and unsteady numerical simulation using fully second-order accuracy in time and space, were the tools employed to meet this purpose. Large-amplitude organized oscillations are reported to occur in the investigated geometry due to fluid-dynamic instability. Detailed flow visualization and unsteady predictions clearly show that the instability process involves a complex coupling of shear layer and recirculating flowfield dynamics. The paper also demonstrates the accuracy of the present calculations.

Velocity characteristics of the flow around a square cross section cylinder placed near a channel wall

Laser-Doppler measurements are reported of the flow around a square cross section cylinder placed at various heights (Y0) above a plane channel wall for a Reynolds number ReH = 1.36 × 104.The thickness of the turbulent boundary layer on the channel wall at the obstacle position, but with it removed from the water tunnel, was equal to 0.8 H, being H the square obstacle height and the free stream turbulence intensity was 6%. The periodic character of the flow in the near wake was characterized by measurements of turbulence spectra in the range 0≤ Y0/H ≤3.3 and the results revealed that regular vortex shedding was suppressed for a gap height less than 0.35 H. Detailed results of time averaged mean flow properties, turbulence intensities and Reynolds stresses revealed the structural differences of the near wakes with and without vortex shedding for YO = 0.5 and YO = 0.25 respectively.

Two-dimensional Numerical Study of Combustion and Pollutants Formation in Porous Burners

Two-dimensional numerical predictions of temperature profiles and pollutants formation in a porous burner are presented and validated through their comparison with available experimental data. The burner under study integrates a heat exchanger where water is heated for household applications. The Navier-Stokes, the energy and the chemical species transport equations are solved and a multislep kinetics mechanism (77 reactions and 26 species) is employed. Thermal nonequilibrium is accounted for and the discrete ordinates method, for the case of isotropic scattering, is used. Centreline temperature predictions are in good agreement with the experimental results. Predicted CO and NO emissions are compared to experimental results for a 5 kW thermal power and several excess air ratios. The model underpredicts the CO emissions and overpredicts the NO emissions. This tendency is stronger for richer mixtures. The effects of the excess air ratio, solid conductivity, convective heat transfer coefficient and radiative properties are investigated.

Comparison of four combustion models for simulating the premixed combustion in inert porous media

This work reports one-dimensional predictions of methane/air fuel combustion in inert porous media using four combustion models: full mechanism (FM, 49 species and 227 elemental reactions), skeletal mechanism (SM, 26 species and 77 elemental reactions), 4-step reduced mechanism (4RM, 9 species) and 1-step global mechanism (1GM). The effects of these models on temperature, species, burning speeds and pollutant emissions are examined. The calculations are compared with available experimental data. It is concluded that the already known limitation of the 1-step global mechanism can be partially eliminated by the present 4-step reduced mechanism. This 4RM model compares very satisfactorily with the full mechanism in the simulation of combustion in porous media. This conclusion is encouraging for the simulation of practical porous media burners because the 4RM model improves the stability of the calculation process and can be used with reduced computational resources and cost.

Numerical Study of Combustion and Pollutants Formation in Inert Nonhomogeneous Porous Media

Abstract Combustion in inert porous media offers an interesting and promissing route towards high power density, high power dynamic range and very low emission of pollutants such as NOx and CO. This work assumes one dimensional combustion in a porous burner but considers the detailed reaction mechanism of methane/air, 27 species and 73 reactions, and also the conductive heat transfer of solid matrix and radiative heat transfer in the porous media. In this work, we present a numerical study which show the effects of excess air ratio, thermal power, solid conductivity and radiative heat transfer on the combustion and pollutants formation in inert nonhomigeneous porous media. It was concluded that NO and CO emission depend mainly on the excess air ratio and thermal power. The peak flame temperature is reduced with the reduction of th solid conductivity resulting in a decrease of NO emission. This important conclusion means that NO emission can be decreased through the utilization of porous medium with small ...

PREDICTION OF EVAPORATING SPRAY IN ANISOTROPICALLY TURBULENT GAS FLOW

Numerical investigation was conducted for a confined evaporating isopropyl alcohol spray issuing into a coflowing, heated turbulent air stream. The Eulerian-Lagrangian stochastic model was used for the spray calculations. The gas phase turbulence was modeled using either the isotropic eddy viscosity model or the second-moment transport model for both Reynolds stresses and heat fluxes. Two droplet dispersion models were studied for the Lagrangian trajectory calculations; the conventional particle-eddy encounter model and the time-correlated dispersion model. In the time-correlated model, gas phase turbulent velocity fluctuations were correlated temporally and directionally between two successive time steps in modeling the droplet dispersion. The droplet evaporation was accounted for by the infinite-conduction evaporation model, where the gas-film variable properties were considered using the one-third rule. Detailed numerical results of the liquid droplet phase, i.e., the droplet mean diameters, mass fluxe...