Horácio A. Vielmo

Numeric and experimental analysis of the turbulent flow through a channel with baffle plates

The present work presents the numeric and experimental analysis of the turbulent flow of air inside a channel of rectangular section, containing two rectangular baffle plates. This is an important problem in the scope of heat exchangers where the characterization of the flow, pressure distribution, as well as the existence and the extension of possible recirculations need to be identified. The differential equations that describe the flow were integrated by the Finite Volumes Method, in two dimensions, employing the Fluent software with the k-e model to describe the turbulence. The mesh is structured, with rectangular volumes. Several boundary conditions were explored, being the more realistic results obtained by prescribing the inlet velocity field and atmospheric pressure at the exit. The obtained results are compared with experimental data, being analyzed and commented the deviations. The velocity field was measured with a hot wire anemometer, and the pressure field with an electronic manometer. The largest variations in the pressure and velocity fields occur in the regions near to the deflectors, as expected.

ANALYSIS OF THE TURBULENT, NON-PREMIXED COMBUSTION OF NATURAL GAS IN A CYLINDRICAL CHAMBER WITH AND WITHOUT THERMAL RADIATION

Abstract This work presents a numerical simulation of the non-premixed combustion of natural gas in atmospheric air in an axis-symmetric cylindrical chamber, focusing on the effect of thermal radiation on the temperature and chemical species concentration fields and the heat transfer. The simulation is based on the solution of the mass, energy, momentum and the chemical species conservation equations. Thermal radiation exchanges in the combustion chamber is computed through the zonal method, and the gas absorption coefficient dependence on the wavelength is resolved by the weighted-sum-of-gray-gases model. The turbulence is modeled by the standard k − ϵ model, and the chemical reactions are described by the E–A (Eddy Breakup–Arrhenius model). The finite volume method is employed to treat the differential equations. Among other results, the solution of the governing equations allows the determination of the region where combustion takes place, the distribution of the chemical species, the velocity fields and the heat transfer rate by convection and radiation. The results indicate that while thermal radiation has a strong effect on the temperature field and heat transfer, its effect on the chemical reactions rates is of less importance. The numerical results are compared to experimental results obtained by Garréton and Simonin (1994).

Numerical analysis of water melting and solidification in the interior of tubes

Latent energy storage systems find applications in many engineering fields, including industrial refrigeration plants, air conditioning installations, recovery of heat in industrial processes, etc. To tackle the design of such systems, it is necessary to have correlations to account for the heat transfer during the melting and solidification of the phase change material (PCM). This work describes and analyzes the results obtained from the numerical simulation of pure water melting and solidification in the interior of tubes, which are typically present in ice banks of air conditioning systems. The shown results consider natural convection, accounting for the inversion in the water density. In the melting process, the considered initial conditions followed the classical Stefan and Neumann approach. The presented simulation results include the evolution of the phase change interface, and of the temperature, density and streamlines fields. Correlations for the Nusselt number and for the melted material volume as functions of time have been proposed.

Comparison of spectral models in the computation of radiative heat transfer in participating media composed of gases and soot

Accurate combustion models are necessary to predict, among other effects, the production of pollutant gases and the heat transfer. As an important part of the combustion modeling, thermal radiation is often the dominant heat transfer mechanism, involving absorption and emission from soot and participating gases, such as water vapor and carbon dioxides. If the radiative heat transfer is not accurately predicted, the solution can lead to poor prediction of the temperature field and of the formation and distribution of the gases and soot. The modeling of the absorption coefficient of the gases is a very complex task due to its highly irregular dependence on the wavenumber. On the other hand, the absorption coefficient of the soot is known to behave linearly with the wavenumber, allowing for a simpler approach. Depending on the amount of soot, the more sophisticated and expensive gas models can be replaced by simpler ones, without considerable loss of accuracy. In this study, the radiative heat transfer for a medium composed of water vapor, carbon dioxide and soot is computed with the gray gas (GG), the weighted-sum-of-gray-gases model (WSGG), and the cumulative wavenumber (CW) models. The results are compared to benchmark line-by-line (LBL) calculations.

A comparison of experimental results of soot production in laminar premixed flames

Abstract Soot emission has been the focus of numerous studies due to the numerous applications in industry, as well as the harmful effects caused to the environment. Thus, the purpose of this work is to analyze the soot formation in a flat flame burner using premixed compressed natural gas and air, where these quasi-adiabatic flames have one-dimensional characteristics. The measurements were performed applying the light extinction technique. The air/fuel equivalence ratiowas varied to assess the soot volume fractions for different flame configurations. Soot production along the flamewas also analyzed by measurements at different heights in relation to the burner surface. Results indicate that soot volume fraction increases with the equivalence ratio. The higher regions of the flamewere analyzed in order to map the soot distribution on these flames. The results are incorporated into the experimental database for measurement techniques calibration and for computational models validation of soot formation in methane premixed laminar flames, where the equivalence ratio ranging from 1.5 up to 8.

Diffusion Flame Stability of Low Calorific Fuels

Mechanisms related to diffusion flame stabilization have been the subject of several studies within the last decades due the industrial and scientific interests. Information on flame stability is of fundamental importance in energy efficiency and safety regarding industrial applications. Thus, an experimental study was performed in order to examine the flame characteristics and regions of stability limits. In this study, a representative burner of industrial applications was employed, which allows the stabilization of several combustion regimes. The lift-off and blow-out flame regimes were investigated for different proportions of carbon dioxide in natural gas. In this way, an analysis of the calorific fuel influence on the flame stability was performed based on the measurements and a comparison with classical literature models was done. The fuel dilution by adding carbon dioxide was found to decrease the soot production, leading to lower flame heights and also, lower lift-off and blow-out limits. Results obtained from this study encourage future works which consider flames in large scale, in order to equate to industrial applications.

Inverse Boundary Design in Heat Transfer Combining Turbulent Convection and Thermal Radiation

This work investigates the solutions of an inverse boundary design problem that has multimode (radiation and convection) heat transfer mechanisms. The problem consists of finding the heat flux distribution required on heaters located on the top and side walls of a two-dimensional enclosure that satisfies both the temperature and heat flux distributions prescribed on the design surface of the enclosure. A turbulent air flow is generated by a fan located inside the chamber. The walls are gray, diffuse emitters and absorbers. The combined heat transfer problem is described by a system of non-linear equations, which is expected to be ill-conditioned as an inverse analysis is involved. The system of equations is solved by an iterative procedure: the basic set of equations relates the radiation transferred between the heater and the design surface, while all the other terms involved in the energy exchange are found from the conditions of the previous iteration. This way, the ill-posed part of the problem (which arises from the design surfaces containing two conditions, and the heater elements being unconstrained) is isolated for a more effective treatment. The solution is obtained by regularizing the ill-conditioned system of equations by means of the truncated singular value decomposition (TSVD) method.

Numerical study of an internal combustion engine intake process using a low Mach number preconditioned density-based method with experimental comparison

When air flows unsteadily in an internal combustion engine through its inlet pipe, chambers and valves, some effects such as friction and inertial forces have direct influence on the volumetric efficiency of the system. The work in this paper aims to investigate numerically and experimentally the pulsating phenomena present in an intake pipe of an internal combustion engine and to discuss the intake jet flow predictions through the novel implementation of a low Mach number preconditioned density-based method, including the three-dimensional modelling of the inlet valve, the inlet pipe and the pulsating effects. The inlet valve movement promotes moderate values of Mach numbers during its opening phase. After the inlet valve closes, the flow is abruptly restricted and a series of pressure waves propagate through the fluid at low Mach numbers. Although the low Mach number preconditioned density-based method is very attractive in this case, the study of the pulsating flow in the internal combustion engine intake systems has not been performed using this method, probably owing to robustness issues and simulation effort. The pressure-based methodology is widely used and, generally, the inlet pipe and pulsating effects are not included in the three-dimensional fluid dynamics simulation. In order to verify the accuracy of the numerical solution, the results are compared with experimental data collected from a bench constructed specifically for this purpose. The numerical results were satisfactory for the amplitudes and the resonance frequencies in the air intake system. Also, different aspects of the jet flow inside the cylinder are shown and discussed.

Spectral Gas Absorption Coefficient Model Effects on Radiative Source Term in a 2D Axisymmetric Diffusion Flame

A calculation of the radiative source term in combustion processes is an important part of the simulation process, because high temperatures are involved and the coupling of radiation to chemistry affects the overall flame characteristics. While relatively simple gas absorption coefficient models have been used in the recent past, it is becoming clearer that more accurate gas models alter the distribution of radiative sources in the flame. To accurately evaluate the radiative losses, it is necessary to use gas models in which the gas absorption coefficient is wavelength dependent. Such analyses can be computationally expensive depending on the particular treatment of the spectral dependence. It is important to understand the relative costs and benefits of different treatment of these effects. In this work, the divergence of the radiative heat flux is calculated for a two-dimensional cylindrical axisymmetric chamber using four different models: a simple gray gas model, the weighted-sum-of-gray-gases (WSGG) model, the spectral line-based weighted-sum-of-gray-gases (SLW) model, and the cumulative wavenumber (CW) model. The gray gas model and the WSGG model are widely used in recent studies and in most commercial software, because they are simple to implement and provide fast results. In general, however, they are not able to accurately predict the radiative losses. On the other hand, the SLW and CW models detail the variations of the absorption coefficient with the wavelength, and can give more accurate answers for the radiative source term, but require bigger computational effort. The divergence of the radiative flux predictions are compared with these four models, using temperature and concentration fields obtained from previous numerical simulations. The overall differences in radiation properties and in the overall cost of computations are detailed.Copyright

Comparison of the Monte Carlo Method Applied to the Absorption-Line Blackbody Distribution With a Conventional WSGG Model

This paper presents results of radiation heat transfer obtained from two gas models: the Absorption-Line Blackbody (ALB) distribution function and a conventional weighted-sum-of-gray-gases (WSGG) model. A typical cylindrical combustion chamber is considered. For the first model, the radiative exchanges are computed with the Monte Carlo, since the ALB distribution function can be directly related to the Monte Carlo cumulative distribution function. For the WSGG model, the radiation heat transfer is solved with the zonal method. Two gaseous mixtures are considered, and represent typical products of stoichiometric combustion of fuel oil and methane. While the Monte Carlo and the zonal solutions match for a gray gas, considerable departures are observed for the gas spectral using the ALB and the WSGG models.Copyright