Eddy Break-up Research Articles

Application of a Detailed Emission Model for Heavy Duty Diesel Engine Simulations

A detailed chemical model describing the formation of soot and NO is applied to simulate emission formation in a heavy duty diesel engine. Cylinder flow and spray development is simulated using an engine CFD code - Speedstar. Combustion is described using a simple eddy break-up model. Modeling of the emission-chemistry/turbulent-flow interaction is based on a flamelet approach. Contrary to a typical flamelet concept, transport equations are solved for mass fractions of soot and NO. The reason being that these major emission constituencies are assumed to change slowly in comparison to typical time scales for chemical processes or transport processes important for combustion. Chemical reactions leading to production and destruction of soot and NO are, however, assumed to be fast. Soot and NO source terms are therefore evaluated from a flamelet library using a presumed probability density function and integrating over mixture fraction space. Results from simulations are compared to engine measurements inform of exhaust emission data and cylinder pressure.

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

A Laminar Flamelet Model for Turbulent Premixed Combustion.

A fully three dimensional numerical model for the combustion process in a spark-ignition engine was established using the combustion submodel described of the flame area evolution. Computations carried out the process of premixed combustion and they were compared with the results using the combustion submodel of the eddy break-up. The combustion in a disc chamber of constant volume was simulated. The results show that the course of flame propagation is well predicted. The calculations for different conditions in a natural-gas engine are performed. It is shown that the effects of fuel-air equivalence ratio and swirl intensity on flame propagation are described. The cylinder pressure histories are agreed with measured one, and the distributions of velocity vectors and temperature in a cylinder are revealed.

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break‐up manipulator

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for turbulent duct flow disturbed by insertion of a complicated body composed of a Large Eddy Break-Up (LEBU) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The spatial distribution of the heat transfer coefficient obtained shows the same features as that observed in a previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in a practical application when a vortex generator is produced by punching from the original plain LEBU plate. It was found that a vortex generator with an open notch works best in augmenting the wall heat transfer and also in suppressing the increase of momentum loss. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 189–200, 1999

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for turbulent duct flow disturbed by insertion of a complicated body composed of a Large Eddy Break-Up (LEBU) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The spatial distribution of the heat transfer coefficient obtained shows the same features as that observed in a previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in a practical application when a vortex generator is produced by punching from the original plain LEBU plate. It was found that a vortex generator with an open notch works best in augmenting the wall heat transfer and also in suppressing the increase of momentum loss. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 189–200, 1999

Numerical modelling of combustion in a zinc flash smelter

The development of a mathematical model for the flash smelting of zinc concentrate is described. A commercial computational fluid dynamics package, CFX4.1, has been used and additional codings for zinc concentrate smelting and combustion of natural gas have been developed and incorporated into the package. In the simulation, gas is combusted using the eddy break-up model and the gas–particle reactions are limited by a combination of heat transfer and mass diffusion. For the zinc concentrate combustion submodel, the concept of a composite particle was adopted to include the contributions from all minerals present in the concentrate. It is assumed that after heating to a temperature of 803 K, the pyrite in the composite particle decomposes and ignites. When the particles are further heated to a temperature of 1873 K, they melt and zinc sulphide vaporizes to form gaseous sulphur and zinc. The sulphur combusts to form sulphur dioxide and heat is released to the gas phase. The rate of vaporization is limited by the supply of heat to the particle. After all the zinc has left the particle, the oxidation of FeS in the concentrate commences and is controlled by the rate of oxygen diffusion to the particle surface. Modelling results of the combustion within the environment of a gas-fired furnace are presented as an example of the simulations performed for flash smelting of zinc concentrate.

CFD modelling of waste heat recovery boiler

A model has been developed of a waste heat recovery boiler utilising typical plant off-gas consisting of both gaseous and particulate combustibles. The model allows the calculation of temperatures of gas and particles within the boiler and hence the likelihood of deposition onto the boiler walls. The model was applied to a typical waste heat boiler geometry, and a typical off-gas composition including a mixture of combustibles (char) and non-combustible particulates. Mixing in the burner region, char burnout and char particle temperature were analysed using the model. Combustion stability was also studied using a simple Eddy break-up model which accounts for combustion kinetics and the results compared with a Mixed-is-burnt model.

The Modeling, Scale and NOx Characteristics of Pre-Vaporized, Premixed Fuel Oil Burner Combustions

This paper presents a direct Reynolds stress, stretched flamelet model for the premixed swirling turbulent reacting flows. The model adopts a statistical presumed joint pdf of turbulent flame stretching and flame wrinkling, together with an established laminar flame data library for fuel oil. The main virtue of the model is that the application of the model is not restricted to a single combustion regime. It is capable of realizably extending to both the fully chemically controlled combustion condition (the laminar flame model) and the fully mixing controlled combustion condition (the eddy break-up model). The scale and NOx characteristics of a pre-vaporized, premixed fuel oil burner have been studied using the model. It is found that combustion confinement has a profound effect on the design of such a burner. There exists an optimum chamber/burner diameter ratio, of about 3:1, at which a premixed blue flame could be stabilized and pollutant emissions are minimized. A very short chamber length adopted in the compact design will result in a higher NOx emission. And there also exists an optimum fuel/air equivalence ratio, of about 0.8. Detailed scale analysis shows that such a burner flame is characterized as flamelet in eddies, with both the turbulence and chemical kinetics playing a role in the combustion process.

Simulation of the Piston Driven Flow Inside a Cylinder With an Eccentric Port

A grid-free Lagrangian approach is applied to simulate the high Reynolds number unsteady flow inside a three-dimensional domain with moving boundaries. For this purpose, the Navier-Stokes equations are expressed in terms of the vorticity transport formulation. The convection and stretch of vorticity are obtained using the Lagrangian vortex method, while diffusion is approximated by the random walk method. The boundary-element method is used to solve a potential flow problem formulated to impose the normal flux condition on the boundary of the domain. The no-slip condition is satisfied by a vortex tile generation mechanism at the solid boundary, which takes into account the time-varying boundary surfaces due to, e.g., a moving piston. The approach is entirely grid-free within the fluid domain, requiring only meshing of the surface boundary, and virtually free of numerical diffusion. The method is applied to study the evolution of the complex vortical structure forming inside the time-varying semi-confined geometry of a cylinder equipped with an eccentric inlet port and a harmonically driven piston. Results show that vortical structures resembling those observed experimentally in similar configurations dominate this unsteady flow. The roll-up of the incoming jet is responsible for the formation of eddies whose axes are nearly parallel to the cylinder axis. These eddies retain their coherence for most of the stroke length. Instabilities resembling conventional vortex ring azimuthal modes are found to be responsible for the breakup of these toroidal eddies near the end of the piston motion. The nondiffusive nature of the numerical approach allows the prediction of these essentially inviscid phenomena without resorting to a turbulence model or the need for extremely fine, adaptive volumetric meshes.

A solution-adaptive mesh procedure for predicting confined explosions

Explosion hazards constitute a significant practical problem for industry. In response to the need for better-resolved predictions for confined explosions, and particularly with a view to advancing safety cases for offshore oil and gas rigs, an existing unstructured, adaptive mesh, finite volume Reynolds-averaged Navier–Stokes computational fluid dynamics code (originally developed to handle non-combusting turbomachinery flows) has been modified to include a one-equation, eddy break-up combustion model. Two benefits accrue from the use of unstructured, solution-adaptive meshes: first, great geometrical flexibility is possible; second, automatic mesh adaptation allows computational effort to be focused on important or interesting areas of the flow by enhancing mesh resolution only where it is required. In the work reported here, the mesh was adaptively refined to achieve flame front capture, and it is shown that this results in a 10%–33% CPU saving for two-dimensional calculations and a saving of between 57% and 70% for three-dimensional calculations. The geometry of the three-dimensional calculations was relatively simple, and it may be expected that the use of unstructured meshes for truly complex geometries will result in CPU savings sufficient to allow an order-of-magnitude increase in either complexity or resolution. © 1998 John Wiley & Sons, Ltd.

Heat Transfer Enhancement of a Turbulent Duct Flow by Means of Vortex Generators Attached to a Large Eddy Break-Up Manipulator.

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for a turbulent duct flow disturbed by insertion of a complicated body composed of a LEBU (Large Eddy Break-Up) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The obtained spatial distribution of heat transfer coefficient shows the same features as that observed in the previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in practical application, when a vortex generator is produced by punching from the original plain LEBU plate. A vortex generator having an open notch was found to work best not only in augmentation of the wall heat transfer but also in suppression of the increase of momentum loss.

Mathematical Modeling of a 2.4 MW Swirling Pulverized Coal Flame

Predictions and measurements of a swirling unstaged, high NOx, pulverized coal flame are presented. The predictions have been obtained by means of relatively simple models for turbulence, turbulent combustion and turbulent nitric oxide chemistry. Turbulence is modeled using the standard k-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. In the NO- chemistry model, thermal-NO and fuel-NO chemical reaction rates are statistically averaged over the fluctuating temperature using the Beta probability density function. Radiation is computed by means of the Discrete Transfer Radiation model. A constant linear absorption coefficient is assumed while scattering is not taken into account. The paper contains a unique, comprehensive comparison between predictions and measurements of a pulverized coal flame issued from an industrial-scale burner. The coal fired was Gottelborn hvBb coal. Attention is paid to velocities, turbulence quantities, temperatures, coal burnout levels and species concentrations of oxygen, carbon monoxide, carbon dioxide, nitrogen oxides precursors and nitrogen oxides. Radiative heat fluxes near the furnace refractory wall and the heat extraction of the cooling loops are addressed also.

Cfd Modelling Of Combustion And Heat Transfer In Compartment Fires

The field modelling of compartment fires incorporates increasingly complex representations of the physical and chemical processes which may occur and these models, in turn, demand detailed evaluation. The influence of alternative descriptions of combustion in the fire source and the introduction of a ray tracing radiation model (discrete transfer ) are assessed in this paper within the framework of progressively refined mesh geometry and by detailed comparison with the experimental measurements in the Steckler compartment fire. The predictions are made with SOFIE, a code specifically developed for building fire prediction. The reported comparisons demonstrate that detailed quantitative predictions of velocity and temperature fields can be performed accurately, given a satisfactory combination of grid resolution and physical modelling, particularly in relation to that of combustion and radiative exchange. Improved accuracy is shown to accompany the implementation of the V' discrete transfer radiation model, together with eddy break-up combustion, in comparison with the adoption of a more simply prescribed heat source. The

Dynamics of isoconcentration surfaces in weak shock turbulent mixing interaction

When developing closures for nonpremixed turbulent combustion ocuuring in supersonic regime, the probability density function is an attractive approach. It is required, however, to find an appropriate strategy to close the diffusive (mixing) part of the problem. Within supersonic turbulent combustion devices, mixing layers interact with shock waves and compression zones. Hence, it is crucial to seek out basic effects related to pressure jumps needing to be accounted for in modeling mixing. Direct numerical simulations are performed to investigate properties of mixing in weakly nonisentropic flows. The simulations correspond to the interaction between a weak shock and a mixing zone. Different conditions for injecting the mixing zone upstream from the weak shock are considered in the simulations. First, it is observed that the appearance of secondary pressure waves attached to the weak shock can be justified in the light of previous studies devoted to shock vortex interactions. Results suggest that the hypothesis of a linear relationship between the frequency of mixing and the inverse of an eddy breakup time fails within the interaction zone. Then, a statistical formalism, suitable to describe isoconcentration surfaces, is chosen to study increase in mixing rate induced by compressions. It is observed that the details of properties of shock turbulent mixing interaction strongly depend on the degree of mixing upstream from the compression. In particular, for a low mixing rate upstream of the pressure jump, the total stretch of surfaces is more controlled by curvature than by in-plane strain rate, whereas blob type injection leads to modifications of stretch induced by strain rate. Moreover, the strain rate becomes negative within the compression zone, and it rapidly relaxes to positive values downstream from the pressure jump, whereas the impact of weak shocks on the curvature of the field is sustained far downstream from the compression zone. Conclusions are drawn for further developments in modeling mixing.

96/05229 Effect of fuel- and furnace-side parameters on the operation of fluidized-bed furnaces

In an enclosure, as all the air inflow is consumed in burning with the excess fuel, the internal fire enters the decay phase, and such process is said flame exhaust. The complicated multistage process from an initial fire growth up to a flame exhaust followed by an external burning is investigated by means of a Large-Eddy-Simulation (LES). Turbulent combustion process is modelled by an Eddy Break-Up concept by using two sequential, semi-global steps for CO prediction. The numerical model solves three dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The critical fuel supply rate needed for flame to exhaust and the time period from the fuel ignition to the appearance of an external flaming in medium-scale facilities are previously obtained experimentally by Chamchine AV, Graham TL, Makhviladze GM, et al. [Experimental studies of under-ventilated combustion in small and medium-scale enclosures. In: Proceedings of the fourth international seminar on fire and explosion hazards; 2003. p. 97–107.], and the general trends predicted by the numerical model follow closely their experimental observation. This model is capable of adequately describing the essential simultaneous phenomena (flame height, soot generation, CO production, convection and radiation) occurring in a room fire. The distinct transient stages of fire development prior to flame exhaust and scenarios of the exhaust are analysed. An external burning is followed after the flame exhaust inside enclosure, and the flame height, Hf, past the ceiling is approximately in an order of the opening height. Even though the flame exhaust takes place under the critical conditions, the heat transferred from the hotter gases and the external fire source poses significant threat to people inside enclosure, and potentially induces an ignition of fuel package exposed near the opening of an enclosure.

Numerical predictions of the carbon burnout performance of coal-fired non-slagging vertical cyclone combustors

A presumed joint probability density function (pdf) model of turbulent combustion is proposed in this paper. The turbulent fluctuations of reactant concentrations and temperature are described using a presumed joint pdf of three-dimensional Gaussian distribution based on first and second-order moments of reactant concentration and temperature. Mean reaction rates in both premixed and diffusion combustion are obtained by mean of integration under the presumed joint pdf. This model is applied to predict turbulent premixed combustion of sudden-expansion flow and turbulent jet diffusion methane/air flame. For turbulent premixed combustion, the predicted results of temperature distribution and maximum temperature using the proposed model agree better with the experiment than that using the conventional eddy-breakup (EBU)–Arrhenius model. For the turbulent jet diffusion methane/air flame, the predicted results of velocity, temperature and species concentrations using the proposed model, the Arrhenius, EBU–Arrhenius, and laminar flamelet models are compared with experiment data. Results obtained with the presumed pdf model and that obtained by the laminar flamelet model both agree well with experiments, while results using the other models have a significant difference. The presumed joint pdf model is used to predict the NO formation process, which also agrees well with the experiment data. A unified turbulent combustion model, in which both effects of turbulent diffusion and chemical dynamics are considered, is established for both premixed and diffusion combustion, especially for the process of NO formation.

On the eddy break-up coefficient

On the eddy break-up coefficient

On the eddy break-up coefficient

Advanced combustion models for turbulent reactive flow are still not at a stage where they are useful for engineering calculations of practical systems such as gas turbine combustors. State-of-the-art methods still use the Eddy Break-Up (EBU) model to give the fast chemistry limit to the reaction rate with a global kinetics formula being used to estimate the kinetically limited rates. While there must continue to be basic reservations about the general correctness of the EBU approach, it has recently been shown that the EBU limit does have a basis in theory for the nonpremixed case. The theoretical result of Bilger for the mixing-limited reaction rate shows that it is proportional to the probability density of the mixture being at stoichiometric. The EBU model, however, takes it as being proportional to the mass fraction of the deficient reactant, but this is in turn a property of the mixture fraction pdf (probability density function) under fast chemistry conditions. The theoretical result can be used to evaluate the correct value of the EBU coefficient, which is usually taken as a constant but with quite widely varying values. In this paper the authors evaluate this theoretical value for the EBU coefficient using two commonlymore » adopted forms of the pdf. Recommendations are made with regard to the best values to use in practice.« less

A Proposal for a New Turbulent Premixed Combustion Model and Its Evaluation. (Development of the Model).

The authors propose a new turbulent premixed combustion model based on a hyperbolic tangent approximation of the reaction progress variable distribution in a one-dimensional laminar flame. Several different conventional models including FL (flamelet), EBU (eddy break up), ED(eddy dissipation) and FS (flame sheet) models are compared with the proposed model. Main features of the proposed model are as follows: (1) The model in constructed from a laminar flamelet part and a distributed flame part. (2) The model is applicable to laminar and turbulent flame analyses, and it does not depend upon the types of turbulent models. (3) Its distributed flame part includes EBU, ED and FS models as approximations.

CFD prediction of coupled radiation heat transfer and soot production in turbulent flames

A novel coupled strategy is presented for predicting soot and gas species concentrations, and radiative exchange in turbulent combustion. The relatively slow processes governing soot formation are described by a model that accounts for radiative loss. In contrast to past studies, it is coupled here to the discrete transfer radiation model (DTRM), incorporating a weighted sum of gray gases (WSGG) solution to the radiative transfer equation, in an elliptic computational simulation. Incorporation of the WSGG solution in the DTRM provides a better representation of the nongray radiative properties of combustion media than that offered by other more straightforward strategies, and without excessive computational expense. Combustion is modelled by an eddy breakup concept and the k−ɛ turbulence model, with temperature evaluated from the solved enthalpy field. This complete strategy—the first reported of its kind—is applied to a confined, turbulent, jet diffusion flame burning methane in air. Confinement of the flame demands that account should be taken of the conjugate heat transfer at the boundaries. Numerical results are compared to experimental measurements of mixture fraction, temperature, and soot volume fraction, and generally good agreement is achieved. Additionally, the computation of radiative exchange is considered in detail.