M. Chrigui

Spray Generated by an Airblast Atomizer under elevated ambient pressures

The objective of this work is to study a spray generated by an airblast atomizer at elevated ambient pressures. The study includes the measurement of a typical sprays integral parameters (average drop diameter and average three-dimensional velocity vector of liquid and gas phases), numerical simulation of spray propagation, and models validation. The numerical study of the spray evolution was carried out using an accurate numerical spray module based on an Euler-Lagrange method accounting for drops evaporation. The experimental data were collected using the phase Doppler and particle image velocimetry techniques. The drop diameter and two velocity components were measured simultaneously using two-dimensional phase Doppler system. These data were used as initial conditions for the numerical simulations (data in the neighborhood of the atomizer exit) and for the validation of the numerical method (data on the spray parameters measured at various distances from the atomizer). The droplet diameter and spray velocity are shown to be influenced by the enhancing of the ambient chamber pressure.

Numerical and Experimental Study of Spray Produced by an Airblast Atomizer Under Elevated Pressure Conditions

This work has two-fold contributions. First, a detailed experimental database that allows an understanding of important aspects of the spray generated by an airblast atomizer, including atomization and spray propagation, at various ambient pressures is provided. Second, as the control, design or optimization tasks are repetitive and costly, the ability of a recently developed numerical spray module (Sadiki et al. 2005) based on a Euler-Lagrangian method to well capture spray properties under elevated pressure is evaluated. Such a module should help to provide early detailed information at moderate costs of processes under study. The experiments have been performed in a pressure chamber equipped with transparent windows allowing optical access to the spray. By means of PIV technique and Phase Doppler instrument the spray properties have been characterized at various ambient pressures (between 1 and 18 bars). Especially three average velocity components and droplet diameters of the spray have been measured. Numerical studies of the spray transport have been achieved by using advanced URANS-based models. An overall agreement between experimental data and numerical simulation points out the accuracy of the evaluation techniques used for the measurement treatment and demonstrated the prediction ability of the mathematical spray model.Copyright

Thermodynamically consistent modelling of gas turbine combustion sprays

In order to support the design procedure and increase the reliability and safety of combustion engines fired with liquid fuel at a reasonable cost, numerical prediction tools well validated by comprehensive experimental data are needed. As there is today enough evidence that Large Eddy Simulation (LES) is able to well capture intrinsically time and space dependent phenomena, LES will be employed. However, in most LES based spray modules for predicting spray combustion the interactions between both phases and between evaporating droplets and combustion are either not adequately considered or not incorporated at all. The objective of this work is to develop and validate a thermodynamically consistent spray module for Large Eddy Simulation that allows describing accurately the essential processes featuring spray combustion in gas turbine combustion chambers. These include besides the injection of liquid fuel, the turbulent droplet dispersion, the vaporization of the droplets and mixture formation and the subsequent spray combustion. In particular, (1) a physically consistent SGS-model describing the influence of droplet diameter and interface transport on the gas phase turbulence as well as the effect of the droplet evaporation on the mass and scalar transport processes (turbulence modulation) has been adapted for LES into an Eulerian–Lagrangian framework. (2) Apart from classical evaporation models valid in atmospheric conditions, an advanced evaporation model, the so called non-equilibrium model, appropriate for gas turbine conditions have been integrated and validated. (3) The chemistry-turbulence interaction under droplet evaporating conditions has been considered according to a presumed (filtered) probability density function while the combustion process itself is described following a tabulated detailed chemistry based on FGM (Flamelet Generated Manifold). (4) All the developed sub-models along with the complete model have been implemented in the working package FASTEST/LAG3D and validated in non-reacting and reacting configurations with available experimental data. Comparisons include exhaust gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit planes. An overall good agreement with experimental data has been achieved. Parts of this contribution has been already reported as mentioned throughout the paper.

Unsteady Euler/Lagrange simulation of a confined bluff-body gas-solid turbulent flow

An unsteady Euler–Lagrangian approach is adopted to predict the gaseous carrier and disperse phases flow dynamics. The turbulence is captured using two different methods, i.e. the unsteady Reynolds averaging based numerical simulation (URANS) and the large eddy simulation (LES). In the latter one, the dynamic Smagorinsky approach is used to model the sub-grid scale stresses. The time-dependent solid particle and gas phase flow properties of a confined bluff-body turbulent flow including two-way coupling effects are evaluated through comparisons with experimental data. The configuration under study features an important recirculation zone and has a mass loading of 22%. So, collision effects are not considered while tracking the disperse phase that consists of glass beads. A thermodynamically consistent turbulence modulation approach is applied for the determination of the source terms that account for the effect of particles on the turbulence level of the carrier phase. Within the URANS technique the dispersion of particles is captured by the Markov sequence approach; this model is modified by integrating a drift factor term while modeling the pressure gradient. A particular emphasis is put on the disperse phase feedback on the carrier phase and coupling procedure within each Eulerian time step along with an unsteady coupling of both codes, the (Eulerian) FASTEST3D and the (Lagrangian) LAG3D codes. Quantitative results of the disperse phase properties as well as those of the carrier phase are presented at different positions around the recirculation zone. The numerical results using both, the LES and/or the URANS delivered comparable results that agree reasonably with experimental data. However, a slight advantage of LES over URANS could be observed.

Numerical Analysis of Spray Dispersion, Evaporation and Combustion in a Single Gas Turbine Combustor

Spray dispersion, evaporation and combustion have been numerically studied in a complex industrial configuration, which consists in a single annular combustor that was experimentally measured by Rolls-Royce-Deutschland Company. Simulations have been achieved using the Eulerian-Lagrangian approach. The computations of the continuous phase have been performed by means of RANS simulations. Though the k-e as well as the Reynolds Stress model (Jones-Musonge) have been used for turbulence modeling. The 3D-computations have been performed in a fully two-way coupling. The effects of turbulence on droplets distribution are accounted for using the Markov sequence dispersion model. The equilibrium as well as the non-equilibrium evaporation model have been applied. In order to account for the combustion, the diffusion flame model is chosen. It relies on the computation of the mixture fraction that has been affected by the presence of vapor source terms. For the interaction of the turbulence with the chemistry, the mixture fraction variance has also been solved. For that purpose a presumed beta-PDF function has been considered. The equilibrium and the flamelet chemistry approaches have been used for the generation of the chemistry tables. The performed simulations have also been compared to commercial CFD-codes. From there one observes, that the obtained results using the mentioned sub-models combination agree most favorably with experimental measurements. One noted that the Reynolds Stress model provided smoother temperature distribution compared to k-e. The flamelet model has been performed using three different scalar dissipation rates. One observes that differences are mainly located at the nozzle exit, where the scalar dissipation rate has got the highest value. Although the comparison between the numerical results and the experimental data was possible only at the combustor exit, due to the limitation on the measurement techniques, one can reiterate that the combination of the following sub-models: thermodynamically consistent model for the turbulence modulation, Langmuir-Knudsen non-equilibrium model for the evaporation, Reynolds Stress Model for the turbulence and flamelet model for the chemistry establish a reliable complete model that seems to allows a better description of reactive multi-phase flow studied in the frame of this work.© 2008 ASME

CFD-Analysis of Conjugate Effects of Turbulence and Swirl Intensity on Spray Combustion in a Single Gas Turbine Combustor

On the basis of an Euler-Lagrangian approach, two examples of CFD analysis of the spray evolution in single gas turbine combustors are investigated to highlight the effects of two of the major controlling factors of the gas turbine combustion. The first example deals with the effects of turbulence properties on droplet dispersion, vaporization and mixing of a non-reacting spray. The second example includes the swirl intensity effects and focuses on the conjugate spray combustion. In contrast to existing works, advanced models for turbulence, evaporation and modulation are combined. The results reveal that 1) an agreement with experiments is better achieved with the non-equilibrium evaporation model; 2) the turbulence intensity influences the efficiency of the droplet interface transport; 3) an increase in swirl degree enhances obviously the mixing rate; 4) from both effects the mixing of gaseous fuel and air is confirmed to become a controlling process of combustion.Copyright

Large Eddy Simulation of Diluted Turbulent Spray Combustion Based on FGM Methodology: Effect of fuel and Mass Loading

A numerical methodology relying on Large Eddy Simulation is used to analyze and evaluate the impact of fuel and mass loading on turbulent spray combustion. To retrieve the flow, mixing and combustion proper-ties, an Eulerian-Lagrangian approach is adopted. The method includes a full two-way coupling between the interacting two phases in presence, while the evaporation process is described by a non-eqnilibrium vaporization model. The carrier phase turbulence is captured by a combustion LES technique in which first order sub-grid scale models are applied.

Evaporation Modeling for Polydisperse Spray in Turbulent Flow

Based on an overview of existing vaporization models, a suggestion for capturing phase transition in a turbulent two phase flow is made. Focus is put on the Uniform Temperature Model (UTM). Comparison between equilibrium and non-equilibrium evaporation models to experimental data is highlighted. Two configurations with different fuels, i.e. different thermodynamic properties, are investigated and the results of both models are validated with the measurements. The configurations exhibit completely different boundary conditions and polydisperse turbulent multiphase flows with different classes and probability distribution of the droplet diameters. Large eddy simulation (LES) and Reynolds averaged numerical simulation (here RANS) models are used to capture the turbulence. In both configurations, results show that non-equilibrium effects influence the vaporization significantly. The UTM with the extension of non-equilibrium, by Langmuir and Knudsen, capture the vaporization well, whereas the equilibrium model over-predicts the volume flux of the liquid phase, i.e. the vaporization process is developing slower in case of equilibrium model. Worth to notice that the mean droplet diameter is between 20 and 40 µm. Thus the ratio of surface to volume is important if compared to larger droplets. Non-equilibrium effects are then correspondingly important and the equilibrium model is not able to describe the phase transition process well.

Toward the Impact of Fuel Evaporation-Combustion Interaction on Spray Combustion in Gas Turbine Combustion Chambers. Part I: Effect of Partial Fuel Vaporization on Spray Combustion

This work aims at investigating the impact of the interaction between evaporation process and combustion on spray combustion characteristics in gas turbine combustion chambers. It is subdivided into two parts. The first part studies how the evaporation process affects the behavior of partially pre-vaporized spray combustion. The second part attempts to answer the question how the fuel evaporation process behaves under premixed combustion conditions.For this purpose an Eulerian-Lagrangian RANS based procedure under a full two-way coupling was used. To describe the 3D-evaporation, two different (equilibrium and non-equilibrium) evaporation models based on the uniform temperature assumption were applied. For the combustion, the conditioned progress variable approach based on the Bray-Moss-Libby model has been adapted and used to account for both premixed and partially premixed combustion. To assess the numerical approach and to analyze the ongoing processes in the first part of this work, a model gas turbine combustor fired by kerosene fuel was considered. It features a partially premixed flame. Among others, the influence of both parameters, the mixing air temperature and the vaporization tube length, on the degree of vaporization and subsequently on spray flame characteristics has been pointed out. An overall agreement with available experimental data was achieved especially using the non-equilibrium evaporation model. With this confidence in place, further parameter studies could be consistently performed and additional information could be extracted to gain insights relevant to understanding the effects under study.

Effect of Evaporation on the Combustion Behaviour of Kerosene Spray Flame

The aim of this paper is to investigate numerically the effects of the evaporation rate on combustion of kerosene sprays in a partially premixed pre-vaporized combustion chamber. The simulations are carried out by means of a Eulerian-Lagrangian approach under consideration of a full two-way coupling. Two different evaporation models (equilibrium and non-equilibrium ansatz) based on the uniform temperature assumption were used. The combustion is described by a BML like model modified for partially premixed combustion description as encountered in LPP (Bray-Moss-Libby Modell) configuration. Dodecane, whose chemistry was described by the flamelet model, was used as a surrogate for kerosene. To isolate the effect of evaporation on spray combustion, liquid droplets have been sprayed at different temperature conditions in a partially premixed pre-vaporized combustor as experimentally investigated in [23]. It turned out that the non-equilibrium performs well and allows achieving an overall agreement with experimental data. With this confidence in place, some parameter studies could be carried out and additional informations, that were not experimentally available, could be extracted to gain insights relevant to understanding the effects under study.Copyright