LM
L. Ma
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1
The multi-flame phenomenon often seen in spray combustion was studied via consideration of two cases from the Delft Spray in Hot Coflow database which have the same fuel (ethanol) but differ in the type of coflow. The two-phase flow was handled with an Eulerian-Lagrangian approach and Large Eddy Simulation with Flamelet Generated Manifolds method was used to study the turbulence and combustion. Simulation results revealed that four spatially separated reaction regions exist in the flame AII and two in the flame HII. These regions are of different types premixed or non-premixed and are formed by different species major fuel (ethanol) or intermediate species e.g. CO. The mechanism underlying the multi-flame structure was investigated in terms of droplet evaporation dispersion convection and reaction. Parametric studies on the effect of spray polydispersity and coflowing air temperature demonstrates an even wider range of flame structures. The 'single flame' structure usually present in the hot-diluted coflow case (HII) was also generated in a case of room temperature air coflow provided the injected droplets are small since representative droplets in all cases with single flame have similar evaporation time scale.
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The multi-flame phenomenon often seen in spray combustion was studied via consideration of two cases from the Delft Spray in Hot Coflow database which have the same fuel (ethanol) but differ in the type of coflow. The two-phase flow was handled with an Eulerian-Lagrangian approach and Large Eddy Simulation with Flamelet Generated Manifolds method was used to study the turbulence and combustion. Simulation results revealed that four spatially separated reaction regions exist in the flame AII and two in the flame HII. These regions are of different types premixed or non-premixed and are formed by different species major fuel (ethanol) or intermediate species e.g. CO. The mechanism underlying the multi-flame structure was investigated in terms of droplet evaporation dispersion convection and reaction. Parametric studies on the effect of spray polydispersity and coflowing air temperature demonstrates an even wider range of flame structures. The 'single flame' structure usually present in the hot-diluted coflow case (HII) was also generated in a case of room temperature air coflow provided the injected droplets are small since representative droplets in all cases with single flame have similar evaporation time scale.
We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.
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We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.
This work studies the influence of coflow conditions on the structure of the Delft Spray in Hot-diluted Coflow (DSHC) flames, using Large Eddy Simulation (LES) techniques. The developed modeling approach was first applied to three different experimental cases for validation. Major properties such as droplet velocity, SMD, gas phase velocity and temperature can be reproduced with good accuracy. Also, the trends of flame lift-off height and flame width with change of coflow conditions were properly captured. Then the study was extended to four more virtual cases, which differ from each other only by coflow temperature or oxygen concentration. The purpose of this step is to eliminate the simultaneous change of multiple parameters as was done in the experiment and to isolate the influences of coflow temperature and O2 concentration, which are important parameters for a MILD furnace design and operation. Two reaction regions (RRs) have been identified in the DSHC flame. The inner RR is created by the premixed reaction of ethanol under hot and fuel rich condition. The outer one is formed by the non-premixed reaction of intermediate fuels, e.g. CO and H2, which are produced in the inner RR. The coflow temperature (Tcf) has a significant influence on the DSHC flames. Increase of flame lift-off height by a factor of five was observed when the coflow temperature was decreased from 1400 K to 1200 K. The flame changed from having a two-RRs structure to a triple flame when the coflow temperature is reduced. The outer RR shifted from non-premixed combustion at high Tcf to premixed combustion at low Tcf. The oxygen mole fraction in the coflow (XO2,cf) alters the role of two RRs. The inner premixed RR is strengthened with increasing XO2,cf. The flame peak temperature is significantly increased in the case with highest XO2,cf. According to Cavaliere and de Joannon's definition [1], all studied cases except the one that has the highest XO2,cf, falls into the MILD regime. But when a restriction of flame peak temperature (Tpeak <1800 K) is also applied, only the case with the lowest XO2,cf and moderate Tcf can be strictly called MILD.
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This work studies the influence of coflow conditions on the structure of the Delft Spray in Hot-diluted Coflow (DSHC) flames, using Large Eddy Simulation (LES) techniques. The developed modeling approach was first applied to three different experimental cases for validation. Major properties such as droplet velocity, SMD, gas phase velocity and temperature can be reproduced with good accuracy. Also, the trends of flame lift-off height and flame width with change of coflow conditions were properly captured. Then the study was extended to four more virtual cases, which differ from each other only by coflow temperature or oxygen concentration. The purpose of this step is to eliminate the simultaneous change of multiple parameters as was done in the experiment and to isolate the influences of coflow temperature and O2 concentration, which are important parameters for a MILD furnace design and operation. Two reaction regions (RRs) have been identified in the DSHC flame. The inner RR is created by the premixed reaction of ethanol under hot and fuel rich condition. The outer one is formed by the non-premixed reaction of intermediate fuels, e.g. CO and H2, which are produced in the inner RR. The coflow temperature (Tcf) has a significant influence on the DSHC flames. Increase of flame lift-off height by a factor of five was observed when the coflow temperature was decreased from 1400 K to 1200 K. The flame changed from having a two-RRs structure to a triple flame when the coflow temperature is reduced. The outer RR shifted from non-premixed combustion at high Tcf to premixed combustion at low Tcf. The oxygen mole fraction in the coflow (XO2,cf) alters the role of two RRs. The inner premixed RR is strengthened with increasing XO2,cf. The flame peak temperature is significantly increased in the case with highest XO2,cf. According to Cavaliere and de Joannon's definition [1], all studied cases except the one that has the highest XO2,cf, falls into the MILD regime. But when a restriction of flame peak temperature (Tpeak <1800 K) is also applied, only the case with the lowest XO2,cf and moderate Tcf can be strictly called MILD.
Flameless combustion technology can provide high efficiency with less emissions. Although spray combustion is prevalent in industrial furnaces, little research has been carried out to investigate ameless spray combustion. This article reports on numerical simulation of the Delft Spray-in-Hot-Coflow (DSHC) burner, which has been constructed to simulate and study ameless combustion of light liquid fuels. Reynolds Averaged Navier-Stokes (RANS) simulation is used with turbulence modeled through either the k-E or the Reynolds stress model (RSM). Moreover, two combustion models have been chosen: the steady amelet and the Flamelet Generated Manifold (FGM). This FGM is constructed from strained steady amelets and an unsteady extinguishing amelet. The interaction between turbulence and combustion is included via the gaseous PDF approach, which means the liquid phase has no direct impact on this interaction. Liquid fuel spray is described by a discrete phase model consisting of various submodels such as atomization, dispersion, and evaporation model. The combination of turbulence, combustion, and discrete phase models is validated by a dataset of liquid and gas-phase velocities as well as gas temperature profiles. Despite small discrepancy in the liquid-phase results, the numerical gas-phase velocities match well the experimental results for both turbulence models. The steady amelet model could not predict the observed liftoff and flame is always attached to the injector while the FGM model can show this liftoff. However, the quantitative prediction of liftoff height needs further investigation as long as the current FGM formulation does not provide accurately the liftoff height and temperature profile near the injector.
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Flameless combustion technology can provide high efficiency with less emissions. Although spray combustion is prevalent in industrial furnaces, little research has been carried out to investigate ameless spray combustion. This article reports on numerical simulation of the Delft Spray-in-Hot-Coflow (DSHC) burner, which has been constructed to simulate and study ameless combustion of light liquid fuels. Reynolds Averaged Navier-Stokes (RANS) simulation is used with turbulence modeled through either the k-E or the Reynolds stress model (RSM). Moreover, two combustion models have been chosen: the steady amelet and the Flamelet Generated Manifold (FGM). This FGM is constructed from strained steady amelets and an unsteady extinguishing amelet. The interaction between turbulence and combustion is included via the gaseous PDF approach, which means the liquid phase has no direct impact on this interaction. Liquid fuel spray is described by a discrete phase model consisting of various submodels such as atomization, dispersion, and evaporation model. The combination of turbulence, combustion, and discrete phase models is validated by a dataset of liquid and gas-phase velocities as well as gas temperature profiles. Despite small discrepancy in the liquid-phase results, the numerical gas-phase velocities match well the experimental results for both turbulence models. The steady amelet model could not predict the observed liftoff and flame is always attached to the injector while the FGM model can show this liftoff. However, the quantitative prediction of liftoff height needs further investigation as long as the current FGM formulation does not provide accurately the liftoff height and temperature profile near the injector.
solver, the Flamelet Generated Manifolds (FGM) model has been implemented, and used to account for the Turbulent-Chemistry Interaction (TCI). We report here a numerical study on the Delft Spray-in-Hot-Colfow (DSHC) flame with this new “sprayFGMFoam” solver. The enthalpy loss effect due to droplet vaporization is considered by employing an additional controlling parameter in the FGM libraries. Analysis of the DSHC experimental data suggests that flash boiling influences the atomization of liquid fuel. This introduces new challenges for modeling the spray atomization process. A conditional injection model is proposed to provide reliable spray boundary conditions for downstream flow and combustion simulation. In this conditional injection model, the droplets have an asymmetric distribution around the spray half angle, in agreement with experimental observations. Also, the possible range of droplet injection angle is conditioned upon the droplet size (mass). Small droplets can be injected to a very wide range of direction, while large droplets move within a small sector centered at the mean spray trajectory. Two cases employing or not the proposed conditional injection model are compared. The results suggest that using the conditional injection model improves the prediction for all the properties examined.
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solver, the Flamelet Generated Manifolds (FGM) model has been implemented, and used to account for the Turbulent-Chemistry Interaction (TCI). We report here a numerical study on the Delft Spray-in-Hot-Colfow (DSHC) flame with this new “sprayFGMFoam” solver. The enthalpy loss effect due to droplet vaporization is considered by employing an additional controlling parameter in the FGM libraries. Analysis of the DSHC experimental data suggests that flash boiling influences the atomization of liquid fuel. This introduces new challenges for modeling the spray atomization process. A conditional injection model is proposed to provide reliable spray boundary conditions for downstream flow and combustion simulation. In this conditional injection model, the droplets have an asymmetric distribution around the spray half angle, in agreement with experimental observations. Also, the possible range of droplet injection angle is conditioned upon the droplet size (mass). Small droplets can be injected to a very wide range of direction, while large droplets move within a small sector centered at the mean spray trajectory. Two cases employing or not the proposed conditional injection model are compared. The results suggest that using the conditional injection model improves the prediction for all the properties examined.