Abstract
This thesis presents an Eulerian multi-fluid CFD model for sprays, which is able to describe droplet breakup and coalescence and size polydispersion as well as the associated size-conditioned dynamics. In order to model the evolution of the polydisperse droplet phase, the population balance equation (PBE) is coupled to the continuity and momentum balance equations. The direct quadrature method of moments (DQMOM) is implemented to simulate the evolution of the droplet size distribution (DSD) due to breakup and coalescence.
The DQMOM-multi-fluid model uses source terms for the first 2N moments of the droplet number distribution as parameters to determine the source terms of the transport equations of the N droplet volume fractions and the N droplet diameters. Transport equations are also solved for phase-average velocities for each droplet phase. Submodels are designed to capture the effects of droplet breakup and droplet-droplet collisions.
The model is applied to calculate local values of droplet sizes and velocities produced by diesel-type, Y-jet, and hollow-cone sprays. The droplet velocity results for the diesel-type spray are well predicted. The droplet size and velocity results for the Y-jet water sprays are less accurate, although this is considered to be due to inaccuracies in the boundary conditions at the nozzle, rather than an error in the model. The collapse of the hollow-cone spray is evident in the predictions, and droplet sizes and velocities are in good agreement with experimental data.
The model has successfully predicted the main features of the sprays, but there are aspects of the model in which improvements can be made.
A computational study of the internal flow field of a large-scale Danfoss pressure-swirl atomizer is also presented. The two-phase flow is modeled using three approaches: 1) a volume of fluid (VOF) method using laminar viscosity only, 2) a VOF method using subgrid-scale turbulence modeling, and 3) a two-fluid Euler/Euler method using the laminar viscosity only. The primary focus of the analysis is on the internal flow characteristics in the swirl chamber. The CFD results compare favorably with experimental data of tangential and axial velocity distributions in the swirl chamber and static wall pressure.
Experiments are carried out in order to obtain local quantities in water sprays from production-scale pressure-swirl and Y-jet atomizers. A two-component phase Doppler anemometry (PDA) system is used for obtaining local values of droplet velocities and sizes. Experimental studies are conducted in sprays produced by nine different single-hole Y-jet atomizers with different operating conditions. Experiments concerned with the effects of atomizer geometry on the spray show that the mixing length should be approximately four times the mixing chamber diameter. Other geometrical variables have relatively little effect, except for the way in which they affect air and water gauge pressures. The investigation into the effects of operating conditions shows that increasing the liquid flow rate or the mass loading ratio both reduce the mean diameters and increase the axial velocity of the spray. The liquid flow rate is the key parameter determining the spray characteristics of the Y-jet atomizers. The spray is less affected by different mass loading ratios.
In supplement, to the PDA measurements, interferometric particle imaging (IPI) measurements are carried out in a hollow-cone spray from a Danfoss pressure-swirl atomizer. IPI is a technique for determining the diameter of transparent and spherical particles in a whole field from out-of-focus particle images. The velocity of each particle is simultaneously determined using particle tracking velocimetry (PTV) on focused images. Results are compared to PDA measurements. In shape and trends the data acquired with IPI and PDA are very similar, however due to different sampling methods employed by the two measuring techniques, IPI yields consistently smaller mean diameters than PDA. The IPI/PTV technique has some advantages over the PDA system as it measures droplet sizes and velocities in a whole field. The main limitation of the IPI/PTV technique is that it cannot be used at high droplet concentrations.
The DQMOM-multi-fluid model uses source terms for the first 2N moments of the droplet number distribution as parameters to determine the source terms of the transport equations of the N droplet volume fractions and the N droplet diameters. Transport equations are also solved for phase-average velocities for each droplet phase. Submodels are designed to capture the effects of droplet breakup and droplet-droplet collisions.
The model is applied to calculate local values of droplet sizes and velocities produced by diesel-type, Y-jet, and hollow-cone sprays. The droplet velocity results for the diesel-type spray are well predicted. The droplet size and velocity results for the Y-jet water sprays are less accurate, although this is considered to be due to inaccuracies in the boundary conditions at the nozzle, rather than an error in the model. The collapse of the hollow-cone spray is evident in the predictions, and droplet sizes and velocities are in good agreement with experimental data.
The model has successfully predicted the main features of the sprays, but there are aspects of the model in which improvements can be made.
A computational study of the internal flow field of a large-scale Danfoss pressure-swirl atomizer is also presented. The two-phase flow is modeled using three approaches: 1) a volume of fluid (VOF) method using laminar viscosity only, 2) a VOF method using subgrid-scale turbulence modeling, and 3) a two-fluid Euler/Euler method using the laminar viscosity only. The primary focus of the analysis is on the internal flow characteristics in the swirl chamber. The CFD results compare favorably with experimental data of tangential and axial velocity distributions in the swirl chamber and static wall pressure.
Experiments are carried out in order to obtain local quantities in water sprays from production-scale pressure-swirl and Y-jet atomizers. A two-component phase Doppler anemometry (PDA) system is used for obtaining local values of droplet velocities and sizes. Experimental studies are conducted in sprays produced by nine different single-hole Y-jet atomizers with different operating conditions. Experiments concerned with the effects of atomizer geometry on the spray show that the mixing length should be approximately four times the mixing chamber diameter. Other geometrical variables have relatively little effect, except for the way in which they affect air and water gauge pressures. The investigation into the effects of operating conditions shows that increasing the liquid flow rate or the mass loading ratio both reduce the mean diameters and increase the axial velocity of the spray. The liquid flow rate is the key parameter determining the spray characteristics of the Y-jet atomizers. The spray is less affected by different mass loading ratios.
In supplement, to the PDA measurements, interferometric particle imaging (IPI) measurements are carried out in a hollow-cone spray from a Danfoss pressure-swirl atomizer. IPI is a technique for determining the diameter of transparent and spherical particles in a whole field from out-of-focus particle images. The velocity of each particle is simultaneously determined using particle tracking velocimetry (PTV) on focused images. Results are compared to PDA measurements. In shape and trends the data acquired with IPI and PDA are very similar, however due to different sampling methods employed by the two measuring techniques, IPI yields consistently smaller mean diameters than PDA. The IPI/PTV technique has some advantages over the PDA system as it measures droplet sizes and velocities in a whole field. The main limitation of the IPI/PTV technique is that it cannot be used at high droplet concentrations.
| Original language | English |
|---|---|
| Place of Publication | Esbjerg |
| Publisher | |
| Print ISBNs | 87-7606-016-0 |
| Publication status | Published - 2007 |