| 1. |
- Grosshans, H., et al.
(författare)
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Sensitivity of VOF simulations of the liquid jet breakup to physical and numerical parameters
- 2016
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Ingår i: Computers & Fluids. - : Elsevier. - 0045-7930 .- 1879-0747. ; 136, s. 312-323
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Tidskriftsartikel (refereegranskat)abstract
- In this paper the characteristics of the primary breakup of a liquid jet is analyzed numerically. We applied the Volumes of Fluids (VOF) approach utilizing the Direction Averaged Curvature (DAC) model, to estimate the interface curvature, and the Direction Averaged Normal (DAN) model, to propagate the interface. While being used for the first time to predict liquid atomization, this methodology showed a high accuracy. The influence of varying the fluid properties, namely liquid-gas density and viscosity ratio, and injection conditions is discussed related to the required grid resolution. Resulting droplet sizes are compared to distributions obtained through the One-Dimensional Turbulence (ODT) model.
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| 2. |
- Järpner, Christoffer, 1988, et al.
(författare)
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An assessment of cooled air cooling for combined cycle gas turbines
- 2013
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Ingår i: 5th International Conference on Applied Energy.
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Konferensbidrag (refereegranskat)abstract
- A triple pressure combined gas turbine cycle reference plant is established targeting technology that is expected to enter service over the next decade. Through the means of exergy analysis major losses occurred in this cycle is discussed, for which exergy destruction due to cooling is singled out as one of the relatively large sources. Several concepts concentrating on the cooling of the cooling air are then evaluated, with the purpose increasing the net efficiency in comparison with the reference plant. The cooling of the cooling flow is only included for the first stage of the turbine. By allowing a full transfer of heat from the cooling flow to the fuel an efficiency improvement of 1.2% in net efficiency is estimated. This estimate is to be seen as a theoretical limit for the one-stage cooling concept, since the fuel temperature is above what can be viewed as feasible. For a concept with a combined intermediate pressure steam heating and heating to a conservatively chosen fuel temperature the benefit is estimated at 0.6%.
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| 3. |
- Kerstein, Alan R., et al.
(författare)
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Parameter dependences of the onset of turbulent liquid-jet breakup
- 2017
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 811
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies have predicted We(-2/5) dependence of the streamwise location at which primary breakup of turbulent liquid jets begins and We(-3/5) dependence of the Sauter mean diameter (SMD) of droplets released at that location, where We is the jet Weber number. Measured deviations from these predictions were attributed to measurement uncertainties and to the simplicity of the analysis, which invoked turbulence inertial-range phenomenology. Here, it is proposed that breakup onset is instead controlled by the residual presence of the boundary-layer structure of the nozzle flow in the near field of the jet. Assuming that the size of the breakup inducing eddy is within the scale range of the log-law region. We(-1) dependence of both the onset location and the SMD at onset is predicted. These dependences agree with the available measurements more closely than those previously predicted. lb predict the dependences on the Reynolds number Re, either the friction velocity in conjunction with the Blasius friction law or the hulk velocity can he used, where the former yields Re-3/8 and Re-1/4 dependence of the onset location and the SMD at onset respectively, while the latter implies no Re dependence of either. 'f he latter result is consistent with the available measurements, but the boundary-layer analysis indicates that the velocity scaling should be based on the friction velocity rather than the bulk velocity, so the origin of the measured lack of Re dependence merits further investigation. A plausible hypothesis is that pressure effects associated with the transition from wall-bounded nozzle flow to jet free-slip boundary conditions induce a transient large-scale flow modification that counteracts the Re dependence of the nozzle flow while preserving the logarithmic flow structure near the jet surface. Notwithstanding the absence of direct evidence supporting this hypothesis, the new analysis and comparisons of its predictions with measurements suggest that transient effects such as the residual influence of the nozzle-flow structure are the likely explanations of the observed parameter dependences.
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| 4. |
- Movaghar, Amirreza, 1987, et al.
(författare)
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A subgrid-scale model for large-eddy simulation of liquid/gas interfaces based on one-dimensional turbulence
- 2019
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Ingår i: ERCOFTAC Series. - Cham : Springer International Publishing. - 1382-4309 .- 2215-1826. ; 26, s. 83-91
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The interface/turbulence interaction between two fluids in a turbulent environment has an important role in many technical processes, e.g. primary liquid atomization in combustion devices. Primary atomization has a significant role in spray formation and its characteristics. The resulting dynamics typically span 4–6 orders of magnitude in length scales, making detailed numerical simulations exceedingly expensive. This motivates the need for modeling approaches based on spatial filtering such as large-eddy simulation (LES). In this paper, a new approach based on One-Dimensional turbulence (ODT) is presented to describe the subgrid interface dynamics. ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps that represent the effects of individual turbulent eddies on property fields. It provides affordable high resolution of interface creation and property gradients within each phase, which are key for capturing the local behavior as well as overall trends. ODT has previously been shown to reproduce the main features of an experimentally determined regime diagram for primary jet breakup. Here a new approach called VODT is presented which produces a size-conditioned as well as a total time rate of generation of droplets for given flow conditions at an interface. At the LES level, the total droplet generation from VODT is interpreted as a rate of mass conversion of LES-resolved liquid into unresolved droplets. Preliminary results of applying VODT to a cell with a planar-shear-layer are discussed at the end of the paper.
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| 5. |
- Movaghar, Amirreza, 1987, et al.
(författare)
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Modeling and numerical study of primary breakup under diesel conditions
- 2018
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 98:2018, s. 110-119
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Tidskriftsartikel (refereegranskat)abstract
- A recently introduced stochastic model for reduced numerical simulation of primary jet breakup is evaluated by comparing model predictions to DNS results for primary jet breakup under diesel conditions. The model uses one-dimensional turbulence (ODT) to simulate liquid and gas time advancement along a lateral line of sight. This one-dimensional domain is interpreted as a Lagrangian object that is advected downstream at the jet bulk velocity, thus producing a flow state expressed as a function of streamwise and lateral location. Multiple realizations are run to gather ensemble statistics that are compared to DNS results. The model incorporates several empirical extensions of the original ODT model that represent the phenomenology governing the Weber number dependence of global jet structure. The model as previously formulated, including the assigned values of tunable parameters, is used here without modification in order to test its capability to predict various statistics of droplets generated by primary breakup. This test is enabled by the availability of DNS results that are suitable for model validation. Properties that are examined are the rate of bulk liquid mass conversion into droplets, the droplet size distribution, and the dependence of droplet velocities on droplet diameter. Quantities of greatest importance for engine modeling are found to be predicted with useful accuracy, thereby demonstrating a more detailed predictive capability by a highly reduced numerical model of primary jet breakup than has previously been achieved.
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| 6. |
- Movaghar, Amirreza, 1987, et al.
(författare)
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Numerical and experimental studies of liquid breakup at the surface of turbulent jets
- 2015
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Ingår i: ICLASS 2015 - 13th International Conference on Liquid Atomization and Spray Systems.
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Konferensbidrag (refereegranskat)abstract
- Primary breakup can be regarded as one of the least developed model components for simulating and predicting liquid jet breakup. This paper presents a numerical investigation of primary breakup of a planar turbulent liquid jet in still air at standard conditions for jet exit Reynolds numbers of 10000 and 23000 and Weber number is varied within the range [102–107]. Due to the limitation of direct numerical simulation (DNS) to moderate Reynolds numbers, the one-dimensional turbulence (ODT) model is used to simulate the jet with high lateral resolution. ODT permits affordable high resolution of interfaces and single-phase property gradients which are essential for capturing the local behavior of the breakup process. ODT is a stochastic model simulating turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. ODT has recently been used by the authors to reproduce the main features of an experimentally determined regime diagram for primary jet breakup. In this study we apply the model to the high Re number regime and compare to new experimental data.
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| 7. |
- Movaghar, Amirreza, 1987, et al.
(författare)
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Numerical investigation of turbulent-jet primary breakup using one-dimensional turbulence
- 2017
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 89, s. 241-254
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Tidskriftsartikel (refereegranskat)abstract
- Primary breakup to form droplets at liquid surfaces is an important fundamental process to study as it determines the initial properties of the dispersed phase, which affect mixing rates, secondary breakup, droplet collisions, and flow separation within the dispersed flow region. Primary breakup can be regarded as one of the least developed model components for simulating and predicting liquid jet breakup. However, it is of paramount importance in many technical applications, e.g. fuel injection in engines and spray painting. This paper presents a numerical investigation of primary breakup of a turbulent liquid jet in still air at standard conditions using the one-dimensional turbulence (ODT) modeling framework. ODT is a stochastic model that simulates turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. An important feature of ODT is the resolution of all relevant scales, both temporal and spatial. The restriction to one spatial dimension in ODT permits affordable high resolution of interfacial and single-phase property gradients, which is key to capturing the local behavior of the breakup process and allows simulations at high Reynolds and Weber numbers that are currently not accessible to direct numerical simulations (DNS). This paper summarizes our extensions of the ODT model to simulate geometrically simple jet breakup problems, including representations of Rayleigh wave breakup, turbulent breakup, and shear-driven breakup. Each jet breakup simulation consists of a short temporal channel section to initialize a turbulent velocity profile at the nozzle exit followed by an adjacent jet section. The simulations are carried out for jet exit Reynolds number of 11,500, 23,000, 46,000 and 92,000 while the Weber number is varied within the range 102–107. We present results on breakup statistics including spatial locations of droplet release, droplet sizes and liquid core length. The results on primary breakup are compared to experimental results and models.
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| 8. |
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| 9. |
- Movaghar, Amirreza, 1987
(författare)
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Numerical study of primary breakup with One-Dimensional Turbulence
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most new engines will use direct injection of the fuel, making the fuel spray a critical part of the engine. Fuel spray behavior controls engine performanceand emissions and yet there is much that remains unknown about how a spray breaks up, mixes, evaporates and burns. In this regard, the aim of this study is the development of a new computational model for primary breakup which is both predictive and efficient. We used a stochastic modeling approach called One Dimensional Turbulence.ODT permits affordable high resolution of interfaces and single-phase property gradients which are essential for capturing the local behaviorof the breakup process. ODT is a stochastic model simulating turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. The occurrence of an eddy itself is affected by the property profiles, resulting in self-contained flow evolution that obeys the applicable conservation laws.
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| 10. |
- Movaghar, Amirreza, 1987
(författare)
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The One-Dimensional Turbulence Model Applied to Spray Atomization
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulation of the spray behavior is an important part of engine research and is critical for combustion optimization. Successful implementation of the advanced modeling tools for sprays is strongly dependent on our current understanding of the physical processes involved. One of the main processes occurring close to the nozzle is primary atomization. It governs the initial size and velocity distribution of droplets formed at the liquid jet surface. This process is not yet fully understood due to challenges in experimental observation of the region close to the nozzle. This has kept the primary atomization as one of the least developed model components in spray simulation and in need of mprovement. In this dissertation, a new primary atomization model is proposed based on the One-Dimensional Turbulence (ODT) model framework. ODT is a stochastic turbulence model simulating turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. This approach provides affordable high resolution at the liquid/gas interface, which is essential for capturing the local behavior of the breakup process. This new approach is assessed under different operating conditions parameterized by the liquid jet Reynolds and Weber numbers. ODT primary atomization results have been provided as an input to a spray model in conventional form to evaluate its predictive capability. These efforts are reported in several manuscripts attached to this dissertation. Furthermore, to better understand the physics behind primary atomization, a canonical simulation configuration is developed that isolates the interaction between surface tension and surrounding turbulence. The ability of the model to capture the breakup is assessed with the available Detailed Numerical Simulation (DNS) data for further improvements. Lastly, a new strategy is proposed to use ODT as a subgrid resolution model in LES/VOF simulations to describe/model unresolved subgrid interface dynamics.
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| 11. |
- Pütz, Michele, 1988, et al.
(författare)
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Numerical simulation of a gasoline spray using one-dimensional turbulence for primary atomization
- 2020
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Ingår i: ICLASS 2018 - 14th International Conference on Liquid Atomization and Spray Systems.
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Konferensbidrag (refereegranskat)abstract
- Predictive and reliable simulations have the potential to constitute a valuable tool for the optimization of spray systems if accurate submodels are developed for the entire range of the governing processes. The primary breakup of the turbulent liquid jet is one the most important mechanisms in sprays, yet the least developed in terms of numerical modeling. The most accurate method to simulate primary breakup is the proper resolution of liquid-gas interfaces and turbulent flow structures. However, a wide range of relevant length and time scales implicate grid requirements that are often prohibitive for real engineering applications. The most widely used method in practice is still the representation of both the continuous liquid core and the dispersed phase by means of discrete Lagrangian particles evolving in and interacting with the Eulerian gas phase. The available models for primary breakup are mainly phenomenological and involve a number of empirical constants. The one-dimensional turbulence (ODT) model is an alternative stochastic approach to model turbulence in flows with a dominant direction of property gradients. The stochastic representation of turbulent eddies on a one-dimensional domain enables high resolution at moderate computational costs. Applications of ODT to atomization revealed a great potential in recent studies. The objective of the present study is to combine ODT as a primary breakup model with a conventional Eulerian-Lagrangian method for the further spray evolution in order to asses ODT as a submodel in full spray models. Our numerical investigations were conducted on the ECN spray G, a gasoline-like, evaporating spray. The results in terms of spray penetration are encouraging, though the applicability of ODT to the transient injection phase and effects on additional spray characteristics require further investigation.
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