Wyniki wyszukiwania - BazTech

1

Studies on the flame structure and drag of a combusting droplet group

Zhou Lixing, Li Ke, Sun Ting

Combustion Engines

|

2023

|

R. 62, nr 2

3--8

EN

Droplet combustion has been studied by experiments and numerical simulation for many years, and most of studies were done about single droplets. As for droplet groups, some theoretical and experimental studies were reported, but less of numerical studies. One of the important characteristics is the drag of combusting droplets, which is closely related to flame structure，and is a sub-model in numerical simulation of spray combustion. There are contradictory research results for the drag of combusting droplets. In the present paper, large-eddy simulation (LES) is used to study the flame structure and drag of a combusting ethanol-droplet group. The results show that there are three combustion modes: fully-enveloped flame, partially-enveloped flame and wake flame in a droplet group, leading to the change of the drag with inlet velocities. It is found that the drag of droplets in the group is much smaller than that of a non-combusting particle in isothermal flows.

2

Numerical study of fuel and turbulence distributions in an automotive-sized scavenged pre-chamber

Bolla Michele, Shapiro Evgeniy, Kotzagianni Maria, Kyrtatos Panagiotis, Tiney Nick, Boulouchos Konstantinos

Combustion Engines

|

2019

|

R. 58, nr 1

61--67

EN

This article presents a numerical study of the fuel and turbulence distributions in a pre-chamber at spark-time. The study has been conducted in the framework of the H2020 Gas-On project, dealing with the development of a lean-burn concept for an automotive-sized gas engine equipped with a scavenged pre-chamber. The test case considered studies a 7-hole pre-chamber with circumferentially-tilted orifices mounted on the cylinder head of a rapid compression-expansion machine (RCEM), consistent with the experimental test rig installed at ETH Zurich. An accurate description of turbulence and fuel distributions are key quantities determining the early flame development within the pre-chamber. Both quantities have an influence on the overall combustion characteristics and therefore on the engine performance. For this purpose, computational fluid dynamics (CFD) is employed to complement experimental investigations in terms of data completeness. The performance of the Reynolds-averaged Navier-Stokes (RANS)-based turbulence model is compared with large-eddy simulation (LES) through ensemble averaging of multiple LES realizations, in which the fuel injection rate evolution into the pre-chamber has been perturbed. Overall, RANS results show that the distributions of the turbulent kinetic energy and fuel concentration at spark-time agree well with the LES ensemble-averaged counterparts. This constitutes a prerequisite in view of the combustion phase and the accuracy reported provides further confidence in this regard.

3

Large-Eddy Simulations of Particle-laden Turbulent Jets

Luniewski M., Kotula P., Pozorski J.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

|

2012

|

Vol. 16, No 1-2

33-51

EN

The work presents an application of Large-Eddy Simulation (LES) for turbulent two-phase flows with dispersed particles. For the simulations of the continuous phase (fluid), an academic, finite volume LES solver was applied and customised. For comparison purposes, also a spectral solver was considered. The LES of fluid was used together with a Lagrangian module for the dispersed phase in the point-particle approximation, including the two-way momentum coupling between the phases. The particle solver has been further developed for parallel computations. The simulations of turbulent, particle-laden round jets were performed. The results for fluid and particle statistics were compared with available reference data.

4

Effects of a fully submerged boulder within a boulder array on the mean and turbulent flow fields: Implications to bedload transport

Papanicolaou A., Kramer C., Tsakiris A., Stoesser T., Bomminayuni S., Chen Z.

Acta Geophysica

|

2012

|

Vol. 60, no. 6

1502-1546

EN

The objective of this coupled experimental and numerical study is to provide insight into the mean and turbulent flow fields within an array of fully submerged, isolated, immobile boulders. Our study showed that the velocity defect law performed well for describing the mean flow around the boulder within the array. A prerequisite, however, was to accurately estimate the spatial variability of u* around the boulder, which was achieved via the boundary characteristics method. The u* exhibited considerable spatial variability within the array and form roughness was shown to be up to 2 times larger than the skin roughness in the boulder near-wake region. Because the boulders bear a significant amount of the flow shear, the available bed shear stress for entrainment of the mobile sediment, τols, near the boulders was roughly 50% lower than the ambient τols. The τols variability induced by the boulders could lead to a threefold overestimation of the sediment transport rate.

5

Large-eddy simulation of katabatic winds. Part 2: Sensitivity study and comparison

Axelsen S.L., van Dop H.

Acta Geophysica

|

2009

|

Vol. 57, no. 4

837-856

EN

The effects of the slope angle, surface buoyancy flux, and background stratification on steady-state katabatic winds are studied using large-eddy simulation (LES). The numerical code was described and validated in a companion paper (Part 1). Our numerical results are interpreted in the light of analytical Prandtl model, and we find that our vertical profiles of the downslope velocity, buoyancy, and the momentum and buoyancy fluxes exhibit many of the features from the analytical solution. On the other hand, there are also differences between the analytical and numerical results due to the assumptions in the analytical model. One of the assumptions is that the Prandtl number is constant throughout the boundary layer. However, the simulations show that this number varies with height, and also that the Prandtl number increases with increasing gradient Richardson number. The immediate benefit of LES over analytical models is its capability of resolving turbulent motions. In our study of the turbulence kinetic energy budgets, we find that the wind shear is the largest production term, and that it is mainly balanced by turbulence dissipation. Near the wind maximum, where the shear vanishes, the turbulence transport is the only production term.

6

Large-eddy simulation of katabatic winds. Part 1: Comparison with observations

Axelsen S.L., van Dop H.

Acta Geophysica

|

2009

|

Vol. 57, no. 4

803-836

EN

Steady-state quasi-one-dimensional large-eddy simulations of slope winds over simple terrain are presented. The model results of up-slope flow are compared to previous simulations by Schumann (1990), and good agreement is found. Modelled downslope winds are compared to meteorological observations from two glaciers. The vertical profiles of velocity and buoyancy agree with the observations, whereas larger discrepancies are found between the modelled and the observed momentum and buoyancy flux profiles. Despite some discrepancies, the model captures the main characteristics of the observed downslope winds fairly well. The numerical model is used in a companion paper (Part II) to study how some external input parameters affect katabatic winds.

7

Nocturnal basin low-level jets: an integrated study

Cuxart J.

Acta Geophysica

|

2008

|

Vol. 56, no. 1

100-113

EN

Low-level jets (LLJs) are a very common feature in the nocturnal stably stratified boundary layer. Many factors can intervene in their generation, linked basically to effects of baroclinity. A special kind of low-level jets is composed by the nocturnal katabatic and basin flows, generated over terrain slopes. A study of observed LLJs in the Duero Basin is shown here, combining observational data and model-ling experiments. Normalized in respect to the maximum wind height, the dynamic characteristics of the jets are similar: a two-layer system, with a stably stratified layer below the jet maximum and a near neutral layer above, with a very stable layer separating them at the level of the wind maximum. There is vertical mixing above and below the jet, and the connection between these layers takes place occasionally in a very turbulent manner.

8

An inconvenient “truth” about using sensible heat flux as a surface boundary condition in models under stably stratified regimes

Basu S., Van De Wiel B., Moene A., Steeneveld G.

Acta Geophysica

|

2008

|

Vol. 56, no. 1

88-99

EN

In single column and large-eddy simulation studies of the atmospheric bound-ary layer, surface sensible heat flux is often used as a boundary condition. In this paper, we delineate the fundamental shortcomings of such a boundary condition in the context of stable boundary layer modelling and simulation. Using an analytical approach, we are able to show that for reliable model results of the stable boundary layer accurate surface temperature prescription or prediction is needed. As such, the use of surface heat flux as a boundary condition should be avoided in stable condi-tions.