Le Cao

Numerical simulation of tropospheric ozone depletion in the polar spring

This study addresses the modeling of the tropospheric ozone depletion event in polar spring where the aim is an improved understanding of the underlying physical and chemical processes. For this purpose, a model is developed and implemented into the open source software Open source Field Operations And Manipulations (OpenFOAM). A detailed chemical reaction mechanism is analyzed with the software package KInetic aNALysis of reaction mechanics (KINAL), and a skeletal mechanism is derived for use in the three-dimensional simulations. The 3D compressible Navier–Stokes equations are solved to predict the effect of turbulent mixing, advection of the fluid, and chemical reactions. Large eddy simulation accounts for the turbulence, and the Smagorinsky model is employed as sub-grid model. The temporal and spatial distributions of the chemical species are captured. The mixing ratio of ozone in the troposphere drops to a value near zero within several days including an “induction stage” and a “depletion stage,” which confirms previous findings. Moreover, the vertical turbulent mixing of air parcels occurs below the height of the polar boundary layer, leading to a nonuniform vertical distribution of the chemical species concentrations. Both the wind speed and the boundary layer stability may affect the boundary layer height, thus influencing the ozone depletion rate.

Modeling and Simulation of Tropospheric Ozone Depletion in the Polar Spring

In polar spring, tropospheric ozone depletion is related to the presence of halogen oxide concentrations in the atmospheric boundary layer. Halogen oxides such as BrO participate in an autocatalytic chemical reaction cycle, leading to the release of Br2 and BrCl from the fresh sea ice. The paper presents the identification of a detailed chemical reaction mechanism for the ozone depletion event, where bromine plays the major role. The heterogeneous reactions in the chemical reaction mechanism are studied in detail, and a sensitivity analysis is performed to identify the importance of each reaction in the mechanism. A skeletal reaction scheme is identified on the basis of the sensitivity analysis,. This skeletal chemical reaction mechanism then is used in a 3-D large eddy simulation (LES) with the Smagorinsky sub-grid model. The configuration studied includes a mountain located at the ground above which the ozone depletion is studied. In this situation, the height of the boundary layer varies, which greatly affects the ozone depletion event.Copyright

Computational sensitivity study of spray dispersion and mixing on the fuel properties in a gas turbine combustor

A swirl stabilized gas turbine burner has been simulated in order to assess the effects of the fuel properties on spray dispersion and fuel-air mixing. The properties under consideration include fuel surface tension, viscosity and density. The turbulence of the gas phase is modeled applying the methodology of large eddy simulation whereas the dispersed liquid phase is described by Lagrangian particle tracking. The exchange of mass, momentum and energy between the two phases is accounted for by two-way coupling. Bag and stripping breakup regimes are considered for secondary droplet breakup, using the Reitz-Diwakar and the Taylor analogy breakup models. Moreover, a model for droplet evaporation is included. The results reveal a high sensitivity of the spray structure to variations of all investigated parameters. In particular, a decrease in the surface tension or the fuel viscosity, or an increase in the fuel density, lead to less stable liquid structures. As a consequence, smaller droplets are generated and the overall spray surface area increases, leading to faster evaporation and mixing. Furthermore, with the trajectories of the small droplets being strongly influenced by aerodynamic forces (and less by their own inertia), the spray is more affected by the turbulent structures of the gaseous phase and the spray dispersion is enhanced. (Less)

Modeling of Auto-Catalytic Halogen Release and Ozone Depletion in Polar Regions

This article concerns the modeling of the tropospheric ozone depletion event in polar spring where the aim is an improved understanding of the underlying physical and chemical processes. For this purpose, a model based on OpenFOAM 1.7.1 is developed, where two-dimensional compressible Navier-Stokes equations are solved numerically by finite volume method to predict the effects of turbulent mixing, advection of the fluid, and detailed chemical reactions. The present chemical reaction mechanism consists of 53 chemical reactions among 33 species to model the auto-catalytic process considering halogen species X, X2, XY, XO, HOX, where X and Y denote halogen atoms - here Br is studied. Large eddy simulation is applied to account for the turbulence and the Smagorinsky model is employed as sub-grid model. Good agreement with literature and experimental data is obtained for the profiles of the chemical species. In particular, the correct time scale of the phenomenon is captured. It is confirmed that the mixing ratio of ozone in the troposphere drops to a value near zero within several days. Moreover, it is shown that the well-mixed air is confined inside the boundary layer. A parameter study shows that the ozone depletion event happens even at reduced initial values of molecular bromine. Also, the study of coupled transport and chemistry shows that the turbulent mixing enhances the ozone depletion in the lowest layer above the earths surface.