OpenFOAM applied to the CFD simulation of turbulent buoyant atmospheric flows and pollutant dispersion inside large open pit mines under intense insolation
Introduction
Extraction and transport of minerals from open pit mines can produce significant emissions of fugitive dust, severely affecting the operations both inside and outside the pit. Throughout the years different techniques have been developed in order to minimize health and environmental issues, with the objective of maximizing the operations. This has increased the interest in understanding the different patterns of pollutant transport inside the atmosphere of open pit mines.
We summarize in Table 1 the main numerical studies of contaminant transport inside open pit mines. In the 1990s Baklanov was one of the first to propose the need to approach the problem from a multiphysics perspective, highlighting the combination of scales involved, and the importance of topography [1], [2]. His work studied the use of ventilators to facilitate the dispersion of contaminants [3] and the effect of surface explosions on the atmosphere of the pit [4]. It verified the existence of inclined currents, defined by the topography inside the pit, controlling the dispersion of contaminants inside and around it, and detected the presence of zones of flow recirculation inside its atmosphere. Shi et al. [5] used a high resolution 3D nonhydrostatic model to simulate the air circulation inside a 2 km wide and 100 m deep pit, and were able to reproduce the intense recirculation inside the cavity, consistent with wind tunnel experiments, which was responsible for maintaining high levels of pollution inside the pit. Their results showed that both mechanical and thermal forcings are important mechanisms controlling the evolution of the atmosphere inside the pit, with fairly strong turbulence (maximum TKE values ∼) produced by the interaction of both processes. Silvester et al. [6] used the CFD code Fluent to study the mechanically forced circulations developed inside the Old Moor open pit (1 km wide and 650 m deep). Without accounting for buoyancy, they showed the existense of strong mechanical shear near the top of the pit (TKE ∼ in that zone), produced by the interaction of the wind that sweeps over the cavity and the internal atmosphere. These numerical experiments highlighted the importance that the geometry of the pit has over the flow (slope angle, aspect ratio, etc.), the existence of intense air flow recirculation inside the pit, and the role played by surface heat flux (that determines the intensity of buoyant currents), all factors controlling the exit of pollutants from open pits. The flow recirculation detected inside the pit by these authors is a well known phenomenon affecting flows over cavities [7], [8], [9], being the aspect ratio of the cavity (length/depth) the key parameter defining the number, size and location of the main rolls inside the cavity.
The environmental problem described before is increased in the case of very large open pit mines subject to strong insolation (annual mean surface radiation >), due to the interaction between intense buoyant currents and the particular complex 3D geometry of these pits. Despite its large scale, the complex features that characterize this circulation, with developed turbulence, buoyant currents and mechanical effects due to topography, are difficult to address with standard meteorological models. There exists consensus that the problem demands appropriate inclusion of multi-physics and multi-scale processes into a Computational Fluid Dynamics (CFD) approach [10], [11]. In particular, intense turbulence and recirculation induced mechanically inside the pit must be correctly simulated [5], as well as buoyancy.
In this work we focus our attention on understanding the interaction between the two main processes that control the air flow inside the internal atmosphere of large open pit mines under intense insolation: mechanical effects due to the surface geometry and convective effects due to buoyancy. Our primary target is to define the main modes of contaminant transport inside and outside Chuquicamata, a very large open pit copper mine located in northern Chile (17′20″S 54′W). Due to its large scale (width ∼4 km, depth ∼1 km, see Section 4) and intense insolation (the mine is located in the Atacama Desert), previous studies cannot be directly applied here.
First, we performed simulations aimed at studying the circulation using a simplified geometry, that conserves the main size of the real pit but with circular symmetry. In second place we used the complex real topography of Chuquicamata and its surroundings. The simplified geometry allows a clearer study of the processes interacting in the formation of the air circulation inside the pit, while the more realistic topography includes the effect of the real geometry. Given the great versatility that it provides, we selected the CFD tool OpenFOAM as development platform, using a solver previously developed [12]. We used Detached Eddy Simulation (DES), to properly simulate turbulence around complex geometries, and included density as an explicit variable in the system of equations in order to improve the treatment of buoyancy. DES combines Large Eddy Simulation (LES) far from walls, to properly solve large atmospheric eddies, and Reynolds Average Navier Stokes (RANS) techniques near walls, to solve complex geometries [13], [14]. Even if the circulation inside closed valleys has been simulated in previous works [6], [15], [16], there is apparently no record that OpenFOAM has been used to study the circulation and pollutant dispersion within very large scale open pit mines subject to intense insolation. In particular, there is no evidence that the new compressible solvers that consider density as a calculation variable (instead of using the Boussinesq approximation) have been used for these applications.
Section 2 of this paper briefly describes the physical problem considered and the numerical treatment. The numerical approach used to configure the simulations was described in detail and validated in a previous work [12]. In Section 3 we present different numerical simulations using an idealized geometry similar to Chuquicamata, considering different boundary conditions in order to investigate the effect of mechanical and buoyant processes over pollutant dispersion separately. In Section 4 we describe the results of the CFD simulation of air flow inside and around Chuquicamata using its real topography, in order to identify the role played by it over the dispersion of contaminants. Section 5 includes the main conclusions taken from the work, and proposes future applications.
Section snippets
Air circulation inside a closed valley
The air flow affecting the dispersion of pollutants inside and around closed valleys, like the ones that interest us, is composed by two main processes that control its evolution:
- •
Slope flows controlled by buoyancy, which will generally be the flow dominating the circulation inside the pit. There exist several references relative to this type of flows [17], [18], [19].
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Mechanical effects produced by the interaction of the external flow with the geometry of the cavity. The approximately conical
Domain
In this first part we used a simplified topography, that retains the same general dimensions of Chuquicamata (see Section 4), representing the pit as an inverted truncated cone with a superior diameter of 4 km, an inferior diameter of 1 km and 1 km deep (Fig. 1b). We selected this simplified symmetric geometry since it allows a clearer study of the processes interacting in the formation of the air circulation inside the pit. The simulation domain consisted of a rectangular region 15 km long, 10 km
Real topography
Chuquicamata is an open pit copper mine located in northern Chile (western slope of the Andes, 17′20″S 54′W, ∼3000 m altitude, Fig. 10a), whose vast dimensions make it one of the largest pits in the world, with more than 4 km long, 3.5 km wide and almost 1 km deep (Fig. 10b). Due to its location in the Chilean desert, Chuquicamata is exposed to high rates of solar radiation all the year ( annual mean surface radiation [34]), fostering the generation of convective currents inside
Conclusions
We have applied a CFD solver to study the turbulent buoyant atmospheric flow inside large open pit mines under intense insolation, and its effect over pollutant dispersion. Using a DES approach we have incorporated buoyancy, stratification, developed turbulence and complex topography in our analysis. Three idealized cases were studied, and two full scale simulations using the complex topography of Chuquicamata were performed.
The simplified conceptual model of the air circulation seen in each
Acknowledgements
This work was carried out during the doctoral research of the first author, with the support of Conicyt under Grant “Beca para Estudios de Doctorado Nacional”. “René Garreaud was partially supported by FONDAP-CONICYT 15110009.” Powered @ NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02), Center for Mathematical Modeling CMM, Universidad de Chile.
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