Análise experimental do escoamento no interior de um túnel de vento portátil utilizado para estimativa de emissões de gases odorantes a partir de superfícies líquidas passivas
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Data
2025-10-31
Autores
Lima, Pâmela Rossoni
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Universidade Federal do Espírito Santo
Resumo
Wastewater treatment plants (WWTPs) are significant sources of odorous gases in urban areas, potentially affecting the well-being and quality of life of nearby populations. Estimating emissions of these gases is a key step in developing control strategies and assessing their environmental impact. Stabilization ponds and primary and secondary settling tanks, characterized by passive liquid surfaces, are among the main emission sources in WWTPs. The Portable Wind Tunnel (PWT) is a device designed to enclose the liquid surface and directly measure emissions from passive liquid surfaces exposed to the atmosphere, as it has the potential to reproduce atmospheric flow conditions over the surface. However, the literature on the physical understanding of the flow within the PWT and the mass transfer phenomena occurring inside the apparatus remains limited. The friction velocity (𝑢∗) at the surface inside the device is a key parameter to ensure that the emission rate measured by the PWT is comparable to that from an open passive liquid surface under atmospheric flow conditions. Therefore, the aim of this study was to investigate the flow characteristics within a portable wind tunnel, evaluating its ability to reproduce representative aerodynamic conditions and its influence on mass transfer over liquid surfaces under turbulent flow. The specific objectives included analyzing the velocity fields inside the device, identifying the flow regime and the need for flow conditioners, and evaluating the influence of operating flow rate on the surface friction velocity through boundary layer analysis within the PWT. To achieve these goals, laboratory experiments were conducted to measure the velocity field inside the PWT using Particle Image Velocimetry (PIV). The results revealed an initially non-uniform flow pattern with extensive recirculation, which was mitigated by installing flow conditioners. At low velocities, the flow was laminar. Increasing the flow rate in the test section resulted in a fully developed turbulent flow analogous to that over a flat plate, allowing the development of a mathematical model to estimate friction velocity under the tested operating conditions. The 𝑢∗ values obtained in the PWT were compared with estimates from an empirical model for passive liquid surfaces in WWTPs. It was observed that, within specific ranges, the PWT is capable of reproducing 𝑢∗ values similar to those found in real wastewater treatment plant scenarios, reaching a maximum compatibility of 23%. Since most values estimated by the empirical model were below the experimental range achieved in the PWT, adjustments allowing operation at lower friction velocities under turbulent flow could significantly improve representativeness, reaching up to 90% compatibility in certain cases. Although the PWT showed partial compatibility with the typical 𝑢∗ values observed in WWTPs, some experimental limitations must be considered. The friction velocities observed in the device were associated with a rigid, smooth surface, unlike the deformable liquid surfaces typical of real WWTP conditions. Moreover, although wave formation was not observed in the wave verification test conducted in a water tank, literature suggests that wave development would be expected for the tested 𝑢∗ range. This discrepancy may be attributed to the short length of the PWT and the influence of sidewalls. Additionally, based on the 𝑢∗ values estimated by the empirical model, a sensitivity analysis was performed using the main models commonly applied to estimate emissions in WWTPs, considering odorous compounds with distinct volatilization patterns. It was found that neglecting the surrounding roughness can lead to significant overestimations of the global mass transfer coefficient (𝐾𝐿), reaching up to 40% for compounds dominated by the liquid phase and up to 33% for those dominated by the gas phase or both. These findings underscore the importance of adequately accounting for aerodynamic surface roughness in emission assessments of WWTPs.
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Túnel de vento portátil , Velocimetria por imagens de partículas , Superfície líquida passiva , Velocidade de fricção , Coeficiente de transferência de massa