Hydraulic assessment of waste stabilization ponds : Comparison of computational fluid dynamics simulations against tracer data
The overall performance of a wastewater treatment plant is in addition to the effectiveness of the biokinetic processes controlled by the flow dynamics. A wide used technique to characterize the water flow dynamics is the use of tracers. At full-scale tracer experiments in wastewater treatment plants provide insufficient information to completely grasp the flow dynamics. Additionally results are often biased by uncontrolled external factors. As alternative, computational fluid dynamic (CFD) models for performance analysis of existing and the design of new wastewater treatment plants have been increasingly used over the last decade. The paper presents the results of a tracer experiment against the output of a CFD model with application to a 7 ha large maturation pond. The tracer study was conducted with the fluorescence dye Rhodamine WT using the stimulus response technique. The three dimensional CFD model was built considering a variable density distribution of spatial grids based on the specific characteristics of the system. For the unsteady flow simulations the turbulence model k- was adopted. Residence time distribution (RTD) generated by both approaches were compared and showed fairly good agreement. The velocity and turbulence profiles of the CFD model were compared with experimental data confirming, although the high computational demand, the robustness of the used CFD model.
Review of practical aspects for modelling of stabilization ponds using Computational Fluid Dynamics
CFD (Computational Fluid Dynamics) is a numerical modelling tool for flow analysis, whose use in research and applications in stabilization ponds has been consolidated over the past few years. However, current literature presents a lack of review articles focusing on this particular application. Based on previous works in the literature, after adaptations, additions and updates, this review presents a new compilation of 28 publications focusing on the use of CFD in non-mechanized stabilization ponds (anaerobic, facultative, maturation and polishing). Emphasis is given to the main features of the models and methods used, as well as the main results and conclusions of these studies. After an introduction to the tool and a summary of the publications, a brief discussion is held on the key points of the modelling practices adopted, with the intention of identifying the main methods used, gaps, bottlenecks and trends, as well as to suggest good modelling practices when possible. It is therefore expected to contribute to the development and consolidation of the CFD technique in modelling of stabilization ponds and assist new researchers to identify possible starting points for future works.
HYPERGOLIC REACTION MECHANISMS OF CATALYTICALLY PROMOTED FUELS WITH ROCKET GRADE HYDROGEN PEROXIDE
Abstract The coupling of the physical and chemical processes controlling the ignition delays (τ ign ) of hypergolic propellants complicates the direct analysis of the transient ignition processes. The transport properties of both the propellants and the ambient gas, and the heat of vaporization and reaction affect τ ign and must be considered experimentally. Furthermore, since hypergolic ignition occurs, by definition, on contact, the mechanics of initial contact and subsequent mixing can affect τ ign and must also be considered. Experimental results from a study of the hypergolic ignition of a catalytically promoted fuel with rocket grade hydrogen peroxide (RGHP) are presented. Statistical analysis of the experimental data was conducted to evaluate the relative effects of RGHP concentration, initial ambient pressure, and the ambient gas on τ ign . At RGHP concentrations of less than 92% in water, stable ignition was difficult to achieve. At sufficiently high Lewis number, an oscillatory ignition process is observed. A phenomenological model comprising mixing, RGHP decomposition, fuel heating and vaporization, and auto-ignition is presented and compared to the experimental results.