Optimization of Mixing and Mass Transfer in In-line Multi-Jet Ozone Contactors Using Computational Fluid Dynamics

Typical ozone mixing and mass transfer calculations are lumped approaches based on ideal operating conditions and can misrepresent behavior in real-life installations. This article models the effect of local hydrodynamics and mixing on the overall mass transfer of ozone into water with the aid of multiphase computational fluid dynamics (CFD). CFD models were validated with measured data from a pipeline ozone contactor installation which was optimized for more rapid, uniform mixing and mass transfer. Results emphasize the sensitivity of mixing quality to nozzle placement, size, orientation and spacing relative to main pipeline diameter and flows.     

Modeling Volumetric Clarifying Filtration of Particulate Matter for Transient Rainfall-Runoff Loadings

Volumetric clarification incorporating filtration with engineered granular media is increasingly used as a viable combined unit operation for separation of rainfall-runoff particulate matter (PM). Such combined unit operations are typically operated at the catchment-level for rainfall-runoff clarification of transient loadings, in contrast to centralized watershed or sewershed regional treatment. Using computational fluid dynamics (CFD), this study models the PM separation by a volumetric clarifying filter (VCF) subject to unsteady event-based hydrologic, hydraulic, mass, and particle size distribution (PSD) loadings. Modeled and measured physical model results indicate that the VCF is capable of PM load reductions and effluent concentrations at or below 30 mg/L. These results, with PM measured as suspended sediment concentration (SSC) represent reductions ranging from 83 to 97% on an event basis. CFD model results predict effluent PM mass loads. Modeled effluent median particle diameter (d50m), as an index for the filtered effluent PSD, reproduces the d50?m from the VCF physical model on an event basis. Filter head loss is examined as a function of flow rate. Despite geometric asymmetry of the multiple radial cartridge configuration tested, hydraulic loading for each individual cartridge is relatively uniform.     

A new dynamic model for predicting transient phenomena in a PEM fuel cell system

In this paper, a mathematical model is developed to simulate the transient phenomena in a polymer electrolyte membrane fuel cell (PEMFC) system. At present many electrochemical models are available to the fuel cell designers to capture steady state behavior by estimating the equilibrium voltage for a particular set of operating conditions, but models capable of describing transient phenomena are scanty. In practical applications such as powertrains of land-based vehicles or submarines, the output power from the fuel cell system undergoes large variations especially during acceleration and deceleration. During such processes, many transient dynamic mechanisms become significant, while simple empirical models are unable to represent the transient dynamics caused by such as diffusion effect and double layer capacitance at the interface between the electrodes and the electrolyte. Hence, a novel dynamic fuel cell model is developed in this paper which incorporates the effects of charge double layer capacitance, the dynamics of flow and pressure in the anode and cathode channels and mass/heat transfer transient features in the fuel cell body. This dynamic model can predict the transient response of cell voltage, temperature of the cell, hydrogen/oxygen out flow rates and cathode and anode channel temperatures/pressures under sudden change in load current. The proposed model is implemented in SIMULINK environment. The simulation results are analyzed and compared to benchmark results. Lab tests are carried out at Connecticut Global Fuel Cell Center and a good agreement is found between tests and simulations. This model will be very useful for the optimal design and real-time control of PEM fuel cell systems.     

Is Coefficient of Variation a Realistic Index for Characterizing Mixing Efficiency in Ozone Applications?

ABSTRACT

The coefficient of variation (COV) of ozone residual is often used to compare the mixing performance of different ozone contacting systems. Multiphase mass transfer CFD modeling is performed and compared with experimental data to investigate the correlation between mass transfer efficiency, a corresponding full cross-sectional spatial COV, a corresponding “grab sample” temporal COV, and a comprehensive uniformity index for mixing for varying sidestream ozone doses. Typical sampling methodology for ozone residual is reviewed and general guidelines for better representative sampling are suggested.     

Socio-technical aspects of remote media control for a NG9-1-1 system

9-1-1 emergency calls mostly involve distress situations that cause people to panic while trying to answer questions or follow instructions given by a dispatcher. To obtain precious information with the least user intervention and reduced stress on the user, there is a need for the dispatcher to have a better control and understanding of the condition or situation at the other end. The increasing growth of smartphones embedded with camera, speaker phone, GPS, microphone and various other sensors, extends their usage from merely making calls to life saving gadgets during critical situations. By integrating these sensor rich smartphones and the rapidly growing VoIP technology, we propose a VoIP based Next Generation 9-1-1 (NG9-1-1) system for remote media control. Specifically, we use Session Initiation Protocol (SIP) in the implementation of the system using a mobile and a PC client. The proposed system on the mobile client accounted for less than 25% of CPU utilization even with video transmission. The average network utilization was about 10 and 72 kbps for audio and video, respectively. With these encouraging results, we believe the proposed remote media control system will facilitate information acquisition and decision making in emergency situations.     

CFD Modeling of Particulate Matter Fate and Pressure Drop in a Storm-Water Radial Filter

This study examined a common form of filtration, a passive radial cartridge filter (RCF) system, to physically separate hetero-disperse particulate matter in rainfall-runoff. The RCF tested utilizes aluminum-oxide coated media with a uniform pumice substrate (d50m = 3.56?mm) gradation. To examine the RCF, this study applied laser diffraction and real-time pressure sensor measurements to validate a computational fluid dynamics (CFD) model to predict particle separation and filter head loss for hetero-disperse particulate matter (PM). Filter hydrodynamics are resolved using a macroscopic approach for the porous media with a k-ε turbulence model coupled with the Ergun equation. PM fate was resolved using a discrete phase model. CFD results closely followed measured data for filtration and head-loss response for all flow rates. With influent PM at 200?mg/L (d50m = 16.3?μm), effluent PM ranged from 32?to?57?mg/L for surface loading rates of 24?to?189?L/m2?min, respectively. There was agreement between measured and modeled data for effluent PM and head loss. CFD postprocessing provided added insight into the mechanistic behavior of the RCF by means of three-dimensional hydraulic profiles, particle trajectories, and pressure distributions, illustrating that a RCF is nonuniformly loaded. As part of design and regulation, such physical testing coupled with modeling is a required precursor to uncontrolled field testing, regular maintenance and certification of a BMP.     

CFD Modeling of a Storm-Water Hydrodynamic Separator

Proliferation of manufactured unit operations to separate particulate matter (PM) transported in storm water has generated interest in modeling of such units beyond conventional “% removal” and “black-box” type evaluations. One such unit operation is a screened hydrodynamic separator (HS) combining settling and to a lesser extent inertial separation by means of an annular static screen. However, performance and mechanistic evaluations of HS units are challenged by loadings with wide PM particle size distributions (PSDs) at a range of flow rates. Wastewater-based methods such as total suspended solids (TSS) and auto samplers further challenge such evaluations. This study utilized computational fluid dynamics (CFD) to model PM separation by a HS coupled with measured PSDs, flow rates [1–125% of hydraulic capacity (Qd) = 15.9?L/s], and head loss through pilot-scale testing with representative sampling and material balances. The static screen was modeled as porous cylinder, a k-ε model accounted for flow turbulence, a Lagrangian discrete phase model examined PM fate, and the resulting CFD model was stable and grid independent. CFD results were validated with pilot-scale data across the flow range with a relative percent difference between model and measured data of less than 10%. For analysis or design, CFD facilitates a spatially distributed examination of PM fate as a function of PSD and flow rate. CFD results are contrasted to conventional methods with PM as a lumped gravimetric index (TSS) and HS as a lumped overflow rate type of best management practice.     

Can a stepwise steady flow computational fluid dynamics model reproduce unsteady particulate matter separation for common unit operations?

Computational fluid dynamics (CFD) is emerging as a model for resolving the fate of particulate matter (PM) by unit operations subject to rainfall-runoff loadings. However, compared to steady flow CFD models, there are greater computational requirements for unsteady hydrodynamics and PM loading models. Therefore this study examines if integrating a stepwise steady flow CFD model can reproduce PM separation by common unit operations loaded by unsteady flow and PM loadings, thereby reducing computational effort. Utilizing monitored unit operation data from unsteady events as a metric, this study compares the two CFD modeling approaches for a hydrodynamic separator (HS), a primary clarifier (PC) tank, and a volumetric clarifying filtration system (VCF). Results indicate that while unsteady CFD models reproduce PM separation of each unit operation, stepwise steady CFD models result in significant deviation for HS and PC models as compared to monitored data; overestimating the physical size requirements of each unit required to reproduce monitored PM separation results. In contrast, the stepwise steady flow approach reproduces PM separation by the VCF, a combined gravitational sedimentation and media filtration unit operation that provides attenuation of turbulent energy and flow velocity.