CFD multiphase modelling for evaluation of gas mixing in an anaerobic digester

Biogas production from municipal and industrial solid and liquid waste has captured the attention of engineers and managers both in the UK and globally due the substantial benefits for achieving environmental protection, energy generation and Green House Gas emission reductions. However, there are number of problems involved in scaling up experimental anaerobic digestion (AD) plants to field level plants. One such problem associated with AD is mixing, which is a vital component to segregate synthesized gas and biomass from digester liquid, to enhance homogeneity and to ensure adequate contact between bacteria and substrate in the AD. Such situations are well suited to Computational Fluid Dynamic (CFD) analysis, where models can be calibrated and validated using the pilot plant and can then be used to accurately simulate the performance of the large-scale reactors. The aim in this work has been to further understand and enhance the use of bubble mixing approaches to improve the performance of future bioreactors. A computational model has been developed to simulate the complex flows occurring in a digester. The paper discusses CFD simulations of a lab scale AD for evaluating mixing characteristics that provides understanding required for developing accurate simulations of mixing conditions in the large-scale systems with the reactor contents simulated for both Newtonian and non-Newtonian case

Tools for Risk Analysis: Updating the 2006 WHO guidelines

This chapter reviews developments since the WHO Guidelines for the safe use of wastewater in agriculture were published in 2006. The six main developments are: the recognition that the tolerable additional disease burden may be too stringent for many developing countries; the benefits of focusing on single-event infection risks as a measure of outbreak potential when evaluating risk acceptability; a more rigorous method for estimating annual risks; the availability of dose-response data for norovirus; the use of QMRA to estimate Ascaris infection risks; and a detailed evaluation of pathogen reductions achieved by produce-washing and disinfection. Application of the developments results in more realistic estimates of the pathogen reductions required for the safe use of wastewater in agriculture and consequently permits the use of simpler wastewater treatment processes

Hydraulic free-surface modelling with a novel validation approach

This work shows that a three-dimensional transient two-phase RANS CFD-VOF model can be used to predict the position of waves and hydraulic jumps within a complex hydraulic flow environment as measured during a series of full-scale experiments. A novel application of LIDAR is used to provide detailed measurements of the position of the water free-surface location during the physical experiments. The test environment is a recreational white-water course that provides a means to vary the flow rates of water and restrict the flow easily as required. Obstructions are added to the channel to create hydraulic jumps and other specific flow features. The influence of these obstructions on the flow has been analysed for size, velocity and position. The results of the study demonstrate that, although computationally intensive, the free-surface CFD approach can reliably predict a range of complex hydraulic flow features in medium/largescale open channel flow conditions. In order to reliably capture the full three-dimensional characteristics of the water free-surface a high resolution mesh (greater than 2.5 million cells) with time-steps in the order of milliseconds is necessary (Simulations presented here represent between 30 and 60 seconds of real-time)

Relationship between healthcare worker surface contacts, care type and hand hygiene: an observational study in a single-bed hospital ward

This study quantifies the relationship between hand hygiene and the frequency with which healthcare workers (HCWs) touch surfaces in patient rooms. Surface contacts and hand hygiene were recorded in a single-bed UK hospital ward for six care types. Surface contacts often formed non-random patterns, but hygiene before or after patient contact depends significantly on care type (P=0.001). The likelihood of hygiene correlated with the number of surface contacts (95% confidence interval 1.1–5.8, P=0.002), but not with time spent in the room. This highlights that a potential subconscious need for hand hygiene may have developed in HCWs, which may support and help focus future hygiene education programmes

Simulating Pathogen Transport within a Naturally Ventilated Hospital Ward

Understanding how airborne pathogens are transported through hospital wards is essential for determining the infection risk to patients and healthcare workers. This study utilizes Computational Fluid Dynamics (CFD) simulations to explore pathogen transport within a six-bed Nightingale hospital ward. Grid independence of a ward model was addressed using the Grid Convergence Index (GCI) from solutions obtained using three fully-structured grids. Pathogens were simulated using source terms in conjunction with a scalar transport equation and a RANS turbulence model. Errors were found to be less than 4% in the prediction of air velocities but an average of 13% was seen in the scalar field. A parametric study into the pathogen release point illustrated that its distribution is strongly influenced by the local velocity field and the degree of mixing present

Multimode morphodynamic model for sediment-laden flows and geomorphic impacts

Sediment-laden flows are a complex solid-fluid interaction process. This study presents a multimode morphodynamic model system combined with shallow water theory and a nonequilibrium assumption for sediment transport. The model system aims to simulate the morphological change caused by sediment-laden flows with various sediment transport modes. It involves three modules: a hydrodynamic module, a sediment transport module, and a morphological evolution module. The hydrodynamic model is governed by modified shallow water equations considering the interaction effects of flow and sediment. A flexible sediment transport model is presented that incorporates a weight coefficient. The model can adaptively choose an appropriate transport mode according to local, real-time flow conditions. Bedload, suspended load, and total mixed sediment load are all involved. The model is solved by a second-order Godunov-type finite-volume method that is robust and accurate. Validation is demonstrated through a series of test cases. The results indicate that the model can attain good agreement with measured data, thereby demonstrating the capabilities of the multimode morphodynamic model system in predicting sediment-laden flows and resulting morphological change

THREE DIMENSIONAL FREE-SURFACE MODELLING WITH A NOVEL VALIDATION APPROACH

We show that that a three-dimensional two-phase CFD-VOF model can reliably predict the position of waves and hydraulic jumps within the complex hydraulic flow environments of a recreational white-water course. The model is validated to predict the full transient behaviour of white-water ‘features’ that form such courses. A novel application of LIDAR provides high quality data for validation of the free-surface location. A RANS CFD model is implemented in this work which incorporates an explicit VOF model to predict the transient free surface behaviour of white-water phenomena. Experimental studies were undertaken in a recreational white-water course channel that provided a means to vary the flow rates of water and restrict the flow easily as required as well as in river with a controlled flow from a dam release. The addition of obstructions within the channels allowed analysis of the impact on the size, position, velocities associated with hydraulic jumps, waves and other key features necessary for white-water kayaking. The study includes an original application LIDAR which has successfully been used as a measurement tool allowing high quality free-surface data in a situation where other available options would be very difficult - either prohibitively expensive or would have had an impact on the flow being measured. LIDAR is not traditionally used to measure the surface of water as typically there is no return (reflection) from the water; however the broken water surface of white-water provides enough signal response to capture the detail of a stationary wave and other features. The results of the study establish that the free-surface CFD approaches can accurately predict the complex hydraulic behaviour in large-scale open channel flows. In order to reliably capture the full three- dimensional characteristics of the water free-surface using the VOF approach a structured, high resolution mesh with time-steps in order of milliseconds is necessary

An Effective Surrogate Tracer Technique for S. aureus Bioaerosols in a Mechanically Ventilated Hospital Room Replica Using Dilute Aqueous Lithium Chloride

Finding a non-pathogenic surrogate aerosol that represents the deposition of typical bioaerosols in healthcare settings is beneficial from the perspective of hospital facility testing, general infection control and outbreak analysis. This study considers aerosolization of dilute aqueous lithium chloride (LiCl) and sodium chloride (NaCl) solutions as surrogate tracers capable of representing Staphylococcus aureus bioaerosol deposition on surfaces in mechanically ventilated rooms. Tests were conducted in a biological test chamber set up as a replica hospital single patient room. Petri dishes on surfaces were used to collect the Li, Na and S. aureus aerosols separately after release. Biological samples were analyzed using cultivation techniques on solid media, and flame atomic absorption spectroscopy was used to measure Li and Na atom concentrations. Spatial deposition distribution of Li tracer correlated well with S. aureus aerosols (96% of pairs within a 95% confidence interval). In the patient hospital room replica, results show that the most contaminated areas were on surfaces 2 m away from the source. This indicates that the room’s airflow patterns play a significant role in bioaerosol transport. NaCl proved not to be sensitive to spatial deposition patterns. LiCl as a surrogate tracer for bioaerosol deposition was most reliable as it was robust to outliers, sensitive to spatial heterogeneity and found to require less replicates than the S. aureus counterpart to be in good spatial agreement with biological results

Testing of a downflow system for high risk infectious disease isolation rooms

Isolation room airflows for infectious diseases are designed to minimise the risk of transmission of airborne pathogens to those outside the room and to protect healthcare workers who tend to the patient. This study considers the risk in the vicinity of the patient and conducts an experimental investigation into a downflow ventilation design to evaluate whether it is capable of providing protection to a healthcare worker. Anemometry and smoke tests are conducted in a mock up room to assess influence of ventilation rate, extract design, heat loads and flow local to a healthcare worker. Results show a good downward flow can be established, but a fan speed capable of delivering 0.35m/s and central extract are required to create a uniform flow. Heat loads and a healthcare worker leaning over the bed both compromise downflow effectiveness; local flow acceleration and exhaust can mitigate to some extent