Recent progress towards hydrodynamic modelling of dense gas-particle flows

In this paper a state-of-the-art review will be presented on hydrodynamic modeling of dense gas-particle flows as encountered in the fluid\ud bed family of gas-solid contactors. After a brief introduction the different classes of fundamental hydrodynamic models will be discussed together with their physical basis and mutual advantages and disadvantages. Thereafter some typical results will be presented on first principles modeling of dense\ud gas-fluidized beds. Finally the conclusions will be presented and areas which need substantial further attention will be indicated

The influence of particle properties on pressure signals in dense gas-fluidised beds: a computer simulation study

A hard-sphere discrete particle model of a gas-fluidised bed was used in order to study the influence of particle properties on pressure signals in dense gasfluidised beds. In the model the gas-phase hydrodynamics is described by the spatially averaged Navier-Stokes equations for two-phase flow. For each solid particle the Newtonian equations of motion are solved taking into account the inter-particle and particle-wall collisions. Pressure fluctuations inside the bed are strongly affected by the coefficients of restitution and friction: the more energy is dissipated in collisions the stronger the fluctuations are. The root mean square of the pressure fluctuations showed an almost linear dependency on the amount of energy dissipated in collisions during the simulation

Discrete particle simulation of bubble and slug formation in a two-dimensional gas-fluidised bed: a hard-sphere approach.

A discrete particle model of a gas-fluidised bed has been developed and in this the two-dimensional motion of the individual, spherical particles was directly calculated from the forces acting on them, accounting for the interaction between the particles and the interstitial gas phase. Our collision model is based on conservation laws for linear and angular momentum and requires, apart from geometrical factors, two empirical parameters: a restitution coefficient and a friction coefficient. A sequence of collisions is processed using techniques which find their application in hard-sphere simulations which are commonly encountered in the field of molecular dynamics. The hydrodynamic model of the gas phase is based on the volume-averaged Navier-Stokes equations. Simulations of bubble and slug formation in a small two-dimensional bed (height 0.50 m, width 0.15 m) with 2400 particles (dp = 4 mm, material: aluminium, p = 2700 kg m¿3) showed a strong dependency of the flow behaviour with respect to the restitution and friction coefficient. A preliminary experimental validation of our model was performed using a small scale "two-dimensional" gas-fluidised bed (height 0.30 m, width 0.15 m, depth 0.015 m) with 850 ¿m ballotini glass particles (p = 2930 kg m¿3) as the bed material. Results compared fairly well with the results of a simulation which was performed with 40,000 particles using realistic values for the restitution and friction coefficients which were obtained from simple independent experiment

Granular dynamics of gas-solid two-phase flows

The study reported in this thesis is concerned with the granular dynamics of gas-solid two-phase flows. In granular dynamics simulations the Newtonian equations of motion are solved for each individual particle in the system while taking into account the mutual\ud interaction between particles and between particles and walls. The gas-phase hydrodynamics is described by the volume averaged Navier-Stokes equations for twophase flow

Economic evaluation of implementation strategies in health care

Economic evaluations can inform decisions about the efficiency and allocation of resources to implementation strategies? strategies explicitly designed to inform care providers and patients about the best available research evidence and to enhance its use in their practices. These strategies are increasingly popular in health care, especially in light of growing concerns about quality of care and limits on resources. But such concerns have hardly motivated health authorities and other decision-makers to spend on some form of economic evaluation in their assessments of implementation strategies. This editorial addresses the importance of economic evaluation in the context of implementation science? particularly, how these analyses can be most efficiently incorporated into decision-making processes about implementation strategies

Estimating the Cost-Effectiveness of Implementation : Is Sufficient Evidence Available?

BACKGROUND: Timely implementation of recommended interventions can provide health benefits to patients and cost savings to the health service provider. Effective approaches to increase the implementation of guidance are needed. Since investment in activities that improve implementation competes for funding against other health generating interventions, it should be assessed in term of its costs and benefits. OBJECTIVE: In 2010, the National Institute for Health and Care Excellence released a clinical guideline recommending natriuretic peptide (NP) testing in patients with suspected heart failure. However, its implementation in practice was variable across the National Health Service in England. This study demonstrates the use of multi-period analysis together with diffusion curves to estimate the value of investing in implementation activities to increase uptake of NP testing. METHODS: Diffusion curves were estimated based on historic data to produce predictions of future utilization. The value of an implementation activity (given its expected costs and effectiveness) was estimated. Both a static population and a multi-period analysis were undertaken. RESULTS: The value of implementation interventions encouraging the utilization of NP testing is shown to decrease over time as natural diffusion occurs. Sensitivity analyses indicated that the value of the implementation activity depends on its efficacy and on the population size. CONCLUSIONS: Value of implementation can help inform policy decisions of how to invest in implementation activities even in situations in which data are sparse. Multi-period analysis is essential to accurately quantify the time profile of the value of implementation given the natural diffusion of the intervention and the incidence of the disease

Large-eddy simulation of a particle-laden turbulent channel flow

Large-eddy simulations of a vertical turbulent channel flow with 420,000 solid particles are performed in order to get insight into fundamental aspects of a riser flow The question is addressed whether collisions between particles are important for the ow statistics. The turbulent channel ow corresponds to a particle volume fraction of 0.013 and a mass load ratio of 18, values that are relatively high compared to recent literature on large-eddy simulation of two-phase ows. In order to simulate this ow, we present a formulation of the equations for compressible ow in a porous medium including particle forces. These equations are solved with LES using a Taylor approximation of the dynamic subgrid-model. The results show that due to particle-uid interactions the boundary layer becomes thinner, leading to a higher skin-friction coefcient. Important effects of the particle collisions are also observed, on the mean uid prole, but even more o on particle properties. The collisions cause a less uniform particle concentration\ud and considerably atten the mean solids velocity prole

The drag force in two-fluid models of gas-solid flows

Currently, the two most widespread methods for modelling the particulate phase in numerical simulations of gas-solid flows are discrete particle simulation (see, e.g., Mikami, Kamiya, & Horio, 1998), and the two-fluid approach, e.g., kinetic theory models (see, e.g., Louge, Mastorakos, & Jenkins, 1991). In both approaches the gas phase is described by a locally averaged Navier-Stokes equation and the two phases are usually coupled by a drag force. Due to the large density difference between the particles and the gas, inter-phase forces other than the drag force are usually neglected, so it plays a significant role in characterising the gas-solid flow. Yasuna, Moyer, Elliott, and Sinclair (1995) have shown that the solution of their model is sensitive to the drag coefficient. In general, the performance of most current models depends critically on the accuracy of the drag force formulation

Cost data in implementation science: categories and approaches to costing

A lack of cost information has been cited as a barrier to implementation and a limitation of implementation research. This paper explains how implementation researchers might optimize their measurement and inclusion of costs, building on traditional economic evaluations comparing costs and effectiveness of health interventions. The objective of all economic evaluation is to inform decision-making for resource allocation and to measure costs that reflect opportunity costs-the value of resource inputs in their next best alternative use, which generally vary by decision-maker perspective(s) and time horizon(s). Analyses that examine different perspectives or time horizons must consider cost estimation accuracy, because over longer time horizons, all costs are variable; however, with shorter time horizons and narrower perspectives, one must differentiate the fixed and variable costs, with fixed costs generally excluded from the evaluation. This paper defines relevant costs, identifies sources of cost data, and discusses cost relevance to potential decision-makers contemplating or implementing evidence-based interventions. Costs may come from the healthcare sector, informal healthcare sector, patient, participant or caregiver, and other sectors such as housing, criminal justice, social services, and education. Finally, we define and consider the relevance of costs by phase of implementation and time horizon, including pre-implementation and planning, implementation, intervention, downstream, and adaptation, and through replication, sustainment, de-implementation, or spread