Donald Giddings

Three-dimensional cerebrospinal fluid flow within the human ventricular system

Cerebrospinal fluid (CSF) is a Newtonian fluid and can, therefore, be modelled using computational fluid dynamics (CFD). Previous modelling of the CSF has been limited to simplified geometric models. This work describes a geometrically accurate three dimensional (3D) computational model of the human ventricular system (HVS) constructed from magnetic resonance images (MRI) of the human brain. It is an accurate and full representation of the HVS and includes appropriately positioned CSF production and drainage locations. It was used to investigate the pulsatile motion of CSF within the human brain. During this investigation CSF flow rate was set at a constant 500 ml/day, to mimic real life secretion of CSF into the system, and a pulsing velocity profile was added to the inlets to incorporate the effect of cardiac pulsations on the choroid plexus and their subsequent influence on CSF motion in the HVS. Boundary conditions for the CSF exits from the ventricles (foramina of Magendie and Lushka) were found using a “nesting” approach, in which a simplified model of the entire central nervous system (CNS) was used to examine the effects of the CSF surrounding the ventricular system (VS). This model provided time varying pressure data for the exits from the VS nested within it. The fastest flow was found in the cerebral aqueduct, where a maximum velocity of 11.38 mm/s was observed over five cycles. The maximum Reynolds number recorded during the simulation was 15 with an average Reynolds number of the order of 0.39, indicating that CSF motion is creeping flow in most of the computational domain and consequently will follow the geometry of the model. CSF pressure also varies with geometry with a maximum pressure drop of 1.14 Pa occurring through the cerebral aqueduct. CSF flow velocity is substantially slower in the areas that are furthest away from the inlets; in some areas flow is nearly stagnant.

Prediction of air/oil exit flows in a commercial aero-engine bearing chamber

Abstract An understanding and optimization of three-dimensional air/oil flows in aero-engine transmission systems forms an integral part of future designs. This especially applies to bearing chambers, which contain a complex two-phase flow formed by the interaction of sealing airflows and lubrication oil. A critical design quantity is the composition of the liquid (oil) and gas (air) phases in the exit flows. Using a previously validated numerical model, the air/oil flow in a commercial bearing chamber is computed with particular focus on the flow exiting the chamber. The division of oil exiting the chamber through the vent and scavenge ports is determined for three shaft speeds and two configurations of the vent port. Comparison with available experimental data shows that consistent trends are predicted, but further model development is necessary in the vicinity of the scavenge port.

Computational fluid dynamics applied to a cement precalciner

Abstract This paper describes a study of the use of computational fluid dynamics (CFD) to investigate the performance of a precalciner vessel at a cement works, In this vessel, limestone, held in suspension, is calcined to calcium oxide and the endothermic reaction is supported by the combustion of coal. Results are presented from a CFD model that contains all the essential features of the precalciner as operated when burning coal. The model fully represents the reactions and fluid dynamics of the precalciner. Previously unidentified features are illustrated. Certain key features at points in the precalciner, where some limited measurements can be made, are compared with the parameters indicated by the computational model. The measurements are consistent with the results calculated by the model indicating fair validation. The CFD data show the following 1 The gases undergo distinct recirculation. 2 The coal particles entering at one inlet have significantly different trajectories and temperature histories from those entering at the second diametrically opposed inlet. 3 There is 90 per cent completion of coal combustion at the exit. 4 73 per cent limestone in the raw meal is calcined to calcined to calcium oxide at the exit from the precalciner. 5 The highest reaction rate of the raw meal is closer to one side of the vessel due to interaction with the gas flows. Future work is proposed which, firstly, will provide further validation of the results so far attained by selective measurements on the precalciner and, secondly, will model the combustion and aerodynamic behaviour of waste-derived fuels in the precalciner vessel, commencing with shredded car tyre chips.

An adaptive RBF finite collocation approach to track transport processes across moving fronts

We develop a radial basis function finite collocation method (RBF-FC) that utilises an adaptive quadtree dataset to cluster nodes around critical features in the domain, for the solution of transport processes occurring in moving front problems.An adaptive quadtree dataset is used to generate an improved distribution of solution centres in the domain, around which local Hermitian collocation systems are formed. In these local systems, the governing PDE and boundary operators of the problem are enforced by collocation. Globally, the systems are linked via reconstruction of the solution variable in terms of the solution value at neighbouring nodes producing a sparse global matrix with a solution cost that scales linearly with the number of nodes in the domain. By generating an adaptive set of nodal points with the quadtree dataset, we can reduce the global solution error in the RBF-FC method whilst maintaining low solution times.The proposed method is validated on a steady-state boundary layer capture problem and a transient advection problem (infinite Peclet number) in a 2D domain. We then couple the method with an interface tracking technique to solve the transient heat transfer in a Hele-Shaw cell viscous fingering problem. We demonstrate the effectiveness of the proposed method for the solution of convection dominated transport in problems across moving fronts.

Numerical Study of Wetting Transitions on Biomimetic Surfaces Using a Lattice Boltzmann Approach with Large Density Ratio

The hydrophobicity of natural surfaces has drawn much attention of scientific communities in recent years. By mimicking natural surfaces, the manufactured biomimetic hydrophobic surfaces have been widely applied to green technologies such as self-cleaning surfaces. Although the theories for wetting and hydrophobicity have been developed, the mechanism of wetting transitions between heterogeneous wetting state and homogeneous wetting state is still not fully clarified. As understanding of wetting transitions is crucial for manufacturing a biomimetic superhydrophobic surface, more fundamental discussions in this area should be carried out. In the present work, the wetting transitions are numerically studied using a phase field lattice Boltzmann approach with large density ratio, which should be helpful in understanding the mechanism of wetting transitions. The dynamic wetting transition processes between Cassie-Baxter state and Wenzel state are presented, and the energy barrier and the gravity effect on transition are discussed. It is found that the two wetting transition processes are irreversible for specific inherent contact angles and have different transition routes, the energy barrier exists on an ideally patterned surface and the gravity can be crucial to overcome the energy barrier and trigger the transition.

Applicability of Mechanical Tests for Biomass Pellet Characterisation for Bioenergy Applications

In this paper, the applicability of mechanical tests for biomass pellet characterisation was investigated. Pellet durability, quasi-static (low strain rate), and dynamic (high strain rate) mechanical tests were applied to mixed wood, eucalyptus, sunflower, miscanthus, and steam exploded and microwaved pellets, and compared to their Hardgrove Grindability Index (HGI), and milling energies for knife and ring-roller mills. The dynamic mechanical response of biomass pellets was obtained using a novel application of the Split Hopkinson pressure bar. Similar mechanical properties were obtained for all pellets, apart from steam-exploded pellets, which were significantly higher. The quasi-static rigidity (Young’s modulus) was highest in the axial orientation and lowest in flexure. The dynamic mechanical strength and rigidity were highest in the diametral orientation. Pellet strength was found to be greater at high strain rates. The diametral Young’s Modulus was virtually identical at low and high strain rates for eucalyptus, mixed wood, sunflower, and microwave pellets, while the axial Young’s Modulus was lower at high strain rates. Correlations were derived between the milling energy in knife and ring roller mills for pellet durability, and quasi-static and dynamic pellet strength. Pellet durability and diametral quasi-static strain was correlated with HGI. In summary, pellet durability and mechanical tests at low and high strain rates can provide an indication of how a pellet will break down in a mill.

Numerical Analysis of Vortex Dynamics in a Double Expansion

The turbulent flow through a 3D diffuser featuring a double expansion is investigated using computational fluid dynamics. Time dependent simulations are reported using the stress omega Reynolds stress model available in ANSYS FLUENT 13.0. The flow topography and characteristics over a range of Reynolds numbers from 42,000 to 170,000 is reported, and its features are consistent with those investigated for other similar geometries. A transition from a chaotic separated flow to one featuring one large recirculation in one corner of the diffuser is predicted at a Reynolds number of 80,000. For a Reynolds number of 170,000 a precessing/flapping motion of the main flow field was identified, the frequency of which is consistent with other numerical and experimental studies.

Wet steam measurement techniques

Abstract In recent years a greater need for power station efficiency has become evident; improving turbine blade efficiency is one of the methods proposed. This efficiency depends upon the wetness of the steam that comes into contact with the blades of the low-pressure turbine stage in general and all turbines in nuclear power generation. Therefore, measurement of the moisture content of the steam in real time in conjunction with an accurate measure of steam velocity can give an overall mass flow re-entering the turbine, allowing for a feedback control. The system can rely on one technique that can measure suspended droplets and wall-bound liquid film, or a combination of techniques can operate together. This work gives a comprehensive review of the different techniques used to measure the moisture content including the liquid film and moisture content and techniques that can give measurements on both simultaneously. Each technique has its strengths and weaknesses, and they were analysed to see which technique works best overall and which techniques can be used together.

CFD multiphase modelling for the nanofluid boiling of the salt solution in a symmetric rectangular boiler

Nano-fluids are found in the literature to enhance the boiling heat transfer and there are modelling case studies demonstrate how these effects occur. In this work a mathematical investigation of nanofluid boiling in a horizontal rectangular symmetric tube is presented, as a component of an exploit to improve heat transfer in vapour absorption refrigeration systems (VARS) via nanofluid. Previous research concentrates on water based fluids or commonly found refrigerants and heat transfer fluids. The nanofluid used in this study is acetone/zinc bromide (ZnBr2) based on the zinc oxide (ZnO) nanoparticles. The process was modelled using ANSYS® Fluent V.15 computational fluid dynamic (CFD) commercially available software using the volume of fluid (VOF) multiphase flow model. An extra user defined code was applied from the literature for boiling of nano-fluids to model the mass transfer on boiling. Different concentration of the nanoparticles (0, 0.1, 0.3, 0.5 & 1 vol. %), the velocities of the fluid (0.005, 0.01 & 0.02 m/s) and temperature of the boiler (330, 333 & 335 K) were simulated to observe the effects of these parameters on the boiling and the phase change of the solution. It was found that the increase in nanoparticles loading leads to an increment in the vapour volume fraction and the heat transfer coefficient because the nanoparticles enhance the heat transfer as a result of increasing the thermal conductivity of the nanofluid. Increasing the boiler temperature increases the vapour volume fraction and decreases the heat transfer coefficient because increasing the vapour with low heat transfer coefficient leads to strike down the heat transfer coefficient of the result. This study is clarifying how the solution with different components and phases behave when its boil and just one of these components evaporate (produce a new phase). The aim of this study is to provide a better mechanism for understanding the heat and mass transfer behaviour of the acetone boiling under the effect of different variables in the boiler.

Impact of Mill Type on Biomass Mill Behavior

With increasing use of biomass in pulverized fuel coal fired power stations, the impact of mill type on biomass size and shape is fundamental in optimizing mill and burner performance. The impact of mill type on the energy consumption, particle size and particle shape of four different biomasses commonly combusted in pulverized fuel boilers was investigated in this paper. Miscanthus, mixed wood, and steam exploded pellets, along with powdered olive cake, were comminuted in a planetary ball mill, Bond ball mill and cutting mill. For pelletized miscanthus, milling showed little impact on the particle size and shape of the pellets, with the milling action only reducing the pellets back to their original particle size distribution. This was also observed for the steam exploded pellets and mixed wood pellets in the cutting mill. For non-densified biomasses, such as olive cake, fines below the screen size should be removed before milling in a hammer mill as they pass straight through the mill, resulting in wasted mill capacity and energy consumption. Pellets should be composed of particles close to the required size for conveyance and combustions, and sphericity and roundness are crucial in determining this size. Olive cake showed the most spherical and round particles, but a coarser milled product size than the steam exploded pellets. Miscanthus and mixed wood pellets showed needle like shape profiles, as well as similar particle size distributions. Optimization of the particle size based on the Stokes shape factor is key to optimizing mill, conveyance and burner performance.