Displacement and mixing ventilation driven by opposing wind and buoyancy

The effect of an opposing wind on the stratification and flow produced by a buoyant plume rising from a heat source on the floor of a ventilated enclosure is investigated. Ventilation openings located at high level on the windward side of the enclosure and at low level on the leeward side allow a wind-driven flow from high to low level, opposite to the buoyancy-driven flow. One of two stable steady flow regimes is established depending on a dimensionless parameter F that characterizes the relative magnitudes of the wind-driven and buoyancy-driven velocities within the enclosure, and on the time history of the flow. A third, unstable steady flow solution is identified. For small opposing winds (small F) a steady, two-layer stratification and displacement ventilation is established. Exterior fluid enters through the lower leeward openings and buoyant interior fluid leaves through the upper windward openings. As the wind speed increases, the opposing wind may cause a reversal in the flow direction. In this case, cool exterior fluid enters through the high windward openings and mixes the interior fluid, which exits through the leeward openings. There are now two possibilities. If the rate of heat input by the source exceeds the rate of heat loss through the leeward openings, the temperature of the interior increases and this flow reversal is only maintained temporarily. The buoyancy force increases with time, the flow reverts to its original direction, and steady two-layer displacement ventilation is re-established and maintained. In this regime, the increase in wind speed increases the depth and temperature of the warm upper layer, and reduces the ventilation flow rate. If, on the other hand, the heat loss exceeds the heat input, the interior cools and the buoyancy-driven flow decreases. The reversed flow is maintained, the stratification is destroyed and mixing ventilation occurs. Further increases in wind speed increase the ventilation rate and decrease the interior temperature. The transitions between the two ventilation flow patterns exhibit hysteresis. The change from displacement ventilation to mixing ventilation occurs at a higher F than the transition from mixing to displacement. Further, we find that the transition from mixing to displacement ventilation occurs at a fixed value of F, whereas the transition from displacement to mixing flow is dependent on the details of the time history of the flow and the geometry of the openings, and is not determined solely by the value of F. Theoretical models that predic t the steady stratification profiles and flow rates for the displacement and mixing ventilation, and the transitions between them, are presented and compared with measurements from laboratory experiments. The transition between these ventilation patterns completely changes the internal environment, and we discuss some of the implications for the natural ventilation of buildings. © 2004 Cambridge University Press.

Dispersion of carbon nanotubes: Mixing, sonication, stabilization, and composite properties

Advances in functionality and reliability of carbon nanotube (CNT) composite materials require careful formulation of processing methods to ultimately realize the desired properties. To date, controlled dispersion of CNTs in a solution or a composite matrix remains a challenge, due to the strong van der Waals binding energies associated with the CNT aggregates. There is also insufficiently defined correlation between the microstructure and the physical properties of the composite. Here, we offer a review of the dispersion processes of pristine (non-covalently functionalized) CNTs in a solvent or a polymer solution. We summarize and adapt relevant theoretical analysis to guide the dispersion design and selection, from the processes of mixing/sonication, to the application of surfactants for stabilization, to the final testing of composite properties. The same approaches are expected to be also applicable to the fabrication of other composite materials involving homogeneously dispersed nanoparticles. © 2012 by the authors; licensee MDPI, Basel, Switzerland.

Influence of film cooling hole angles and geometries on aerodynamic loss and net heat flux reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (α), compound angle (β ), hole inlet geometry and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR = 0.16, 0.64 and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different α and β but with the same angle between the mainstream and coolant flow directions (angle κ). This relationship is assessed through experiments by testing two sets of cylindrical holes with different α and β : one set with κ = 35°, another set with κ = 60°. The data confirm the stated relationship between α, β, κ and the aerodynamic mixing loss. The results show that the designer should minimise κ to obtain the lowest loss, but maximise β to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α =35.0°, β =0°) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is ≈ 50 % of the loss of the fan-shaped hole at IR = 0.64 and 1.44. Copyright © 2011 by ASME.

Physicians in leadership: the association between medical director involvement and staff-to-patient ratios

In a hospital environment that demands a careful balance between commercial and clinical interests, the extent to which physicians are involved in hospital leadership varies greatly. This paper assesses the influence of the extent of this involvement on staff-to-patient ratios. Using data gathered from 604 hospitals across Germany, this study evidences the positive relationship between a full-time medical director (MD) or heavily involved part-time MD and a higher staff-to-patient ratio. The data allows us to control for a range of confounding variables, such as size, rural/urban location, ownership structure, and case-mix. The results contribute to the sparse body of empirical research on the effect of clinical leadership on organizational outcomes.

Thermal stratification produced by plumes and jets in enclosed spaces

The airflow and thermal stratification produced by a localised heat source located at floor level in a closed room is of considerable practical interest and is commonly referred to as a 'filling box'. In rooms with low aspect ratios H/R ≲ 1 (room height H to characteristic horizontal dimension R) the thermal plume spreads laterally on reaching the ceiling and a descending horizontal 'front' forms separating a stably stratified, warm upper region from cooler air below. The stratification is well predicted for H/R ≲ 1 by the original filling box model of Baines and Turner (J. Fluid. Mech. 37 (1968) 51). This model represents a somewhat idealised situation of a plume rising from a point source of buoyancy alone-in particular the momentum flux at the source is zero. In practical situations, real sources of heating and cooling in a ventilation system often include initial fluxes of both buoyancy and momentum, e.g. where a heating system vents warm air into a space. This paper describes laboratory experiments to determine the dependence of the 'front' formation and stratification on the source momentum and buoyancy fluxes of a single source, and on the location and relative strengths of two sources from which momentum and buoyancy fluxes were supplied separately. For a single source with a non-zero input of momentum, the rate of descent of the front is more rapid than for the case of zero source momentum flux and increases with increasing momentum input. Increasing the source momentum flux effectively increases the height of the enclosure, and leads to enhanced overturning motions and finally to complete mixing for highly momentum-driven flows. Stratified flows may be maintained by reducing the aspect ratio of the enclosure. At these low aspect ratios different long-time behaviour is observed depending on the nature of the heat input. A constant heat flux always produces a stratified interior at large times. On the other hand, a constant temperature supply ultimately produces a well-mixed space at the supply temperature. For separate sources of momentum and buoyancy, the developing stratification is shown to be strongly dependent on the separation of the sources and their relative strengths. Even at small separation distances the stratification initially exhibits horizontal inhomogeneity with localised regions of warm fluid (from the buoyancy source) and cool fluid. This inhomogeneity is less pronounced as the strength of one source is increased relative to the other. Regardless of the strengths of the sources, a constant buoyancy flux source dominates after sufficiently large times, although the strength of the momentum source determines whether the enclosure is initially well mixed (strong momentum source) or stably stratified (weak momentum source). © 2001 Elsevier Science Ltd. All rights reserved.

Study by Hall probe mapping of the trapped flux modification produced by local heating in YBCO HTS bulks for different surface/volume ratios

The aim of this report is to compare the trapped field distribution under a local heating created at the sample edge for different sample morphologies. Hall probe mappings of the magnetic induction trapped in YBCO bulk samples maintained out of thermal equilibrium were performed on YBCO bulk single domains, YBCO single domains with regularly spaced hole arrays, and YBCO superconducting foams. The capability of heat draining was quantified by two criteria: the average induction decay and the size of the thermally affected zone caused by a local heating of the sample. Among the three investigated sample shapes, the drilled single domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field. Finally, a simple numerical modelling of the heat flux spreading into a drilled sample is used to suggest some design rules about the hole configuration and their size. © 2005 IOP Publishing Ltd.

Investigation of the dilution process for measurement of particulate matter from spark-ignition engines

Measurements of particulate matter (PM) from spark ignition (SI) engine exhaust using dilution tunnels will become more prevalent as emission standards are tightened. Hence, a study of the dilution process was undertaken in order to understand how various dilution related parameters affect the accuracy with which PM sizes and concentrations can be determined. A SI and a compression ignition (CI) engine were separately used to examine parameters of the dilution process; the present work discusses the results in the context of SI exhaust dilution. A Scanning Mobility Particle Sizer (SMPS) was used to measure the size distribution, number density, and volume fraction of PM. Temperature measurements in the exhaust pipe and dilution tunnel reveal the degree of mixing between exhaust and dilution air, the effect of flowrate on heat transfer from undiluted and diluted exhaust to the environment, and the minimum permissible dilution ratio for a maximum sample temperature of 52°C. Measurements of PM concentrations as a function of dilution ratio show the competing effects of temperature and particle/vapor concentrations on particle growth dynamics, which result in a range of dilution ratios-from 13 to 18-where the effect of dilution ratio, independent of flowrate, is kept to a minimum. This range of dilution ratios is therefore optimal in order to achieve repeatable PM concentration measurements. Particle dynamics during transit through the tunnel operating at the optimal dilution ratio was found statistically insignificant compared to data scatter. Such small differences in number concentration may be qualitatively representative of particle losses for SI exhaust, but small increases in PM volume fraction during transit through the tunnel may significantly underestimate accretion of mass due to unburned hydrocarbons (HCs) emitted by SI engines. The fraction of SI-derived PM mass due to adsorbed/absorbed vapor, estimated from these data, is consistent with previous chemical analyses of PM. © 1998 Society of Automotive Engineers, Inc.

Numerical modeling of deep soil mixing excavation support

The finite element method (FEM) is growing in popularity over the pressure diagram/hand calculation method for analysis of excavation systems in general and deep soil mixing excavations in particular. In this paper, a finite element analysis is used to study the behavior of a deep mixed excavation. Through the use of Plaxis (a FEM software program), the construction sequence is simulated by following the various construction phases allowing for deflections due to strut or anchor installation to be predicted. The numerical model used in this study simulates the soil cement columns as a continuous wall matching the bending stiffness of the actual wall. Input parameters based on laboratory tests and modeling assumptions are discussed. An example of the approach is illustrated using the Islais Creek Transport/Storage Project in San Francisco, California. Copyright ASCE 2006.

Deep mixing for excavation support: Design issues

Deep mixing is effectively used in excavations both in conjunction with and in substitution of traditional techniques, where it results in more economical and convenient solutions for the stability of the system and the prevention of seepage. Deep mixed walls constructed as part of a soldier pile and tie-backs system act also as temporary support, prevent seepage like a sheet pile wall, but require a lower amount of steel. The deep mixed treatment can also contribute to the stability of the wall system against deep-seated failures. Although deep mixing is currently used for excavation control in numerous projects, no standard procedure has been developed and the different applications have not been evaluated. As this technique emerges as a more economical and effective alternative to traditional excavation shoring, there is a need for guidelines describing proven procedures for evaluation of design, analysis and construction. This paper presents comparisons in the design of excavation support using deep mixing and other traditional techniques. Issues important for design, analysis, and construction of deep mixed excavation walls are also discussed.

On Mixing and Shaking : Structure and Dynamics of Turbulent Stratified Flames

The drive for low emission combustion systems encourages applications using premixed flames. Yet in many applications, considerations of flame stability or mixing times lead to systems with neither premixed nor diffusion flames, which are often called technically premixed or stratified flames. In this talk we discuss the current state of understanding of the effect of mixing and extent of stratification on the structure, microstructure and dynamics of selected turbulent stratified flames. Over the past few years, a significant database of scalar and velocity data has been built to analyze the effects of unmixedness on local and global flame structure. Microscale studies of the flame structures show in detail how the effect of local stratification affects (or not!) the flame structure, flame surface density and scalar dissipation rates, and production of selected species. The experiments place exacting demands on current spectroscopic diagnostics, and reveal the progress and limits to our understanding of turbulent flames in general. The dynamics of stratified flames with respect to instabilities is also shown to be very rich, as the particular shape of the flames and the stabilization points are is significantly affected by the fuel distribution, modifying the rate and location of heat release, and thus the coupling with the surrounding acoustics and determining the onset of self-excitations.

Mixing and internal dynamics of droplets impacting and coalescing on a solid surface.

The coalescence and mixing of a sessile and an impacting liquid droplet on a solid surface are studied experimentally and numerically in terms of lateral separation and droplet speed. Two droplet generators are used to produce differently colored droplets. Two high-speed imaging systems are used to investigate the impact and coalescence of the droplets in color from a side view with a simultaneous gray-scale view from below. Millimeter-sized droplets were used with dynamical conditions, based on the Reynolds and Weber numbers, relevant to microfluidics and commercial inkjet printing. Experimental measurements of advancing and receding static contact angles are used to calibrate a contact angle hysteresis model within a lattice Boltzmann framework, which is shown to capture the observed dynamics qualitatively and the final droplet configuration quantitatively. Our results show that no detectable mixing occurs during impact and coalescence of similar-sized droplets, but when the sessile droplet is sufficiently larger than the impacting droplet vortex ring generation can be observed. Finally we show how a gradient of wettability on the substrate can potentially enhance mixing.

Influence of film cooling hole angles and geometries on aerodynamic loss and net heat flux reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (a), compound angle (b), hole inlet geometry, and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR=0.16, 0.64, and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different a and b but with the same angle between the mainstream and coolant flow directions (angle k). This relationship is assessed through experiments by testing two sets of cylindrical holes with different a and b: one set with k=35 deg, and another set with k=60 deg. The data confirm the stated relationship between α, β, k and the aerodynamic mixing loss. The results show that the designer should minimize k to obtain the lowest loss, but maximize b to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α=35.0 deg, β=0 deg) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan, and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is≈50% of the loss of the fan-shaped hole at IR=0.64 and 1.44. © 2013 by ASME.

Seasonal variations in stable isotope ratios of two biomanipulation fishes and seston in a large pen culture in hypereutrophic Meiliang Bay, Lake Taihu

This paper reports on seasonal changes in stable carbon and nitrogen isotope ratios of seston and muscle tissue of silver carp and bighead carp during 2004 and 2005, focusing primarily on the carbon sources and trophic relationships among phytoplankton, zooplankton and silver carp and bighead carp in a large fish pen of Meiliang Bay (Lake Taihu, China). delta C-13 showed a minimal value in March 2005 and a maximal value in August 2005 in seston both inside and outside the pen, whereas delta N-15 of seston showed the minimum in winter and the maximum during algal blooms. A positive correlation between delta C-13 of silver carp and that of seston suggested that temporal variation Of delta C-13 in seston was preserved in fish via the food chain. The differences of delta C-13 among seston, zooplankton and muscle tissue of silver carp and bighead carp ranged only 0.2-1.7%, indicating that plankton production was the primary food source of filter-feeding fishes. According to a mass balance model, we estimated that the contributions of zooplankton to the diets of silver carp and bighead carp were 45.7% and 54.3%, respectively, based on the delta N-15 values of zooplankton and planktivorous fishes. (C) 2007 Elsevier B.V. All rights reserved