262 research outputs found
Turbulent heat transfer in spacer-filled channels: Experimental and computational study and selection of turbulence models
Heat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100–2000, encompassing both steady laminar and early-turbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions
Natural convection cooling of a hot vertical wall wet by a falling liquid film
The system studied is a plane channel delimited by two vertical walls, one of which is imposed an arbitrary temperature profile and may be partially or totally wet by a liquid film, while the other is adiabatic. Air from the environment flows along the channel, driven by buoyancy forces. Its mass flow rate depends on the hydraulic resistances and on the distribution of temperature and humidity (hence, density) along the channel, which, in turn, depends on the heat and mass transfer between hot wall and humid air. Due to evaporative or boiling mass transfer, the liquid film, if present, shrinks as it descends along the hot wall, and may be completely dried out at some height. A simplified computational model of the above system was developed and applied to the prediction of relevant quantities, such as the total energy subtracted to the hot wall, for a range of conditions (hot wall temperature and its distribution; film flow rate; ambient air temperature and humidity; channel height and thickness; localized hydraulic resistance)
Optimization of net power density in Reverse Electrodialysis
Reverse Electrodialysis (RED) extracts electrical energy from the salinity difference between two solutions using selective ion exchange membranes. In RED, conditions yielding a large net power density (NPD) are generally desired, due to the still large cost of the membranes. NPD depends on a large number of physical and geometric parameters. Some of these, for example the inlet concentrations of concentrate and diluate, can be regarded as “scenario” variables, imposed by external constraints (e.g., availability) or chosen by different criteria than NPD maximization. Others, namely the thicknesses HCONC, HDIL and the velocities UCONC, UDIL in the concentrate and diluate channels, can be regarded as free design parameters and can be chosen so as to maximize NPD. In the present study, a simplified model of a RED stack was coupled with an optimization algorithm in order to determine the conditions of maximum NPD in the space of the variables HCONC, HDIL,UCONC, UDIL for different sets of “scenario” variables. The study shows that an optimal choice of the free design parameters for any given scenario, as opposed to the adoption of standard fixed values for the same parameters, may provide significant improvements in NPD
Social geography of rhinoscleroma and qualitatively and quantitatively abnormal cell-mediated immunity
Rhinoscleroma is a progressive chronic granulomatous disease of the upper respiratory tract that may extend to the tracheobronchial tract. It is common belief that the pathology is determined by Klebsiella Rhinoscleromatis. In the authors' opinion, the infection with Klebsiella Rhinoscleromatis may not represent the only etiopathogenic factor of the disease. Rhinoscleroma is reported in many countries, but has a peculiar social and geographic distribution, in that it assumes an endemic character only in some regions of the Middle East, West Russia, North Africa, Indonesia, Central and South America. In Europe, most of the cases are reported in Poland, Hungary and Romania. In Italy, Rhinoscleroma is almost exclusively located in the southern and island regions. Rhinoscleroma is predominantly reported in rural areas, in the presence of poor socio-economic conditions, which according to many authors would be a co-factor triggering the disease. In this article, the authors review some inconsistencies in etiology, histology and epidemiology of Rhinoscleroma. Based on the overall picture, they propose that intrinsic factors, possibly of genetic origin, may give rise to the disease, and suggest possible lines of research to distinguish between extrinsic and intrinsic factors as determinants for Rhinoscleroma
Cfd investigation of spacer-filled channels for membrane distillation
The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models
Fluid-structure interaction and flow redistribution in membrane-bounded channels
The hydrodynamics of electrodialysis and reverse electrodialysis is commonly studied by neglecting membrane deformation caused by transmembrane pressure (TMP). However, large frictional pressure drops and differences in fluid velocity or physical properties in adjacent channels may lead to significant TMP values. In previous works, we conducted one-way coupled structural-CFD simulations at the scale of one periodic unit of a profiled membrane/channel assembly and computed its deformation and frictional characteristics as functions of TMP. In this work, a novel fluid-structure interaction model is presented, which predicts, at the channel pair scale, the changes in flow distribution associated with membrane deformations. The continuity and Darcy equations are solved in two adjacent channels by treating them as porous media and using the previous CFD results to express their hydraulic permeability as a function of the local TMP. Results are presented for square stacks of 0.6-m sides in cross and counter flow at superficial velocities of 1 to 10 cm/s. At low velocities, the corresponding low TMP does not significantly affect the flow distribution. As the velocity increases, the larger membrane deformation causes significant fluid redistribution. In the cross flow, the departure of the local superficial velocity from a mean value of 10 cm/s ranges between -27% and +39%
Experimental investigation of two-side heat transfer in spacer-filled channels
In Membrane Distillation (MD), spacers support the membranes and promote mixing, thus reducing temperature polarization. Their efficient design requires a knowledge of the distribution of the local heat transfer coefficient h and of its dependence on Reynolds number, spacer geometry and flow-spacer relative orientation. In previous work, we applied Thermochromic Liquid Crystals (TLC) and digital image processing to the measurement of h distributions for different spacer configurations; data were used to validate CFD simulations and select turbulence models. For constructive reasons, the test section allowed only one-side heat transfer, while in most MD configurations (e.g. spiral-wound modules) heat transfer occurs from both sides of the feed water channels. Analytical and numerical solutions show that changing from one-side to two-side heat transfer deeply affects h values. This motivated the design and construction of an improved test section in which a hot channel is sandwiched between two cold channels, and twin cameras and lighting equipment allow the simultaneous acquisition of TLC images on both walls. This paper describes this new test section and the experimental technique, discusses measurement uncertainty, and presents preliminary results
Olfactory evaluation in obstructive sleep apnoea patients
The sense of smell has a high impact on the quality of life. The aim of the present study was to investigate olfactory dysfunction in patients with obstructive sleep apnoea syndrome (OSAS) and correlate the severity of disease with olfactory dysfunction. The relationships between nasal obstruction, nasal mucociliary cleareance and olfactory tests were also evaluated. Sixty patients with a diagnosis of OSAS were enrolled and underwent olfactory function evaluation. In all patients olfactory performance was tested with the Sniffin’ Sticks method. Mucociliary transport times and anterior rhinomanometry were performed to identify eventual nasal obstruction and deficits in nasal mucociliary clearance. Olfactory dysfunction was present in 22 (36.6%) patients of the study group: of these, hyposmia was present in 19 (86.4%) and anosmia in 3 (13.6%). The mean TDI score in the study group was 30. A strong correlation between the olfactory dysfunction and severity of sleep apnoea measured using the AHI was found. Patients with OSA would seem to have a high incidence of olfactory dysfunction. The degree of olfactory dysfunction appears to be related to the severity of disease. However, other co-factors such as nasal obstruction and reduced mucociliary clearance might also play a role in of the aetiology of this condition
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Large-eddy simulation of turbulent flows with heat transfer in simple and complex geometries
Some basic aspects of turbulence, transition to turbulence, and turbulence modelling, are summarized in Chapter 1 (Introduction). Emphasis is put on the increasing understanding of turbulent phenomena made possible by recent advances in the theory of dynamical systems; on the concept of "coherent structure"; and on the parallel evolution of computing power and computational fluid dynamics.
Chapter 2 is a survey of the turbulence modelling technique known as Large Eddy Simulation (LES). The equations governing fluid flow and scalar transport are introduced; the direct simulation of turbulent flows and its limitations are briefly outlined; and the concept of LES, with the related topics of decomposition, filtering and subgrid modelling, is discussed. The state of the art in LES is reviewed in the last three sections, under the separate headings of proposed subgrid models; wall boundary conditions; and applications presented in the literature. An attempt is made to give the most complete and updated possible account of the subject; work carried out from the early 'Seventies up to now is considered. Emphasis is put more on physical models and corresponding performances than on numerical methods and computational details.
In Chapter 3, the finite-volume numerical techniques used in the present work are presented and discussed. Emphasis is put on those aspects and options which bear more relevance for the accuracy and quality of the results, such as the pressure-velocity coupling algorithm, the discretization of advective terms and the treatment of centred (co-located), body-fitted grids. The architecture and the basic features of the computer code Harwell-FL0W3D, Release 2, are outlined, while the modifications introduced in order to implement the Smagorinsky subgrid model and the appropriate boundary conditions for LES are described in detail.
In Chapters 4-6 results are presented and discussed for the basic geometries studied.
Chapter 4 deals with the flow between indefinite parallel plates (plane channel), one of which heated with a uniform heat flux. For this basic geometry, a detailed study is presented on the influence of numerical options (grid size, time step and time-stepping method, pressure-velocity coupling, discretization of the advective terms); model parameters (Smagorinsky constant, subgrid Prandtl number, near-wall damping); Reynolds number; and alternative wall boundary conditions. The issues of initial conditions, numerical transients and statistical processing of the results are also discussed with some depth.
In Chapter 5, computations are presented for a plane channel having one of the walls roughened by transverse square ribs. An extensive literature review of experimental and numerical studies on this geometry is included. The parametrical study is limited here to the influence of grid size and Reynolds number; LES results are presented in detail for a reference case, and are compared with experimental flow and heat transfer data.
Chapter 6 is dedicated to the geometry of cross-corrugated ducts, representative of storage-type air preheaters for fossil-fuelled power stations. Flow and heat transfer predictions from direct simulation and LES are presented; they are compared with experimental results and with numerical predictions obtained by a standard and a low-Reynolds number version of the k-s turbulence model.
Finally, Chapter 7 summarizes the main conclusions which can be drawn from the above studies. Emphasis is put on the basic issue of LES applicability to engineering problems of practical interest, and of its feasibility using a commercial, general-purpose (though highly sophisticated) computer code. A critical comparison with more conventional turbulence modelling approaches is outlined, and 'weak spots', or issues requiring further clarifications, are pointed out for future studies.
The work includes an extensive bibliography with almost 400 references, and an appendix on the tensorial formulation of the governing equations of fluid dynamics in general domains
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