A. Viedma

Heat transfer and pressure drop for low Reynolds turbulent flow in helically dimpled tubes

Three-dimensional helically dimpled tubes have been experimentally studied in order to obtain their heat transfer and isothermal friction characteristics. Using water and ethylene glycol as test fluids, a wide range of fluid flow conditions was covered: 2000<Re<100,000 and 2.5<Pr<100. An experimental study of 10 tubes with different geometric forms (dimple height h/d ranging from 0.08 to 0.12 and helical pitch p/d, from 0.65 to 1.1) offers insight into the influence of manufacturing parameters on tube thermohydraulic behaviour. The large amount of experimental data have been correlated so as to obtain easy to use expressions for Fanning friction factors and Nusselt numbers as functions of flow and geometry non-dimensional parameters. Performance evaluation criteria, commonly used in the enhanced heat transfer literature, were calculated in order to assess the real benefits offered by dimpled tubes.

Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow

Abstract This paper presents the experimental results carried out in dimpled tubes for laminar and transition flows and completes a previous work of the authors focused on the turbulent region. It was observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region. Employing water and ethylene glycol as test fluids, the following flow range was covered: x ∗ =10 −4 –10 −2 and Ra=106–108. The experimental results of isothermal pressure drop for laminar flow showed dimpled tube friction factors between 10% and 30% higher than the smooth tube ones. Moreover, it was perceived that roughness accelerates transition to critical Reynolds numbers down to 1400. Correlations for the laminar friction factor f=f(Re,h/d) and for the critical Reynolds Recrit=Recrit(h/d) are proposed. The hydraulic behaviour of dimpled tubes was found to depend mainly on dimple height. In mixed convection, high temperature differences in the cross section were measured and therefore heat transfer was evaluated by a circumferentially averaged Nusselt number. Experimenal correlations for the local and the fully developed Nusselt numbers Nu x = Nu x (x ∗ ,Ra) and Nu ∞ = Nu ∞ (Ra) are given. Results showed that at low Rayleigh numbers, heat transfer is similar to the smooth tube one whereas at high Rayleigh, enhancement produced by dimpled tubes can be up to 30%.

Extended Willis circle model to explain clinical observations in periorbital arterial flow

A fluid-dynamic model of the circle of Willis and its periorbital links with the external carotid arteries has been established and tested. It is based on anatomic data and takes Doppler measurements as flow input conditions. The model explains, on fluid-dynamic grounds, the clinical observations of periorbital reverse flow and arrival pulse time delay. It also obtains the velocity and pressure pulse at any point of the studied area. This allows the comparison between the normal or healthy condition and the flow distribution when an internal carotid is externally or pathologically occluded. Several combinations of the communicating artery sizes are explored to obtain the reduced cerebral flow. The combination of the communicating diameters can lead to insufficient irrigation which can be hydrodynamically assessed. No other physiological response is included, and the results must be considered as a minimum assured. These results show the need for a common evaluation of the alternative paths and explain some paradoxes found in literature.

Performance Evaluation of a Zero-Fouling Reciprocating Scraped-Surface Heat Exchanger

An innovative self-cleaning shell-and-tube heat exchanger is presented. Inside each interior tube (through which the product flows) a scraping rod is fitted. This rod moves in reciprocal manner and the scraping elements mounted on the rod fully clean the tube wall surface. Additionally, the macroscopic displacements of the flow, induced by the insert device motion, promote high flow mixing. Consequently, tube-side heat transfer coefficients are enhanced. Thermal-hydraulic and scraping power measurements are performed in laminar regime for motionless and dynamic conditions. An extended performance evaluation criterion is proposed, in order to balance the augmentation of heat transfer and the increased power consumption (pumping and scraping power) of the device. This study allows stating guidelines for the operation of the device, concluding that the performance of the heat exchanger is irrespective of the velocity ratio. The scraper can be used intermittently, or at the minimum scraping frequency that ensures fouling mitigation.

Eulerian and Lagrangian description of a pulsatile, partially turbulent flow

A detailed flow field analysis is carried out downstream of a Björk-Shiley prosthetic aortic valve, placed in a Great Circulation simulator. Mean periodic velocity and turbulence are obtained by means of selective sampling through enabling windows. Flow mapping includes three different heights and three meridional planes (i.e. nine diameters). Some effort is devoted to study cycle-to-cycle variations, and to remove them for determining true turbulence values. A lagrangian interpretation, based upon the convection of vorticity produced at the aortic prosthesis, is needed to understand the flow pattern behaviour.

CFD Modeling of Legionella's Atmospheric Dispersion in the Explosive Outbreak in Murcia, Spain

ABSTRACT Cooling towers, among other equipment, could have an important atmospheric impact, becoming a source of pollutants or biological agents. The most important, due to its frequency and importance of the outbreaks, is Legionella. Since its discovery in 1976 in Philadelphia, PA, several outbreaks have been reported causing tens of deaths. The most important one took place in the city of Murcia, Spain, in 2001, with more than 600 cases. In the present work, a validated numerical model using the computational fluid dynamics code ANSYS Fluent is employed to simulate the dispersion of the drift from the cooling tower causative of the outbreak in the urban environment of Murcia in the days of highest emissions. The results of the model are compared with the results of the epidemiological investigation carried out by the Epidemiology Service (Consejería de Sanidad, Murcia, Spain). The main objective of this model is to predict a cooling tower influence area, and what will help to reduce environmental and personal impact in case of an infection of its water or to improve the resources used to find the focus of infection after an outbreak has taken place. The model was previously validated using data from an experimental cooling tower installation.

Numerical analysis of the vacuum infusion process for sandwich composites with perforated core and different fiber orientations

The vacuum infusion is a process usually applied to manufacture large structures of composite materials, such as wind turbine blades. The specific stiffness and weight ratio required by these structures can be achieved by manufacturing sandwich composites. The forecast by numerical simulation of the resin infusion flow is an indispensable tool to design and optimize the manufacturing process of composite. Present work analyzes by numerical simulation the mold filling process of a sandwich composites, performed by fiberglass plies with different fiber orientations and a perforated core. The flow through a single perforation of the core is analyzed and the influence of the permeability values of fiberglass on the volume flow through core perforations is determined. In order to reduce the computing costs, a transfer function to simulate the flow through the perforations is developed and integrated in the numerical code by computational subroutines. A 3D numerical modeling of a sandwich composite, in which the flow through the core perforations is simulated via computational subroutines, is carried out and experimentally validated.

Numerical study of the vacuum infusion process for laminated composites with different fiber orientations

The vacuum infusion is a process usually applied to manufacture large structures of composite materials, such as wind turbine blades. This work analyzes the macroscopic resin flow through a laminate of fiberglass plies with different orientations, during the filling stage of the vacuum infusion process to manufacture two different pieces. The pressure inside the mold, velocity vectors, and the resin inlet mass flow are studied through a three-dimensional numerical modeling under non-steady conditions validated experimentally. The numerical model simulates each ply of the laminate like an individual porous media and takes into account the stacking sequence of the laminate. The influence of the permeability values of the distribution media and of the fiberglass laminate on the evolution of resin infusion is analyzed. The numerical model reproduces the effects of the stacking sequence and race tracking on the resin flow front.

A Validated CFD Methodology to Obtain the Total Pressure Loss Coefficients in Internal Compressible Flow at Junctions

A global methodology has been developed and validated to obtain the total pressure loss coefficient in internal compressible flow at T-type junctions. This methodology is based on the calculation of the thermo-fluid properties extrapolated to the branch axes intersection, once the straight pipe friction losses numerically calculated have been subtracted from the total energy losses. For this purpose, a steady adiabatic compressible one-dimensional flow with friction mathematical model has been applied to the results obtained by numerical simulation using the commercial finite volume code FLUENT. A 90 degree T-type junction has been studied and the predicted loss coefficient has been related to the extrapolated Mach number in the common branch and to the mass flow rate ratio between branches at different flow configurations, in both combining and dividing flows. The numerical results have been compared with experimental results and published data in open literature. In general, a good agreement is obtained. The correlations obtained will be applied as boundary condition in one-dimensional global simulation models of fluid systems in which these components are present.Copyright

Vorticity profiles and streak-lines in axially perturbed coflowing jets

The development of coherent structures formed between coflowing vertical jets at moderate Reynolds numbers has been studied experimentally. By means of phase-locked LDV and flow visualization it has been possible to produce data enough to describe the flow subjected to the effects of axial forcing. Flow mapping includes axial and radial phase averaged velocity profiles, vorticity profiles, streamlines and streak-lines. Vorticity dynamics inferred from these data as well as streak-lines shows a good agreement with flow visualization. Mixing between the streams is also analyzed, improving with higher levels of forcing amplitude.