Tube électronique | EPFL Graph Search

vignette|upright=0.7|Lampe double-triode de fabrication russe. Un tube électronique (thermionic valve en anglais ou vacuum tube aux États-Unis), également appelé tube à vide ou même lampe, est un composant électronique actif, généralement utilisé comme amplificateur de signal. Le tube à vide redresseur ou amplificateur a été remplacé dans beaucoup d'applications par différents semi-conducteurs, mais n'a pas été remplacé dans certains domaines comme l'amplification de forte puissance ou des hyperfréquences. Historique Historique des tubes électroniques Principe Structure générale Les tubes électroniques désignent les composants qui utilisent des électrodes, placées dans le vide ou dans un gaz rare, isolées entre elles par ce milieu, et comprenant au moins une source d'électrons. Une enveloppe résistante à la température isole l'ensemble de l'extérieur. Bien que l'enveloppe soit généralement en verre, les tubes de puissance utilisent souvent la céramiq

Heat transfer and flow visualization of falling film condensation on tube arrays with plain and enhanced surfaces

Daniel Gstöhl

The tubes in shell-and-tube condensers, widely used in refrigeration and chemical process industries, are subjected to condensate inundation from the neighboring tubes. The aim of the present investigation is to study the effect of condensate inundation on the thermal performance of an vertical array of horizontal tubes with plain and enhanced surfaces. The experimental approach is split in two parts: measurement of the heat transfer coefficients and visualization of the flow patterns of the condensate falling between the tubes. Refrigerant R-134a was condensed at a saturation temperature of 304K on tube arrays with up to ten tubes at pitches of 25.5, 28.6, and 44.5mm. Four commercially available copper tubes with a nominal diameter of 19.05mm and 544mm in length were tested: a plain tube, a 26 fpi / 1024 fpm low finned tube (Turbo-Chil) and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Measurements were performed at three nominal heat flux levels up to 60kW/m2 with liquid overfeed corresponding to film Reynolds numbers up to 3000. The test section offers full visual access to study the flow patterns of the condensate. Furthermore, the large experimental database is unique in that true local heat transfer coefficients were measured as opposed to tube length averaged values in previous studies. With little liquid inundation the tubes with 3D enhanced surface structures outperform the low finned tube. Increasing liquid inundation deteriorates the thermal performance of the 3D enhanced tubes, while it has nearly no affect on the low finned tube, resulting in a higher heat transfer coefficient for the low finned tube at high film Reynolds numbers. Large differences in condensate flow patterns were observed. For the 3D enhanced tubes the ideal flow modes (droplet, column and sheet mode) were observed, while the flow was very unstable for the other two types of tubes. For the 3D enhanced tubes oscillations occurred in the sheet mode at film Reynolds numbers such that liquid left the array of tubes sideways. A heat transfer model for an array of 3D enhanced tubes based on these visual observations was proposed, including the effects of liquid lost by the sideways slinging phenomenon. The measurements were predicted within a mean error of 3% and a standard deviation of 13% by this model.

EPFL2004

Falling film evaporation on a single tube and on a tube bundle

Jean-François Roques

This study is a contribution for the improvement of horizontal falling film heat exchangers. This type of evaporator has the potential to be widely used in the petrochemical industry, for the sea water desalination, or in the large refrigeration systems. An experimental test facility has been constructed in order to study the evaporation of the liquid refrigerant R-134a flowing on a vertical array of horizontal copper tubes with a length of 0.55 meter and a diameter of 19.1mm. Special care has been taken to achieve uniform liquid distribution along the top tube. Four types of tubes were tested including a plain tube, two tubes with mechanically enhanced boiling surfaces (Turbo-BII HP and Gewa-B) and a porous coated tube (High-Flux). Tube arrays with three different tube pitches were tested. Measurements were performed at three different nominal heat flux levels over a wide range of liquid overfeed. For comparison, heat transfer coefficients in pool boiling conditions were measured. The experimental parameters are the following: the liquid film Reynolds numbers from 0 to 3000, heat fluxes between 20 and 60kW/m2 and tube pitches from 22.3 and 25.5mm. The heat transfer coefficients have been measured locally at the mid point of each of the ten tubes. The study shows that there are two types of fundamental behavior characterized by a particular film Reynolds number: Over a threshold value of this Reynolds number, the nucleate boiling in the film governs the heat transfer and this heat transfer coefficient is independent of the Reynolds number. Below this threshold value, the heat transfer coefficients drop drastically due to the formation of local dry patches. The heat transfer coefficient during falling film evaporation was found to be approximately 1.3 times larger than the heat transfer coefficient during pool boiling and this multiplier was found to be dependant on the heat flux and tube spacing but not the flow mode. Finally, the understanding of the physical phenomena governing the falling film evaporation of liquid refrigerants has been improved. Furthermore, a method for predicting the film Reynolds number at the onset of dry patch formation has been developed and a heat transfer correlation for boiling iii falling films proposed. These represent significant improvements for the design of falling film evaporators.

EPFL2004

Rapid processing of net-shape thermoplastic planar random composite preforms

Patrick Blanchard, Simon Jespersen, Jan-Anders Månson, Véronique Michaud, Daniel Schmäh

A novel thermoplastic composite preforming and moulding process is investigated to target cost issues in textile composite processing associated with trim waste, and the limited mechanical properties of current bulk flow- moulding composites. The thermoplastic programmable powdered preforming process (TP-P4) uses commingled glass and polypropylene yarns, which are cut to length before air assisted deposition onto a vacuum screen, enabling local preform areal weight tailoring. The as-placed fibres are heat- set for improved handling before an optional preconsolidation stage. The preforms are then preheated and press formed to obtain the final part. The process stages are examined to optimize part quality and throughput versus processing parameters. A viable processing route is proposed with typical cycle times below 40 s (for a plate 0.5 × 0.5 m2, weighing 2 kg), enabling high production capacity from one line. The mechanical performance is shown to surpass that of 40 wt.% GMT and has properties equivalent to those of 40 wt.% GMTex at both 20°C and 80°C.

Springer Netherlands2009

Falling film evaporation on a single tube and on a tube bundle

Jean-François Roques

EPFL2004

Heat transfer and flow visualization of falling film condensation on tube arrays with plain and enhanced surfaces

Daniel Gstöhl

EPFL2004