1,720,967 research outputs found
Boiling heat transfer performance and phenomena of Al2O3-water nano-fluids from a plain surface in a pool
No full textBoiling heat transfer characteristics of nano-fluids with nano-particles suspended in water are studied using different volume concentrations of alumina nano-particles. Pool boiling heat transfer coefficients and phenomena of nano-fluids are compared with those of pure water, which are acquired on a smooth horizontal flat surface (roughness of a few tens nano-meters). The experimental results show that these nano-fluids have poor heat transfer performance compared to pure water in natural convection and nucleate boiling. On the other hand, CHF has been enhanced in not only horizontal but also vertical pool boiling. This is related to a change of surface characteristics by the deposition of nano-particles. In addition, comparisons between the heat transfer data and the Rhosenow correlation show that the correlation can potentially predict the performance with an appropriate modified liquid-surface combination factor and changed physical properties of the base liquid. (c) 2005 Elsevier Ltd. All rights reserved
Applicability of Nano-fluids for a Thermal Hydraulic System : Boiling Heat Transfer Characteristics of Al2O3 Nano-fluids
No full textVisualization of the subcooled flow boiling of R-134a in a vertical rectangular channel with an electrically heated wall
No full textA digital photographic study of subcooled flow boiling with R-134a was performed in a vertical rectangular channel heated from one side to observe near-wall structures. Primary attention was given to bubble formation and bubble coalescence in the bubble layer. The visualization made possible a detailed observation of the characteristic near-wall region, consisting of vapor remnants, an interleaved liquid layer, and coalesced adjacent bubbles. In particular, the visualization provided clearer insights about bubble coalescence behaviors during flow nucleate boiling. In addition, it is shown that the near-wall bubble layer of nucleate boiling beneath vapor clots is extinguished and, afterwards, the heated surface is locally covered by large vapor films, at CHF. (C) 2004 Elsevier Ltd. All rights reserved
A photographic study on the near-wall bubble behavior in subcooled flow boiling
No full textThe behavior of near-wall bubbles in subcooled flow boiling has been investigated photographically for water flow in vertical, one-side heated, rectangular channels at mass fluxes of 500, 1500, 2000 kg.m(-2).s(-1) under atmospheric pressure. Primary attention was given to the bubble coalescence phenomenon and the structure of the near-wall bubble layer. The number of near-wall bubbles increased with the increase in the heat flux. At sufficiently high heat fluxes (>60-70% CHF), three characteristic layers were observed in the heated channel: (a) a superheated liquid layer with small bubbles attached on the heated wall, (b) a flowing bubble layer consisting of large coalesced bubbles over the superheated liquid layer, and (c) the liquid core over the flowing bubble layer. In addition, the existence of a liquid sublayer under coalesced bubbles was identified photographically. According to visualization, the CHF mechanism for the present experimental condition could be related to the formation of large vapor clots resulting from coalescences of bubbles and the evaporation of the superheated liquid layer beneath those clots. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved
Critical heat flux performance for flow boiling of R-134a in vertical uniformly heated smooth tube and rifled tubes
No full textIn the present paper, critical heat flux (CHF) experiments for flow boiling of R-134a were performed to investigate the CHF characteristics of four-head and six-head rifled tubes in comparison with a smooth tube. Both of rifled tubes having different head geometry have the maximum inner diameter of 17.04 mm while the smooth tube has the average inner diameter of 17.04 mm. The experiments were conducted for the vertical orientation under outlet pressures of 13, 16.5, and 23.9 bar, mass fluxes of 285-1300 kg/m(2)s and inlet subcooling temperatures of 5-40 degrees C in the R-134a CHF test loop. The parametric trends of CHF for the tubes show a good agreement with previous understanding. In particular, CHF data of the smooth tube for R-134a were compared with well-known CHF correlations such as Bowring and Katto correlations. The CHF in the rifled tube was enhanced to 40-60% for the CHF in the smooth tube with depending on the rifled geometry and flow parameters such as pressure and mass flux. In relation to the enhancement mechanism, the relative vapor velocity is used to explain the characteristics of the CHF performance in the rifled tube. (c) 2005 Elsevier Ltd. All rights reserved
Visualization of a principle mechanism of critical heat flux in pool boiling
No full textA new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor "holes" made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes. (c) 2005 Elsevier Ltd. All rights reserved
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