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Ebullition en écoulement d'azote liquide dans des microtubes. I. Le début de l'ébullition nucléée, l'instabilité de l'écoulement diphasique et la chute de pression lors de l'écoulement diphasique.

Flow boiling of liquid nitrogen in micro-tubes. I. The onset of nucleate boiling, two-phase flow instability and two-phase flow pressure drop.

Auteurs : QI S. L., ZHANG P., WANG R. Z., et al.

Type d'article : Article

Résumé

This paper is the first portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents mainly include the onset of nucleate boiling (ONB), two-phase flow instability and two-phase flow pressure drop. At ONB, mass flux drops suddenly while pressure drop increases, and apparent wall temperature hysteresis in the range of 1.0-5.0 K occurs. Modified Thom model can predict the wall superheat and heat flux at ONB. Moreover, stable long-period (50-60 s) and large-amplitude oscillations of mass flux, pressure drop and wall temperatures are observed at ONB for the 1.042 and 1.931 mm micro-tubes. Block phenomenon at ONB is also observed in the cases of high mass flux. The regions for the oscillations, block and stable flow boiling are classified. A physical model of vapour patch coalesced at the outlet is proposed to explain the ONB oscillations and block. Vapour generation caused by the flash evaporation is so large that it should be taken into account to precisely depict the variation of mass quality along the micro-tube. The adiabatic and diabatic two-phase flow pressure drop characteristics in micro-tubes are investigated and compared with four models including homogeneous model and three classical separated flow models. Contrary to the conventional channels, homogeneous model yields better prediction than three separated flow models. It can be explained by the fact that the density ratio of liquid to vapour for nitrogen is comparatively small, and the liquid and vapour phases may mix well in micro-tube at high mass flux due to small viscosity of liquid nitrogen, which leads to a more homogeneous flow. Part II of this study will focus on the heat transfer characteristics and critical heat flux of flow boiling of liquid nitrogen in micro-tubes (see this Bulletin, reference 2008-1081). [Reprinted with permission from Elsevier. Copyright, 2007].

Détails

  • Titre original : Flow boiling of liquid nitrogen in micro-tubes. I. The onset of nucleate boiling, two-phase flow instability and two-phase flow pressure drop.
  • Identifiant de la fiche : 2008-1082
  • Langues : Anglais
  • Source : International Journal of Heat and Mass Transfer - vol. 50 - n. 25-26
  • Date d'édition : 12/2007

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