Effect of inclination on heat transfer coefficient during flow boiling in a mini-channel
Résumé
An experimental study of heat transfer during R245fa flow boiling is carried out in a 1.6 mm inner diameter circular channel (198 original data points). The test section is composed of a sapphire tube coated with ITO, which enables a total transparency of the evaporator in the visible spectrum. The acquisition of the outer wall temperature field is ensured by means of an infrared camera and the heat transfer coefficient is calculated by means of a local thermal model. The effect of the inclination on the heat transfer is presented and discussed for various vapour qualities and mass velocities and a saturation temperature of 81°C, corresponding to a Bond number of 4.1. For each experimental condition, the mean heat transfer coefficient is calculated for various inclinations from the vertical downward flow (-90°) to the vertical upward flow (+90°). The variation of the heat transfer coefficient are first presented and analyzed in the horizontal case. The heat transfer coefficient variations with the flow parameters are interpreted in terms of nucleate and convective boiling and these observations are confirmed by the visualisation of the nucleation phenomenon in the evaporator. The inclination has almost no effect on the heat transfer and this lack of effect is finally interpreted in terms of shift between the annular to intermittent flow pattern transition coupled with the nucleate to convective boiling transition.
Origine : Fichiers produits par l'(les) auteur(s)