Experimental And Theoretical Research On The Characteristics Of Falling Film Evaporation On The Horizontal Tubes In Refrigerating System

Under the background of the energy-saving and emission reduction policy at home and abroad, based on the reduction technology of refrigerant, falling film evaporation outside horizontal tubes in the refrigerating system was studied. Falling film evaporation was regarded as the object of the research and its flow and heat transfer theory has been analyzed and summarized to lay the theoretical foundation of analyzing the experimental phenomena.The thickness of liquid film outside horizontal tube is very important in the process of falling film evaporation. Therefore, this paper established a two-dimensional physical model with using R134 a, under the condition of ignoring heat transfer with phase change, to study the distribution of liquid film thickness. The simulation results showed that the film thickness was not evenly distributed. First the film thickness varied from thick to thin, then from thin to thick, and the film thickness was the smallest at about 150°from the vertical. The film thickness increased with the increase of inlet velocity of refrigerant; the thickness of liquid film was independent on tube type in the same inlet velocity on the condition of this paper.In practice, working fluid from the outlet of compressor contains lubricating oil, as the working fluid flows into the evaporator, lubricating oil will inevitably affect heat transfer performance of the evaporator. Concerning this issue, through transforming the experimental system and using R134 a as working fluid, three different tube types was studied to analyze the impact of different oil contents on evaporation of falling film on horizontal enhanced tube. Experiments were conducted for R134 a at saturation temperature 6? with sprinkle densities of 0.13, 0.17 and 0.21kg?s^-1?m^-1, oil contents of 0.5%, 1.2% and 5.1% respectively.Through analysis and comparison, it was founded that for horizontal smooth and 02# enhanced tube, as a whole, the outside heat transfer coefficient increased with oil contents in the same sprinkle density. However, for the 01# enhanced tube at low sprinkle density, the outside heat transfer coefficient increased with the increase of oil content, up to a certain optimum oil content, and then decreased with increasing oil content. At high sprinkle density, the change of oil content impacted on the outside heat transfer coefficient inconspicuously.At higher oil content, there was a suitable sprinkle density which can make the outside heat transfer coefficient to reach the maximum for the smooth tube. For 01# and 02# enhanced tube, sprinkle density had little impact on the outside heat transfer coefficient.At low oil content, it was not obvious that the outside heat transfer coefficient was affected by sprinkle density for the smooth tube. For 01# enhanced tube, the outside heat transfer coefficient increased with increasing sprinkle density, up to a certain value. then decreased with the increase of sprinkle density. For 02# enhanced tube, the outside heat transfer coefficient increased with the sprinkle density.