Heat transfer measurements on ammonia forced convection boiling in vertical tubes

This paper tackles a subject of present interest. It contributes to the promotion of small capacity refrigerating systems working with ammonia - an ecological and high efficient refrigerant - as an alternative to the traditional commercial refrigeration and air conditioning systems working with CFC refrigerants. Since a literature survey showed that available theoretical methods have not been enough experimentally validated, the authors focussed on constructing a test facility, namely a test loop, assisted by a data acquisition system. Experiments have been run in order to develop a data bank referring to the heat transfer of ammonia inside an electrically heated tube. The local heat transfer coefficient for boiling ammonia was measured in 10 different locations along the tube and the flowing regime was visualized in 3 locations, using segments of glass tube. Sequences of computerized collected data (from pressure transducers, thermocouples, platinum RTD temperature sensors, and ultrasound flowmeter) suffered a digital filtration. A comparison between theoretical and experimental results indicated that Steiner-Taborek's and Gungor-Winterton's correlation makes the best predictions for ammonia's boiling local heat transfer coefficient. In addition, the authors experimentally determined the fluid coefficient value to be used in Kandlikar's correlation in order to extend its applicability for ammonia boiling. It was showed the influence that the main parameters have on the boiling process, over a range of the input parameters. Both the developing of the two-phase flow mechanisms and the heat transfer coefficients characterizing ammonia boiling inside tubes, are strongly influenced by those parameters.