Heat transfer measurements on ammonia forced convection boiling in vertical tubes

Abstract

Abstract This paper tackles a subject of present interest. It contributes to the promotion of small capacity refrigerating systems working with ammonia – an ecologically and highly efficient refrigerant – as an alternative to the traditional commercial refrigeration, heat pumps and air-conditioning systems working with CFC refrigerants. Since a literature survey showed that available theoretical methods have not been sufficiently experimentally validated, the authors focused 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 for 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 three locations, using segments of glass tube. Sequences of computerized collected data (from pressure transducers, thermocouples, platinum RTD temperature sensors, and an ultrasound flowmeter) were treated with a digital filtration. A comparison between theoretical and experimental results indicated that the Steiner–Taborek and Gungor–Winterton correlations make the best predictions for the local heat transfer coefficient of boiling ammonia. In addition, the authors experimentally determined the fluid coefficient to be used in Kandlikar's correlation in order to extend its applicability to ammonia boiling. It is shown 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.