| To search alternative refrigerants has been one of the urgent issues in the refrigeration and air conditioning industry for the sake of reducing ozone depletion and global warming effect. R161as a refrigerant with good environmental acceptability (GWP=12, ODP=0), good thermo-physical properties, good heat transfer and flow characteristics, and highly similar thermo-physical properties to R22, has attracted broad attention as a substitute of R22. The study on boiling heat transfer characteristic of an alternative refrigerant with oil presence inside tubes is very necessary to further analyze its performance and the application potential. In addition, enhanced tubes with small diameter have been utilized widely in evaporators of air-conditioners to reduce the cost and improve efficiency of evaporator. So far, research on the heat transfer of R161/Oil mixture inside small diameter enhanced tube has not been conducted at home and abroad, and the related data and reference models are limited in open literatures. To predict the boiling heat transfer and pressure drop characteristics of R161/Oil mixture inside such scale tubes is important for promoting the substitute of R22by R161and promoting practical application of mini-scale tubes in compact and effective evaporator design.This paper investigated the boiling heat transfer and pressure drop characteristics of R161and R161/Oil mixture inside7mm O.D. horizontal enhanced tube by experimental and theoretical methods. Experimental apparatus is reformed at first based on the experimental working conditions, and the effect of various working conditions on the boiling heat transfer was analyzed between R161and R161/Oil mixture. In addition, new correlation to predict the boiling heat transfer coefficients of R161/Oli mixture inside enhanced tube was developed. The main contents and results are summarized as follows:(1) The experimental apparatus was reformed for the boiling heat transfer of refrigerant-oil mixture inside enhanced tube.(2) The boiling heat transfer and pressure drop characteristics of R161in7mm O.D. horizontal enhanced tube were investigated at the saturation temperature range of-5～8℃, the heat flux range of11.75～52.94kW/m2, and the mass flux range of100～250kg/m2·s. Results showed that:1) the local heat transfer coefficients of R161firstly increased with the vapor quality and up to the maximum value when the quality rose to about0.7～0.8, then decreased for the occurrence of dry-out;2) the heat transfer coefficients increased with the mass flux and the heat flux;3) The pressure drop increased strongly with the mass flux;4) The pressure drop increased with the heat flux;5) The pressure drop increased with the decreasing of saturation temperature.(3) The boiling heat transfer and pressure drop characteristics of R161/Oil mixture in7mm O.D. horizontal enhanced tube were investigated at oil concentration range of1.5%～5%, the saturation temperature range of-5～8℃, the heat flux range of11.75～52.94kW/m2, and the mass flux range of100～250kg/m2·s. The test results showed that:1) at the low vapor quality, the local heat transfer coefficients of R161/Oil mixture increased with the increasing of oil concentration;2) at the high vapor quality, the local heat transfer coefficients of R161/Oil mixture decreased rapidly with the increasing of oil concentration.3) The pressure drop increased with the oil concentration;4) The pressure drop increased with the heat flux and mass flux;5) The influence of oil on pressure drop decreased with the increasing of mass flux. A new correlation to predict the local boiling heat transfer of R161/Oil mixture inside small enhanced tubes was developed based on the local properties of refrigerant-oil mixture and structure of enhanced tube, and the predicted data agrees with the experimental data within the deviation of15%. |
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