The heat exchanger with small diameter tubes can always has a higher heat transfer coefficient. In order tocharacterize the heat transfer of ammonia in heat exchangers, an experimental apparatus was built up toinvestigate the heat transfer characteristics of ammonia during flow boiling in a horizontal small diametertube. In this study, the test section is made of an AISI 316L stainless steel tube with internal diameter of 4mm. The experiments were performed at the saturation temperatures of -5 oC, the mass flux range is from 50to 100 kg m~(-2) s~(-1), and the heat flux range is from 7to 21 kW m~(-2). The results show that the heat transfercoefficient increases withvapor quality. And the mass flux and heat flux all have positive effect on the heattransfer coefficient, that is, the heat transfer coefficient increases with the rises of mass flux and heat flux.
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