INVESTIGATION ON BOILING HEAT TRANSFER CHARACTERISTICS OF R404A IN A 7.49-mm SMOOTH HORIZONTAL TUBE

Experimental investigation on flow boiling heat transfer characteristics of R404A (R125/R143a/R134a) was carried in a smooth horizontal tube of 7.49-mm inner diameter. The tested conditions were similar to those present in the evaporator of supermarket display cases, refrigerated cabinets, laboratory freezers, etc. The heat transfer coefficients along the length of the test section were experimentally measured under varied heat flux conditions between 4 and 19 kW m(-2), mass flow rate between 2.5 and 4.5 g s(-1) (57 to 102 kg m(-2) s(-1)), and working pressure between 4.4 and 6.1 bar. The flow patterns for the tested conditions were stratified, stratified-wavy, and near-annular flows, which were confirmed using the Kattan-Thome-Favrat flow pattern maps. The experiments revealed that under the aforementioned operating condition, the nucleate boiling mechanism was predominant in the heat transfer coefficient. Further, unlike high mass flow rate conditions, the presence of nucleation was witnessed even at higher vapor qualities.