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In the present study, the boiling flow of R1234yf refrigerant in a horizontal helically coiled tube is investigated to experimentally measure the heat transfer coefficient and pressure drop. A well-instrumented apparatus consisting of the test evaporator and other necessary equipment is used in this investigation. A preheater and a post-evaporator are used to achieve the required vapor qualities and test conditions. Experimental investigations are carried out for a straight tube of 8.7 mm inner diameter and 1,200 mm length as well as helically coiled tubes with three different coil diameters of 10.95, 13.45, and 15.95 cm. For each experiment, different characteristic parameters such as refrigerant flow rate, outside tube surface temperature at various locations, heat flux, and pressure drop within the test section are measured. The results reveal that under the flow conditions studied in this work, heat transfer coefficient is increased up to 299% for the helically coiled tube compared to the horizontal straight tube. In addition, it is observed that for a constant coil pitch, pressure drop increases up to 227% occurring at the minimum coil diameter of 10.95 cm. Moreover, it is found that among the available models, Gungor and Winterton correlation predicts the experimental data with the least error. [ABSTRACT FROM AUTHOR]

Heat transfer coefficient and pressure drop of the refrigerant R134a flow boiling in metal-foam filled annular tubes are studied. Three test tubes are analyzed. One tube is plain annular and the others are filled with 10 and 20 PPI copper foam pieces. The three test tubes are dimensionally similar with a length of 30 cm, outer and inner diameters of 14.3 mm and 4.7 mm, respectively. The experiments are performed for mass flux ranging from 10 to 80 kgm−2s−1 and vapor quality of 0.13–0.85. Also, a constant and uniform heat flux of 5.7 kWm−2 is applied to the test tube in all of the experiments. It is shown that boiling heat transfer coefficient and pressure drop of the refrigerant profoundly increases using metal foam. Furthermore, they are increased by an increment in pore density, and in particular, heat transfer coefficient is enhanced up to 220% using 20 PPI metal foam. Evidently, both heat transfer and pressure drop are proportionally varying with mass flux. Although these parameters generally increase with an increment in vapor quality, heat transfer coefficient decrement is observed at high vapor quality and low mass flux values. Based on the test data, and due to the lack of existing correlation to span over the testing conditions in this experiment, two new correlations are proposed. New correlations can predict boiling heat transfer coefficient and two-phase pressure drop of refrigerant in the metal-foam filled annular tubes with good accuracies. • Heat transfer and pressure drop of R134a flow boiling in metal-foam filled annular tubes are studied. • Effects of flow parameters and metal foam properties on flow boiling are investigated. • Heat transfer and pressure drop are profoundly increased by inserting metal foam pieces. • The experimental results are compared to the available correlations. • New correlations are proposed to estimate boiling HTC and two-phase pressure drop in metal-foam filled annular tubes. [ABSTRACT FROM AUTHOR]