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Beth Neel

Flow Boiling Heat Transfer Characteristics in Horizontal, Three-Dimensional Enhanced Tubes

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Publication date
2019/1
Journal
Volume
12
Issue
5
Pages
927
Publisher
Multidisciplinary Digital Publishing Institute
Description
An experimental investigation was conducted to explore the flow boiling heat transfer characteristics of refrigerants R134A and R410A inside a smooth tube, as well as inside two newly developed surface-enhanced tubes. The internal surface structures of the two enhanced tubes are comprised of protrusions/dimples and petal-shaped bumps/cavities. The equivalent inner diameter of all tested tubes is 11.5 mm, and the tube length is 2 m. The experimental test conditions included saturation temperatures of 6 C and 10 C; mass velocities ranging from 70 to 200 kg/(m 2 s); and heat fluxes ranging from 10 to 35 kW/m 2, with inlet and outlet vapor quality of 0.2 and 0.8. It was observed that the enhanced tubes exhibit excellent flow boiling heat transfer performance. This can be attributed to the complex surface patterns of dimples and petal arrays that increase the active heat transfer area; in addition, more nucleation sites are produced, and there is also an increased interfacial turbulence. Results showed that the boiling heat transfer coefficient of the enhanced surface tubes was 1.15–1.66 times that of the smooth tubing. Also, effects of the flow pattern and saturated temperature are discussed. Finally, a comparison of several existing flow boiling heat transfer models using the data from the current study is presented.
Beth Neel

Investigation on Flow Condensation of Refrigerant in Annulus of Smooth and Enhanced Tube-in-Tube Heat Exchanger

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Publication date
2019/1/9
Volume
55
Issue
1
Pages
223-234
Publisher
Springer Berlin Heidelberg

An experimental investigation of R22 and R410A condensation outside a horizontal smooth tube, a herringbone tube and a newly developed 3D enhanced heat transfer (1EHT) dimple tube has been conducted. The herringbone tube has a fin root diameter of 11.43 mm, a helical angle of 21.3 °, 48 fins with a fin height of 0.262 mm and an apex angle of 36 °; the 1EHT tube has an inner diameter of 11.5 mm with a dimple enhancement; while the smooth tube has an inner diameter of 11.43 mm; and all the tubes have an outer diameter of 12.7 mm. Experiments were performed for a constant saturation temperature of 45°C; with a constant inlet vapor quality of 0.8 and a constant outlet vapor quality of 0.1; for a mass flux ranging from 5 kg/(m2 s) to 250 kg/(m2 s). In addition, annular side condensation experiments were performed using an outer shell tube with outer diameters of 17 mm and 25.4 mm. Heat transfer performance varied with mass flux. At a low mass flux the enhanced dimple tube had the smallest heat transfer coefficient; while at higher values of mass flux, the smooth tube had the smallest heat transfer coefficient. Finally, the effect of average vapor quality on the heat transfer coefficient was also investigated. Characteristic analysis was performed in order to account for the various phenomena found in this series of experiments. Annular side heat transfer performance combined with pressure drop measurements reveal that the herringbone tube generally had a better heat transfer performance than the other tubes, and can be a good choice for use in annular side condensation applications.

Beth Neel

Condensation Heat Transfer of R410A on Outside of Horizontal Smooth and Three-Dimensional Enhanced Tubes

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Publication date
2019/2/1
Journal

Volume
98
Pages
1-14

Experimental investigations of flow condensation on the outside surface of one smooth tube and two enhanced surface tubes with the same outer diameter of 12.7 mm were conducted. The refrigerant flowed in an annular space between the test tube and the outer tube. The newly developed 2EHT three-dimensional surfaces have multiple enhancement patterns made up of staggered petal arrays and parallel wavy channels. Tests were performed using refrigerant R410A over a mass flux range of 75–250 kg m−2 s−1, at average saturation temperatures ranging from 35 to 45°C. For the smooth tube, the external heat transfer coefficient decreases first and then flattens out gradually at low mass fluxes. Relative significance of inertia force might be the major reason. New prediction correlations for smooth tubes based on the Nusselt’s theory were presented, having all data points within ± 4% error band. The effects of saturation temperature, mass flux and vapor quality on heat transfer coefficients for all the tested tubes were analyzed. In addition, the condensation heat transfer coefficient ratio of the 2EHT-2 tube to a smooth tube is only 0.86–0.94 for mass fluxes ranging from 100 to 250 kg m−2 s−1. This phenomenon can be explained by the retention of condensate and surface tension effects. New correlations for film condensation in the annular channel outside the 2EHT tubes were also developed, showing a good agreement with experimental data.

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