Frontal Air Velocity Research Articles

Numerical Analyses of Effects of Tube Shape on Performance of a Finned Tube Heat Exchanger

Th is article discusses the eff ects of tube geometry on the performance of a multi-row fi nned tube heat exchanger having herringbone wavy fi ns. Th e air-side heat transfer and pressure drop characteristics of the heat exchanger were predicted numerically and the tube-side heat transfer coeffi cient and pressure drop were calculated using commonly adopted equations for turbulent fl ow in a smooth tube. Th e numerical calculations were performed in three dimensions for three frontal air velocities of 1.0, 2.0, and 3.0 m/s, yielding hydraulic diameter Reynolds numbers from 297 to 999. Investigated were fi ve tube geometries, including a round tube, three elliptical oval tubes, and a fl at oval tube. Th e round tube has a 15.875 mm (⅝ ) outside diameter and serves as the baseline tube. Th e four oval tubes were made by reforming the baseline round tube and have the same perimeter as that of the round tube. Th e aspect ratios of the three elliptical tubes are 2.00, 3.00, and 4.29, respectively, and the aspect ratio of the fl at oval tube is 3.00. As the tube aspect ratio is increased, the air-side heat transfer coeffi cient and pressure drop decrease, but the water-side heat transfer coeffi cient and pressure drop increase. At 2.0 m/s frontal velocity, the 3.00 aspect ratio elliptical tube yields a 6.9% lower air-side heat transfer coeffi cient and a 45.9% lower air pressure drop than the round tube. As compared to the 3.00 aspect ratio elliptical tube, the same aspect ratio fl at tube has a 0.6% higher air-side heat transfer coeffi cient and a 2.5% higher air pressure drop.

저속 영역에서 루버휜이 장착된 평판관형 알루미늄 열교환기의 공기측 전열 성능에 대한 실험적 연구

The heat transfer and pressure drop characteristics of heat exchangers with louver fins were experimentally investigated. The samples had small fin pitches (1.0 mm to 1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low as 0.3 m/s). At a certain Reynolds number (critical Reynolds number), the flattening of the heat transfer coefficient curve was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch ratio (L_p/F_p) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. It is suggested that, for proper assessment of the heat transfer behavior, the louver pattern in addition to the flow characterization need to be considered. The heat transfer coefficient increased as the fin pitch decreased. At low Reynolds numbers, however, the trend was reversed. Possible explanation is provided considering the louver pattern between neighboring fins. Different from the heat transfer coefficient, the friction factor did not show the flattening characteristic. The reason may be attributed to the form drag by louvers, which offsets the decreased skin friction at a low Reynolds number. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted 92 % of the heat transfer coefficient an d 90 % of the friction factor within ± 10%.

Numerical Predictions of Wavy Fin Coil Performance

The wavy fin-and-tube heat exchanger is widely used in the air-conditioning industry. Hence, the ability of numerical codes to predict the thermo-hydraulic performance of this heat exchanger geometry is of considerable interest. Here, a numerical study has been conducted to predict the air-side heat transfer and pressure drop characteristics of a 2-row finned tube heat exchanger having a herringbone wavy fin geometry. The calculations were performed using the FLUENT CFD code. The calculations were performed in three dimensions and account for conduction within the fins. The computational domain was chosen to cover the entire fin length in the air flow direction. The calculations are implemented for the frontal air velocities between 1.0 and 3.0 m/s yielding hydraulic diameter Reynolds number from 380 to 1,150. The uniqueness of this work is inclusion of the tubes in the wavy channel geometry, and use of a three-dimensional (3D) calculation model. The flow and thermal fields in this finned tube heat exchanger as well as the temperature distribution of the fin are presented and described. The fin efficiency is numerically determined and compared with that given by the Schmidt equation (1966) and found to agree within {+-}2.3%. Predictions were performed for a herringbone channelmore » without and with tubes. The tubes considerably increase the pressure drop, but the effect on the average heat transfer coefficient is much smaller. The calculation results are compared with test data taken by Kang and Webb (1997) for a test sample having the same geometry. The FLUENT over predicts the air-side heat transfer coefficient by 13% and pressure drop by 31%.« less

Heat transfer and friction characteristics of fin-and-tube heat exchangers

An experimental study has been carried out to investigate the heat transfer and pressure drop characteristics of fin-and-tube exchangers with plate, wavy and louvered fin surfaces. In all, 36 samples of heat exchangers, including 12 plate-fin, 12 wavy-fin and 12 louvered-fin geometries, were tested. Results are presented as plots of friction factor f and Colburn j factor against Reynolds number in the range 300–2000. Additionally, the dimensional heat transfer coefficient and pressure drop are also presented against frontal air velocity. Finally, various comparison methods were adopted to evaluate the air side performance of the plate, wavy and louver fin heat exchangers.

Performance of Dehumidifying Heat Exchangers With and Without Wetting Coatings

Limited previous work has shown that use of special hydrophilic coatings will provide lower air pressure drop in finned tube heat exchangers operated under dehumidifying conditions. However, no detailed work has been reported on the effect of different coating types, or different fin surface geometries on the wet pressure drop. In this study, wind tunnel tests were performed on three different fin geometries (wavy, lanced, and louver) under wet and dry conditions. All dehumidification tests were done for fully wet surface conditions. For each geometry, the tests were performed on uncoated and coated heat exchangers. For all three fin geometries, the wet-to-dry pressure drop ratio was 1.2 at 2.5 m/s frontal air velocity. The coatings have no influence on the wet or dry heat transfer coefficient. However, the wet surface heat transfer coefficient was 10 to 30 percent less than the dry heat transfer coefficient, depending on the particular fin geometry. The effect of the fin press oil on wet pressure drop was also studied. If the oil contains a surfactant, good temporary wetting can be obtained on an uncoated surface; however, this effect is quickly degraded as the oil is washed from the surface during wet operation. This work also provides a critical assessment of data reduction methods for wet surface operation, including calculation of the fin efficiency.