Heat Transfer Performance of Finless Heat Exchanger Using Airfoil-shaped Tubes with Extended Leading or Trailing Edge Section
Performance improvement of heat exchangers realizes the energy saving. Therefore, achievement of high heat transfer performance is a key issue in the design of compact heat exchangers that are widely used in many practical applications. Until now, various studies have been made for improvement of the air-side heat transfer performance of the typical finned tube heat exchangers as the air-side thermal resistance generally comprises over 70-90% of the total thermal resistance. However, as it is necessary to make good use of novel ideas in order to find a clue to design much higher-performance heat exchangers, our research group has been proposing finless tube heat exchangers.
In the previous study, numerical analysis was made for one unit of proposed two-row finless symmetric airfoil-shaped tube heat exchanger, then the heat transfer performance of this case was compared with that of the flat tube and the asymmetric airfoil-shaped tube case. The numerical results showed that the heat transfer performance becomes much higher for the symmetric airfoil-shaped tube case compared to the flat tube or the asymmetric airfoil-shaped tube case when the tube streamwise pitch is decreased.
In this study, in order to improve the heat transfer performance of symmetric airfoil-shaped tube heat exchanger, a two dimensional numerical simulation is performed for estimating the pressure drop penalty and the heat transfer performance of a proposed symmetric airfoil-shaped tube heat exchanger with extended leading or trailing edge section in order to increase heat transfer area per unit volume. The effects of Reynolds number and geometrical parameters such as streamwise tube pitch and spanwise tube pitch are systematically examined and the obtained results are compared with those of the airfoil-shaped tube heat exchanger without extended sections. The numerical result of the flow field shows that the extended trailing edge section plays an important role in suppression of the deflection flow which induces high pressure loss. From the view point of heat transfer, the present case shows better performance in terms of heat transfer rate per unit temperature difference because of extending heat transfer area. As a result, the present cases show better heat transfer performance than the cases of airfoil-shaped tube without extended sections, when they are evaluated in terms of heat transfer rate per unit temperature difference and unit volume at the same pumping power per unit volume.