Heat transfer is a critical aspect of industrial processes. Generally, industrial processes rely on fossil fuels as the main source of energy. Due to the environmental impacts of the massive consumption of fossil fuels, several stringent measures have been enacted to promote efficient energy utilization. Among the available fossil fuels, natural gas has been increasingly used in various industrial applications owing to its minimal air pollution effects. Unfortunately, natural gas is non-renewable and requires high energy efficiency to relieve pressure from resource depletion. One way of enhancing energy efficiency is by recovering the sensible and latent heat in the moist air, which is challenging due to the presence of non-condensable gases. Therefore, the development of effective methods for enhancing the condensation process is highly desirable.
To date, several condensation heat transfer enhancement methods have been developed. Among them, using extended surfaces and changing the geometry of the heat exchangers have been identified as the promising solutions for enhancing and maintaining the latent heat transfer owing to their advantages. Extended surfaces come in different shapes and parameters, meaning heat exchangers can have different parameters and shapes. Despite being common and effective, extended surfaces are less reliable specifically due to the disadvantages of welding and interface fitting on a smooth tube – the commonly used manufacturing methods.
Unlike conventional extended surfaces, recent research revealed that the three-dimensional (3D) finned tubes directly machined onto smooth surfaces have no contact thermal resistance and can effectively increase the heat transfer coefficient by breaking the boundary layer near the tube wall. Nevertheless, the condensation heat transfer of moist air outside the 3D fined tubes has not been fully explored. Equipped with this knowledge, researchers at Chongqing University: Mr. Yu-heng Gu (PhD candidate), Professor Qiang Liao, Associate Professor Min Cheng, Professor Yu-dong Ding, and Professor Xun Zhu studied the condensation heat transfer characteristics of moist air outside a 3D finned tube under forced convection. The aim was to provide a reference guideline for the design of a high-efficiency heat exchanger. Their work is currently published in the International Journal of Heat and Mass Transfer.
In their approach, the 3D finned tube had an extra-large extended surface. The authors conducted several experiments under atmospheric pressure conditions to investigate the effects of different parameters: moist air velocity, steam mole fraction, the temperature of the inlet cooling water, and the moist air temperature. The feasibility of the presented approach was validated by comparing the results to those of a smooth tube tested under the same conditions.
Results showed that the 3D finned tube outperformed the smooth tube under all the tested conditions. For instance, it achieved a higher heat transfer coefficient up to 3.1 times higher for the single-phase convective heat transfer and 1.7 times for the moist air condensation heat transfer case at 0.05 steam mole fraction. For both 3D fined and smooth tubes, the heat transfer coefficient increased with an increase in the moist air velocity and steam mole fraction. On the other hand, the heat transfer coefficient decreased with an increase in the inlet cooling water temperature and moist temperature. Furthermore, despite the pronounced condensate retention than the smooth tube, the 3D finned tube still increased the heat transfer coefficient by breaking the flow boundary layer outside the wall.
In summary, the scientists reported enhancing the condensation heat transfer of moist air using 3D fined tubes under different conditions. Despite a lower increase in the heat transfer coefficient than the increase in the heat transfer area, the 3D finned tube still exhibited remarkable results in terms of heat transfer coefficient compared to the smooth tube. In a statement to Advances in Engineering, the authors said their study provided important advances on the influence of 3D fined tube on condensation heat transfer and would provide a guideline for high-efficiency heat exchanger design.
Yu-heng Gu, Qiang Liao, Min Cheng, Yu-dong Ding, & Xun Zhu (2020). Condensation heat transfer characteristics of moist air outside a three-dimensional finned tube. International Journal of Heat and Mass Transfer, 158, 119983.