Study of R134A Boiling Heat Transfer in a Pool Outside Two Bilaterally Enhanced Tubes

The heat transfer tube is widely regarded as the most critical component in heat exchangers, playing a pivotal role in determining the overall efficiency of thermal systems. Enhancing the pool boiling heat transfer performance of these tubes is, therefore, of utmost importance for improving the overall heat exchange process. In this study, to achieve enhanced thermal performance, two types of bilaterally enhanced tubes (designated as E1 and E2 tubes) with different surface structures were designed and fabricated. These tubes were specifically engineered to improve the heat transfer characteristics under pool boiling conditions. The design of these tubes incorporates a threaded structure on the inner surface, which serves to disrupt the boundary layer and enhance the fluid mixing inside the tube. Meanwhile, the outer surface of the tubes is equipped with a complex array of fins, designed to significantly increase the surface area available for heat transfer. To evaluate the effectiveness of these enhancements, heat transfer experiments were conducted on both types of bilaterally enhanced tubes under pool boiling conditions. Additionally, the experimental data obtained from these enhanced tubes were compared with that of a smooth tube, which was made of the same material and had the same diameter, in order to assess the performance improvements. In the experiments, R134a was used as the working fluid, and the tests were performed under saturated temperature conditions of 6 °C. The refrigerant R134a flowed on the external surface of the heat exchange tubes, while deionized water circulated inside the tubes. Under the condition of constant heat flux, the heat flux density was maintained at 30 kW/m2, and the water-side flow velocity ranged from 1 to 3 m/s. Under the condition of constant water-side inlet temperature, the water inlet temperature was controlled at 12°C. The results show that the overall heat transfer coefficient and boiling heat transfer coefficient on the refrigerant side of the bilaterally enhanced tubes can reach up to five times that of the smooth tube. The complex fin array on the outer surface of the enhanced tubes significantly increased the effective heat transfer area and the number of vaporization cores, promoting bubble growth and timely detachment, which in turn facilitated efficient heat transfer. Three different heat transfer coefficient correlations were selected to predict the heat transfer coefficient for the smooth tube. The most accurate boiling heat transfer coefficient correlation was identified and then modified using the experimental data from the enhanced tubes in this study. This resulted in a new predictive correlation for the boiling heat transfer coefficient applicable to the novel bilaterally enhanced tubes studied here. The modified correlation can predict over 90% of the heat transfer coefficient data points within an error range of ±20%.