Performance assessment of passive enhancement methods during in-tube condensation and heat pipe applications

Abstract

To understand the fundamental of flow condensation phenomenon and enhance its performance, a thorough study on the liquid-vapor phase change heat transfer is targeted by means of macroscale level experiments and numerical simulations. An application-based investigation to understand the interfacial physics which relates with the thermal performance during micro pulsating heat pipe operations is also targeted in the present work. At first, a proposal for enhancement of flow condensation is made by developing tube with discrete internal structure. Mass flux from 75 to 215 kg m-2s-1, vapor quality from 0.1 to 0.9 and condensing temperature from 35 °C to 40 °C has been considered as the experimental range using R134a refrigerant in an in-house experimental set up. The heat transfer characteristics of the newly developed tubes with internal micropillar (MP) tube are compared with the two different commercially available microfin (MF1 and MF2) tubes and smooth (SM) tube. The overall performance is calculated using performance evaluation factor (PEF) which is defined to evaluate the thermal-hydraulic performance of the tubes. Using the same, it has been observed that the newly developed MP tube has a PEF value greater than unity which suggest augmentation of heat transfer coefficient at the expense of lesser pressure drop penalty. Comparison of the flow patterns of the refrigerant inside the newly developed tube has been made to confirm the fluidic facts behind enhancement. With a target of understanding the interfacial dynamics in flow condensation, Volume of Fluid (VOF) based three dimensional numerical simulations have been performed with R134a refrigerant flowing inside horizontal smooth and enhanced tubes. At first, the hemispherical rib structure on the circumferential surface of the tube with square pitch variations of 2.5 mm, 4 mm and 5 mm is simulated and compared with the smooth tube. Liquid vapor interfaces at different axial and longitudinal locations for both smooth and hemispherical rib tube are presented to better understand the condensation process. The results indicate that the vapor quality and liquid film thickness inside structured tube are lower than the same observed inside smooth tube. Also, the average heat transfer coefficient and pressure drop for both smooth and enhanced tubes are reported and interestingly, the tube with a 2.5 mm square pitch has shown twice the heat transfer coefficient with minimal increase in the pressure drop when compared with the smooth tube. Further, considering the optimum pitch as 2.5 mm, the numerical simulation has also been performed to investigate the effect of various structures on the thermo-fluidic characteristics during the flow condensation of representative R134a refrigerant. Along with a perfectly smooth surface, four different surface structures, i.e., hemispherical ribs, conical fins, axial, and circumferential continuous protrusions (tunnels and huddles) on the inner surface of the tube are tried to understand the heat transfer enhancement mechanism. The effect of structures on the flow behaviour is analysed, and the presence of directional condensate drainage near the protrusions is observed. The qualitative and quantitative examinations of interfacial structures at different axial and longitudinal sections are also presented to better understand the distinctive condensation phenomenon for smooth and enhanced tubes. The spatial and time-averaged vapor fraction and liquid film thickness show lower values in the case of enhanced tubes compared to the smooth surface for all tested operating conditions. Furthermore, the hemispherical rib structure showed the highest heat transfer coefficient among the tested structures, whereas a tube with circumferential protrusions (huddles) results in maximum pressure drop during flow condensation. The benefits of heat transfer enhancement appear to be more than the pressure drop penalty for tubes with a conical fin structure and axial tunnels.