Air Injection Impact on Thermal Performance of Vertical Tube with Helical Corrugations: Upward Flow

In this experimental work, the 2-phase air-water non-boiling ascending fluid flow in a vertical tube with helical corrugations has been investigated. The results showed that the head loss values decreased with an increase of the volume fraction. Also, by comparing the head loss values for each corrugation pitch, it can be observed that as the corrugation pitch goes down, the head loss values significantly increase. As a result, the intensity of vapors increases perpendicular to the main flow of water, which leads to an increase in the intensity of disturbance in the flow, and then the head loss increases. The Nusselt number goes down when the volume fraction experiences an increment. Looking at the figures related to Nusselt number, it is easy to see that the curves are drawn for a constant air flow rate. Consequently, an increase in volume fraction (VF) is equal to a decrease in the water flow rate. By reducing the water flow rate, the intensity of the main flow is reduced the intensity of turbulence is also reduced and the heat transfer coefficient is reduced. As a result, the amount of heat transfer has increased due to air injection. It should be noted that pipes with the largest corrugation pitch had the best Cost-benefit ratio (C.B.R.) factor values (which means the lowest value). This means that air injection in larger corrugation pitch tubes was more beneficial than in smaller corrugation pitch tubes.