In this experimental investigation, the heat transfer and pressure drop of helical tubes with various helical diameters have been studied considering air injections. The tube was rested in vertical form and was put under constant heat flux. The flow had a downward form and the air was injected into the water stream outside the helical tube. According to the achievements, air injection has a notable impact on the heat transfer coefficient of each helical tube. The results showed that employing air bubbles could increase the Nusselt Number by up to 14%. To make an acceptable comparison among all states, the Cost Benefit Ratio (C.B.R.) factor was evaluated. The results showed that pipes with a bigger diameter had the best C.B.R factor values. It means that the air injection in the tubes with larger diameters was more beneficial than tubes with smaller helix diameters. The best value was attained for helix diameter of 18 cm and VF of 0.33 with a C.B.R factor of 0.84. Also, the worst value was 1.18 for a helix diameter of 10 cm.
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