The Journal of Science and Engineering Applications (JSEA)

Abstract

This study investigates the effects of air injection on heat transfer performance in helical coil tube heat 
exchangers. Experiments were conducted varying air injection rates (0-50 L/min), injection locations 
(0%, 33%, 66% of coil length), coil diameters (300 mm, 450 mm), and water flow rates (0.1-0.5 kg/s). 
Results show significant heat transfer enhancement with air injection, achieving a maximum Nusselt 
number increase of 84.7% (±4.4%) at 50 L/min air injection. The optimal configuration, balancing heat 
transfer enhancement and pressure drop, occurred at 30 L/min air injection, 33% injection location, and 
300 mm coil diameter, yielding a 62.2% (±3.8%) increase in Nusselt number and a thermal performance 
factor of 1.32. Air injection efficacy was inversely proportional to water flow rate, with the thermal 
performance factor decreasing from 1.45 to 1.18 as water flow increased from 0.1 to 0.5 kg/s. Novel 
correlations for Nusselt number and friction factor were developed, valid for Reynolds numbers 5,000
50,000 and Prandtl numbers 3.5-6.5, with mean absolute errors of 7.2% and 8.5% respectively. For a 10 
kW heat duty, the optimized air-injected system required 38.3% less heat transfer area compared to a 
conventional helical coil heat exchanger, at the cost of 34.9% higher pumping power. These findings 
provide valuable insights for designing and optimizing air-injected helical coil heat exchangers, 
potentially improving energy efficiency in various industrial applications.