Optimisation and evaluation of NTU and effectiveness of a helical coil tube heat exchanger with air injection

•Thermal performance of helical coil heat exchanger was experimentally considered.•Effect of air bubble injection on the thermal performance of HEX was investigated.•NTU and effectiveness were intensively studied under wide range of Qa,Qs,QcandΔT.•NTU and effectiveness significantly increase with air injection.•Air injection and shell flow rates were optimised.