Experimental and Computational Fluid Dynamics Studies on Straight and U-Bend Double Tube Heat Exchangers with Active and Passive Enhancement Methods

In this work, the authors wanted to demonstrate the possibility to increase the heat transfer efficiency by using simple wire coil inserts to create turbulent flow in the boundary layer as well as air blowing into the annulus of the pipe. Experimental investigations were carried out for four heat exchanger constructions, i.e., plain double tube, turbulized double tube, plain U-bend double tube, U-tube with turbulator, plain double tube and U-bend double tube working with two-phase flow conditions (air-water mixture). For these geometries, the values of pressure drop and heat flux were determined. The comparison of heat transfer efficiency was performed based on NTU-epsilon method. The computational fluid dynamics (CFD) technique was also employed to investigate the influence of coiled wire inserts and air bubbly injection at heat transfer, pressure drops and overall efficiency in double tube heat-exchangers. For this purpose, six configurations of heat exchangers were meshed and simulated at various flow conditions by using the CFD package Autodesk CFD 2016 (Students & Educators version).