The work is focused on optimizing a counter-current heat exchanger under laminar conditions using von Kármán vortices. Cylindrical obstacles placed in the hot channel generate vortices to mix the fluid and enhance the heat transfer between the hot and cold fluids. Numerical analysis is aimed at examining the influence of cylinders position and their diameter on the value of local and global heat transfer coefficient. Simulations are performed using one cylinder at the center of the hot channel, then two cylinders placed diagonally. The results show that increasing the diameter of the cylinder placed at the center of the cavity leads to an enhancement in heat transfer. However, it also results in a proportional increase in pressure drop. The results also demonstrate that it is preferable to position two small cylinders diagonally and closer to the walls to promote the mixing of the cold fluid within the thermal boundary layer with the hot fluid at the center of the cavity. Additionally, the horizontal gap between the two cylinders has a significant role in terms of local heat transfer. The geometry, therefore, should depend on the desired objective: whether one aims to increase heat transfer at a specific location or to enhance overall heat transfer along the heat exchanger.