In this paper, a porous media based computational fluid dynamic (CFD) model has been developed for the topology enhancement of a plate fin heat exchangers. The huge development in metal additive manufacturing has allowed topology optimization to find its way into engineering applications especially for optimizing mechanical structures. Using topology optimization in 3d heat and mass transfer problems requires huge computational time; therefore coupling it with CFD simulations can reduce this. However, existing CFD models cannot be coupled with topology optimization. The CFD model must make it possible to create a uniform mesh despite the initial geometry complexity and also to swap the cells from fluid to solid and vice versa. In this paper, a porous media approach compatible with topology optimization criteria is developed. It consists of modeling the fluid and solid regions in the heat exchangers as porous medium having consecutively high and low porosity. The switching from fluid to solid cells required by topology optimization is simply done by changing cell porosity using a user defined function. This model is tested on a plate and fin heat exchanger and validated by comparing its results to experimental data and simulations results. Furthermore, this model is used to perform a material reallocation based on local criteria to enhance a plate and fin heat exchanger. The enhanced fin uses 20% less materials than the first while the pressure drop is reduced by about 13% for the same heat duty.