A common tool for the determination of the thermal characteristics of fin-and-tube heat exchangers is the experimental testing. However, experimental testing is not feasible considering the cost and the labor-time. One alternative to the experimental testing is the utilization of Computational Fluid Dynamics (CFD) to predict the thermal characteristics of these kinds of radiators. However, CFD models are also not suitable to be used as a design tool since the considerably amount of computational power and the computational time required due to the complex geometric structures of the fins. This issue becomes problematic when the large-scale heavy-duty radiators are considered. One solution for this issue is to utilize a reduced model to simulate the airflow within the fin structures based on porous flow. In this study, a methodology to model the thermal characteristics of a radiator is presented. A porous flow based reduced model is developed to model the airflow within the fin structure. Utilizing the reduced model, a 3-D CFD analysis of a radiator with straight fins is performed. Both the pressure drop and the temperature of the cold fluid, which is air, and the hot fluid, which is water, are determined. It is observed that by the introduction of the reduced model, the computational time and the computational power required decreases drastically, which enables this CFD model as a design tool for radiator design.