A simplified approach which utilizes an isotropic porous medium model has been widely adopted for modeling the flow through a compact heat exchanger. With respect to situations where the compact heat exchangers are partially installed inside a channel, such as the application of recuperators in an intercooled recuperative engine, the use of an isotropic porous medium model needs to be carefully assessed because the flow passing through the heat exchanger is very complicated. For this purpose, in this study the isotropic porous medium model is assessed together with specific pressure-velocity relationships for flow field modeling inside a rectangular channel with a built-in double-U-shaped tube bundle heat exchanger. Firstly, experiments were conducted using models to investigate the relationship between the pressure drop and the inlet velocity for a specific heat exchanger with different installation angles inside a rectangular channel. Secondly, a series of numerical computations were carried out using the isotropic porous medium model and the pressure-velocity relationship was then modified by introducing correction coefficients empirically. Finally, a three-dimensional (3-D) direct computation was made using a computational fluid dynamics (CFD) method for the comparison of detailed flow fields. The results suggest that the isotropic porous medium model is capable of making precise pressure drop predictions given the reasonable pressure-velocity relationship but is unable to precisely simulate the detailed flow features.