An experimental apparatus was designed, built and used to study the behavior of transient forced convection in a rectangular duct with and without barriers due to a sinusoidal heat input at the inlet. to simulate the electric heating and cooling inside the computer. A wide range of Reynolds number (400 less-than-or-equal-to Re less-than-or-equal-to 20,000) and inlet frequencies (0.01 Hz less-than-or-equal-to beta less-than-or-equal-to 0.08 Hz) was covered in this experimental study for both laminar and turbulent flow. The experiments are carried out in the thermal entrance region with the fully developed velocity profile. The temperature variation at several locations along the duct was measured and recorded, and the experiment results showed that the oscillation at different locations are the sine variation with the same period as the heat input, while its amplitude decayed exponentially along the duct. The decay indexes are obtained from the experimental investigation under different conditions and presented in tabular and graphical forms.