In this article, a transient performance of a plate-fin cross-flow heat exchanger with convergent-divergent longitudinal vortex generators is investigated. The effect of flow geometry is taken into account to analyze the transient forced convection heat transfer in a designed heat exchanger. The time-dependent Nusselt number and dissipation energy criterion are experimentally measured in 4- and 8-kW heater powers for various Reynolds numbers between 42,000 and 60,000 for the hot and cold fluids. In order to present the quality of the heat exchanger, the general empirical equations of the time-dependent Nusselt number and friction factor were derived as a function of the Reynolds number corresponding to fin geometry parameters. Following this, the transient behavior of the heat exchanger according to the change in the inlet and outlet temperatures of the hot and cold fluids was analyzed. The results showed that the variations of the time-dependent dissipation energy criterion increase with the increase in the Reynolds number. The appropriate correlations are proposed to predict the heat transfer and friction characteristics of the transient performance for the presented configuration, which indicates the designed heat exchanger has good heat conduction.