Active generative cooling based on supercritical hydrocarbon fuel is regarded as one of the most promising methods for scramjet thermal management. A large eddy simulation method is used to explore the transient change law of the initial flow， and heat transfer behaviors of supercritical hydrocarbon fuel is in rectangular channels. An oscillation effect can be proved in the convective heat transfer process， i.e.， the temperature and velocity distributions fluctuate strongly. By monitoring the velocity variation rule of 10^-4 s time scale， the approximate trigonometric frequency and amplitude are present in this oscillation. At the initial heating condition， the high-temperature fluid， which is close to the heated wall， flows into the low-temperature region under the effect of buoyancy force. As the fluid temperature gradually reaches the pseudo-critical value， its thermo-physical properties change dramatically. The friction factor oscillates as well as the fluid kinetic energy. Correspondingly， the enhanced vortex is induced away from the heated wall. Over time， the enhanced vortex begins to move to the heated wall and the low-temperature fluid impinges on the wall. Consequently， the thermal transport is enhanced.