The fluid mechanics and heat transfer of zero-mean oscillatory flows at low frequencies and large amplitudes in essentially two-dimensional short channels are studied numerically. The results show that intracycle oscillations do also occur, and that such oscillations are different from those for long ducts. In the case of sudden pressure changes as the flow reverses at the ends, the intracycle oscillations are found to occur in the acceleration phase of the oscillatory cycle and are responsible for the heat transfer enhancement. These results compare favorably with previously obtained experimental data.