New phenomena have been observed experimentally in a novel rapid mixing process, where a confined mixing layer in a pipe is periodically forced, especially under strong forcing. In the confined configuration without forcing, there are two incommensurable peaks in the scalar power spectrum: one corresponds to the well-known Kelvin-Helmholtz instability, but the other is not known, having a narrow-frequency band around a nearly constant frequency that is independent of the mean flow velocity. With strong forcing within the narrow-frequency band, the spreading angle of the mixing layer is not restrained by the Well-known saturation phenomenon, but could even achieve a quasi-180 degrees immediately downstream of the trailing edge, exhibiting all extraordinarily fast mixing. The mixing layer not only biases to the low-speed side as the conventional mixing layer does, it could also be symmetric to the splitterplate and even bias to the high-speed side. These phenomena can be observed only when the forcing is within the narrow-frequency band. We also find that the streamwise vortices, which result from interaction between the primary vorticity and the vorticity originating from the streamwise corner flow between the splitterplate and sidewall immediately downstream of the trailing edge of the splitterplate, could play an important role for these new observations. These streamwise vortices are very sensitive to initial periodic forcing at the narrow-frequency band and amplify very fast. The results indicate that the confined mixing layer in a pipe is a very interesting flow. (c) 2005 American Institute of Chemical Engineers AIChE J, 52: 111-124, 2006.