  We introduce the planar helical flows on three dimensional torus and study
the dissipation enhancement of such flows. We then use such flows as transport
flows to solve the three dimensional advective Kuramoto-Sivashinsky and
Keller-Segel equations. The global well-posedness of the Kuramoto-Sivashinsky
equation is achieved when the linearized operator does not have growing mode in
the direction orthogonal to the flow. The global classical solution of the
three dimensional Keller-Segel is ensured for any size of the torus with
arbitrarily large initial data.
