A heat exchanger comprises at least one tube or plate pair defining a fluid flow passage which is reduced in height across a portion of its width. A turbulizer comprising a plurality of rows of convolutions is received inside the fluid flow passage in either the low pressure drop or high pressure drop orientation. The turbulizer includes convolutions of reduced height in order to at least partially fill the reduced-height portions of the fluid flow passage and thereby reduce bypass flow. In some preferred embodiments of the invention, heat exchanger tubes or plate pairs define fluid flow passages which are reduced in height along their edges, and the turbulizer is similarly reduced in height along its edges.