The plate-fin heat exchanger includes a first fluid pathway running along a first axis, a second fluid pathway running along a second axis perpendicular to the first axis, and a blocker bar having an inlet face and an outlet face. The blocker bar inlet face coincides with a heat exchanger face. The blocker bar is at an inlet of the second fluid pathway and receives a second fluid. The outlet face of the blocker bar is at the inlet of the second fluid pathway. The blocker bar includes a set of spaced apart pores that extend from the inlet face to the outlet face.