A heat exchanger includes a core defining a first passageway configured for a first fluid to flow through and a second passageway configured for a second fluid to flow through. The core includes a plurality of unit cells coupled together. Each unit cell of the plurality of unit cells includes a sidewall at least partly defining a first passageway portion, a second passageway portion, a plurality of first openings for the first fluid to flow through, and a plurality of second openings for the second fluid to flow through. Each unit cell of the plurality of unit cells is configured to enable the first fluid to combine and divide in the first passageway portion. Each unit cell is further configured to enable the second fluid to combine and divide in the second passageway portion.