A heat exchanger is provided that can include furcating unit cells coupled with each other. Each of the unit cells can be elongated along an axis and include a sidewall that defines annular ring openings on opposite ends of the unit cell along the axis. The sidewall also can define undulating annular rings between the annular ring openings and axially separated from each other along the axis. The sidewall can further define angled openings into the unit cell both above and below each of the undulating annular rings. At least a first opening of the annular ring openings and the angled openings can be configured to be an inlet to receive a first fluid into the unit cell and at least a second opening of the annular ring openings and the angled openings configured to be an outlet through which the first fluid exits the unit cell.