Abstract: According to various aspects, exemplary embodiments are provided of heat exchangers and applications. In an exemplary embodiment, a heat exchanger can include at least a first feature array and a second feature array, a channel in an interior of the heat exchanger, through which a fluid can flov, an inlet for the fluid to enter the channel, an outlet for the fluid to exit the channel. The channel may include at least one surface and said first feature array and said second feature array are positioned on the at least one surface of the channel, the fluid configured to flow from said inlet, through said channel to said outlet, and the first and second feature arrays have different densities.