An example apparatus is disclosed that includes a base and a wickless capillary driven constrained vapor bubble heat pipe carried by the base. The wickless capillary driven constrained vapor bubble heat pipe includes a capillary, and the capillary has a longitudinal axis and a cross-sectional shape orthogonal to the longitudinal axis. The cross-sectional shape includes a first curved wall, a second curved wall, a first corner between a first straight wall and a second straight wall, and a second corner between a third straight wall and a fourth straight wall.