Forced convection boiling occurs when a fluid undergoing a phase change from liquid to vapor is forced to move over or inside a heated surface by an external force like a pump. It exhibits characteristics of both convection and pool boiling. Internal forced convection boiling, also called two-phase flow, is more complex as both the liquid and vapor phases flow together inside the heated surface with no free space for the vapor to escape. The flow patterns within can range from a total liquid flow to different vapor-liquid mixtures to a total vapor flow, depending on the relative amounts of each phase present.