ABSTRACT
This chapter examines the fluid flow process in a heat pipe. The two-dimensional nature of the flow process is emphasized, and one-dimensional approximations suitable for design purposes are discussed. Flow in a heat pipe is a rather complex process in which the liquid and vapor phases move in counterflow to each other. Although the liquid and vapor phases are nominally saturated at their common interface, they may be superheated or subcooled at locations remote from the interface. Frictional resistance at flow boundaries produces a pressure drop in both the liquid and the vapor streams. The addition of mass to a flow stream increases the rate of momentum flow, causing a pressure drop in addition to that resulting from friction. These molecules then collide with adjacent liquid molecules, eventually assuming the direction and velocity of the bulk liquid flow.
