The capillary limit relates to the fundamental phenomenon governing
heat pipe operation which is the development of capillary pressure
differences across the liquid-vapor interfaces in the evaporator and
condenser. When the driving capillary pressure is insufficient to
provide adequate liquid flow from the condenser to the evaporator, dry
out of the evaporator wick will occur. Generally, the capillary limit is
the primary maximum heat transport limitation of a heat pipe and is
given by