Abstract
The purpose of this article is to show that the limiting case of the Carnot vapor cycle with a temperature difference of only a few degrees between hot and cold reservoirs, can be interpreted as the idealized heat pipe (Carnot heat pipe) for transporting heat, utilizing the boiling condensation processes. The observation of the basic relationship (definition of absolute temperature) Q
1
/ Q
2
= T
1
/ T
2
for Carnot cycle reveals that the Carnot heat engine becomes a heat transfer device when the temperature difference is very small. The Clapeyron—Clausius equation for the infinitesimal (differential) Carnot vapour cycle becomes the genesis for the operating principle of the heat pipe and two-phase thermosyphon. The Carnot heat pipe is independent of the system and working fluid. Within the context of the idealized Carnot heat pipe, it is shown that thermodynamics, fluid mechanics and heat transfer (thermal sciences) are closely coupled in heat pipe operation. The present observation is based on various expositions of classical thermodynamics available in the historical literature. It is of interest to note that the Carnot vapor cycle provides the basic principle for heat pipe and two-phase thermosyphon in addition to the classical cases of heat engine, refrigeration machine and heat pump.