4. Operating limits

The operation of heat pipes and thermosyphons is based on the coexistence, in conditions close to saturation, of the vapor and liquid phases of a heat transfer fluid. A first obvious operating condition is the fluid temperature range between the triple point and the critical point (figure 10 ). In addition, the strong evolution of the fluid's thermophysical properties with temperature leads to the appearance of phenomena blocking the transferred flux at a maximum value. We present these operating limits in order of appearance as the operating temperature rises.