Axial heat transfer rate can be limited by a number of factors (Figure 1):

• Viscous limit: at the low pressures the friction losses in vapor channel can limit the circulating fluid flow rate;
• Sonic limit: the velocity of vapor cannot exceed the sonic or choking velocities;
• Cooling limit: the capacity of the heat sink to provide effective cooling of the condenser is often a major heat pipe heat transfer limitation, which depends on number conditions (variant of heat removal from condenser by radiation is shown in Figure 1);
• Entrainment limit: entrainment of liquid by the vapor must be avoided, or induced shear stress can slow down liquid back flow and also stimulate the transfer of entrained liquid droplets in the vapor to the condenser which can lead to evaporator dry-out;
• Capillary limit: every wick has certain pumping capacity threshold for specified liquid and heat pipe design (see Section 311);
• Boiling limit: if the liquid superheat in the evaporator part of the wick exceeds that required for incipient nucleation of the liquid then boiling process can result in the formation of large vapor bubbles in the wick that block liquid circulation.

Figure 1 Operation domain: maximum heat transfer rate as a function of operating temperature. Key: 1, viscous limit; 2, sonic limit; 3, cooling limit; 4, entrainment limit; 5, capillary limit; 6, boiling limit

Figure 1 illustrates diagrammatically how these six factors combine to give the operational domain for a given design of heat pipe. In practice the heat pipe heat transfer capacity in the operational range generally is bounded by the capillary or/and cooling limits (especially for high temperature heat pipes). However, the viscous, sonic and entrainment limits are often encountered during the startup from the liquid or frozen state. For detail analysis and analytical representations of the limits the reader is referred to Faghri (1995), Chi (1976), Brennan and Kroliczek (1979), Reay (2006), Peterson (1994), Silverstein (1992), Ivanovskii et al. (1982).