In a thermosyphon, the vapor flows from the evaporator at the bottom of the thermosyphon to the condenser at the top of the thermosyphon. At the same time, the condensed liquid drains back to the evaporator along the wall (driven by gravity). The flooding limit is reached when the vapor velocity in the thermosyphon is high enough that shear stresses prevent liquid from returning to the evaporator. Like the viscous, sonic, and entrainment limits, the flooding limit is related to the vapor velocity, and is more significant at lower temperatures. The reason is that the vapor pressure and vapor density decrease as the temperature is lowered, so the vapor velocity must increase to carry the same power.
One flooding correlation that is often used was developed by Faghri.
The first step is to define the Bond number, a dimensional number that measures the importance of surface tension forces versus body forces:
ID Internal Diameter, m
ρL Liquid density, kg/m3
ρV Vapor density, kg/m3
g gravity or acceleration, m/s2
σ Surface tension, N/m
The flooding constant, KFlooding, is defined as:
The Flooding Limit, qFlooding, is then:
AVapor Heat pipe vapor space area, measured perpendicular to the flow, m2
λfg Latent heat, liquid to vapor, J/kg
θ Tilt from vertical (valid for small tilts)
As a rule of thumb, always operate with powers less than 0.75 of the flooding limit.
Note that loop thermosyphons are sometimes used to eliminate the flooding limit, since the vapor and liquid flows are separated from each other.
Return to Heat Pipe Limits
 A. Faghri, Heat Pipe Science and Technology, CRC Press, pp. 387-397, 1995.