In a heat pipe the vapor flows from the evaporator to the condenser, with countercurrent liquid flow from the condenser to the evaporator. The entrainment limit is reached when the vapor velocity in the heat pipe is high enough to shear liquid from the wick. Like the viscous, sonic, and flooding limits, the entrainment 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.
The entrainment limit is given by:
qEntrain Entrainment limit, W
AVapor Heat pipe vapor space area, measured perpendicular to the flow, m2
λfg Latent heat, liquid to vapor, J/kg
σ Surface tension, N/m
ρV Vapor density, kg/m3
rc Wick pore radius, m
As a rule of thumb, always operate with powers less than 0.75 of the entrainment limit.
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