TABLE OF CONTENTS In this section, the theory underpinning the evaluation of the terms in equations 2.1 and 2.2 and the determination of the operating limits shown in Fig. 2.1 is discussed.
The pressure difference, ?P g, due to the hydrostatic head of liquid may be positive, negative or zero, depending on the relative positions of the condenser and evaporator. The pressure difference may be determined from:
where ? l is the liquid density (kg/m 3), g the acceleration due to gravity (9.81 m/s 2), l the heat pipe length (m), and ? the angle between the heat pipe and the horizontal ( ? is positive when the condenser is lower than the evaporator).
Molecules in a liquid attract one another. A molecule in a liquid will be attracted by the molecules surrounding it and, on average, a molecule in the bulk of the fluid will not experience any resultant force. In the case of a molecule at or near the surface of a liquid, the forces of attraction will no longer balance out and the molecule will experience a resultant force inwards. Because of this effect, the liquid will tend to take up a shape having minimum surface area, in the case of a free falling drop in a vacuum this would be a sphere. Due to this spontaneous tendency to contract, a liquid surface behaves rather like a rubber membrane...
