Liquid motion in the wick depends on the dynamic

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: he tube by extruding axial grooves along its inner surface, but this approach presents manufacturing difficulties. The performance of a wick depends on its structure. The characteristics of a wick can be changed by changing the size and the number of the pores per unit volume and the continuity of the passageway. Liquid motion in the wick depends on the dynamic balance between two opposing effects: the capillary pressure, which creates the suction effect to draw the liquid, and the internal resistance to flow as a result of friction between the mesh surfaces and the liquid. A small pore size increases the capillary action, since the capillary pressure is inversely proportional to the effective capillary radius of the mesh. But decreasing the pore size and thus the capillary radius also increases the friction force opposing the motion. Therefore, the core size of the mesh should be reduced so long as the increase in capillary force is greater than the increase in the friction force. Note that the optimum pore size will be different for different fluids and different orientations of the heat pipe. An improperly designed wick will result in an inadequate liquid supply and eventual failure of the heat pipe. Capillary action permits the heat pipe to operate in any orientation in a gravity field. However, the performance of a heat pipe will be best when the capillary and gravity forces act in the same direction (evaporator end down) and will be worst when these two forces act in opposite directions (evaporator end up). Gravity does not affect the capillary force when the heat pipe is in the horizontal position. The heat removal capacity of a horizontal heat pipe can be doubled by installing it vertically with evaporator end down so that gravity helps the capillary action. In the opposite case, vertical orientation with evaporator end up, the performance declines considerably relative the horizontal case since the capillary force in this case must work against the gravity force. Most heat pipes are cylindrical in shape. However, they can be manufactured in a variety of shapes involving 90° bends, S-tur...
View Full Document

This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

Ask a homework question - tutors are online