Design Parameter for Assessing Wicking Capabilities of Heat Pipes

Abstract

The increasing use of heat pipes as efficient thermal power-transfer devices for the thermal control of highpower communication satellites suggests that there is a need for some parametric index to compare the thermal characteristics of different heat pipe structures. It is shown that the maximum heat-transfer rate sustainable by a heat pipe with a given working fluid is proportional to a wick parameter N w , which is a function of the wick geometry and structural design alone. Expressions for N w are derived and numerical values are tabulated for various wick structures used in heat pipe applications. Validity of this parameter is confirmed by comparison with over 70 measurements gathered from various sources. It is also shown that N w may be evaluated for heat pipes with composite wick structures, and an example is given. Nomenclature A ( = cross-sectional ara of liquid-flow channel b = width of grooves Dg = equivalent hydraulic diameter D p = particle diameter g = gravitational constant h^ = latent heat of evaporation ij = summation indices K p = wick permeability i = length £ m = effective transport length l t = total heat pipe length £ w = width of wick, or nominal inside circumference of heat pipe M = number of wick sections N = number of wick layers 'N g = liquid parameter [ Eq. (11) ] N w = wick parameter (Fig. 3) AP G = gravitational pressure force APg = pressure drop in liquid phase AP 5 = capillary pumping pressure Q = heat-transfer rate r c = equivalent minimum meniscus radius t w = wick thickness a p = groove pitch ratio, pitch/£ a w = groove depth ratix), t w jb e = porosity Pi = liquid viscosity pg -liquid density or = surf ace tension 0 = angle of inclinatsion .6 = contact angle \l/ = gravity factor [ Eq. (9) ] Subscripts a = adiabatic section c = condenser end e = evaporator end ij = indices max = maximum value