Detailed model for transient liquid flow in heat pipe wicks

Abstract

Transient liquid flow in a low-temperature, homogeneous wick heat pipe is investigated experimentally and analytically for pulsed heat load conditions. The continuous distribution of the liquid in the wick is modeled, where previous models had assumed a uniformly saturated structure. A heat pipe with beryllium walls was used to obtain transient measurements of the saturation distribution in the wick structure using x-ray radiography. Analytical and experimental results are presented for the axial liquid distribution in the wick as a function of time. Transient liquid distributions for the model and experiment compare favorably. These results show that significant reductions in saturation may occur in the evaporator region for higher heat loads. These reductions affect the wick-flow properties and must be included in the liquid-flow analysis. Nomenclature A -area, m 2 /(5) -moisture diffusivity, kg-m/s, Eq. (6) / = source term, s~l, Eq. (5) K = full permeability, m 2 K r -relative permeability L = length, m m = mass of liquid in pores, kg m -mass flow rate, kg/s P = pressure, Pa P c = capillary pressure, Pa Q = heat transfer rate, w S = saturation (ratio of liquid volume to void volume) 5, = irreducible saturation T = temperature, °C t -time, s x -position coordinate, m e = wick porosity A = latent heat of vaporization, J/kg IJL = dynamic viscosity of liquid, kg/m -s p = density of liquid, kg/m 3 a = surface tension, N/m </> = reduced saturation (<£ = S -5/1 -S,)

A quasi-3D analysis of the thermal performance of a flat heat pipe

“…In their analysis, the convection heat transfer in the wick was neglected, assuming pure conduction in the wick. A detailed transient liquid flow analysis of a homogeneous heat pipe wick operating at low temperature was performed experimentally and analytically by Ambrose et al [11]. They observed significant reductions in the wick's saturation at high heat loads, thus affecting the wick-flow properties.…”

“…Investigations on the transient operation of heat pipes have been reported for various external operating conditions [10][11][12][13][14][15][16]. Chang et al [10] presented a finite difference analysis for predicting the transient operating characteristics of low temperature heat pipes.…”

Dynamic test method for determining the thermal performances of heat pipes

“…Suman et al [15] took quite many effects into account and established a 1-D model to describe the transient thermal behavior of a micro-grooved heat pipe with respect to several parameters. Besides, many studies of the transient operation of heat pipes have been reported [16][17][18][19][20]. In these works, transient behaviors of heat pipes are addressed experimentally, mathematically, or numerically.…”

Heat and Mass Transport in Heat Pipe Wick Structures

“…10,11 Liquid flow studies in porous media have included anisotropic permeability, 12 buoyancy induced flows, 13 and dependence of flow properties on saturation. 14 Visual observations of liquid flow including images of vapor formation have also been obtained. 15,16 Porosity, which is the ratio of pore volume to total volume, is another parameter that affects liquid flow in porous media and can be determined from a variety of methods, with varying levels of accuracy.…”