Capillary flow properties of mesh wicks

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,)

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“…(6) based on the measured capillary flow properties. 9 A curve fit of the falling capillary pressure data shown in Fig. 2 gives dPJdS.…”

Section: Theorymentioning

confidence: 99%

“…Details of the saturation measurement using x-ray radiography are given in Ref. 9. Uncertainty of this measurement varies with the value of 5, but is about 20% for S…”

Section: Beryllium-wall Heat Pipe Testsmentioning

confidence: 99%

Detailed model for transient liquid flow in heat pipe wicks

Ambrose

Chow

Beam

1991

Journal of Thermophysics and Heat Transfer

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“…To measure in-plane intrinsic permeability of meshes, researchers have developed methods considering stacks of meshes submerged in a liquid to understand how different characteristics of the mesh, such as the number of layers, wire diameter and spacing, crimping factor, mesh thickness, and stacking density, can influence single-phase flow through the mesh. [23][24][25][26][27][28][29][30] The results of these studies have contributed to the development of models, such as the modified Blake-Kozeny equation, which can predict the intrinsic permeability of metal meshes for certain configurations. However, both the measurement methods and mathematical models may not be accurate for very thin stacks of mesh and do not address meshes with free surfaces.…”

Section: Introductionmentioning

confidence: 99%

Permeability of Single‐Layer‐Free‐Standing Meshes at Varying Capillary Pressure via a Novel Method

Shattique

Giglio

Dede³

et al. 2023

Adv Materials Inter

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The permeability of mesh wicks is important for various applications, including two‐phase heat transfer. However, the understanding of the permeability of single‐layer, free‐standing mesh wicks, with liquid–gas interfaces on both sides, is limited. A novel and simpler method is presented to determine the permeability of a free‐standing wick and apply it to a representative mesh. This method involves modifying the capillary pressure via elevation and simultaneously measuring the permeability to determine the permeability‐capillary pressure relationship. When applied to a copper mesh with plain weave having undergone surface cleaning, the permeability is found to decrease as capillary pressure for deionized water increases. A dimensional analysis is presented to generalize this data for other mesh sizes with similar weaves and fluids. The behavior of mesh in application is modeled, based on the integration of Darcy's law with an analytic function fit to measured data, and parametric studies are conducted to investigate the superficial velocity of liquids through the mesh under varying driving pressures, transport lengths, and liquid viscosity, based on the obtained capillary pressure–permeability relationship. This study provides valuable insights into the transport properties of mesh wicks, with potential applications in fields such as electronics cooling, electrochemical devices, and fluid purification technologies.

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“…Generally, the liquid used for this measurement completely wets the solid surface (contact angle, θ ∼ 0°). The R c of the medium is then calculated from the capillary pressure using the Young–Laplace equation: The most widely used method to characterize the capillary pressure of porous media is the rate of rise (fall) method, − where the rise (fall) of liquid in a porous medium is recorded with time. The rate of liquid progression is fit to theoretical models based on solutions to the continuity and momentum equations.…”

Section: Introductionmentioning

confidence: 99%

Measurement of Capillary Radius and Contact Angle within Porous Media

2015

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The pore radius (i.e., capillary radius) and contact angle determine the capillary pressure generated in a porous medium. The most common method to determine these two parameters is through measurement of the capillary pressure generated by a reference liquid (i.e., a liquid with near-zero contact angle) and a test liquid. The rate of rise technique, commonly used to determine the capillary pressure, results in significant uncertainties. In this study, we utilize a recently developed technique for independently measuring the capillary pressure and permeability to determine the equivalent minimum capillary radii and contact angle of water within micropillar wick structures. In this method, the experimentally measured dryout threshold of a wick structure at different wicking lengths is fit to Darcy's law to extract the maximum capillary pressure generated by the test liquid. The equivalent minimum capillary radii of different wick geometries are determined by measuring the maximum capillary pressures generated using n-hexane as the working fluid. It is found that the equivalent minimum capillary radius is dependent on the diameter of pillars and the spacing between pillars. The equivalent capillary radii of micropillar wicks determined using the new method are found to be up to 7 times greater than the current geometry-based first-order estimates. The contact angle subtended by water at the walls of the micropillars is determined by measuring the capillary pressure generated by water within the arrays and the measured capillary radii for the different geometries. This mean contact angle of water is determined to be 54.7°.

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Capillary flow properties of mesh wicks

Cited by 8 publications

References 6 publications

Detailed model for transient liquid flow in heat pipe wicks

Detailed model for transient liquid flow in heat pipe wicks

Permeability of Single‐Layer‐Free‐Standing Meshes at Varying Capillary Pressure via a Novel Method

Measurement of Capillary Radius and Contact Angle within Porous Media

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