Thermophysical Properties of Biporous Heat Pipe Evaporators

Abstract

Thirty biporous slugs with 3 different cluster diameters and 5 different particle diameters (15 combinations with 2 repetitions) and 12 monoporous slugs with 6 different particle diameters were sintered from spherical copper powder, and thermophysical properties were measured. The neck size ratio for all the particles was approximately 0.4. The porosity of monoporous samples was found to be independent of particle diameter and was equal to 0.28, and the porosity of biporous samples was found to be independent of cluster and particle diameters, and was equal to 0.64. The liquid permeability and maximum capillary pressure of small pores were found to be a linear function of the particle diameter. Similarly, vapor permeability was found to be a linear function of the cluster diameter. The thermal conductivity of monoporous samples was measured to be 142±3W∕mK at 42±2°C, and it was independent of particle diameter. The thermal conductivity of biporous samples was found to be a function of cluster to particle diameter ratio.

Recent Advances in Vapor Chamber Transport Characterization for High-Heat-Flux Applications

“…Atabaki and Baliga [124] reviewed several thermal conductivity models for two-phase mixtures and developed a modified empirical correlation based on experimental data for sintered metal powders [123,124]. Similar testing of biporous and monoporous sintered copper powders performed by Catton and coworkers [64,125] revealed that the effective medium theory model [126] discussed by Carson et al [127], which assumes a random dispersion of both material phases, provides an upper bound on effective thermal conductivity. Li and Peterson [128] reviewed models for predicting the effective conductivity of layers of wire screen, and performed experiments using sintered screen to validate a proposed analytical model as a function of the mesh number and wire diameter.…”

“…In the bubble-point method [136], the capillary pressure is determined based on the minimum pressure required to penetrate gas through the sample; a bubble will first pass through the largest pore, providing a conservative estimate. Conversely, the maximum capillary pressure can be determined by measuring the pressure head sustained by a plug of wick material holding a liquid column against gravity [64,125,137]. An alternative method is to measure the transient rate of rise of liquid in a sample to predict the capillary pressure and permeability based on Washburn's equation [138], as first described with respect to heat pipe wick materials by Adkins and Dykhuizen [132].…”

“…A detailed description of the geometric and thermophysical properties of biporous wicks tested was presented in [64]. In [36], critical heat flux was defined as the heat flux at which the minimum thermal resistance was measured.…”

“…The cluster diameter, which governs the wick permeability to exiting vapor [64], was used to estimate the approximate pore size for the biporous wick structures. The predicted heat flux for a given superheat is evaluated and compared against 428 experimental data points, as shown in Figure 16a.…”

Permeability of mono- and bi-dispersed porous media

“…In Table 3, analytically (equation [6][7][8] and experimentally obtained distribution parameters are compared. As shown in this table, they match each other well.…”