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Research Papers: Micro/Nanoscale Heat Transfer

# Latent Heat Fluxes Through Soft Materials With Microtruss Architectures

[+] Author and Article Information
Matthew J. Traum

Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

Peter Griffith

Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

Edwin L. Thomas

Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

William A. Peters1

Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139peters@mit.edu

1

Corresponding author.

J. Heat Transfer 130(4), 042403 (Mar 17, 2008) (11 pages) doi:10.1115/1.2818760 History: Received July 31, 2006; Revised June 20, 2007; Published March 17, 2008

## Abstract

Microscale truss architectures provide high mechanical strength, light weight, and open porosity in polymer sheets. Liquid evaporation and transport of the resulting vapor through truss voids cool nearby surfaces. Thus, microtruss materials can simultaneously prevent mechanical and thermal damage. Assessment of promise requires quantitative understanding of vapor transport through microtruss pores for realistic heat loads and latent heat carriers. Pore size may complicate exegesis owing to vapor rarefaction or surface interactions. This paper quantifies the nonboiling evaporative cooling of a flat surface by water vapor transport through two different hydrophobic polymer membranes, $112–119μm$ (or $113–123μm$) thick, with microtruss-like architectures, i.e., straight-through pores of average diameter of $1.0–1.4μm$ (or $12.6–14.2μm$) and average overall porosity of 7.6% (or 9.9%). The surface, heated at $1350±20Wt∕m2$ to mimic human thermal load in a desert (daytime solar plus metabolic), was the bottom of a $3.1cm$ inside diameter, $24.9cm3$ cylindrical aluminum chamber capped by the membrane. Steady-state rates of water vapor transport through the membrane pores to ambient were measured by continuously weighing the evaporation chamber. The water vapor concentration at the membrane exit was maintained near zero by a cross flow of dry nitrogen $(velocity=2.8m∕s)$. Each truss material enabled $13–14°C$ evaporative cooling of the surface, roughly 40% of the maximum evaporative cooling attainable, i.e., with an uncapped chamber. Intrinsic pore diffusion coefficients for dilute water vapor $(<10.4mole%)$ in air ($P$ total $∼112,000Pa$) were deduced from the measured vapor fluxes by mathematically disaggregating the substantial mass transfer resistances of the boundary layers $(∼50%)$ and correcting for radial variations in upstream water vapor concentration. The diffusion coefficients for the $1.0–1.4μm$ pores (Knudsen number $∼0.1$) agree with literature for the water vapor-air mutual diffusion coefficient to within $±20%$, but for the nominally $12.6–14.2μm$ pores (Kn $∼0.01$), the diffusion coefficient values were smaller, possibly because considerable pore area resides in noncircular, i.e., narrow, wedge-shaped cross sections that impede diffusion owing to enhanced rarefaction. The present data, parameters, and mathematical models support the design and analysis of microtruss materials for thermal or simultaneous thermal-and-mechanical protection of microelectromechanical systems, nanoscale components, humans, and other macrosystems.

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## Figures

Figure 1

SEMs of microtruss simulant surfaces (top panels) and edges exposed by microtoming (bottom panels). Left hand side panels: Nucrel® ; right hand side panels: Hytrel® . Magnifications: top left panel, 4000×; top right panel, 500×.

Figure 3

Typical temperature-time histories (corrected for ambient temperature) for evaporative cooling of an aluminum surface using a closed chamber (negative control), an open chamber (positive control), or microtruss simulant materials. The absolute latent and fractional accomplished cooling (defined in the text) are also shown.

Figure 4

Cumulative mass of water vapor transported from the evaporation chamber as affected by time. The instantaneous flux of coolant vapor through the microtruss simulant pores is obtained from the first derivative of the curves shown.

Figure 5

Schematic cross section of the evaporation chamber to illustrate chirality of the thermal buoyancy driven flows and the radial concentration gradient across the upstream face of the microtruss

Figure 6

Comparison of experimental (light gray) rates of water vapor mass transport from the evaporation chamber for the four experiments with microtruss stimulant barriers, with rates predicted by increasingly refined mass transfer models (various fills)

Figure 2

Schematic (not to scale) of apparatus for quantitative study of evaporative cooling of surfaces by modulation of latent heat carrier flow using barrier materials with microtruss and nanotruss architectures. Dotted BLs represent an average location because turbulence agitates the fluid boundaries.

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