Latent heat fluxes through nano-engineered porous materials

Abstract

Micro- and nano-scale truss architectures provide mechanical strength, light weight, and breatheability in polymer barriers. Liquid evaporation and transport of resulting vapor through truss voids (pores) cools surfaces covered by the barriers, suggesting the possibility for simultaneous protection of humans from mechanical and thermal stresses. Design of real systems employing this technique requires quantitative understanding of vapor transport within the truss pores and accompanying latent heat lift under ambient temperature and pressure. One application is desert Soldier protection. Need exists to clarify whether smaller pore diameters affect surface cooling and water vapor transport owing to fluid rarefaction or surface interactions. Contrasting previous studies where pressure within capillaries of fixed diameter was modulated, in this thesis Knudsen Number (Kn) was systematically varied by changing pore diameter at constant pressure (one atmosphere). Cooling efficacy was assessed for porous membranes with pore diameters ranging from 39 to 14,400 nm, varied in regular increments. Evaporative cooling experiments simulated combined daytime desert solar and metabolic thermal load on humans by heating an evaporation chamber partially filled with liquid water and capped with a porous membrane.

(cont.) Hot, dry gas was swept over the membrane, simulating desert ambient conditions. By continuously weighing the entire evaporation apparatus, intrinsic pore diffusion coefficients for dilute water vapor in air were deduced for each membrane by correcting for upstream and downstream boundary layer mass transfer resistances. Pore diameter impact on evaporative cooling of an underlying surface by water vapor transport across two types of porous polymer membranes with micro/nano-scale truss architecture was quantified. This research showed that transition diffusion regime theory predicted observed transport rates to better than + 35% for pore diameters between 14,400 nm and 60 nm (0.01 < Kn < 3). Despite low membrane porosity, substantial Fractional Accomplished Cooling (up to 60% maximum achievable) was demonstrated via latent heat transport. The absolute magnitude of achieved surface cooling was 3.7 K to 14.0 K. An engineering design correlation was developed linking latent heat transport at various Knudsen Numbers (pore diameters) to evaporative cooling efficacy. Results of this research inform design of porous mechanical barriers that permit evaporative cooling of underlying surfaces.