Latent Heat Fluxes Through Nano-engineered Porous Barriers for Evaporative Cooling and Human Protection

Latent and sensible heat carried through porous barrier materials by vapor transmission can make an appreciable contribution to the total transported energy, augmenting purely conductive heat transfer. Using a latent heat carrier concentration gradient, thermal energy can even be moved without, or opposed to, a temperature gradient, which provides the opportunity to neutralize incoming heat and/or cool a heat source. This process is relevant to soldiers for maintaining normal body temperature in a hot desert environment despite metabolic heat generation and exterior solar radiation.

“Nano-trusses” are polymeric materials whose ordered networks of micro- to nano-scale rods, struts, cells, and channels provide light weight, high porosity, and extraordinary absorption of mechanical energy. These porous soft materials show promise for integrating blast and ballistic protection with evaporative cooling. Design of practical integrated systems requires knowledge of tradeoffs in the strength, porosity, weight, and cooling efficacy of the nano-structure, which demands quantitative understanding of how pore size, shape, order, distribution, and orientation impact evaporative cooling of surfaces overlaid by the nano-trusses.

An apparatus was developed for experimentally determining the evaporative surface cooling efficacy of diverse porous membranes. Apparatus steady state heat balances were closed to within +12.4% to +3.3% at 1400 +/- 70 W/m^2, which mimics the daytime solar and metabolic heat load on desert soldiers. Initial studies on porous membranes of similar thicknesses (~120μm) gave appreciable cooling i.e., 13.4-14.0°C, versus 35.2°C maximum (no membrane), despite their low total porosity (7.5-11.2%) and tiny pore sizes (as low as 1.0 µm). Therefore, these membranes can provide substantial solid material to incorporate other nano-engineered capabilities, e.g., high mechanical strength. The experimental technique also quantifies apparatus-independent porous membrane mass transfer coefficients.

Further investigation of the heat and mass transfer performance of membranes with deep-nano pore features is ongoing using a series of Millipore polycarbonate membranes. These membranes contain straight-through pores with constant pore diameters from 50 nm to 12,000 nm while all other physical features (i.e. thickness, porosity, material of composition) are fixed from sample to sample. Ongoing work to improve the experimental apparatus and increase the sophistication of the associated heat- and mass-transfer model is expected to improve understanding of the underlying transport mechanisms.