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ABSTRACT
Silica aerogel is an excellent thermal insulator for high-speed aircraft, but there is little research on it in a high-temperature and complex-pressure environment. This research aims to evaluate the thermal insulation performance of silica aerogel monoliths with different porosities under large temperature differences and pressure gradients. We established an experimental system to measure and analyze the hot surface temperature response by fixing the heat flux and the cold surface temperature at transient pressure conditions. An unsteady-state heat transfer model considering gas flow is developed. The effective thermal conductivity of silica aerogels with 79.55 ~ 90.91% porosity is measured at different temperature differences between cold and hot surfaces (127 ~ 512 K), near-vacuum (<10 Pa), and transient pressure conditions. The results demonstrated that silica aerogel with 90.91% porosity showed the best thermal insulation performance when the temperature differences were over 500 K, while the aerogel with 79.55% porosity became the best when the temperature differences were less than 500 K. In addition, both the temperature and pressure difference affect the thermal insulation performance: the energy transport caused by gas flow affects the dynamic temperature response when gas permeability is of the order of 10−15 m2; the thermal insulation performance is improved by increasing gas permeability and pressure difference when gas flow and heat transfer directions are opposite.
Nomenclature
| aA | = | thermal diffusivity, m2·s−1 cross-section area, m2 |
| c | = | specific heat, J·kg−1·K−1 |
| cp | = | specific heat capacity at a constant pressure, J·kg-1·K-1 |
| cv | = | specific heat capacity at a constant volume, J·kg-1·K-1 |
| Cdp | = | gas coefficient, m nanopore diameter, m |
| = | mean pore diameter of the nanopore, m | |
| Df,Dt | = | area fractal dimension |
| f | = | volume fraction |
| h | = | convective heat transfer coefficient, W∙m−2∙K−1 |
| I | = | electric current, A |
| Ja | = | ideal gas volume flow, m3∙s−1 |
| Jv | = | gas volume flow, m3∙s−1 |
| Kg | = | gas permeability, m2 |
| k | = | 1st derivative of θ (0, t) to |
| lg | = | mean free path of air molecule, m |
| L0 | = | representative length of the nanopore, m |
| Lt | = | actual length of the nanopore, m |
| m | = | mass of molecule, kg |
| MmnN | = | molar mass of gas molecule, kg mean refractive index total number of the nanopores |
| PPe | = | gas pressure, Pa heating power, W |
| = | average pressure of gas, Pa heat flux, W·m−2 total gas flow flux, m3·s−1 gas constant temperature, K | |
| uU | = | velocity, m·s−1 voltage, V |
| t | = | time, s |
| X/Y/Z | = | coordinate direction |
| Greek symbols | = | |
| βγθδ | = | mean Rosseland extinction coefficient γ=cp/cv excess temperature, K thickness of specimen, m |
| ΔPΔT | = | pressure difference, Pa temperature difference, K |
| εҡΒɅρσ | = | the emissivity of the material Boltzmann constant, J·K−1 phonon mean free path, m density, kg·m−3 Stefan-Boltzmann constant, W·m−2·K−4 |
| λνμ | = | thermal conductivity, W·m−1·K−1 longitudinal sound velocity, m·s−1 dynamic viscosity, N∙s∙m−2 |
| φ | = | porosity |
| Subscripts and superscripts | = | |
| abulkc | = | aerogel bulk porous medium of silica aerogel conduction, cold surface |
| cold, bottom | = | cold bottom surface |
| cold, top | = | cold top surface |
| efgg,c | = | effective glass-fiber gas gas-contributed heat conduction and the solid-gas coupling heat transfer |
| h | = | hot surface |
| ppmaxpminp-pr | = | aerogel particle maximum pore diameter minimum pore diameter nanoparticle of contact aerogel radiation |
| refs | = | reference skeleton |
| v0 | = | additive initial |
Disclosure statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.