Thermal Insulation Provided by Vertical,Annular,Air-Filled Cavities

Abstract

Steady-state rates of heat transfer outwards across vertical air-filled annular cavities, with aspect ratios (i.e. height divided by annular gap) which ranged from 16·0 to 70·3, were measured for several pressures of the contained air between 10^-3and 760 torr. The chosen temperature difference between the isothermal cylindrical walls ranged from 20 to 180 K. For constant values of the temperature difference the gaseous conductive, convective and radiative components of the heat leaks across the air gap to the outer cylindrical wall were distinguished and evaluated using a technique which employed the air pressure as the variable. Convection of the air ensued throughout the whole height of the cavity when the Rayleigh number attained a value of 1·6 × 10⁴. The optimal separation for maximum thermal resistance greatly exceeded the 6 mm annular gap recommended for chimneys in British Standard 4076:1966 (1)‡.