Thermal Properties and Applications of High-Temperature Aircraft Insulation

Abstract

Fundamental principles of heat transfer in fibrous insulations are discussed. Thermal conductivity of a h ightemperature insulat ing felt is plotted against m e a n t e m perature at various densit ies , and the conductivity data are analyzed to determine the contribution by each of the mechanisms of heat transfer to the total conductivity. Radiation and air conduct ion account for nearly all the conductivity at h igh temperatures . Conductivity due to radiation varies approximately as the cube of the absolute temperature, linearly wi th the effective fiber diameter, and inversely as the density of the insulat ion. Conductivity due to air conduct ion depends only o n temperature and cannot be controlled by varying the physical properties of the insulat ion. Thermal conductance of a given in sulating felt may be reduced by increasing either thickness or density, the former being more efficient from the standpoint of m i n i m u m insulat ion weight . An internal reflective foil does no t appreciably reduce the conductance of an insulat ing felt. Applications of h igh-temperature insulat ion to protect aircraft structures are i l lustrated in specific heat-transfer problems. Criteria are discussed for evaluating the performance of a je t -engine insulat ing blanket . Computations are carried o u t tha t show the relationship between engine and fuselage temperatures , the conductance of the felt, the emissivity of the enclosing foil, and the air flow rate between the blanket and the fuselage, for fixed a m bient condit ions. Problems tha t i l lustrate thermal funct ion of insulat ion under transient heat-flow condit ions are described and solved with m i n i m u m mathemat ica l detail .