Definition Thermal insulation

car exhausts require form of heat barrier, high performance exhausts ceramic coating applied

solid materials chosen insulation have low thermal conductivity k, measured in watt-per-meter per kelvin (w·m·k). thickness of insulation increased, thermal resistance increases.

for insulated cylinders, critical radius must reached. before critical radius reached added insulation increases heat transfer. convective thermal resistance inversely proportional surface area , therefore radius of cylinder, while thermal resistance of cylindrical shell (the insulation layer) depends on ratio between outside , inside radius, not on radius itself. if outside radius of cylinder increased applying insulation, fixed amount of conductive resistance (equal 2*pi*k*l(tin-tout)/ln(rout/rin)) added. however, @ same time, convective resistance reduced. implies adding insulation below critical radius increases heat transfer. insulated cylinders, critical radius given equation

r

c
r
i
t
i
c
a
l



=


k
h




{\displaystyle {r_{critical}}={k \over h}}

this equation shows critical radius depends on heat transfer coefficient , thermal conductivity of insulation. if radius of insulated cylinder smaller critical radius insulation, addition of amount of insulation increase heat transfer.