What is the mode of heat transfer in vacuum? Define absorptivity, reflectivity and transmissivity.

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written 8.4 years ago by

teamques10 ★ 68k

Radiation is the mode of heat transfer in vacuum.
When incident radiation (G) also called irradiation (defined as the total incident radiation on a surface from all direction per unit time and per unit area of surface; expressed in W/m2 and denoted by G) impinges on a surface, three things happen; a part is reflected back (Gr), a part is transmitted through (Gt), and the remainder is absorbed (Ga), depending upon the characteristics of the body, as shown in figure.
By the conservation of energy principle,
Ga + Gr + Gt = G

Dividing both sides by G, we get

GaG+ GrG+ GtG= GG

α+ρ+τ=1

where,

α = absorptivity (or fraction of incident radiation absorbed),

ρ = reflectivity (or fraction of incident radiation reflected),

τ = transmissivity (or fraction of incident radiation transmitted).
When the incident radiation is absorbed it is converted into internal energy.

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