-
7/30/2019 (34)-radiation-fundamentals-3.pdf
1/17
FUNDAMENTALS (3)
Associate Professor
IIT Delhi
E-mail: [email protected]
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
2/17
Absorption, Reflection and
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
3/17
Absorption, Reflection and
= ,re ,a s , r
=, , r
There is no net effect of the reflection,
has the effect of increasing the internal
Surface absorption and reflection are
MECH/IITD
respons e or our percep on o co or
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
4/17
Absorptivit The absorptivity is a property that determines
the
fraction of the irradiation absorbed by a surface Surface
exhibits selective absorption with
respect to the wavelength and direction of the
incident radiation oes no epen muc on sur ace empera ure
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
5/17
Absorptivit),,(I abs,i, =
Spectral directional absorptivity:
),,(I
,,
i,
,
Spectral hemispherical absorptivity:
)(G
)(abs,
=
Total hemis herical absor tivit
MECH/IITD G
Gabs
=
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
6/17
Reflectivit
incident radiation reflected by a surfaceGref
-
G=
In addition to depending on the direction of the
incident radiation it also de ends on the
direction of the reflected radiation
Surface ma be idealized as diffuse or s ecular
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
7/17
S ectral Directional S ectral Hemis herical
Reflectivity Reflectivity
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
8/17
(a)
(b)
Different types of reflection from
a surface: (a) actual or irregular,
,or mirrorlike(c)
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
9/17
Transmissivit Spectral hemispherical transmissivity)(G tr,
G tr
)(G
G
transmissivity as
MECH/IITD For a semitransparent medium, ++ = 1
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
10/17
The s ectral directional absor tivit and s ectral
directionalreflectivity of a surface are defined, respectively, as
the
absorbed and reflected fractions of the intensity of
radiation
The s ectral hemis herical absor tivit and s ectral
hemispherical reflectivity of a surface are defined as
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
11/17
Kirchoffs law
surface temperature Ts within which
Ts
G
1
A2E1
MECH/IITD
3
E3
2
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
12/17
Kirchoffs Law (contd) ,
surface forms a blackbody cavity Accordin l re ardless of its
orientation the
irradiation experienced by any body in the cavity
is diffuse and equal to emission from ablackbody at Ts i.e. G =
Eb(Ts)
Under steady-state conditions,
T1 = T2 = T3 =---- = Ts
and net energy transfer to each surface is zero
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
13/17
Kirchoffs Law (contd)
about body 1, GA E T A = 0
E1(Ts)/ 1= G = Eb(Ts)
,
E1(Ts)/ 1 = E2(Ts)/ 2 = ------- = Eb(Ts)
===
1
2
2
1
1
MECH/IITD =
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
14/17
The Gra Surface
TE
)T,(E)T,(
Accepting the fact that the spectral
=
2 2/
2
0
2/
0e,
,
ddsincos)T,,,(I
directional emissivity and absorptivity
are always equal.
2 2/
,
0 0
b,
ddsincos)T,,,(
ddsincos)T,(I
= 2
0
2/
0
ddsincos
The irradiation is diffuse: I is independent of,
MECH/IITD
,
The surface is diffuse: , and , is independent of,
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
15/17
1. The irradiation corresponds to emission from a blackbodyat
the surface temperature T, in which case
G = E,b (,T) and G = Eb(T)
2. The surface is gray ( and are independent of)
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
16/17
Spectral distribution of Spectral distribution of
the spectral absorptivity
of a surface
the spectral irrradiation
at the surface
MECH/IITD
-
7/30/2019 (34)-radiation-fundamentals-3.pdf
17/17
Gra Surface
MECH/IITD
