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relative THC in case of CT, can be considered an equivalent to its value
for the conventional finned channels.
According to [6] the finning efficiency coefficient
η
f
accounts for both
heat realeasing surface development and fins thermal efficiency:
η
p
= 1
−
1
t
+
2
h
p
t
th
h
p
√
2
Bi
h
p
√
2
Bi
ξ,
where Bi
=
αδ
f
/λ
f
is the Biot number;
ξ
is a possible heat release
correction factor for the channel external wall, assumed to equal unity in
real cases.
In the CC case, thermal and geometric parameters will be considered
separately while calculating
η
f
. The heat effect is mostly generated by fins
with the height
h
1
located on the heat releasing surface, while the so-called
forming fins (of
h
2
height) on the opposite wall can operate, i.e. receive and
release heat, only via the contact spots with
h
1
fins. Taking into account
the real ranges of the fin pitch
t
= 2
.
5
. . .
7
and the height
h
1
= 3
. . .
10
,
the area and the heat transfer of these contact spots can be neglected,
especially with regard to the possible contact resistance. Thus, only the
height
h
1
of the fins on the heat releasing surface should be considered
while calculating
η
f
.
Apart from the fins side surface
h
1
the heat release can be produced by
non-contacting end surfaces, which can be accounted for with an additional
coefficient
ξ
end
. Then the formula for calculating a finning coefficient will
take the following form
η
p
= 1
−
1
t
+
2
h
1
t
th
h
1
√
2
Bi
h
1
√
2
Bi
ξ
end
,
where
t
=
t/δ
f
1
,
h
1
=
h
1
/δ
f
1
are the relative values of finning parameters,
i.e. the pitch and height (
t, h
1
),
δ
f
1
is the fins thickness on the heat releasing
surface.
A simple analysis can reveal that the contribution factor for the fins
end surfaces can be expressed via the relative fraction of the free surface
S
f
and the relative height
h
1
using the following dependence
ξ
end
= 1 + 0
.
5
S
p
h
1
,
where
S
p
= (1
−
1
/ t
)
is the relative fraction of the free side surface on
the heat releasing wall, non-contacting with the coupled fins.
ISSN 0236-3941. HERALD of the BMSTU. Series Mechanical Engineering. 2015. No. 2 49