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Fig. 8. Effect of the fins crossing
angle on the efficiency criterion
K
Q
at various Biot numbers (
Re
= 10
4
;
ˉ
h
1
= 4 ˉ
h
ˉ
h
;
ˉ
t
p
= 4 ˉ
t
ˉ
t
):
curves
1
— Bi = 0.4; curves
2
—
Bi = 0.04
Fig. 9. Dependency of the efficiency criterion
K
Q
on the finning pitch at various
angles and Biot numbers (
Re
= 10
4
;
ˉ
h
1
= 4 ˉ
h
ˉ
h
):
curves
1
(Bi = 0.4) —
•
—
β
= 20
◦
; —
β
= 30
◦
;
×
—
β
= 45
◦
; curves
2
(Bi = 0.04) —
∗
—
β
= 20
◦
; —
β
= 30
◦
;
N
—
β
= 45
◦
)
3–4-fold; when Bi decreases from 0.4 to 0.04,
K
Q
increases 1.7–2-fold
(Fig. 8). At the same time, when
β
is less than 20. . . 30
◦
the
K
Q
criterion
demonstrates tenuous and inefficient values, even when the Biot number
has favourable values.
When the fins are positioned with small intervals, the efficiency index
K
Q
can reach values from 3 to 10 units. However, as the finning pitch
t
increases, the efficiency index
K
Q
decreases, which is particularly
noticeable at low (favourable) Bi values (Fig. 9).
Both quantitative and qualitative values of
β
angles and Biot numbers
make a significant contribution to the dependency of the efficiency
K
Q
on
the heat releasing fins height
h
1
. For instance, in case of the favourable
(optimistic) value Bi
= 0
.
04
and the fins height
h
1
growing from 1 to 4,
the criterion
K
Q
increases from 1 to 4 and nearly stabilizes afterwards.
In case of the pessimistic values Bi
= 0
.
4
, the efficiency rate does not
exceed 1.5. . . 5.5 units, noteceably decreases as
h
1
grows along the total
range of values, with low sensitivity to the finning pitch ratio (Fig. 10).
This estimation can be used to select finning parameters when designing
cooling channels.
52
ISSN 0236-3941. HERALD of the BMSTU. Series Mechanical Engineering. 2015. No. 2