
13-11Thermal Sensors
Table 13.4 shows a collection of formulae for calculating the Nusselt number. e uid properties (kine-
matic
vi
scosity
ν an
d
Pr
andtl
nu
mber
Pr
)
sh
ould
be ch
osen
at th
e
av
erage
te
mperature
T
av
between the
he
ater
t
emperature
T + ∆T a
nd
t
he
uid
t
emperature
T:
T T
T
av
= +
2
(13.18)
In
th
e
ca
se
of na
tural
co
nvection,
th
e
Nu
sselt
nu
mber
de
pends
on th
e
Gr
ashof
nu
mber
Gr
,
wh
ich
de
scribes
t
he
i
nuence
o
f
b
uoyancy
f
orces:
Gr =
∆g L T
β
3
2
v
(13.19)
where
g
i
s
t
he
a
cceleration
d
ue
to t
he
g
ravity
(
9
.81 m s
−2
)
β
t
he
t
hermal
e
xpansion
c
oecient
o
f
t
he
uid
∆T
i
s
t
he
t
emperature
d
ierence
b
etween
t
he
h
ot
uid
a
nd
t
he
a
mbient
e
a
verage
N
usselt