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previous cases
St =)76{ 1 - exp(-t*)}
From Eqs.(61) • and (62), the approximate solution of Eq.(59) will
m for
for -ln[1-{1-exp(-46.0{x*Pr°•42(u .25}1.2)}0.67] < t*
St = [ 1 - exp(-46.0{x*Pr°•4202(4 )°•25}1.2)]0.33
-ln[1-{1-exp(-46.0{x Pr°.42(u )0.25}1.2)}0.67] > t*
St ={ 1 - exp(-t*)}k
Then, • the transient heat transfer coefficient is given by
for -ln[1-{1-exp(=46.6{x*Pro.42(u )o.25}1.2)}0•67] < t*
*2
co
co
10 [ 1 - exp(-46..0{x Pr°.42(u)°.25}1.2)]0.33
for -ln[1-{1-exp(-46.0{x )0.25}1.2)}0.67j>t*
co
(62)
be given
.(63)
(64)
(65)
h= --------------------------2 (66)
{ 1 - exp(-t*)}
The transient heat transfer coefficient calculated by Eqs.(65) and (66) is
shown in Fig.9 in h vs. x~(Vw/u2)°•25Pr°•42 plot. The steady state heat
transfer coefficient can be obtained by putting w }0 in Eq.(65)
Nu st= 0.228 Rex0.6pr°•33(66)
The ratio between asymptotic value of transient heat transfer coeffi-
cient and steady state one is derived from Eqs.(65) and (67)
128
•
by