KURENAI : Kyoto University Research Information Repository

*0.4.2
hNux Pro,i7(u)0.1
_a _a co
hst - Nust-3.58 ----------------------------------------------------------------
1 - exp(-46.0{x pr°•42(u )o.zs}i.z)]o.33(68)
IL. 8 TURBULENT FLOW FOR Pr = 1
When Prandtl number is not so small, thermal boundary layer penetrates
into turbulent boundary layer of velocity field. Under this circumstance,
heat transfer process is dominated by turbulent heat diffusivity. For
laminar flow, heat transfer process is characterized by thermal conductivity
A, which is unchanged for both transient and steady state. On the other
hand, for turbulent flow, one faces the problem whether the turbulent diffu-
sivity of heat,a., for steady state can be applied to transient case. The
turbulent diffusivity of heat for transient state may not be same as that
for steady state, when the time scale of interest for transient heat transfer
is of same order as time scale for turbulence, Q,'/u', where Q,' is a scale
of eddy and u' is turbulent velocity. However, this time scale for turbu-
lence.is considered to be very small. Therefore for practical purpose,
it can be postulated that the steady state turbulent diffusivity of heat
can be applicable in analysing transient heat transfer without serious errors.
In fact, for step rise of wall temperature or wall heat flux, the transient
heat transfer coefficients calculated based on above assumption agree well
with experimental data [10-12,21-26]. In present analysis, above assump-
tion has been adopted.
As generally accepted for turbulent boundary layer analyses, the velo-
city and temperature distributions can be characterized by 1/7 th power law
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