Cryogenic Convection Heat Transfer.pdf

文档介绍：Cryogenic Convection Heat Transfer
„ Involves process of heat transfer between q
solid material and adjacent cryogenic fluid
f = TT mean
„ Classic heat transfer problem (Newton’s law)
2 m& f
q(kW/m ) = h (Ts –Tf)
„ Configurations of interest

„ = TT
Internal forced flow (single phase, f mean ) q
„ Free convection (single phase, f = TT ∞)
„ Internal two phase flow
„ Pool boiling (two phase)
„ Understanding is primarily empirical leading to
correlations based on dimensionless numbers
„ Issue is relevant to the design of: Liquid
T
„ Heat exchangers f
Ts
„ Cryogenic fluid storage
„ Superconducting s
„ Low temperature instrumentation
USPAS Short Course Boston, MA 6/14 to 6/18/2010 1
Single phase internal flow heat transfer
Forced
T D ,Tm f
Convection s &
Q
Classical fluid correlations
„ The heat transfer coefficient in a classical fluid system is generally
correlated in the form where the Nusselt number,
hD
where, NuD ≡ and D is the characteristic length
k f
„ For laminar flow, NuD = constant ~ 4 (depending on .)
„ For turbulent flow (Re > 2000)
D μ C
mn Pr ≡ pf
D = fNu D = C D PrRePr),(Re and (Prandtl number)
k f
„ Dittus-Boelter Correlation for classical fluids (+/- 15%)
4 2
5 5
NuD =
Note that fluid properties should be
+TT fs
computed at T (the “film temperature”): T ≡
f f 2
USPAS Short Course Boston, MA 6/14 to 6/18/2010 2
Johannes Correlation (1972)
„ Improved correlation
specifically for helium
(+/- %)
−
4 2 ⎛ T ⎞
Nu = 5 5 ⎜ s ⎟
D D ⎜⎟
⎝ Tf ⎠
„ Last factor takes care of
temperature dependent
properties
„ Note that one often does
not know Tf, so iteration
may be necessary.
Example
USPAS Short Course Boston, MA 6/14 to 6/18/2010 3
Application: Cryogenic heat exchangers
„ Common types of heat exchangers used
in cryogenic systems P1 , T1
P , T
„ Forced flow single phase fluid-fluid 2 2
„